Brazing techniques for the fabrication of biocompatible …and vacuum brazing [15, 16]. A recent,...
Transcript of Brazing techniques for the fabrication of biocompatible …and vacuum brazing [15, 16]. A recent,...
Accepted Manuscript
Brazing techniques for the fabrication of biocompatible carbon-based electronicdevices
Nicholas V Apollo Desmond Lau Arman Ahnood Alastair Stacey KumaraveluGanesan Samantha G Lichter Kate Fox Javad Foroughi Hamish Meffin Gordon GWallace Ray Baughman Steven Prawer David J Garrett
PII S0008-6223(16)30413-4
DOI 101016jcarbon201605047
Reference CARBON 11011
To appear in Carbon
Received Date 12 February 2016
Revised Date 18 May 2016
Accepted Date 20 May 2016
Please cite this article as NV Apollo D Lau A Ahnood A Stacey K Ganesan SG Lichter KFox J Foroughi H Meffin GG Wallace R Baughman S Prawer DJ Garrett Brazing techniquesfor the fabrication of biocompatible carbon-based electronic devices Carbon (2016) doi 101016jcarbon201605047
This is a PDF file of an unedited manuscript that has been accepted for publication As a service toour customers we are providing this early version of the manuscript The manuscript will undergocopyediting typesetting and review of the resulting proof before it is published in its final form Pleasenote that during the production process errors may be discovered which could affect the content and alllegal disclaimers that apply to the journal pertain
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Brazing techniques for the fabrication of biocompatible carbon-based electronic devices Nicholas V Apollo1 Desmond Lau2 Arman Ahnood1 Alastair Stacey1 Kumaravelu Ganesan1 Samantha G Lichter1 Kate Fox3 Javad Foroughi4 Hamish Meffin5 Gordon G Wallace4 Ray Baughman6 Steven Prawer1 David J Garrett1 1 School of Physics The University of Melbourne Victoria 3010 Australia 2 School of Applied Sciences RMIT 3 Centre for Additive Manufacturing School of Engineering RMIT University Melbourne Australia 4 Intelligent Polymer Research Institute ARC Centre of Excellence for Electromaterials Science AIIM Facility Innovation Campus University of Wollongong Australia 5 National Vision Research Institute Australian College of Optometry Victoria 3053 and Department of Optometry and Vision Science University of Melbourne Victoria 3010 Australia 6 Alan G MacDiarmid NanoTech Institute University of Texas at Dallas Richardson TX 75083 USA Corresponding author David Garrett e-mail dgarrettunimelbeduau
Abstract
Prototype electronic devices have been critical to the discovery and demonstration of the
unique properties of new materials including composites based on carbon nanotubes (CNT)
and graphene However these devices are not typically constructed with durability or
biocompatibility in mind relying on conductive polymeric adhesives mechanical clamps or
crimps or solders for electrical connections In this paper two key metallization techniques
are presented that employ commercially-available brazing alloys to fabricate electronic
devices based on diamond and carbonaceous wires Investigation of the carbonmdashalloy
interfacial interactions was utilized to guide device fabrication The interplay of both chemical
(ldquoadhesiverdquo) and mechanical (ldquocohesiverdquo) forces at the interface of different forms of carbon
was exploited to fabricate either freestanding or substrate-fixed carbonaceous electronic
devices Elemental analysis in conjunction with scanning electron microscopy of the carbonmdash
alloy interface revealed the chemical nature of the Ag alloy bond and the mechanical nature of
the Au alloy bond Electrical characterization revealed the non-rectifying nature of the
carbonmdashAu alloy interconnects Finally electronic devices were fabricated including a Au
circuit structure embedded in a polycrystalline diamond substrate
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1 Introduction
Owing to their unique combination of biological mechanical electrochemical electrical
thermal and optical properties carbon-based materials such as graphene carbon nanotubes
(CNT) carbon fiber (CF) and diamond have attracted significant scientific and industrial
interest Exploiting CNT and graphene properties in micro- and macroscale structures is of
significant interest for engineering research and product development [1-3] For example
high strength and high conductivity fabrics based on these materials could enable a variety of
applications from lightweight body armor to space elevators [4 5] Several solutions to the
problem of scale have been demonstrated for both graphene and CNT films Spinning CVD-
grown CNT forests into a continuous yarn has enabled conductive high-strength structures
with micro-scale diameters and macro-scale lengths [3] Control of mechanical strength and
electricalthermal conductivity of CNT yarns is established by adjusting component CNT
lengths as well as the spinning angles of the fibers [6 7] Twisted andor coiled structures of
CNT yarns have been used to fabricate electrochemical as well as electrolyte-free CNT
torsional actuators (artificial muscles) with large-stroke and high work-capacity [8 9] Cell
culture dishes containing CNT yarns have shown that neurons grow and respond to electrical
stimulation delivered by the yarn [10] As bulk carriers of electricity and data CNT yarns
have been evaluated as replacements for standard Cu and Al wires due to their light weight
(alleviating stress on joint connections) ballistic conduction (lack of scattering reduces risk of
Joule heating) and capacity to handle high frequencies [11] Expanding upon the application
of long-distance electricity transfer CNT yarns have been used to build wireless data transfer
networks that were mechanically-resilient and displayed frequency-independent resistive
behavior [12] As a demonstration of their favorable electrochemical characteristics CNT
yarns have been used as an alternative to the standard CF electrochemical sensor for
neurotransmitters due to an intrinsic ability to resist surface fouling [13]
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All of the aforementioned research and much that was not mentioned was performed using
prototype devices containing silver epoxy gold paste solder or mechanical clamps to create
electrical connections to carbonaceous wires While these connections served their respective
experimental purposes their use in fabrication does not generally yield scalable or integrated
electronic devices nor are these connection methods resilient or biocompatible enough for
consideration as materials for biomedical devices There have been several methods
developed for bonding a single CNT to metals as well as joining a single CNT to another
CNT end-to-end [14] However making electrical connections to larger (micro rather than
nano) CNT composites and yarns is a different challenge Current approaches include
mechanical clamps Ag- and Au-based epoxy adhesives carbon solder ultrasonic welding
and vacuum brazing [15 16] A recent very exciting approach involves use of transition
metal soldering alloys to join carbon wires using standard solder conditions (eg 350 degC in
air) [17] While it is a technique that will certainly revolutionize the utility of carbonaceous
wires the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known
cytotoxicity of these metals [18 19] We have recently developed a technique to create a
hermetic diamond capsule using biocompatible gold alloys [20] Additionally we have
demonstrated a method for the construction of hermetic biocompatible feedthroughs for a
retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21] In the
present paper we extend our previous work and describe the construction of diamond-based
electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads
Finally we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn
CF and graphene fiber) with the option to incorporate them into diamond circuit substrates
The methods we describe constitute an important toolkit to allow for CNT-based and other
similar carbonaceous ldquosuper materialsrdquo to be incorporated into electronic devices and
integrated with traditional surface-mount electronics with specific focus on biocompatible
systems for implantation into the body
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11 Brazing to carbon
Active brazing a variation of vacuum brazing utilizes an alloy containing an active
component that reacts chemically with a relatively inert surface such as diamond or ceramics
[22 23] Transition metals are of particular interest due to electron vacancies in their d-
orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with
carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti
V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of
other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active
brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle
and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)
though the electrical and mechanical properties were not fully characterized [16] The
formation of interfacial TiC particles has also been shown to significantly increase the tensile
strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements
[27] In the present work commercially-available ABAs were investigated to determine their
suitability to make electronic connections to several carbonaceous materials including
polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based
carbon fiber (CF) to enable various types of electronic devices (Fig 1)
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Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes
In the first technique described here ABAs were used to create embedded circuit boards in
PCD wherein the solidified ABA forms the interconnects and contact pads following
mechanical polishing The second metallization system involves making free-standing Au
contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance
of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)
cos
1131
Where θ is the contact angle between the substrate and liquid metal WA is the work of
adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion
force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1
describes the balance of forces at the liquid metalsolid substrate interface wherein liquid
metals (intrinsically very high cohesion force materials) tend to create large contact angles on
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6
most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one
facet of this work we use active brazing to modify WA and create robust electrical
connections to carbonaceous materials such as PCD and CF In another technique we exploit
the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion
forces within the liquid metal droplet to create a freestanding carbonaceous wire with
metallic connections Broadly speaking the techniques described here are applicable to the
development of devices in which a strong biocompatible and non-rectifying connection to
carbon-based materials is required
2 Experimental
21 Surface interactions brazing alloys and carbonaceous materials
211 Sample preparation and wettability studies
Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a
thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser
(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were
placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a
graphite substrate with a carbonaceous wire The elemental composition and melting points of
all braze alloys tested is listed in Table 1
ABA (Manufacturer)
Ag () Au () Cu () Al () Ti () Ni () MP (degC)
Ag-ABA (Wesgo)
9275 - 5 1 125 - 912
Au-ABA (Morgan)
- 964 - - 06 3 1003
TiCuNi (Wesgo)
- - 15 - 70 15 960
Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study
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Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
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8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
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Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
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purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
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freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
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Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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1
Brazing techniques for the fabrication of biocompatible carbon-based electronic devices Nicholas V Apollo1 Desmond Lau2 Arman Ahnood1 Alastair Stacey1 Kumaravelu Ganesan1 Samantha G Lichter1 Kate Fox3 Javad Foroughi4 Hamish Meffin5 Gordon G Wallace4 Ray Baughman6 Steven Prawer1 David J Garrett1 1 School of Physics The University of Melbourne Victoria 3010 Australia 2 School of Applied Sciences RMIT 3 Centre for Additive Manufacturing School of Engineering RMIT University Melbourne Australia 4 Intelligent Polymer Research Institute ARC Centre of Excellence for Electromaterials Science AIIM Facility Innovation Campus University of Wollongong Australia 5 National Vision Research Institute Australian College of Optometry Victoria 3053 and Department of Optometry and Vision Science University of Melbourne Victoria 3010 Australia 6 Alan G MacDiarmid NanoTech Institute University of Texas at Dallas Richardson TX 75083 USA Corresponding author David Garrett e-mail dgarrettunimelbeduau
Abstract
Prototype electronic devices have been critical to the discovery and demonstration of the
unique properties of new materials including composites based on carbon nanotubes (CNT)
and graphene However these devices are not typically constructed with durability or
biocompatibility in mind relying on conductive polymeric adhesives mechanical clamps or
crimps or solders for electrical connections In this paper two key metallization techniques
are presented that employ commercially-available brazing alloys to fabricate electronic
devices based on diamond and carbonaceous wires Investigation of the carbonmdashalloy
interfacial interactions was utilized to guide device fabrication The interplay of both chemical
(ldquoadhesiverdquo) and mechanical (ldquocohesiverdquo) forces at the interface of different forms of carbon
was exploited to fabricate either freestanding or substrate-fixed carbonaceous electronic
devices Elemental analysis in conjunction with scanning electron microscopy of the carbonmdash
alloy interface revealed the chemical nature of the Ag alloy bond and the mechanical nature of
the Au alloy bond Electrical characterization revealed the non-rectifying nature of the
carbonmdashAu alloy interconnects Finally electronic devices were fabricated including a Au
circuit structure embedded in a polycrystalline diamond substrate
MANUSCRIP
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2
1 Introduction
Owing to their unique combination of biological mechanical electrochemical electrical
thermal and optical properties carbon-based materials such as graphene carbon nanotubes
(CNT) carbon fiber (CF) and diamond have attracted significant scientific and industrial
interest Exploiting CNT and graphene properties in micro- and macroscale structures is of
significant interest for engineering research and product development [1-3] For example
high strength and high conductivity fabrics based on these materials could enable a variety of
applications from lightweight body armor to space elevators [4 5] Several solutions to the
problem of scale have been demonstrated for both graphene and CNT films Spinning CVD-
grown CNT forests into a continuous yarn has enabled conductive high-strength structures
with micro-scale diameters and macro-scale lengths [3] Control of mechanical strength and
electricalthermal conductivity of CNT yarns is established by adjusting component CNT
lengths as well as the spinning angles of the fibers [6 7] Twisted andor coiled structures of
CNT yarns have been used to fabricate electrochemical as well as electrolyte-free CNT
torsional actuators (artificial muscles) with large-stroke and high work-capacity [8 9] Cell
culture dishes containing CNT yarns have shown that neurons grow and respond to electrical
stimulation delivered by the yarn [10] As bulk carriers of electricity and data CNT yarns
have been evaluated as replacements for standard Cu and Al wires due to their light weight
(alleviating stress on joint connections) ballistic conduction (lack of scattering reduces risk of
Joule heating) and capacity to handle high frequencies [11] Expanding upon the application
of long-distance electricity transfer CNT yarns have been used to build wireless data transfer
networks that were mechanically-resilient and displayed frequency-independent resistive
behavior [12] As a demonstration of their favorable electrochemical characteristics CNT
yarns have been used as an alternative to the standard CF electrochemical sensor for
neurotransmitters due to an intrinsic ability to resist surface fouling [13]
MANUSCRIP
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3
All of the aforementioned research and much that was not mentioned was performed using
prototype devices containing silver epoxy gold paste solder or mechanical clamps to create
electrical connections to carbonaceous wires While these connections served their respective
experimental purposes their use in fabrication does not generally yield scalable or integrated
electronic devices nor are these connection methods resilient or biocompatible enough for
consideration as materials for biomedical devices There have been several methods
developed for bonding a single CNT to metals as well as joining a single CNT to another
CNT end-to-end [14] However making electrical connections to larger (micro rather than
nano) CNT composites and yarns is a different challenge Current approaches include
mechanical clamps Ag- and Au-based epoxy adhesives carbon solder ultrasonic welding
and vacuum brazing [15 16] A recent very exciting approach involves use of transition
metal soldering alloys to join carbon wires using standard solder conditions (eg 350 degC in
air) [17] While it is a technique that will certainly revolutionize the utility of carbonaceous
wires the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known
cytotoxicity of these metals [18 19] We have recently developed a technique to create a
hermetic diamond capsule using biocompatible gold alloys [20] Additionally we have
demonstrated a method for the construction of hermetic biocompatible feedthroughs for a
retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21] In the
present paper we extend our previous work and describe the construction of diamond-based
electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads
Finally we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn
CF and graphene fiber) with the option to incorporate them into diamond circuit substrates
The methods we describe constitute an important toolkit to allow for CNT-based and other
similar carbonaceous ldquosuper materialsrdquo to be incorporated into electronic devices and
integrated with traditional surface-mount electronics with specific focus on biocompatible
systems for implantation into the body
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4
11 Brazing to carbon
Active brazing a variation of vacuum brazing utilizes an alloy containing an active
component that reacts chemically with a relatively inert surface such as diamond or ceramics
[22 23] Transition metals are of particular interest due to electron vacancies in their d-
orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with
carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti
V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of
other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active
brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle
and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)
though the electrical and mechanical properties were not fully characterized [16] The
formation of interfacial TiC particles has also been shown to significantly increase the tensile
strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements
[27] In the present work commercially-available ABAs were investigated to determine their
suitability to make electronic connections to several carbonaceous materials including
polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based
carbon fiber (CF) to enable various types of electronic devices (Fig 1)
MANUSCRIP
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5
Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes
In the first technique described here ABAs were used to create embedded circuit boards in
PCD wherein the solidified ABA forms the interconnects and contact pads following
mechanical polishing The second metallization system involves making free-standing Au
contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance
of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)
cos
1131
Where θ is the contact angle between the substrate and liquid metal WA is the work of
adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion
force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1
describes the balance of forces at the liquid metalsolid substrate interface wherein liquid
metals (intrinsically very high cohesion force materials) tend to create large contact angles on
MANUSCRIP
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6
most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one
facet of this work we use active brazing to modify WA and create robust electrical
connections to carbonaceous materials such as PCD and CF In another technique we exploit
the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion
forces within the liquid metal droplet to create a freestanding carbonaceous wire with
metallic connections Broadly speaking the techniques described here are applicable to the
development of devices in which a strong biocompatible and non-rectifying connection to
carbon-based materials is required
2 Experimental
21 Surface interactions brazing alloys and carbonaceous materials
211 Sample preparation and wettability studies
Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a
thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser
(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were
placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a
graphite substrate with a carbonaceous wire The elemental composition and melting points of
all braze alloys tested is listed in Table 1
ABA (Manufacturer)
Ag () Au () Cu () Al () Ti () Ni () MP (degC)
Ag-ABA (Wesgo)
9275 - 5 1 125 - 912
Au-ABA (Morgan)
- 964 - - 06 3 1003
TiCuNi (Wesgo)
- - 15 - 70 15 960
Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study
MANUSCRIP
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7
Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
MANUSCRIP
T
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8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
MANUSCRIP
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9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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2
1 Introduction
Owing to their unique combination of biological mechanical electrochemical electrical
thermal and optical properties carbon-based materials such as graphene carbon nanotubes
(CNT) carbon fiber (CF) and diamond have attracted significant scientific and industrial
interest Exploiting CNT and graphene properties in micro- and macroscale structures is of
significant interest for engineering research and product development [1-3] For example
high strength and high conductivity fabrics based on these materials could enable a variety of
applications from lightweight body armor to space elevators [4 5] Several solutions to the
problem of scale have been demonstrated for both graphene and CNT films Spinning CVD-
grown CNT forests into a continuous yarn has enabled conductive high-strength structures
with micro-scale diameters and macro-scale lengths [3] Control of mechanical strength and
electricalthermal conductivity of CNT yarns is established by adjusting component CNT
lengths as well as the spinning angles of the fibers [6 7] Twisted andor coiled structures of
CNT yarns have been used to fabricate electrochemical as well as electrolyte-free CNT
torsional actuators (artificial muscles) with large-stroke and high work-capacity [8 9] Cell
culture dishes containing CNT yarns have shown that neurons grow and respond to electrical
stimulation delivered by the yarn [10] As bulk carriers of electricity and data CNT yarns
have been evaluated as replacements for standard Cu and Al wires due to their light weight
(alleviating stress on joint connections) ballistic conduction (lack of scattering reduces risk of
Joule heating) and capacity to handle high frequencies [11] Expanding upon the application
of long-distance electricity transfer CNT yarns have been used to build wireless data transfer
networks that were mechanically-resilient and displayed frequency-independent resistive
behavior [12] As a demonstration of their favorable electrochemical characteristics CNT
yarns have been used as an alternative to the standard CF electrochemical sensor for
neurotransmitters due to an intrinsic ability to resist surface fouling [13]
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All of the aforementioned research and much that was not mentioned was performed using
prototype devices containing silver epoxy gold paste solder or mechanical clamps to create
electrical connections to carbonaceous wires While these connections served their respective
experimental purposes their use in fabrication does not generally yield scalable or integrated
electronic devices nor are these connection methods resilient or biocompatible enough for
consideration as materials for biomedical devices There have been several methods
developed for bonding a single CNT to metals as well as joining a single CNT to another
CNT end-to-end [14] However making electrical connections to larger (micro rather than
nano) CNT composites and yarns is a different challenge Current approaches include
mechanical clamps Ag- and Au-based epoxy adhesives carbon solder ultrasonic welding
and vacuum brazing [15 16] A recent very exciting approach involves use of transition
metal soldering alloys to join carbon wires using standard solder conditions (eg 350 degC in
air) [17] While it is a technique that will certainly revolutionize the utility of carbonaceous
wires the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known
cytotoxicity of these metals [18 19] We have recently developed a technique to create a
hermetic diamond capsule using biocompatible gold alloys [20] Additionally we have
demonstrated a method for the construction of hermetic biocompatible feedthroughs for a
retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21] In the
present paper we extend our previous work and describe the construction of diamond-based
electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads
Finally we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn
CF and graphene fiber) with the option to incorporate them into diamond circuit substrates
The methods we describe constitute an important toolkit to allow for CNT-based and other
similar carbonaceous ldquosuper materialsrdquo to be incorporated into electronic devices and
integrated with traditional surface-mount electronics with specific focus on biocompatible
systems for implantation into the body
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4
11 Brazing to carbon
Active brazing a variation of vacuum brazing utilizes an alloy containing an active
component that reacts chemically with a relatively inert surface such as diamond or ceramics
[22 23] Transition metals are of particular interest due to electron vacancies in their d-
orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with
carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti
V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of
other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active
brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle
and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)
though the electrical and mechanical properties were not fully characterized [16] The
formation of interfacial TiC particles has also been shown to significantly increase the tensile
strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements
[27] In the present work commercially-available ABAs were investigated to determine their
suitability to make electronic connections to several carbonaceous materials including
polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based
carbon fiber (CF) to enable various types of electronic devices (Fig 1)
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5
Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes
In the first technique described here ABAs were used to create embedded circuit boards in
PCD wherein the solidified ABA forms the interconnects and contact pads following
mechanical polishing The second metallization system involves making free-standing Au
contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance
of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)
cos
1131
Where θ is the contact angle between the substrate and liquid metal WA is the work of
adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion
force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1
describes the balance of forces at the liquid metalsolid substrate interface wherein liquid
metals (intrinsically very high cohesion force materials) tend to create large contact angles on
MANUSCRIP
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6
most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one
facet of this work we use active brazing to modify WA and create robust electrical
connections to carbonaceous materials such as PCD and CF In another technique we exploit
the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion
forces within the liquid metal droplet to create a freestanding carbonaceous wire with
metallic connections Broadly speaking the techniques described here are applicable to the
development of devices in which a strong biocompatible and non-rectifying connection to
carbon-based materials is required
2 Experimental
21 Surface interactions brazing alloys and carbonaceous materials
211 Sample preparation and wettability studies
Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a
thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser
(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were
placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a
graphite substrate with a carbonaceous wire The elemental composition and melting points of
all braze alloys tested is listed in Table 1
ABA (Manufacturer)
Ag () Au () Cu () Al () Ti () Ni () MP (degC)
Ag-ABA (Wesgo)
9275 - 5 1 125 - 912
Au-ABA (Morgan)
- 964 - - 06 3 1003
TiCuNi (Wesgo)
- - 15 - 70 15 960
Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study
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7
Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
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8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
MANUSCRIP
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9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
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28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
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3
All of the aforementioned research and much that was not mentioned was performed using
prototype devices containing silver epoxy gold paste solder or mechanical clamps to create
electrical connections to carbonaceous wires While these connections served their respective
experimental purposes their use in fabrication does not generally yield scalable or integrated
electronic devices nor are these connection methods resilient or biocompatible enough for
consideration as materials for biomedical devices There have been several methods
developed for bonding a single CNT to metals as well as joining a single CNT to another
CNT end-to-end [14] However making electrical connections to larger (micro rather than
nano) CNT composites and yarns is a different challenge Current approaches include
mechanical clamps Ag- and Au-based epoxy adhesives carbon solder ultrasonic welding
and vacuum brazing [15 16] A recent very exciting approach involves use of transition
metal soldering alloys to join carbon wires using standard solder conditions (eg 350 degC in
air) [17] While it is a technique that will certainly revolutionize the utility of carbonaceous
wires the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known
cytotoxicity of these metals [18 19] We have recently developed a technique to create a
hermetic diamond capsule using biocompatible gold alloys [20] Additionally we have
demonstrated a method for the construction of hermetic biocompatible feedthroughs for a
retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21] In the
present paper we extend our previous work and describe the construction of diamond-based
electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads
Finally we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn
CF and graphene fiber) with the option to incorporate them into diamond circuit substrates
The methods we describe constitute an important toolkit to allow for CNT-based and other
similar carbonaceous ldquosuper materialsrdquo to be incorporated into electronic devices and
integrated with traditional surface-mount electronics with specific focus on biocompatible
systems for implantation into the body
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4
11 Brazing to carbon
Active brazing a variation of vacuum brazing utilizes an alloy containing an active
component that reacts chemically with a relatively inert surface such as diamond or ceramics
[22 23] Transition metals are of particular interest due to electron vacancies in their d-
orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with
carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti
V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of
other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active
brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle
and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)
though the electrical and mechanical properties were not fully characterized [16] The
formation of interfacial TiC particles has also been shown to significantly increase the tensile
strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements
[27] In the present work commercially-available ABAs were investigated to determine their
suitability to make electronic connections to several carbonaceous materials including
polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based
carbon fiber (CF) to enable various types of electronic devices (Fig 1)
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5
Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes
In the first technique described here ABAs were used to create embedded circuit boards in
PCD wherein the solidified ABA forms the interconnects and contact pads following
mechanical polishing The second metallization system involves making free-standing Au
contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance
of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)
cos
1131
Where θ is the contact angle between the substrate and liquid metal WA is the work of
adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion
force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1
describes the balance of forces at the liquid metalsolid substrate interface wherein liquid
metals (intrinsically very high cohesion force materials) tend to create large contact angles on
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6
most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one
facet of this work we use active brazing to modify WA and create robust electrical
connections to carbonaceous materials such as PCD and CF In another technique we exploit
the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion
forces within the liquid metal droplet to create a freestanding carbonaceous wire with
metallic connections Broadly speaking the techniques described here are applicable to the
development of devices in which a strong biocompatible and non-rectifying connection to
carbon-based materials is required
2 Experimental
21 Surface interactions brazing alloys and carbonaceous materials
211 Sample preparation and wettability studies
Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a
thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser
(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were
placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a
graphite substrate with a carbonaceous wire The elemental composition and melting points of
all braze alloys tested is listed in Table 1
ABA (Manufacturer)
Ag () Au () Cu () Al () Ti () Ni () MP (degC)
Ag-ABA (Wesgo)
9275 - 5 1 125 - 912
Au-ABA (Morgan)
- 964 - - 06 3 1003
TiCuNi (Wesgo)
- - 15 - 70 15 960
Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study
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7
Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
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T
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8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
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9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
MANUSCRIP
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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4
11 Brazing to carbon
Active brazing a variation of vacuum brazing utilizes an alloy containing an active
component that reacts chemically with a relatively inert surface such as diamond or ceramics
[22 23] Transition metals are of particular interest due to electron vacancies in their d-
orbitals with greater numbers of vacancies leading to greater reactivity (overlap) with
carbonrsquos p-orbitals [24] In the case of carbon substrates the active componentmdashtypically Ti
V or Crmdashforms a carbide interface layer that acts as a surfactant to enable the wetting of
other filler metals and produce non-rectifying electrical contacts [25 26] A Ag-based active
brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle
and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS)
though the electrical and mechanical properties were not fully characterized [16] The
formation of interfacial TiC particles has also been shown to significantly increase the tensile
strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements
[27] In the present work commercially-available ABAs were investigated to determine their
suitability to make electronic connections to several carbonaceous materials including
polycrystalline diamond (PCD) CNT yarns graphene oxide (GO) fibers and PAN-based
carbon fiber (CF) to enable various types of electronic devices (Fig 1)
MANUSCRIP
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5
Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes
In the first technique described here ABAs were used to create embedded circuit boards in
PCD wherein the solidified ABA forms the interconnects and contact pads following
mechanical polishing The second metallization system involves making free-standing Au
contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance
of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)
cos
1131
Where θ is the contact angle between the substrate and liquid metal WA is the work of
adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion
force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1
describes the balance of forces at the liquid metalsolid substrate interface wherein liquid
metals (intrinsically very high cohesion force materials) tend to create large contact angles on
MANUSCRIP
T
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6
most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one
facet of this work we use active brazing to modify WA and create robust electrical
connections to carbonaceous materials such as PCD and CF In another technique we exploit
the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion
forces within the liquid metal droplet to create a freestanding carbonaceous wire with
metallic connections Broadly speaking the techniques described here are applicable to the
development of devices in which a strong biocompatible and non-rectifying connection to
carbon-based materials is required
2 Experimental
21 Surface interactions brazing alloys and carbonaceous materials
211 Sample preparation and wettability studies
Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a
thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser
(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were
placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a
graphite substrate with a carbonaceous wire The elemental composition and melting points of
all braze alloys tested is listed in Table 1
ABA (Manufacturer)
Ag () Au () Cu () Al () Ti () Ni () MP (degC)
Ag-ABA (Wesgo)
9275 - 5 1 125 - 912
Au-ABA (Morgan)
- 964 - - 06 3 1003
TiCuNi (Wesgo)
- - 15 - 70 15 960
Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
7
Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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5
Fig 1 - ABAs were used to fabricate two basic electronic device platforms (a) a circuit board embedded into a polycrystalline diamond (PCD) substrate and (b) a freestanding carbonaceous wire with electrical connections on one or both ends The aforementioned technologies had lead to the fabrication of medical devices containing active carbon electrodes such as (c) conductive diamond electrodes and (d) carbonaceous wire electrodes
In the first technique described here ABAs were used to create embedded circuit boards in
PCD wherein the solidified ABA forms the interconnects and contact pads following
mechanical polishing The second metallization system involves making free-standing Au
contacts to carbonaceous fibers using a graphite ldquolift-offrdquo method which exploits the balance
of cohesive and adhesive forces of the liquid metalgraphite interface (Equation 1)
cos
1131
Where θ is the contact angle between the substrate and liquid metal WA is the work of
adhesion (adhesion force) between the substrate and the liquid metal and σL is the cohesion
force (liquid-vapour surface tension) within the liquid metal droplet [28] Equation 1
describes the balance of forces at the liquid metalsolid substrate interface wherein liquid
metals (intrinsically very high cohesion force materials) tend to create large contact angles on
MANUSCRIP
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6
most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one
facet of this work we use active brazing to modify WA and create robust electrical
connections to carbonaceous materials such as PCD and CF In another technique we exploit
the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion
forces within the liquid metal droplet to create a freestanding carbonaceous wire with
metallic connections Broadly speaking the techniques described here are applicable to the
development of devices in which a strong biocompatible and non-rectifying connection to
carbon-based materials is required
2 Experimental
21 Surface interactions brazing alloys and carbonaceous materials
211 Sample preparation and wettability studies
Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a
thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser
(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were
placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a
graphite substrate with a carbonaceous wire The elemental composition and melting points of
all braze alloys tested is listed in Table 1
ABA (Manufacturer)
Ag () Au () Cu () Al () Ti () Ni () MP (degC)
Ag-ABA (Wesgo)
9275 - 5 1 125 - 912
Au-ABA (Morgan)
- 964 - - 06 3 1003
TiCuNi (Wesgo)
- - 15 - 70 15 960
Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study
MANUSCRIP
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7
Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
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8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
MANUSCRIP
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9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
MANUSCRIP
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
MANUSCRIP
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
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28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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6
most substrates unless σL lt WA where contact angles lt90˚ are considered ldquowettingrdquo In one
facet of this work we use active brazing to modify WA and create robust electrical
connections to carbonaceous materials such as PCD and CF In another technique we exploit
the lack of adhesion and wetting between graphitic carbon and Au as well as the cohesion
forces within the liquid metal droplet to create a freestanding carbonaceous wire with
metallic connections Broadly speaking the techniques described here are applicable to the
development of devices in which a strong biocompatible and non-rectifying connection to
carbon-based materials is required
2 Experimental
21 Surface interactions brazing alloys and carbonaceous materials
211 Sample preparation and wettability studies
Thermal grade polycrystalline diamond (PCD) substrates (TM-250 Element Six) with a
thickness of 250 microm were laser-cut into 5 times 5 mm squares with a 532 nm NdYAG laser
(Oxford Lasers) Approximately 65 mg of either TiCuNi Ag ABA or Au ABA paste were
placed ontop of either the PCD substrate the PCD substrate with a carbonaceous wire or a
graphite substrate with a carbonaceous wire The elemental composition and melting points of
all braze alloys tested is listed in Table 1
ABA (Manufacturer)
Ag () Au () Cu () Al () Ti () Ni () MP (degC)
Ag-ABA (Wesgo)
9275 - 5 1 125 - 912
Au-ABA (Morgan)
- 964 - - 06 3 1003
TiCuNi (Wesgo)
- - 15 - 70 15 960
Table 1 Elemental composition and melting point (MP) of the braze alloys used in this study
MANUSCRIP
T
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7
Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
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T
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8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
MANUSCRIP
T
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9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
MANUSCRIP
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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T
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
T
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
MANUSCRIP
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
T
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
T
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
7
Samples were placed in a brazing furnace under vacuum and heated until the braze was
visibly melted and then held at the melting temperature for 1 minute Upon cooling samples
were imaged using either an optical microscope or scanning electron microscope (SEM)
Contact angles were measured using the low-bond axisymmetric drop shape analysis in the
Droplet-Analysis package in ImageJ image processing software [29 30]
212 Elemental analysis of brazemdashcarbon interface
Some diamond samples were laser milled with deep grooves prior to brazing enabling
visualization and characterization of the interface with both SEM and energy-dispersive X-ray
spectroscopy (EDX)(Supplementary Information Fig S1) The metalcarbonaceous wire
interfaces were studied by embedding brazed samples in epoxy resin (Araldite) and
mechanically-polishing them to reveal the cross-section (Fig 2)
Fig 2 ndash Embedding and polishing method utilized to visualize the carbonmdashbraze interface during both SEM and EDX analysis
SEM images were acquired and EDX was used to identify elemental composition at the
interface of both Ag-ABACF and Au-ABACF joints
213 Raman spectroscopy
Raman spectroscopy (inVia Raman Renishaw) was used to investigate the effects of high
temperature brazing on the microstructure of CF bundles CNT yarns and reduced liquid
crystalline graphene oxide fiber (rLCGO) A 514 nm green laser with a power of 50 (15
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
MANUSCRIP
T
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9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
MANUSCRIP
T
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
MANUSCRIP
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
MANUSCRIP
T
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
T
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
T
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
MANUSCRIP
T
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
8
mW) was used for CF bundles while a power of 5 (15 mW) was used for 15 microm CNT
yarns and 40 microm rLCGO fiber to avoid ablation damage Samples were scanned before and
after high temperature brazing (1003-1030˚C) The laser was focused on the center of the
sample and an exposure time of 60 seconds was used Acquired signals were processed in
Matlab using a linear baseline correction procedure followed by Gaussian fitting to resolve
peaks
22 Device fabrication and electrical characterization
221 PCD circuit substrates
Thermal grade PCD substrates (TM-500 Element Six) with a thickness of 500 microm were laser
engraved using a 532 nm NdYAG laser (Oxford lasers) at a pulse frequency of 10 kHz A
boiling solution of sodium nitrate and sulfuric acid was used to remove debris from the
engraved pattern followed by 5 minutes of ultrasonication in acetone The PCD substrates
were rinsed with isopropanol and deionized water then dried with nitrogen Groove width
was correlated with laser milling power using scanning electron microscopy (SEM) images
ABA paste was spread over the PCD substrate Samples were transferred to a 10 times 10 mm
graphite wafer placed inside an electron beam deposition system (Thermionics) and
evacuated to 1 times 10-6 Pa The electron beam was focused onto the graphite substrate rather
than the diamond sample and current was increased until both the graphite and PCD were
glowing red ultimately leading to the ABA melting and spreading over the substrate and into
the laser-milled patterns Current was decreased over the course of 1 minute and the samples
were cooled over the course of 1 hour in a N2 environment The brazed samples were
removed from the deposition chamber and mechanical polishing was performed with a
diamond-embedded polishing plate (Struers Inc) using deionized water as a coolant The
samples were polished until embedded Ag ABA or Au ABA tracks were revealed and
residual metals between patterns removed ensuring no shorting between interconnects
MANUSCRIP
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9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
MANUSCRIP
T
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
MANUSCRIP
T
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11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
MANUSCRIP
T
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
T
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
9
Resistance between isolated interconnects was measured to ensure no shorting (eg gtMΩ
resistance) between unconnected tracks If shorting did occur due to residual unpolished
metal mechanical polishing was continued until it was removed The fabrication of AuPCD
circuit boards requires an additional brazing step to enable Au wetting (Fig 3)
Fig 3 - Construction of embedded-in-diamond Au ABA circuit structures using a Ag ABA adhesion layer (a-b) circuit patterns were laser cut into the diamond substrate and the surface treated with a combination of sulfuric acid and sodium nitrate (c) Ag-ABA paste was applied to the surface melted in an electron beam deposition system (d) The electron beam was continuously applied until a majority of Ag evaporated from the surface leaving behind a conformal film for Au adhesion (e) A Au-based paste was applied to the surface and melted with the aforementioned method (f) (g) The substrate was mechanically-polished revealing Au circuit structures (h) Finally connections or components were attached with laser welding soldering or wirebonding if necessary
222 Freestanding carbonaceous wires
MWNT forest was synthesized by catalytic chemical vapor deposition using acetylene gas as
the carbon source Carbon nanotubes in the 400 um tall forests typically had diameters of 10
nm The CNT yarns were drawn from the forest by pulling and twisting as described by
Zhang et al [31] Graphene oxide (GO) was prepared from intercalated graphene flakes The
liquid crystalline state of graphene oxide was exploited to wet-spin conductive fibers as
described previously [32] Unsized CF bundles having filament diameters of 7 microm were
MANUSCRIP
T
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10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
MANUSCRIP
T
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
T
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13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
10
purchased from Goodfellow On a graphite substrate two mounds of ABA paste were placed
on opposite sides of a 10 times 10 mm graphite wafer Either a bundle of CFs (Goodfellow) a
single CNT yarn (15 microm) or a graphene oxide fiber (50 microm) was placed across the mounds
and pressed into them Additionally one PtIr wire (75 microm Goodfellow) was placed into each
of the ABA paste mounds The graphite substrate was transferred to an electron beam
deposition system (Thermionics) and evacuated to 1 times 10-6 Pa The electron beam was
focused onto the graphite substratemdashas described above for PCD circuit board constructionmdash
and the current was increased until the substrate was glowing red The aforementioned
process is illustrated in Fig 4
Fig 4 - (a) Fabrication of freestanding metalABACNT yarn wires Mounds of ABA paste were placed onto a graphite substrate and either CNT yarns CFs or graphene oxide fibers were pressed into them PtIr wires were pressed into the end for testing (b) The graphite substrate was heated until the Au formed beads on the surface (c) Upon cooling the entire wiremetal structure was easily lifted from the surface
Once melting of the ABA was observed the current was ramped down over 1 minute and the
samples were cooled for 30 minutes in an N2 environment Au-ABA samples were easily
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
11
removed from the graphite substrate due to lack of reaction while considerable force was
needed to remove the Ag ABA and TiCuNi samples
223 Integrated PCDcarbonaceous electrode devices
Our group has previously published a method for integrating PCD substrates with conductive
diamond electrodes to produce hermetic feedthroughs [21] Integrating carbonaceous wire
electrodes in this case 7 microm diameter CF was done manually as depicted in Fig 5
Fig 5 - Method for placing individual CF into laser cut groove in PCD substrate and securing the CF with braze paste The braze paste is then ldquoscreen-printedrdquo with a squeegee to remove the excess braze from the surface of the PCD to minimize shorting upon braze melting
PCD substrates were laser machined as described above to create grooves for CF filaments to
rest in Next individual CF filaments were manually placed into the grooves and Ag ABA
paste was spread over the entire surface The surface was wiped with a mini squeegee
(cleanroom cloth wrapped around syringe needle) to remove excess braze paste from the
surface Finally the sample was heated to melt the braze as described above
224 Electrical characterization of devices
The resistance of the Au ABA interconnects embedded in the polished PCD substrates were
measured using a 2-way probe station and a multimeter (Keithley Instruments) Transmission
line devices were fabricated to correct for contact resistance The electrical behavior of the
MANUSCRIP
T
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12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
MANUSCRIP
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
MANUSCRIP
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
12
freestanding PtIrAuCNT yarn structures was characterized by performing voltage sweeps (-1
to +1 V) with a potentiostat (eDAQ) while measuring current
3 Results and Discussion
31 Embedded-in-diamond circuit substrates
311 Investigating PCD-braze interactions
Three different commercially-available active brazing alloy (ABA) pastes were assessed for
their ability to wet and bond to polycrystalline diamond (PCD) substrates The elemental
composition of these brazes is included in Table 1 of the Experimental section Ag ABA was
found to spread and adhere strongly to PCD which was likely facilitated by the formation of
an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig
6a) SEM and EDX elemental analysis of the PCDAg ABA interface reveals a Ti-rich region
of ~34 microm adjacent to the Ag ABA joint (Supplementary Information Fig S1) Conversely
Au ABA did not spread on the PCD surface upon melting instead forming a droplet with a
111deg contact angle on the surface (Fig 6b)
Fig 6 - (a) Ag ABA wets PCD and bonds strongly (b) Au ABA does not wet PCD forming a droplet that makes ~111deg contact angle with the surface (c) TiCuNi braze wets the PCD surface but does not bond The cooled TiCuNi droplet is easily pulled from the PCD surface with forceps (d) Au ABA wets the PCD following a previous Ag ABA brazing step implemented to create a conformal adhesion layer
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
13
Though the manufacturers have included Ti as the active component in the Au ABA this
braze is unable to spread on carbonaceous materials Considering the elemental composition
of Au ABA (964 Au) this effect is partially explained by the very low miscibility and lack
of reactivity of the Au-C system [33] Furthermore Ti is presumably an inappropriate active
component for Au-based ABA due to the precipitation of graphitic carbon (Cg) that occurs in
the TiC-Au melt [33]
rarr + 132
Equation 2 is driven in the forward direction by the formation of a Au-Ti intermetallic which
is unsurprising considering the large difference in electronegativity (X) between Au
(XAu=254) and Ti (XTi=154) [34] In the case of Ag ABA formation of a Ag-Ti intermetallic
is a possibility but the electronegativity differences are less pronounced between Ti and Ag
(XAg=193) Traditionally a diffusion barrier such as Pt would be introduced to prevent this
reaction TiCuNi a braze used previously for the encapsulation of alumina ceramic-based
medical devices [35] wetted the PCD surface upon heating but did not bond and was easily
removed with forceps upon cooling (Fig 6c) Differently we observed TiCuNi wetting and
bonding to both graphite substrates and CF bundles (Supplementary Information Fig S2)
This finding is consistent with what has been shown previously for TiCuNirsquos interaction with
carbon-carbon composites [25] as well as for bulk graphite [36] The lack of bonding
between TiCuNi and PCD is possibly explained by the thermal expansion mismatch between
the braze alloy and the material The stress induced by thermal mismatch (σTM) can be
described theoretically by Equation 3 [37]
13 ∙ ∆ (3)
Where α is the coefficient of linear thermal expansion (CTE) of either the substrate or the
braze E is the elastic modulus of the ldquocoatingrdquo (braze in this instance) and ∆T is the
temperature differential approximately 1000 K for the difference between brazing
MANUSCRIP
T
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14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
T
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
MANUSCRIP
T
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
14
temperature and room temperature According to manufacturer data (Element Six) the PCD
used in this work has a CTE of 1-44 ppm K-1 while pyrolytic and natural graphite substrates
have a larger characteristic CTE of 283 ppm K-1 [38] The TiCuNi braze has a CTE of 203
ppm K-1 a value closer to that recorded for graphite [25] Further investigation of the
TiCuNiPCD system is warranted though beyond the scope of the present work In
consideration of our aimmdashto fabricate a biocompatible biostable metalcarbon substrate
compatible with standard joining technologies (ie wirebonding laser welding reflow
soldering etc)mdashthe AuPCD system was the main focus In order to create a AuPCD system
a Ag ABA brazing step was introduced to produce a metallic conformal adhesion layer for a
subsequent brazing step with Au ABA leading to significantly enhanced wetting (Fig 6d) If
a metal is unable to both wet and bond to PCD it is not suitable for the creation of embedded
thick- film conductors or contact pads required for the embedded-in-diamond circuit
structures described in the following sections Additionally the ubiquity of Au contacts in
semiconductor and electronics fabrication makes the AuPCD system a practical device
substrate
312 Device fabrication and characterization
The entire fabrication process for these devices is illustrated above in the Experimental
section A 532 nm NdYAG laser was used to create microscopic grooves in a PCD substrate
prior to brazing and mechanical polishing processes (Fig 7a) The resultant brazedpolished
devices were imaged using optical microscopy (Fig 7b) The interconnects achieved so far
with this technique range between 6-15 microm in width Lower milling powers lead to less
reproducibility perhaps due to issues with focusing on the surface which would be more
pronounced with lower laser powers (Fig 7c) Smaller and more reproducible features are
likely possible following more extensive laser parameter optimization including adjustment
of the gaseous environment during laser milling [39] Only AgPCD and AgAuPCD devices
MANUSCRIP
T
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
MANUSCRIP
T
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
T
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
MANUSCRIP
T
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
T
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
T
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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15
were fabricated due to the poor adhesion of TiCuNi braze and the poor wetting of pure Au
ABA on PCD substrates Ag-ABA spreads and flows during heating to fill in laser-milled
structures having three dimensions which demonstrates a significant advantage over
conventional ldquoline-of-sightrdquo thin film metallization procedures Additionally the brazing
process eliminates the extra annealing step to form TiC at the interface [40] The resistance of
Au ABA interconnects embedded in PCD was measured using a transmission line set-up to
correct for contact resistance The relationship between the resistance of a 500 microm length Au
interconnect and laser power utilized to mill the track is shown in Fig 7d
Fig 7 ndash Construction and characterization of embedded-in-diamond circuit substrates (a) PCD substrates were laser milled to create grooves for interconnects or contact pads (b) following brazing and polishing circuit boards with embedded microstructures realized A single contact is visualized with light source from above (middle) and from below (right) The diamond structure is clearly visible when the sample is illuminated from below (c) An approximate correlation (R2=0816) between laser milling power and the width of the resultant groove was found (d) An approximate relation (R2=0922) between track resistance and laser milling power was also found
The 3D nature of the interconnect leads to a significant decrease in electrical resistance
compared with a thin film metal contact according to R=ρLA where lsquoRrsquo is resistance lsquoρrsquo is
material resistivity lsquoLrsquo is interconnect length and lsquoArsquo is interconnect cross-sectional area In
MANUSCRIP
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
MANUSCRIP
T
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
MANUSCRIP
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
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28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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16
addition to stability and biocompatibility these embedded-in-diamond circuit boards are ideal
for thermal management applications due to diamondrsquos extremely high thermal conductivity
(1-2 kWmK) [41] The ability to integrate the ABA interconnects with standard joining
technologies is paramount Both varieties of braze are easily joined via laser welding
wirebonding or reflow soldering (Supplementary Information Fig S3)
32 Freestanding carbonaceous wires
321 Investigating braze-wire interactions
The active brazing of carbonaceous structures such as rLCGO fiber and CNT yarn has not
been studied as far as we are aware As a result the surface interaction of these materials was
investigated initially using PCD as a substrate PCD was originally chosen due to its low
electrical conductivity making it an adequate support structure for electrical characterization
However the interaction between PCD and Au though non-wetting is significant enough to
prevent the facile ldquolift-offrdquo of a freestanding wire Fig 8ab depicts an attempt to braze an
individual rLCGO fiber to a PCD substrate It is clear that Au ABA is unable to wet either
material though some bonding between Au ABA and PCD occurs A similar effect was
observed for the CNT yarn in which the Au ABA formed spherical beads on the surface of
the wire (Supplementary Information Fig S4) Ag ABA however displayed wetting
behavior similar to that observed for PCD exhibiting capillary wetting behavior and a contact
angle of ~404deg (Fig 8c)
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
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26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
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17
Fig 8 ndash Surface interactions of rLCGO fibers on PCD substrates with either Au ABA (a) or Ag ABA (c)
In the interest of creating freestanding carbonaceous wires with metallic contacts we
investigated whether the lack of wetting between a graphite substrate and braze alloy could be
exploited to facilitate a lift-off process in which graphite functioned as a sacrificial substrate
(Fig 9) It has been observed previously that surface roughness can lead to large increases in
contact angle during brazing making graphite a reasonable choice to perhaps further diminish
the wetting behavior [42]
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
T
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
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28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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18
Fig 9 ndash Overview of the interaction between Ag (b-e) and Au (f-k) braze alloys and a CF bundle comprised of 7 microm filaments using a sacrificial graphite substrate (a) A graphite substrate was laser-milled to create 500 microm wells (bc) Ag ABA was found to wet and spread on the graphite substrate upon heating (d) SEM imaging of the cross section reveals a splaying of the fiber bundle indicating a chemicaladhesive reaction in which a Ti-rich region was found around each 7 um filament (e) (fg) Au ABA neither wetted nor bonded to the graphite substrate instead enclosing the CF bundle within the liquid metal droplet as revealed by the cross-sectional SEM (h) (i) Individual CF filaments have been packed together by the cohesion forces within the liquid Au droplet (k) though some Ti appears interspersed within the bundle (j)
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
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ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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19
A graphite substrate was laser milled in an attempt to control separation distance between the
contacts as well as the amount of ABA paste applied (Fig 9a) By laser milling the substrate
a number of geometries are possible for the freestanding wires Ag ABA was able to both wet
and bond strongly to the graphite substrate (Fig 9bc) A cross-sectional image of the Ag
ABACF interface revealed separation of the CF bundle into individual 7 microm filaments (Fig
9d) EDX of the polished interface revealed rings of Ti around the fibers and intimate contact
with Ag perhaps suggesting a TiC reaction product (Fig 9e)Au ABA had minimal reaction
with the graphite substrate forming spherical contacts and enabling the CFAu ABA structure
to be easily lifted from the substrate (Fig 9fg) Additionally the Au ABA droplets formed a
seal around the bundle effectively compressing the filaments together (Fig 9h) However if
enough force is applied the Au contact can be removed from the CF bundle though this was
not observed for CNT yarn devices This effect is likely proportional to the ratio of wire
diameterAu droplet diameter geometry in which a smaller ratio leads to enhanced strength of
the bond due to a rise in the contact surface area
322 Device fabrication and characterization
A ldquolift-offrdquo method was developed to create free-standing carbonaceous wires with Au or
AuPtIr wire contacts Of the three varieties of ABA paste examined Au ABA is preferred
because it does not form a strong bond to the graphite substrate following a melting cycle so
the entire wire can be lifted off of the substrate with the contacts intact preventing damage to
the wire Both Ag ABA and TiCuNi bonded strongly to the graphite substrate preventing
facile lift-off and sometimes leading to mechanical damage of the fiber whenever subsequent
lift-off was attempted Au does not bond to the graphite for reasons explained above When
the graphite substrate was heated the Au-ABA formed a spherical metallic contact enclosing
both the carbonaceous fiber and a metallic wire within it (Fig 10a)
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
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24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
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25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
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ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
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28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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20
Fig 10 ndash SEM imaging and electrical characterization of free-standing carbonaceous wires with Pt wire contacts (a) SEM image of the Au contact between the PtIr wire and the 20 microm diameter CNT yarn (b) 2300X magnification of the junction between the Au ABA contact and the CNT yarn (c) 370X magnification of the Au contact on rLCGO fiber (d) Optical photograph of PtIrAuCNT yarn system during electrical measurement (e) Linear (Ohmic) I-V curves of several CNT yarns and one rLCGO fiber sample
The Au ABA droplet was found to enclose both CNT yarn (Fig 10b) and rLCGO fibers (Fig
10c) This technique is a potential improvement over mechanical clamps especially
considering the diameters of the fibers studied here (7-50 microm) because mechanical stress from
clamps would likely lead to fraying or damage Furthermore brazed contacts at either end of a
woven or spun material such as CNT yarn prevents unwinding and generally helps to keep
the fiber intact Mechanical clamps which apply large forces to the end of the yarn can lead
to fraying and damage especially at smaller yarn diameters[15] Voltage sweeps (-1 to +1 V)
were performed while current was recorded across the PtIrCNT yarn (Fig 10d) suggesting a
linear (non-rectifying) contact in the tested voltage range (Fig 10d) The resistivity ρ of the
CNT yarns in this work was comparable to those of similar size and length measured by
Jasinghe et al[43] We measure an average ρ of 43times10-3 Ωcm for the PtIrAuCNT yarn
structures and Jasinghe et al report a ρ of 17-3times10-3 Ωcm for pristine CNT yarns While
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
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21
preliminary measurements suggest that contact resistances are negligible further investigation
is needed to estimate the contact resistance through the contact It was of interest to determine
whether this process modified the carbon structure of the wire materials To investigate this
phenomenon Raman spectroscopy was performed on all 3 varieties of carbonaceous wire
following high temperature (gt1000 degC) brazing with Au ABA (Fig 11)
Fig 11 -- Raman spectroscopy of CNT yarns (ab) rLCGO fibers (cd) and CF bundles (ef) before and after brazing (g) Chart illustrating change in DG intensity with brazing The D-peak (or disorder peak) is located around 1330-1350 cm-1 and the G-peak (or graphite band) is located around 1580-1590 cm-1
The before and after Raman spectra of CFs reveals very little change in the microstructure
aside from the loss of a broad peak at around 2750 cm-1 following brazing (Fig 11ab) This
peak known as Grsquo is described as an overtone of the D-peak [44] The presence of a
prominent D-peak in CFs based on polyacrylonitrile (PAN) is typically attributed to poor
graphitization and is never present in pure crystallized graphite [44] The Grsquo peak appears
quite prominent in both CF spectra presented here though there appears to have been some
MANUSCRIP
T
ACCEPTED
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22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
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23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
26
[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
MANUSCRIP
T
ACCEPTED
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27
[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
28
[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
22
moderate increase in graphitization following brazing Additionally the loss of the D-peak
overtone (the Grsquo peak) potentially suggests the increased crystallinity of the material due to
increased graphitization However it is important to note that the graphitization process
during commercial CF manufacturing occurs at around 2600 ˚Cmdashmuch higher than is used to
create Au contacts in this study [44] The Raman spectra of the rLCGO fiber revealed several
peaks typically seen in the spectra of graphite carbon fibers and graphene oxide (Fig 11cd)
Some major changes occurring following brazing include (1) loss of a peak at ~1100 cm-1
(2) increase in DG intensity ratio and (3) loss of a ldquoshoulderrdquo on the Grsquo peak It is possible
that the loss of these peaks is the result of the loss of a surface functionality as a result of high
temperature treatment such as a C-O group However further investigation is required to
acquire a full understanding of the structural changes occurring not only as a result of the high
temperature processes in the presence of various metals In an attempt to improve the thermal
and electrical properties of CNTs and composites gamma irradiation and high temperature
treatments have been previously investigated [45] The CNT yarn spectra suggest a decrease
in the intensity ratio of DG peaks following brazing suggesting the decrease of ldquodisorderrdquo in
the lattice (Fig 11e-g) A similar result was observed by Pieoli et al following thermal
treatment of CNT yarns [45] Given that electrical properties are retained it is suggested that
the vacuum brazing processes described here do not have deleterious effects on carbonaceous
wires
33 Integrating diamond substrates with carbonaceous electrodes
Two varieties of carbonaceous electrodes were integrated with PCD circuit boards (i)
nitrogen-doped ultrananocrysalline diamond (N-UNCD) and (ii) CF bundles The respective
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
23
processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
24
milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
MANUSCRIP
T
ACCEPTED
ACCEPTED MANUSCRIPT
25
to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
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processes for integrating these materials with a suitable substrate differ considerably For N-
UNCD stimulating electrodes our group has demonstrated a method to grow the conductive
diamond layer atop the insulating PCD followed by shaping and isolation with a laser [21]
This method produces a hermetic electrode array for implantable neural prosthetic
applications However the integration of this array with standard electronic components
requires a metallic interface utilizing the embedded circuit board technology described here
A Micro-CT image demonstrates the complex three-dimensional assembly of embedded braze
interconnects and contact pads utilized to contact conductive diamond electrodes (dotted
circle) in a full electrode array device (Fig 12a)
Fig 12 ndash Integrated carbonaceous electrode devices (a) micro-CT image of a PCD circuit board with N-UNCD electrodes The electrodes are contacted by ABA and also used to create fan-out tracks that are embedded into the PCD substrate (bc) A single CF electrode is integrated into a PCD substrate enabling a high density array of penetrating electrodes
Incorporating carbonaceous wire electrodes into electronic devices is less elegant and
currently requires the manual placement of individual wire electrodes into designated laser-
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milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
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to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
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milled grooves in the PCD substrate Next the suitable brazing paste is applied though a
screen-printing technique is required to bypass the mechanical polishing step Avoiding
mechanical polishing is important for delicate small and fragile electrode materials such as
those described in this work However because the brazing pastes are comprised of both
metal beads and a polymeric binder the functional volume of the interconnect is reduced
upon melting (Fig 12b) Despite the challenges associated with fabrication we have managed
to produce a CF array embedded in diamond (Fig 12c) We are currently investigating
methods for protecting the CFs and other carbonaceous wires during mechanical polishing to
enable more complex robust device architectures
4 Conclusion
Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires
(CNT yarns CF bundles and rLCGO fibers) have been described with biocompatibility of all
components paramount For PCD electronics Ag ABA is used due to its wetting and covalent
bonding to diamond via reactive formation of TiC though Ag is susceptible to corrosion in
physiological saline [46] Au ABA can be used following creation of an Ag ABA interfacial
layer as it does not wet PCD Our group has recently confirmed the biocompatibility of the
AgAu ABAPCD system [20] Both Au and Ag contact pads of the embedded PCD circuit
boards are compatible with commercially-available solders and are easily laser-welded for
solder-free joining The non-wetting method of enclosing carbonaceous wires was used to
create strong low-resistance and non-rectifying connections between PtIr wires and
carbonaceous wires Structural and interfacial examinations of these structures lend insight
into the forces at play during both the wetting and non-wetting fabrication of carbonaceous
devices We exploit surface tension effects of two commercially-available active brazing
alloys to create two different device architectures
(1) The wetting technique was used in conjunction with mechanical polishing to create
robust circuit substrates with embedded Ag or Au interconnects and contact pads The option
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to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
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to braze a variety of carbonaceous wires or conductive diamond electrodes into the substrate
illustrates the utility of the method for enabling carbon-based electrode arrays
(2) The non-wetting technique was developed to enable freestanding carbon wires with Au or
AuPtIr wire contacts This technique is interesting for the creation of not only free-standing
carbonaceous conductors but also for neural probes or sensors for both central and peripheral
nervous system integration We have previously described methods to fabricate neural
stimulation and recording electrodes from carbonaceous wires described here such as rLCGO
fiber which demonstrated both in vitro and in vivo functionality [47]
Acknowledgements
Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for
insightful technical conversations NVA is supported by a MMI-CSIRO Material Science
PhD Scholarship DJG is supported by ARC DECRA grant DE130100922This research was
supported by the Australian Research Council (ARC) through its Special Research Initiative
(SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA) JF is
supported by the Australian Research Council under Discovery Early Career Researcher
award (Javad Foroughi DE12010517) GGW acknowledges ARC Laureate Fellowship and
support through the ARC Centre of Excellence for Electromaterials Science
Supplementary Information Available Figures S1-S4 This material is available free of charge online References
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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
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[1] Li Y-L Kinloch I Windle A Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis Science 2004304(5668)276-8 [2] Jiang K Wang J Li Q Liu L Liu C Li C et al Superaligned Carbon Nanotube Arrays Films and Yarns A Road to Applications Adv Mater 201123(9)1154-61 [3] Ko F Gogotsi Y Ali A Naguib N Ye H Yang G et al Electrospinning of continuous carbon nanotube‐filled nanofiber yarns Adv Mater 200315(14)1161-5 [4] Mylvaganam K Zhang LC Ballistic resistance capacity of carbon nanotubes Nanotechnology 200718(47)475701 [5] Pugno N The role of defects in the design of space elevator cable From nanotube to megatube Acta Mater 200755(15)5269-79 [6] Jakubinek MB Johnson MB White MA Jayasinghe C Li G Cho W et al Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns Carbon 201250(1)244-8 [7] Behabtu N Green MJ Pasquali M Carbon nanotube-based neat fibers Nano Today 20083(5)24-34 [8] Lima Mr Li N de Andrade M Fang S Oh J Haines CS et al Electrically chemically and photonically powered torsional and tensile actuation of hybrid carbon nanotube yarn muscles Science 2012338(6109)928-32 [9] Foroughi J Spinks G Wallace G Oh J Kozlov M Fang S et al Torsional Carbon Nanotube Artificial Muscles Science 2011334(6055)494-7 [10] Galvan Garcia P Keefer E Yang F Zhang M Fang S Zakhidov A et al Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns Journal of biomaterials science Polymer edition 200718(10)1245-61 [11] Lekawa Raus A Kurzepa L Peng X Koziol K Towards the development of carbon nanotube based wires Carbon 201468597-609 [12] Abbas S Sevimli O Heimlich M Esselle K Kimiaghalam B Foroughi J et al Microwave Characterization of Carbon Nanotube Yarns For UWB Medical Wireless Body Area Networks IEEE Trans Microwave Theory Tech 201361(10)3625-31 [13] Schmidt A Wang X Zhu Y Sombers L Carbon Nanotube Yarn Electrodes for Enhanced Detection of Neurotransmitter Dynamics in Live Brain Tissue ACS Nano 20137(9)7864-73 [14] Roberts GS Singjai P Joining carbon nanotubes Nanoscale 20113(11)4503-14 [15] Schauerman CM Alvarenga J Staub J Forney MW Foringer R Landi BJ Ultrasonic Welding of Bulk Carbon Nanotube Conductors Adv Eng Mater 2014 [16] Wu W Hu A Li X Wei J Shu Q Wang K et al Vacuum brazing of carbon nanotube bundles Mater Lett 200862(30)4486-8 [17] Burda M Lekawa-Raus A Gruszczyk A Koziol KK Soldering of Carbon Materials Using Transition Metal Rich Alloys ACS Nano 20159(8)8099-107 [18] Cortizo MC De Mele MFL Cytotoxicity of copper ions released from metal Biol Trace Elem Res 2004102(1-3)129-41 [19] Ganguly BB Talukdar G Sharma A Cytotoxicity of tin on human peripheral lymphocytes in vitro Mutation Research Letters 1992282(2)61-7 [20] Lichter SG Escudieacute MC Stacey AD Ganesan K Fox K Ahnood A et al Hermetic diamond capsules for biomedical implants enabled by gold active braze alloys Biomaterials 201553(0)464-74 [21] Ganesan K Garrett DJ Ahnood A Shivdasani MN Tong W Turnley AM et al An all-diamond hermetic electrical feedthrough array for a retinal prosthesis Biomaterials 201435(3)908-15 [22] Sung J Sung M The brazing of diamond International journal of refractory amp hard metals 200927(2)382-93
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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
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[23] Zhu WJ Wang J Liu LB Liu HS Jin ZP Leinenbach C Modeling and simulation of the TiC reaction layer growth during active brazing of diamond using DICTRA Comput Mater Sci 20137874-82 [24] Artini C Muolo ML Passerone A Diamond-metal interfaces in cutting tools a review J Mater Sci 201247(7)3252-64 [25] Singh M Asthana R Shpargel TP Morscher GN Active metal brazing and characterization of brazed joints in titanium to carbonndashcarbon composites Materials science amp engineering A Structural materials properties microstructure and processing 2005412(1-2)123-8 [26] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface II Titanium contacts A carbide-forming metal Physical Review B 199245(20)11975 [27] Song X-r Li H-j Zeng X Brazing of CC composites to Ti6Al4V using multiwall carbon nanotubes reinforced TiCuZrNi brazing alloy J Alloys Compd 2016664175-80 [28] Ricci E Novakovic R Wetting and surface tension measurements on gold alloys Gold Bull 200134(2)41-9 [29] Abragravemoff MD Magalhatildees PJ Ram SJ Image processing with ImageJ Biophotonics international 200411(7)36-43 [30] Stalder AF Melchior T Muumlller M Sage D Blu T Unser M Low-bond axisymmetric drop shape analysis for surface tension and contact angle measurements of sessile drops Colloids Surf Physicochem Eng Aspects 2010364(1)72-81 [31] Zhang M Atkinson KR Baughman RH Multifunctional carbon nanotube yarns by downsizing an ancient technology Science 2004306(5700)1358-61 [32] Jalili R Aboutalebi S Esrafilzadeh D Shepherd R Chen J Aminorroaya Yamini S et al Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide Towards Multifunctional Textiles Adv Funct Mater 201323(43)5345-54 [33] Frage N Froumin N Dariel MP Wetting of TiC by non-reactive liquid metals Acta Mater 200250(2)237-45 [34] Miedema A The electronegativity parameter for transition metals heat of formation and charge transfer in alloys Journal of the less common metals 197332(1)117-36 [35] Vanhoestenberghe A Donaldson N Lovell N Suaning G Hermetic encapsulation of an implantable vision prosthesis-combining implant fabrication philosophies Conference Paper IFESS [36] Lipa M Chappuis P Deschamps P Brazed graphite for actively cooled plasma-facing components in Tore Supramdashdescription tests and performance Fusion Sci Technol 199119(4)2041-8 [37] Soliman H Waheed A Effect on differential thermal expansion coefficient on stresses generated in coating Journal of Materials Science amp Technology(China)(USA) 199915(5)457-62 [38] Martin W Entwisle MF Thermal expansion of graphite over different temperature ranges J Nucl Mater 196310(1)1-7 [39] Dubey A Yadava V Experimental study of NdYAG laser beam machiningmdashAn overview J Mater Process Technol 2008195(1)15-26 [40] Tachibana T Williams B Glass J Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface I Gold contacts A non-carbide-forming metal Physical Review B 199245(20)11968 [41] Woumlrner E Wild C Muumlller-Sebert W Locher R Koidl P Thermal conductivity of CVD diamond films high-precision temperature-resolved measurements Diamond Relat Mater 19965(6)688-92
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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9
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[42] Wulf E Bachmann H Moehwald K Eifler R Maier H Moumlhwald K The influence of brazing temperature and surface roughness on the wettability of reactive brazing alloys Int J Mater Res 2014105(3)240-8 [43] Jayasinghe C Amstutz T Schulz M Shanov V Improved Processing of Carbon Nanotube Yarn Journal of Nanomaterials 201320131-7 [44] Melanitis N Galiotis C Tetlow PL Characterization of PAN-based carbon fibres with laser Raman spectroscopy J Mater Sci 199631(4)851-60 [45] Pierlot A Woodhead A Church J Thermal annealing effects on multi-walled carbon nanotube yarns probed by Raman spectroscopy Spectrochimica acta Part A Molecular and biomolecular spectroscopy 2014117598-603 [46] Sarkar NK Fuys RA Stanford JW The chloride corrosion behavior of silver-base casting alloys J Dent Res 197958(6)1572-7 [47] Apollo NV Maturana MI Tong W Nayagam DA Shivdasani MN Foroughi J et al Soft Flexible Freestanding Neural Stimulation and Recording Electrodes Fabricated from Reduced Graphene Oxide Adv Funct Mater 201525(23)3551-9