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PeakForce - Scanning Microwave Impedance Microscopy
Teddy Huang1 and Oskar Amster2
1Sr. Application Scientist, Bruker Nano Surfaces, [email protected]
2Director of Sales and Marketing, PrimeNano Inc, [email protected]
PeakForce - Scanning Microwave Impedance Microscopy
Teddy Huang1 and Oskar Amster2
1Sr. Application Scientist, Bruker Nano Surfaces, [email protected]
2Director of Sales and Marketing, PrimeNano Inc, [email protected]
p-epi p-epi
n+ n+
n-LDD n-LDD
n-channel n-channel
p p p-epi
n+
n-LDD
n-channel
p
sMIM-C
Modulus
sMIM-R
Adhesion
Outline
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• Bruker nano-electrical measurements.
• PeakForce nano-electrical modes.
• Scanning Microwave Impedance Microscopy.
• Case studies:
Regular sMIM.
PeakForce sMIM.
• Summary.
AFM Nano-electrical Measurements
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Sample
Sample chuck
DC bias
Amplifier, filter and
gain stage
to A/D
AC bias
• Nano-electrical characterization: the probe and the
sample are parts of the electrical circuit.
• Bring macroscopic measurements to nanoscale.
• Bruker provides a versatile array of electrical
techniques for a multitude of applications.
Conductivity/Resistivity C-AFM, TUNA, SSRM
Electric Field EFM
Charge EFM, SCM
Surface Potential /
Work Function KPFM
Carrier Density SCM, SSRM
Piezoelectricity PFM
C-AFM: Conductive AFM
TUNA: Tunneling AFM
SSRM: Surface Spreading Resistance Microscopy
EFM: Electric Force Microscopy
KPFM: Kelvin Probe Force Microscopy
SCM: Scanning Capacitance Microscopy
PFM: Piezoelectric Force Microscopy
Bruker Nano-electrical Measurements
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SSRM, 1.2 x 0.6 um scan, cross-sectioned MOS transistors, log scale
KPFM, 2 x 1 um scan, potential map on InP nanowire with 3V electrical bias between contacts
SCM, 4 x 2 um scan, carrier diffusion of cross-sectioned double-diffused SiC MOSFET
PeakForce Tapping (2009, Bruker)
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• Probe is modulated at 1~2 kHz, allowing for imaging at high scan rate and high pixel resolution
• Feedback setpoint: maximum force or peak force of the tapping cycle.
• Sinusoidal ramping: direct force control of imaging forces with ultra-low setpoints (< 50 pN).
• Linear force control: automatic image optimization, ScanAsyst
• A triggered force curve at every tapping cycle: PeakForce QNM (Quantitative Nano Mechanics)
Adhesion Modulus Height
Brush polymer
2 nm
Sample courtesy: S. Sheiko, UNC, Chapel Hill
https://www.youtube.com/watch?v=wjguTT0rGXM
• Mechanical & Electrical properties are measured simultaneously
• Impossible in contact mode, as forces are too high.
• Higher resolution vs. contact mode.
PeakForce-enabled Electrical Measurements
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Electrical
Leclere et. al. Nanoscale, 2012, 4, 2705
Integration with PeakForce Tapping for inaccessible, delicate
samples and adds correlated nanomechanical data PeakForce-TUNA
PeakForce-SSRM
PeakForce-KPFM
Height Adhesion Current
700x700 nm scan size
• Improve tip lifetime with hard samples
• Decrease sample wear with soft samples
• Improve resolution due to sharper tips & less sample damage
407 S California Ave, suite 5 Palo Alto, California 94306 Phone: +1 (650) 300-5115 Email: [email protected]
Oskar Amster
Introduction to Electrical Scanning Probe Microscopy Measurements Using
Microwave Impedance Microscopy
Who is PrimeNano, Inc?
• Instrument company focused on imaging and metrology for research and industry
• Founded in 2010 in Palo Alto, CA
• Patented technology spun out of Stanford University Applied Physics Dept.
• ScanWaveTM is an electrical module integrated with Bruker’s Icon AFM.
9
Probe the Nano Scale
STM
Atomic scale density of states
Conductive AFM
Spreading resistance
Electrostatic Force Microscope /Kelvin Probe Microscope
Work function / Capacitive Coupling
Scanning Gate Microscope
Current flow path
Scanning Capacitance Microscope
Doping level in semiconductor
Microwave
Microwave Impedance Microscope
Local (s, e) Probe of electrical properties
What is missing?
10
What is sMIM? – A mode for AFM
• Direct measurement of electrical properties – Image local variation of e (permittivity) and s
(conductivity) – Measure carrier type and carrier concentration
for doped semiconducting samples
• Compatible with all materials – Images dielectrics, insulators, semiconductors &
metals
• Compatible with typical AFM imaging modes: – contact, tapping mode, and PeakForce Tapping
• Sub-surface sensitivity – Can image through ~100+ nm overlayers
11 11
Why Microwaves?
• Optics (light) have poor contrast
Silicon (poor conductor, s = 0.0016)
Aluminum (good conductor, s = 3.5e7)
Sapphire (Al2O3, e ~ 9) Glass(SiO2, e ~ 4) sMIM of SiO2 in Si2N3
• Microwaves have high contrast
sMIM of Al dots on SiO2
High Contrast between metals and insulators
SiO2 SiO2
12
Basic Theory (electrical model)
C = e*(A/d) R = (1/s)*(d/A)
How do do we get to this from Microwaves?
13
3. Relation of physical parameters (ε,σ) to lumped element model
4. Changes in ε and σ are seen as changes in
C & R respectively
2. Probe-sample impedance, Ztip, can be
expressed as a lumped element model.
1. Probe/sample interface has its own impedance Metal probe Oxide interface
Sample
(leaky capacitor) capacitor and
resistor in parallel
Z Z Z
Z Z Z
Contrast Mechanism (How Does it Work?)
1. 3GHz Microwave is generated by the ScanWave electronics
Complex impedance is made up of 2 components:
Ztip = Z(Re) + Z(Im)
2. At the probe/sample interface there is a microwave reflection due to a variation in the system
impedance from 50 ohm.
Z(Re) Resistance Z(Im) Capacitance
ScanWave presents these two components as output signals from
the electronics, (sMIM-R and sMIM-C)
By interfacing with the scanning AFM we can synchronize the the output sMIM signals with the
topography and display two images representing the variation in permittivity and conductivity
Ztip
14
Contrast Mechanism (How Does it Work?)
1. 3GHz Microwave is generated by the ScanWave electronics
2. At the probe/sample interface there is a microwave reflection due to a variation in the system
impedance from 50 ohm.
Image Reflection as tip scans.
Real reflection
x
MIM-Re MIM-Im
Imaginary reflection
y
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Shielded Cantilever Probe
(e,s) (e,s)
20 μm
Shielding is important to reduce stray coupling Low loss, low capacitance Sharp tip (~ 50 nm) Batch fabrication
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sMIM: Basic Operation
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• 6 Channels of data available simultaneously with Topography • sMIM-C; sMIM-R • dC/dV Phase & Amplitude • dR/dV Phase & Amplitude
• sMIM-C; sMIM-R Non-contact imaging (dC/dZ; dR/dZ) • sMIM-C; sMIM-R with PeakForce Tapping
6 output channels
Compatibility of Materials
Semiconductors
Silicon FinFETs
Semiconductor/Metallic
Tre
nch
gat
e
Gat
e o
xid
e la
yer
Emitter region
Gate contact
Emitter region
Common emitter/Source metal
Insulators/Dielectrics
Oxide buried under SiN
Compact disk
SiO2
SiO2
SiO2
SiO2
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Series of bulk insulator samples
• sMIM-C is proportional to ε (permittivity)
– Measure dielectric constant over wide range
sMIM-C proportional to Capacitance
C R C = e*(A/d)
Im(z)
Geometric term = probe tip & distance
0
0.2
0.4
0.6
0.8
1
1.2
1 10 100 1000
sMIM vs Dielectric Constant
Prox. Mod Results
System Model
Dielectric Constant
sMIM
Sig
nal
(V
)
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0.1
0.2
0.3
0.4
0.5
1E+15 1E+16 1E+17 1E+18 1E+19 1E+20
sMIM vs Doping Level
Acceptor Concentration (/cm3)
sMIM
Sig
nal
(V
)
sMIM-C proportional to Capacitance
IMEC staircase studies
• Linear response with log doping concentration
– Sensitivity from 1014 atoms-cm3 – to 1020 atoms-cm3
Metal
Oxide
Semiconductor
Depletion
layer Vgate
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ScanWave Electronics Capacitance Sensitivity
Measure Au dot on thickest oxide layer
Smallest Au area – 0.1fF
Sensitivity of 0.15af/mV (calculated)
Measured NIST Capacitance standard using ScanWave
sMIM Image
Line profile from capacitance standard
0.3aF RMS
Schematic of NIST C standard
• 4 different Au dot sizes
• 4 different SiO2 step sizes
• Resulting in 16 different Capacitors
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.1fF
ScanWave on Bruker ICON Instrument
PrimeNano bracket
Probe
interface
module
Microwave
Electronic
Module
• ScanWaveTM has easy integration with the Bruker instruments
• PrimeNano has made a custom probe interface module for the ICON
• Probe interface bracket uses same scanner placement and screw holes as the SCM electrical module
• PrimeNano probe interface module uses standard Bruker pins to secure to the scanner
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Case Studies: Contact sMIM
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• Buried Structures.
• Cobalt Surface-Modified γ-Fe2O3.
• Inverted Gate Bipolar Transistor (IGBT).
• Static Radom Access Memories (SRAM).
• CMOS Image Sensor.
sMIM-C
SiO2
SiO2
SiO2
SiO2
sMIM-C sMIM-R
Phase overlays on height sensor
Polished Si3N4 film, buried SiO2 patterned structures
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• Surface was polished to eliminate residual
topographic features.
• SiO2 and Si3N4 are both insulating, no variation in
sMIM-R over the sample.
• Permittivity difference between SiO2 (ε = 3.9) and
Si3N4 (ε = 7.5), sMIM-C shows different capacitive
response. sMIM-R
SiO2 SiO2
sMIM-C SiO2
SiO2
sMIM is a near-field technique: long range sensitivity to local permittivity variation
sMIM on SRAM Sample
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sMIM-C sMIM dC/dV Amplitude sMIM dC/dV Phase Height Sensor
n+ n+
n-LDD n-LDD
n-channel n-channel
p p
n+
n-LDD
n-channel
p • Image variation of local permittivity.
• SCM modulates sample without
knowing the DC properties. Phase overlays on Height Sensor
)(
)(
Vdd
AVC
dep
Si
oxox
oxMOS
e
ee
• Capable of resolving semiconductor specification.
sMIM on SRAM Sample
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sMIM dC/dV Phase sMIM dC/dV Amplitude sMIM-C
Phase overlays on height sensor
sMIM on SRAM Sample
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Phase overlays on height sensor
• Sharp phase contrast on the
topographically featureless region.
• sMIM resolves transitions in carrier
type < 20 nm.
sMIM on Vertical Insulated Gate Bipolar Transistor (IGBT)
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• sMIM-C shows device structure.
• sMIM carrier profiling resolves electronic structures.
• sMIM provides a level of information that usually
requires both the SEM and SCM to provide the full level
of device structural detail.
sMIM dC/dV Amplitude sMIM dC/dV Phase
p
n
SEM capability N-Substrate
Poly
-Si
(p-type)
base
(n-
type)
Tren
ch gate
Gate
oxid
e laye
r
Emitter region
Gate contact
sMIM-C
1 um
1 um 1 um
p
n
n-Type
p-Type
Highly
Or
Un-
doped
Phase x Amplitude
1 um
This IGBT was prepared by ChipWorks
Substrate
Base
sMIM on CMOS Image Sensor
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N-type cathode
P-type implant
P-type pinning
• Small feature size
• Doping gradient on p-doped region
• Pinning layer thickness and spacing
0.7 um
1 um
0.8 um
P-type pinning
140 nm
~100 nm
sMIM dC/dV Phase
sMIM dC/dV Amplitude
~75 nm
sMIM-C Doping gradient
Doping gradient
This CMOS sensor was prepared by ChipWorks
Cobalt Surface-Modified γ-Fe2O3
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960 nm x 960 nm
1.Magnetic materials
2. Inhomogeneous conductivity
Height Sensor sMIM-C sMIM-R
PeakForce-TUNA MFM
Cobalt Surface-Modified γ-Fe2O3
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sMIM-R sMIM dR/dV Amplitude
sMIM-C sMIM dC/dV Amplitude sMIM dC/dV Phase
• Carrier profiling for
nanoparticle film.
• dC/dV shows phase
domains.
sMIM dR/dV Phase
• dR/dV shows phase
domains.
• Nonlinear dielectric
properties.
Cobalt Surface-Modified γ-Fe2O3
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sMIM-C
sMIM-R
sMIM dC/dV Phase
sMIM dR/dV Phase
sMIM dC/dV
Phase on Height
sensor
sMIM dR/dV
Phase on Height
sensor
• Sharp transition from one phase to another: ~ 10 nm.
• dC/dV and dR/dV do not necessarily have the same phase
distribution.
• Different electrical properties within one particle domain.
Voltage Sweeping: sMIM-C vs Voltage (C-V characteristics). sMIM-R vs Voltage (R-V characteristics).
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• Direct measurement of the DC signals allows for local CV and RV sweeping.
• Reversed phases expect reversed slopes, confirmed by CV and RV sweeping.
sMIM dC/dV Phase
Sample Bias (V)
-1.0 -0.5 0.0 0.5 1.0
sMIM
-C (
mV
)
-10
-5
0
5
10
1
2
1
2
sMIM dR/dV Phase
Sample Bias (V)
-1.0 -0.5 0.0 0.5 1.0sM
IM-R
(m
V)
-20
-10
0
10
1 3
1
3
Case Studies: PeakForce-sMIM
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1) First trace/retrace: cantilever tracks surface topography with
PeakForce tapping.
2) Cantilever ascends to Lift height.
3) Second trace/retrace: cantilever profiles topography while collecting
sMIM data.
Contact time
Force vs. time
sMIM vs. time
• Capture sMIM data averaged over a full tapping circle.
• Capture sMIM signal during contact time are measured
• Tip oscillates at 1kHz. Contact time is typically 20 – 200 µs
One-path measurement, similar to PeakForce TUNA
Two primary approaches to integrate PeakForce tapping with sMIM
Interleave scanning, similar to PeakForce-KPFM
PeakForce-sMIM on Carbon Nanotube
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• Convenient for delicate samples.
• Differentiation of CNTs with insulating, semiconducting,
and metallic properties .
• No need to make electrical contacts.
• Simultaneously mapping mechanical properties.
sMIM-R
PeakForce-sMIM on IGBT
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N-Substrate
Poly-
Si
(p-type)
base
(n-type)
Common emitter/source
metal
• PeakForce Tapping eases the scanning on the rough region,
challenging for contact mode.
• PeakForce-sMIM increases tip lifetime.
• Higher resolution due to a sharper tip.
6 µm
Height Sensor
sMiM-C
Emitter
Base
Ga
te o
xid
e
Tre
nc
h g
ate
Common emitter/source metal
Topography from
contact-sMIM
PeakForce-sMIM on SRAM
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• PeakForce Tapping advantages.
• High data quality, no flattening on sMIM-C channel.
• Resolve detailed electronic variations on sMIM-C and dC/dV channels.
sMIM-C overlays on Height Sensor dC/dV Phase overlays on Height Sensor dC/dV Amplitude overlays on Height Sensor
sMiM-C
n-channel
n-LDD
p
n+
PeakForce-sMIM on SRAM
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sMIM dC/dV Phase
sMIM dC/dV Amplitude
sMIM-C
• High lateral resolution in the dC/dV phase image.
• The p-n junction is well defined.
• The depletion region edges are resolved even
below the probe tip radius.
Summary
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• Bruker AFMs are platforms for comprehensive electrical measurements, and PeakForce
Tapping extends the applications of these nano-electrical modes.
• sMIM is a powerful tool for direct measurement of material electric properties on various
materials with resolution in the 10’s of nm’s.
• The integration of sMIM with PeakForce Tapping expands electrical measurement to
otherwise inaccessible, delicate samples and adds correlated nanomechanical data.
n+
n-LDD
n-channel
p
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If you need Bruker AFM help:
Atomic Force Microscope Technical Support Group
Phone: +1 800-873-9750
E-mail: [email protected]
Website: www.bruker.com
Bruker Support: http://brukersupport.com/
Resources: www.nanoscaleworld.bruker-axs.com
Bruker Probes: www.brukerafmprobes.com
Expert Training: http://www.bruker.com/service/education-training/training-courses/afm-optical-training-courses.html
Teddy Huang, Ph.D.
Sr. Applications Scientist, Electricity and Electrochemistry
Bruker Surfaces Business
112 Robin Hill Road, Santa Barbara, CA 93117
Email: [email protected]
Tel: (805) 967-2700 x2431; Fax: (805) 967-7717
Thank you for your attention!
Oskar Amster
Director of Marketing
PrimeNano, Inc.
407 S. California, suite 5
Palo Alto, CA 94306
Tel: 650-300-5115; Fax: 650-300-5200
If you need PrimeNano sMIM help:
PrimeNano Technical Support
Phone:650-300-5115
E-mail: [email protected]
Website: www.primenanoinc.com
Cross-section analysis
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sMIM dC/dV Phase sMIM dR/dV Phase