Valente - SRF thin films produced by energetic condensation

Anne-Marie VALENTE-FELICIANO

J. Spradlin, L. Phillips, C. Reece, B. Xiao, X. Zhao

R.A. Lukazew, D. Beringer

D. Gu

K. Seo

4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity

OUTLINE

Approach

Nb growth on Sapphire

Nb growth on MgO

Nb growth on Cu surfaces

Concluding Remarks

4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

Approach


Engineered Nb films:

Growing and characterizing niobium films with controlled deposition energy and substrate temperature via Electron Cyclotron Resonance (ECR) and High Power Impulse Magnetron Sputtering (HIPIMS) in self-sputtering mode.Monitoring in-situ crystal character dependence on substrate properties and deposition parameters.

Film growth approach in 3 sequential phases:

Film nucleation on the substrate (Nb, Al2O3, Cu; single crystal & polycrystalline)Growth of an appropriate template for subsequent deposition of the final

RF surfaceDeposition of the final surface optimized for minimum defect density.

Engineered Nb films & SIS Multilayer Structures

In the framework of a collaboration with surrounding universities, JLAb is exploring 2 opportunities:

Tailoring for optimum RF performance

Careful characterization of the attained composition and

microstructure

Close association with resulting rf surface impedance and

superconducting properties


Connecting Structure & Performance for SRF Surfaces

involves understanding

of

The chemistry of the involved speciesReactivityStoichiometric sensitivityReaction process temperatures

Crystal structure dependence on substrate structureInfluence of deposition energy on resulting structureSensitivity to the presence of contaminating speciesStabilization of desired film against subsequent degradation

Characterization of deposited film surfaces In-situ crystallographic structure characterization – (RHEED), Scanning Tunneling Microscopy (STM)Large area crystallographic structure – X-ray diffraction (XRD)10 nm-scale crystallographic texture within ~ 50 nm of surface – Electron backscatter diffraction (EBSD) Topography – stylus profilometry, atomic force microscopy (AFM), optical profilometryNear surface (< 8 nm) chemistry – X-ray photoelectron spectroscopy (XPS)Micro-contaminant defects – Secondary ion mass spectrometry, with standards (SIMS)Structural cross-section of film – Transmission electron microscopy (TEM), Focused Ion Beam (FIB).



• surface self-diffusion• competitive grain growth• defect density• back-sputtering• preferential sputtering rates

Grain boundaries have an effect on surface resistance.

Film quality is dependent on deposition technique:

• surface roughness• defect density• Back-sputtering from energetic condensation


Measurement with the SIC cavity (TE011 sapphire-loaded cylindrical Nb cavity) Surface impedance as a function of magnetic field and temperature from 1.9 K to 4.8 K.Normal state surface impedance at 10 K, from which the surface value of electronic mean free path and surface Hc1 can be determined.Superconducting penetration depth, λ, at low field will be measured by carefully tracking the cavity frequency with temperature as the sample temperature is swept slowly back and forth across the transition temperature (SIC sensitivity: 30 Hz/nm) while the rest of the cavity is held at 2 K.

Tc – easy coarse measure of intra-grain quality of the film RRR – convenient assessment of aggregate defect density


High-energy deposition techniques

Crystalline defects, grains connectivity and grain size may be improved with an higher substrate temperature which provides higher surface mobility (important parameter is Tsubstrate/Tmelting_of_film)

However the Cu substrate does not allow heating

The missing energy may be supplied by ion bombardment In bias sputter deposition a third electron accelerates the noble gas ions, removing

the most loosely bound atoms from the coating, while providing additional energy for higher surface mobility

“Structure Zone Model”

– Other techniques allow working without a noble gas, by ionizing and accelerating directly the Nb that is going to make up the coating

– These techniques allow also to obtain “conformal” coatings that follow the surface profile better filling voids.


Film growth


The grain size of a polycrystalline film is affected by:• The substrate temperature during deposition (high for large grains)• Adatom diffusivity (high)• The annealing temperatures (high)• The deposition flux (low)• The impurity content (low)• The film thickness (high)* Energy of the deposited atom (high)• Energy of bombarding ions/atoms (high)* Tm of Material (low)• The materials class (metals)

Energetic Condensation via ECRNiobium vapour produced by an e-beam gun is ionized by an ECR process. The Nb ions can be accelerated to the substrate by an appropriate bias. Energies in excess of 100 eVcan be obtained.

Generation of plasma

3 essential components:Neutral Nb vaporRF power (@ 2.45GHz)Static B ERF with ECR condition

Why ECR?No working gas

High vacuum ie. reduced impurities

Singly or “quasi-singly charged” ions

Controllable deposition energy

90 deposition flux

(Possible to help control the crystal structure)

Excellent bonding

No macro particles

Faster rate (Conditional)

Wu, G., et al. J. Vac. Sci. Technol. A Vol. 21, No. 4, (2003)

m

eB


Energetic vacuum deposition by ECR plasma


Nucleation studies in UHV deposition system at the College William & Mary

In-situ observation of the nucleation and subsequent growth with coating parameters, annealing… on single crystal and polycrystalline substrates.

UHV system with in-situ RHEED & STM


Samples produced on different substratesInsulating:

Al2O3 (11-20)Al2O3 (11-20)MgO(100)SrTiO3

Metallic:Cu (100)Cu (110)Cu (111)

Fine grain CuLarge grain Cu

Typical vacuum during plasma in ECR system:<5x10-8 Torr4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

ECR growth rate

Films with thicknesses up to 4mm with 0V bias and 2mm with 120V bias have been produced

No problem of de-lamination for thick films even when important lattice mismatch as on Cu and MgO

Only peel-off issue on Cu when heated to temperatures higher than 400°C due to roughening (P. Zeppenfeld et al., PRL 62,1, Jan 89)

Troughening Cu (111)~400°CTroughening Cu (110), (100)>700°C

Adhesion to substrate

Growth rates from 15 nm/min to 135 nm/mi. have been achieved2’ 270nm with -120V bias 45’ 4mm with 0V bias and 2mm with -120V bias

Nb films creationECR films coated with different bias voltages (0 to 150V, 300W RF input power) on Al2O3 ( a- & c- plane), Cu2O and Cu (substrate heat-treated in-situ and coating at same temperature).Magnetron sputtered films on a-plane sapphire @ 600°C (after 1h annealing @ 600°C ) with thicknesses up to 600nm (7h), 60-120W, PAr= 1x10-3Torr.Metallic substrate surface preparation need to be optimized to allow in-situ RHEED and STM observations.


Niobium epitaxy on sapphireAnticipated epitaxial relationships between Nb (bcc) and Al2O3 (hcp)

A. R. Wildes et al., Thin Solid Films, 401 7 (2001)

Sapphire as a substrate is a suitable proving ground for niobium thin film studies due to the low lattice mismatch (~1.9-12%) and comparable rates of thermal expansion.



ECR, bias -120V, 20 minutes, Bake-out (18hrs) and coating at 450ºC

Nb/a-Al2O3 (11-20)

150 x 150 μm, 1 μm resolution, CI Avg. 85

Nb/Al2O3 (11-20), magnetron sputtered @ 600°C50nm

RMS Roughness=4.84 nmRRR = 96

After 50 nm

600 nm

XRD pole figures about Nb(110)peak indicate (011) texturing along the 2 possible in-plane orientations in the thinner films while a strong preferred orientation was observed in the

thicker film, consistent with the observed surface morphology.

Strain Evolution & Structure

In heterostrained growth occurs wherein the film’s lattice spacing deviates from bulk equilibrium values. As the film grows thicker, the strain may be relieved by the formation of lattice defects such as vacancies or dislocations. RHEED(Reflection High Energy Electron Diffraction) is a surface sensitive technique that was used to characterize strain evolution in this case. The niobium lattice parameter at the surface can be determined by analyzing the spacing of the characteristic streaks.

RHEED

•RHEED images were collected for epitaxial niobium for varying thicknesses and growth parameters.•Image processing with MATLAB allows for a systematic way to abstract information related to surface crystallinityand morphology.•A specific image can be used to calculate the lattice parameter corresponding to the top most layer of the sample using the following equation: a*= 2pW/lL•Curve-fitting models are then applied to obtain quantitative information to extract the in-plane strain and lattice parameters


Structure vs. bias voltage -Al2O3 (11-20)

Bias [V] 0V -30 -60 -120 -150

Tc [K] 9.10 9.26 9.10 9.2 9.00

DTc [K] 0.08 0.02 0.20 0.04 0.10

RRR 132 141 179.9 179.8 152.6

PF (110) (110) (110) (110) (110)

Orientation spread [°]

0.281 0.244 0.183 0.115 0.161

15mm x 15mm, 0.1mmCI =0.835000x

0.025°Tc

DTc


Structure vs. Bake Temperature-Al2O3 (11-20)

Tbake[°C] 360 700 900

Tc [K] 9.20 9.19 9.23

DTc[K] 0.04 0.03 0.02

RRR 179.8 213.3 71.5

PF (110) (110) (110)

Orientation spread [°]

0.115 0.025 3.575

Thickness [nm] 840 1140 270

(Bias -120V)



ECR, bias -120V, 20 minutes, bake-out (1.5hrs) and coating @450ºC

Nb/c-Al2O3 (0001)

250 x 250 μm, 2 μm resolution, CI Avg. 30

CI =0.24

Sapphire (0001) substrate

0V -60V -150V

0.357 °

CI =0.58

CI =0.20

CI =0.54

CI =0.50

CI =0.69

CI =0.82 0.320° 0.269°

15 0 m

m x

150m

m, 1 m

m ste

p50

0x

500

0x

15 mm

x 15

mm

, 0.1 m

m ste

p


Nb/Al2O3 (0001) substrate, -150VODF project:

C:\...\ECR-Nb-0.5-c-Al2O3-042\PF

(110)-0.rw 1

Pole figure: 110 Raw

Intensities:

Psi Phi Intensity

Min 10.0 57.5 0.000

Max 0.0 352.5 71609.760

Dimension: 2D

Projection: Schmidt

Scale: Root

Colour map: Default

Contours: 100

Intensity Colour

1 7.020

26 4745.437

51 18258.699

76 40546.807

100 70198.765

Grid settings:

Psi Phi

First 0 0

Last 90 360

Step 10 10

ODF project:

C:\...\ECR-Nb-0.5-c-Al2O3-042\PF

(200)-0.rw 1


Intensities:

Psi Phi Intensity

Min 0.0 2.5 0.000

Max 55.0 202.5 1521.000

Dimension: 2D

Projection: Schmidt

Scale: Root

Colour map: Default

Contours: 100

Intensity Colour

1 0.149

26 100.794

51 387.817

76 861.219

100 1491.030

Grid settings:

Psi Phi

First 0 0

Last 90 360

Step 10 10

ODF project:

C:\...\ECR-Nb-0.5-c-Al2O3-042\PF

(211)-0.rw 1


Intensities:

Psi Phi Intensity

Min 0.0 52.5 0.000

Max 20.0 207.5 4096.000

Dimension: 2D

Projection: Schmidt

Scale: Root

Colour map: Default

Contours: 100Intensity Colour

1 0.402

26 271.434

51 1044.378

76 2319.233

100 4015.293

Grid settings:

Psi Phi

First 0 0

Last 90 360

Step 10 10

ODF project:

C:\...\ECR-Nb-0.5-c-Al2O3-042\PF

(222)-0.rw 1


Intensities:Psi Phi Intensity

Min0.0 172.5 0.000

Max0.0 317.5 4761.000

Dimension: 2D

Projection: Schmidt

Scale: Root

Colour map: Default

Contours: 100

Intensity Colour

1 0.467

26 315.502

51 1213.936

76 2695.769

100 4667.189

Grid settings:

Psi Phi

First 0 0

Last 90 360

Step 10 10

40 50 60 70 80 90 100 1102Theta-Omega (°)

0

2500

10000

22500

40000

62500

Inte

nsity (

counts

)

(110

) Nb

c-A

l2O

3

(222

) Nb

(110) (200)

(222)


Structure vs. bias voltage -Al2O3 (0001)Bias [V] 0 -30 -60 -120 -150

Tc [K] 8.85 9.21 9.28 9.24 9.22

DTc[K] 0.1 0.02 0.04 0.02 0.05

RRR 84 51 108 100 76

PF (111) (111) (111) (110) (110)

Thickness [nm] 0.88 1.02 1.33 1.14 0.53

Tc

DTc


Nb epitaxy on Al2O3

Nb films deposited with higher bias voltages on (0001) sapphire grow along (110) not (111) as anticipated.Due to higher ion incident energy? T. Wagner et al., J. Mat. Res. Vol. 11, nº5, pp. 1255-1264 (1996), Mat. Res. Symp. Proc, Vol. 440, pp.151-156, (1997)MBE Nb films on (0001) sapphire grow along (111) at 900°C, but (110) at 1100°C

I n all cases, Nb films grow along (110) on a-plane sapphire (11-20) for both magnetron sputtered film s and ECR films with different bias voltages.

RRR values from 8 to 213 have been obtained by varying the incident ion energy with a bias voltage, the cleanliness of the interface and ad-mobility of the atoms on the surface with baking and coating substrate temperatures.


Nb/MgO (100)

ECR, bias -120V, 30 minutes, Bake-out (24h) and coating at 360ºC

10.8% mismatch


RMS Roughness = 6.6nm

Nb/MgO



30 nm Nb/MgO

600 nm Nb/MgO

400nm

Magnetron sputtering 600nm , 600C, 7h, RRR~47 , ECR 620nm , 360°C, 30’, RRR~50


150 mm x 150mm, 1mm step500x

-150V

0.173°

-120V

75mm x 75mm, 1mmCI =0.161000x

0V -60V

CI =0.28

-30V


CI =0.42CI =0.19 CI =0.2

Nb/MgO (100)

Nb/Mgo(100), 700°C bake 6h, 360°C coating

15mm x 15mm, 0.1mm stepCI =0.455000x

RRR ~ 1151.34mm


Structure vs. bias voltage MgO(100)Bias 0V -30V -60 -90 -120 -150

Tc [K] 9.1 0.37 9.2 9.36 9.20 9.05

DTc 0.11 0.01 0.04 0.04 0.09 0.20

RRR 33 51 59 44 81 50

PF (110) (110) (110) (110) (110) (110)

Thickness [mm] 0.34 0.51 0.94 1.17 1.26 0.62

Tc

DTc


Nb growth on MgO

I n all cases, Nb films grow along (110) on MgO (110) for both magnetron sputtered films and ECR films with different bias voltages.

Despite a large lattice mismatch, RRR values up to 115 have been obtained by varying the incident ion energy, the baking and coating substrate temperatures.


Nb/Cu

ECR, bias -120V, 20 minutes, Bake-out (18hrs) and coating at 450ºC


Single crystal Cu substrates

(100) (110) (111)

0.733° 0.357 ° 0.432°

CI =0.58CI =0.59CI =0.51

50x 1500 mm x 1500mm, 25 mm step

Lattice mismatch with(100) 8.5%


150mm x 150mm, 1mmCI =0.49500x

15mm x 15mm, 0.1mmCI =0.245000x

15mm x 15mm, 0.1mmCI =0.065000x

150mm x 150mm, 1mmCI =0.24500x

150mm x 150mm, 1mmCI =0.21500x

15mm x 15mm, 0.1mmCI =0.785000x

Nb(110)//Cu(100) Nb(100)//Cu(110) (Nb(110)//Cu(111)

Nb hetero-epitaxy on Cu


150 mm x 150mm

Nb/Cu (100)

CI =0.46

CI =0.53

CI =0.28

Nb/Cu (111)

Nb/Cu (110)

-150V

1000mm x 1000mm, 50mmCI =0.1500x

150mm x 150mm, 1mmCI =0.09500x

15mm x 15mm, 0.1mmCI =0.275000x

150mm x 150mm, 1mmCI =0.14500x

-120V-60V-0V

150 mm x 150mmCI =0.34500x

CI =0.11

CI =0.12


Nb/Cu2O – High Resolution TEM


Effect of Bias Voltage for Nb/Cu2O

Obvious advantage: no noble gas for plasma creation

Sample tests: good RRR and Tc, 100-nm grain size, lower defect density and smooth surfaces

60eV

90eV 4000X4000 µm2

3-D Profilometer ImagesTEM Images

20 30 40 50 60 70 80 90 100 110

0

5000

10000

15000

20000

Inte

nsity)

2 theta

110 Nb

200 Cu

211 Nb 220 Nb311 Cu

NbOx

Sample: JLAB-ECR-028B

20 30 40 50 60 70 80 90 100 110

-2000

0

2000

4000

6000

8000

10000

12000

14000

16000

Inte

nsity

2 theta

110 Nb

200 Cu

220 Nb 311 Cu211 Nb

111 Cu

NbOx

Sample:JLAB-ECR-026A

200 Nb 220Cu


XRD pole figures show the presence of growth features (rotation the poles of 70°)

Dependence on thickness, ion incident energy and substrate temperature?


Nb/Cu – High Resolution TEM


150 x 150 μm, 1 μm resolution,CI Avg. 0.71



Bias -120V, 2mm Bias 0V , 4mm

Typical Cu substrate

Effect of Bias Voltage for Nb/Cu

Nb/Cu, fine grain – effect of Bias voltage

750mm x 750mm, 10mmCI =0.04100x

CI =0.05

-30V-60V -90V

-120V -150V

CI =0.25

150mm x 150mm, 1mm step500x

CI =0.32

CI =0.2CI =0.11


Ab-normal growth

750mm x 750mm, 10mm 100x

-120V, bake@360°CCI =0.09

-120V, bake@700°CSome peel-off due to Cu roughening

CI =0.29

Nb/Cu, fine grain – effect of bake temperature


Nb/Cu, large grain

CI =0.22

-30V -120V

-150V

750mm x 750mm, 10mm step100x

CI =0.2

CI =0.26

1500 mm x 1500 mm, 10mm step50x


Equi-axial growthTo be verified by cross-sectionEBSD

1st SIC Measurements on Nb/Cu2O & Cu


Nb growth on Cu substratesHetero-epitaxial relationship between Nb and Cu have been verified with ECR.

Films have been grown on both oxidized and metallic Cu surfaces.As a function of the bias voltage and substrate temperature, it appears that the film structure can be tailored from columnar growth to equi-axial growth in the case of epitaxy. Cross-section observation with EBSD and TEM are necessary to establish it.RRR measurements on films grown on Cu need to be performed for direct correlation with RF measurements


CONCLUDING REMARKSJLab in collaboration with surrounding universities (College William & Mary, Old Dominion University and Norfolk State University) is pursuing two opportunities to create viable superconducting RF cavity surfaces to reduce the cost framework of SRF accelerators and to reach higher gradients and allow operation of SRF structures at 4K.

One of them is to understand and develop niobium films with bulk-like performance by elucidating the functional dependence of film-grown Nb crystal texture, intra-grain defect density, and grain boundary impurities on SRF performance. Studies on the correlation of the surface resistance of films produced by energetic condensation ECR with surface and material properties of the film as a function of incident ion energy and substrate temperature are underway.

Nb films have been grown on a variety of substratesHigh RRR values have been achieved on Al2O3 and MgO substrates.

The Nb film structure can be tuned from columnar growth, ab-normal to equi-axial growth by varying the incident ion energy with the substrate temperature for temperatures lower than if using thermal energy only.


SRF Thin Films Collaboration

Under DOE HEP Grant ARRA & U.S. DOE Contract No. DE-AC05-06OR23177

Films creation:Jlab: A-M Valente-Feliciano, J. Spradlin, L. PhillipsW&M: A. Lukaszew, D. Beringer, S. ….

Material and RF characterization:Jlab: A-M Valente-Feliciano, J. Spradlin, L. Phillips, X. Zhao, B. Xiao, A. WuW&M: A. Lukaszew, D. Beringer, S. , R. Outlaw, O. TrofimovaNSU: K. SeoODU: H. Baumgart, D. GuBlack Labs LLC: R. Crooks


SIC Measurements for CED & ECR films


Cu thermal roughening

50mm x 50mmZ Range = 1.694 µm

RMS = 203.79 nmRa = 156.50 nm

Z Range = 241.08 nmRMS = 22.568 nm

Ra = 18.114 nm


Cu thermal roughening

Heat Treatment@ 750 °C

Z Range = 681.45 nmRMS = 33.8 nm

Ra = 25.1 nm


Sample 018 – Heat Treated 450 °C Sample 016 – Heat Treated 750 °C


NbTiN, NbN, Mo3Re, V3Si coatingswith Reactive Sputtering and

High Power Pulse Magnetron Sputtering& MgO, AlN, Al2O3 coating with reactive, RF sputtering

New UHV Multi-technique deposition system under commissioning @ JLab

A unique, versatile thin film deposition system enabling multiple coating techniques in-situ

Designed to enable rapid exploration of the production parameter space of:

Nb filmsAlternative material films like NbN, NbTiNS-I-S multilayer structures based on these compounds

Thin Films: Alternative Materials


Alternative Materials:

NbTiN, NbNNb3Sn, V3Si, Mo3Re

Substrates :

Single crystal NbPoly crystalline NbThick Nb/Cu films

Insulator:

Al2O3, MgO, AlNAccording to

lattice mismatch between

I and S materials

Multilayer:S/I/Nb & S/I/S/I/…/S/I/Nb

Study of growth modes of Superconductor/ Insulator

&Insulator/superconductor

Multilayers & Alternative Materials


3D Epitaxial Relationship of Nb and a-plane Sapphire

Note: Twoequivalents both satisfy “3D-Registry”

Nb [111]

Al2O3 [0001]

1800

[100]

[010]

Nb (011)

Nb* (011)

[001]

[1010]

[101]

Nb (0,1,-1) // Al2O3(1,1,-2,0)

Nb [1,1,1] // Al2O3 [0001]

Nb [1,0,-1] // Al2O3[1,0,-1,0]

4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF

Superconductivity - A-M Valente-Feliciano

IPF from Plane Normal [001] IPF from Transverse Direction [010]Nb/MgO (100)

62 °/<773>

~90° / <101>

(IPF from Transverse Direction [010])


SRF Cavities: state of the art

Breakdown fields close to the de-pairing limit of 50 MV/m for NbBest Nb cavities approaching their intrinsic limit at Hmax = HC

Nb has the highest Tc among all pure metals and the highest Hc1≈150 mT among all superconductorsMeissner state can exist up to H = Hc


Thin Films: niobium –state of the art

Attaining even bulk-like performance with Nb film would enable

major system simplifications.

Deposited films, once a production process is developed, may offer

the highest level of quality assurance and

thus reliable performance.

1.5 GHz Nb/Cu cavities, sputtered w/ Krat 1.7 K (Q0=295/Rs)

Nb

Nb/Cu

• The maximum field of Nb/Cu cavities is not limited by anyintrinsic phenomena.

• However, the residual resistance suffers from a steepincrease at high field (threshold ~15 MV/m).

• Several possible causes have been investigated. The mostprobable sources are: surface defects, hydrogen content.

CERN 2000


Thin Films applications:Nb, Alternative Materials & Multilayers

Use of SRF Thin Films

Bulk Nb Nb film (>1mm thick)

Multi-LayersS-I-S-I-S

SubstrateSubstrateCu, Al …

Single LayerNb, high k

Bulk-like performance Nb film major system simplifications.highest level of quality assurance and reliable performance.Use of substrates with higher thermal conductivity

NbN, Nb3Sn, MgB2, S-I-S-I-S…

Accessible almost only via deposited or synthesized films.

high k superconducting filmsfunction at higher temperatures or higher fields Suppression of vortex entry in multilayer structures

for cavity operation at 4.2K or higher


Valente - SRF thin films produced by energetic condensation

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