http://www.surfacetreatments.it/thinfilms RF and structural characterization of SRF thin films produced by energetic condensation (AnneMarie Valente - 30') Speaker: AnneMarie Valente - Jefferson Lab - Newport News (VA) USA | Duration: 30 min. Abstract In the past years, energetic vacuum deposition methods have been developed in different laboratories to further improve thin film technology for superconducting cavities. Energetic condensation produces high density plasma with singly or quasi-singly charged ions and allows growth modes that favour a better structure and performance in thin films. In the framework of a collaboration with surrounding universities, JLab is pursuing energetic condensation deposition particularly via Electron Cyclotron Resonance (ECR). As part of this study, the influence on the material and RF properties of Nb thin film of the deposition parameters like energy, coating temperature, interface with the substrate is investigated. The film surface and structure analyses are conducted with various techniques like X-ray diffraction, Transmission Electron Microscopy, Auger Electron Spectroscopy and RHEED. The microwave properties of the films are characterized on 50 mm disk samples with a 7.5 GHz surface impedance characterization system. This paper presents results on RF and cryogenic measurements in correlation with surface and material characterization for Nb films produced on insulating and metallic substrates. Emerging opportunities for developing multilayer SRF films with a new deposition system will also be highlighted.

1. 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

2. 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

5. Growth of an appropriate template for subsequent deposition of the final RF surface

8. Reactivity

9. Stoichiometric sensitivity

10. Reaction process temperatures

11. Crystal structure dependence on substrate structure

12. Influence of deposition energy on resulting structure

13. Sensitivity to the presence of contaminating species

14. Stabilization of desired film against subsequent degradationDeposition Control involves understanding of 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 profilometry Near 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). 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

15. Connecting Structure & Performance for SRF Surfaces Grain boundaries have an effect on surface resistance. Film quality is dependent on deposition technique: • surface roughness • defect density • Back-sputtering from energetic condensation Simulations under development • surface self-diffusion • competitive grain growth • defect density • back-sputtering • preferential sputtering rates 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

17. (TE011 sapphire-loaded cylindrical Nb cavity) Surface impedance as a function of magnetic field and temperature from 1.9 K to 4.8 K.

18. Normal state surface impedance at 10 K, from which the surface value of electronic mean free path and surface Hc1 can be determined.

19. 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.

20. Tc – easy coarse measure of intra-grain quality of the film

22. These techniques allow also to obtain “conformal” coatings that follow the surface profile better filling voids. “Structure Zone Model” 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

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

24. Energetic Condensation via ECR Niobium 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 eV can be obtained. Generation of plasma 3 essential components: Neutral Nb vapor RF 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) 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

25. Energetic vacuum deposition by ECR plasma 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

26. 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 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

27. Samples produced on different substrates Metallic: Cu (100) Cu (110) Cu (111) Fine grain Cu Large grain Cu Insulating: Al2O3 (11-20) Al2O3 (11-20) MgO(100) SrTiO3 Typical vacuum during plasma in ECR system:<5x10-8 Torr 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

28. Nb films creation ECR 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. ECR growth rate Growth rates from 15 nm/min to 135 nm/mi. have been achieved 2’ 270nm with -120V bias 45’ 4mm with 0V bias and 2mm with -120V bias Adhesion to substrate 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°C Troughening Cu (110), (100)>700°C 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

29. Niobium epitaxy on sapphire Anticipated 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. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

30. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano Nb/a-Al2O3 (11-20) 150 x 150 μm, 1 μm resolution, CI Avg. 85 ECR, bias -120V, 20 minutes, Bake-out (18hrs) and coating at 450ºC

31. Nb/Al2O3 (11-20), magnetron sputtered @ 600°C 50nm 600 nm RMS Roughness=4.84 nm RRR = 96 After 50 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.

32. Strain Evolution & Structure RHEED In hetero-epitaxial growth, the bulk lattice parameters of the film and the substrate are not commensurate.Thus, strained 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 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 crystallinity and 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 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

34. DTc0.025° 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

35. Structure vs. Bake Temperature- Al2O3 (11-20) (Bias -120V) 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

36. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano Nb/c-Al2O3 (0001) 250 x 250 μm, 2 μm resolution, CI Avg. 30 ECR, bias -120V, 20 minutes, bake-out (1.5hrs) and coating @450ºC

37. Sapphire (0001) substrate 500x 15 0 mm x 150mm, 1 mm step CI =0.20 CI =0.58 CI =0.54 CI =0.69 0.357 ° CI =0.24 5000x 15 mm x 15mm, 0.1 mm step CI =0.50 0.269° 0.320° CI =0.82 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

38. Nb/Al2O3 (0001) substrate, -150V (110) (200) (222) (110) Nb c-Al2O3 (222) Nb 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

40. DTc4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

41. Nbepitaxy on Al2O3 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. 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 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. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

42. Nb/MgO (100) 10.8% mismatch ECR, bias -120V, 30 minutes, Bake-out (24h) and coating at 360ºC 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

43. Nb/MgO RMS Roughness = 6.5nm 600 nm Nb/MgO RMS Roughness = 6.6nm 30 nm Nb/MgO RMS Roughness = 1.42nm 400nm Magnetron sputtering 600nm , 600C, 7h, RRR~47 , ECR 620nm , 360°C, 30’, RRR~50 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

44. 150 mm x 150mm, 1mm step 500x Nb/MgO (100) 0V -150V -120V -60V -30V CI =0.42 CI =0.19 CI =0.2 75mm x 75mm, 1mm CI =0.16 1000x CI =0.28 0.173° 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

45. Nb/Mgo(100), 700°C bake 6h, 360°C coating 15mm x 15mm, 0.1mm step CI =0.45 5000x RRR ~ 115 1.34mm 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

47. DTc4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

48. 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. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

49. Nb/Cu ECR, bias -120V, 20 minutes, Bake-out (18hrs) and coating at 450ºC 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

50. Single crystal Cu substrates 50x 1500 mm x 1500mm, 25 mm step CI =0.58 CI =0.59 CI =0.51 0.733° 0.357 ° 0.432° Lattice mismatch with (100) 8.5% 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

51. Nb hetero-epitaxy on Cu 150mm x 150mm, 1mm CI =0.21 500x 150mm x 150mm, 1mm CI =0.49 500x 150mm x 150mm, 1mm CI =0.24 500x 15mm x 15mm, 0.1mm CI =0.06 5000x 15mm x 15mm, 0.1mm CI =0.78 5000x 15mm x 15mm, 0.1mm CI =0.24 5000x 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

52. 15mm x 15mm, 0.1mm CI =0.27 5000x 150 mm x 150mm -150V -120V -60V -0V CI =0.12 150mm x 150mm, 1mm CI =0.09 500x CI =0.46 Nb/Cu (100) CI =0.53 150mm x 150mm, 1mm CI =0.14 500x Nb/Cu (110) CI =0.28 CI =0.11 1000mm x 1000mm, 50mm CI =0.1 500x 150 mm x 150mm CI =0.34 500x Nb/Cu (111) 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

53. Nb/Cu2O – High Resolution TEM 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

54. 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 3-D Profilometer Images TEM Images 90eV 4000X4000 µm2 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

55. XRD pole figures show the presence of growth features (rotation the poles of 70°) Dependence on thickness, ion incident energy and substrate temperature? 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

56. Nb/Cu – High Resolution TEM 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

57. Effect of Bias Voltage for Nb/Cu 150 x 150 μm, 1 μm resolution, CI Avg. 0.71 Typical Cu substrate Bias 0V , 4mm Bias -120V, 2mm 120 x 150 μm, 1 μm resolution, CI Avg. 0.23 50 x 75 μm, 1 μm resolution, CI Avg. 0.16

58. Nb/Cu, fine grain – effect of Bias voltage -30V -90V -60V CI =0.05 CI =0.25 CI =0.32 750mm x 750mm, 10mm CI =0.04 100x -150V -120V 150mm x 150mm, 1mm step 500x CI =0.2 CI =0.11 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano Ab-normal growth

59. Nb/Cu, fine grain – effect of bake temperature 750mm x 750mm, 10mm 100x -120V, bake@700°C Some peel-off due to Cu roughening CI =0.29 -120V, bake@360°C CI =0.09 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

60. Nb/Cu, large grain -30V 750mm x 750mm, 10mm step 100x -120V CI =0.26 CI =0.22 -150V CI =0.2 Equi-axial growth To be verified by cross-section EBSD 1500 mm x 1500 mm, 10mm step 50x 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

61. 1st SIC Measurements on Nb/Cu2O & Cu 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

62. Nb growth on Cu substrates Hetero-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 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

63. Concluding Remarks JLab 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 substrates High 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. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

64. SRF Thin Films Collaboration Films creation: Jlab: A-M Valente-Feliciano, J. Spradlin, L. Phillips W&M: A. Lukaszew, D. Beringer, S. …. Material and RF characterization: Jlab: A-M Valente-Feliciano, J. Spradlin, L. Phillips, X. Zhao, B. Xiao, A. Wu W&M: A. Lukaszew, D. Beringer, S. , R. Outlaw, O. Trofimova NSU: K. Seo ODU: H. Baumgart, D. Gu Black Labs LLC: R. Crooks Under DOE HEP Grant ARRA & U.S. DOE Contract No. DE-AC05-06OR23177 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

65. SIC Measurements for CED & ECR films 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

66. Cu thermal roughening 50mm x 50mm Z Range = 1.694 µm RMS = 203.79 nm Ra = 156.50 nm Z Range = 241.08 nm RMS = 22.568 nm Ra = 18.114 nm 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

67. Cu thermal roughening Heat Treatment@ 750 °C Z Range = 681.45 nm RMS = 33.8 nm Ra = 25.1 nm 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

68. Sample 018 – Heat Treated 450 °C Sample 016 – Heat Treated 750 °C 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

70. Alternative material films like NbN, NbTiN

71. S-I-S multilayer structures based on these compoundsNbTiN, NbN, Mo3Re, V3Si coatings with Reactive Sputtering and High Power Pulse Magnetron Sputtering & MgO, AlN, Al2O3 coating with reactive, RF sputtering

72. Thin Films: Alternative Materials 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

73. Multilayers & Alternative Materials Study of growth modes of Superconductor/ Insulator & Insulator/superconductor Insulator: Al2O3, MgO, AlN According to lattice mismatch between I and S materials Alternative Materials: NbTiN, NbN Nb3Sn, V3Si, Mo3Re Substrates : Single crystal Nb Poly crystalline Nb Thick Nb/Cu films Multilayer: S/I/Nb & S/I/S/I/…/S/I/Nb 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

74. 3D Epitaxial Relationship of Nb and a-plane Sapphire Note: Two equivalents both satisfy “3D-Registry” 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

75. Nb/MgO (100) IPF from Plane Normal [001] IPF from Transverse Direction [010] (IPF from Transverse Direction [010]) 62 °/<773> ~90° / <101> 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

76. SRF Cavities: state of the art Nb has the highest Tc among all pure metals and the highest Hc1≈150 mT among all superconductors Meissner state can exist up to H = Hc Breakdown fields close to the de-pairing limit of 50 MV/m for Nb Best Nb cavities approaching their intrinsic limit at Hmax = HC 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

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

79. Several possible causes have been investigated. The most probable sources are: surface defects, hydrogen content. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano

81. highest level of quality assurance and reliable performance.

83. Suppression of vortex entry in multilayer structures for cavity operation at 4.2K or higher high k superconducting films NbN, Nb3Sn, MgB2, S-I-S-I-S… Accessible almost only via deposited or synthesized films. 4th International Workshop on thin Films and New Ideas for Pushing the Limits of RF Superconductivity - A-M Valente-Feliciano