This document summarizes a presentation on nano-mechanical and microstructural characterization of niobium thin films deposited by magnetron sputtering. Key points discussed include: - Four niobium coatings were produced by varying the applied bias voltage and substrate material. - Nanoindentation testing revealed the coatings' hardness is not affected by the substrate at depths less than one tenth the coating thickness. - Analysis of deformation mechanisms using FIB sectioning showed columnar grain sliding and recrystallization during plastic deformation. - A thin surface oxide layer of around 10nm was found, which could impact the coatings' superconducting performance.

1. Material Science and Technology Research Group (STM) DipartimentodiIngegneriaMeccanica e Industriale Università Roma TRE Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Edoardo Bemporad, Marco Sebastiani, Fabio CarassitiGiulia Lanza and Enzo Palmieri The fourth international Workshop on: THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY October 4-6, 2010 Legnaro National Laboratories (Padua) ITALY

2. www.stm.uniroma3.it STM Roma TRE channel on YouTube Image Gallery Agilent Web seminar Interviews and videos on daily life at STM group Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 2 Marco Sebastiani, Legnaro National Laboratories October 4 2010

3. www.lime.uniroma3.it Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 3 Marco Sebastiani, Legnaro National Laboratories October 4 2010

4. Resonant Cavities for particle accelerators NiobiumCoated (PVD)Coppercavity Bulk Niobium Very low Surfaceelectricalresistance (n a 1,8 K) Lowercosts Higherthermalstability But… Significantlylowersuperconductingperformances–WHY?? Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 4 Marco Sebastiani, Legnaro National Laboratories October 4 2010

6. Roughness of the substrate;

7. Morphology of coating/substrate interface;

8. Coating thickness and thickness homogeneity;

9. Coating density

10. Grain size and crystals growth angle;

11. Thickness and composition of the surface oxide layer

12. Residual stress

13. All these aspects are related to the deposition parameters;1999, Physica C 316 153–88 2005, Thin Solid Films 489 56–62 1997 , J. Appl. Phys. 81 (10) 6754 1998, Appl. Surf. Sci. 126 219–30 2005, J. Appl. Phys. 97 083904 2006, Physica C 441 79–82

14. Introduction Quality control measurement (RRR and Tc) are usually performed on Nb film deposited on Quartz substrate RRR and Tc measured on Quartz are very useful as threshold parameters; Nevertheless, characterization methods are absolutely necessary to verify what are the micro-structural differences between coatings which are grown either on quartz or on copper Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 6 Marco Sebastiani, Legnaro National Laboratories October 4 2010 Supercond. Sci. Technol. (21 (2008) 125026)

15. Gap ofknowledge 7 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 A comprehensive chemical, morphological, and mechanical surface characterization can be useful, in order to find out the appropriate correlation function among process parameters/microstructure/surface properties and superconducting properties In particular, the influence of the applied bias voltage on the functional and nano-mechanical properties of sputtered Nb thin films still needs to be deeply investigated. Supercond. Sci. Technol. (21 (2008) 125026)

16. Aimof the work 8 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 To verify the influence of the applied bias voltage and nature of the substrate on the microstructural and nano-mechanical properties of MS-PVD Niobium thin films for use in superconducting resonant cavities for particle accelerators; To establish a correlation between the functional performance (RRR and Tc) and the observed microstructural/mechanical properties; To verify how nano-mechanical characterization can help in the characterization and quality control of such coatings Supercond. Sci. Technol. (21 (2008) 125026)

17. Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 9 Marco Sebastiani, Legnaro National Laboratories October 4 2010

18. Marco Sebastiani, Legnaro National Laboratories October 4 2010 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Hardness – Scale of indentation 10 N 10 mN Load Nano-hardness Micro-hardness Macro-hardness 10

19. Roma TRE nanohardness tester Berkovichtip Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 11 Marco Sebastiani, Legnaro National Laboratories October 4 2010 early 2008 - CSM option - Lateral force measurement - Nano- positioninng stage

20. Instrumented Indentation Testing Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 12 Marco Sebastiani, Legnaro National Laboratories October 4 2010

21. F F F h h h Depth Sensing Indentation describe materials behaviour: Elastic to Plastic Deformation Plastic Elastic - Plastic Elastic Stress-Strain curves Indentation curves Indentation prints Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 13 Marco Sebastiani, Legnaro National Laboratories October 4 2010

22. Quasi-static Instrumented Indentation Testing Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 14 Marco Sebastiani, Legnaro National Laboratories October 4 2010

23. Nano hardness maps Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 15 Marco Sebastiani, Legnaro National Laboratories October 4 2010

24. Stiffness is determined NOT from the unloading curve, but continuously from the ratio of the amplitude of the imposed force oscillation (F0) to the amplitude of the resulting displacement oscillation (z0) Continuous stiffness measurement (CSM): in depth H and E variation, viscoelasticity Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 16 Marco Sebastiani, Legnaro National Laboratories October 4 2010

25. Nanoindentation with CSM W. C. Oliver, G. M. Pharr, J. Mater. Res., Vol. 19, No. 1, Jan 2004 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 17 Marco Sebastiani, Legnaro National Laboratories October 4 2010

26. 80 nm thin ALD coating TiO2-Al2O3 coating on silver substrate (Roma Tre). CSM: analysis of thin coatings Pt TiO2 Al2O3 Intrinsic hardness of the top layer Silver TEM, 120kV, 380.000x dual-layer coating Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 18 Marco Sebastiani, Legnaro National Laboratories October 4 2010

27. Tip rounding Surface roughness and chemistry Thermal Drift Initial penetration depth Instrument compliance Indenter geometry Specimen preparation and mounting Piling-up / sinking-in Substrate effect Indentation size effect Residual stress Factors affecting results Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 19 Marco Sebastiani, Legnaro National Laboratories October 4 2010

29. How to separate real microstructural effects from artifacts?

30. How to identify the actual deformation/failure mechanisms?

31. Must validate nanoindentation measurements

32. materials behavior during indentation would not be considered with just continuum theory

33. The solution relies in the coupled and synergic use of high resolution microscopy techniques and indentation testingNano-mechanical and microstructural characterization of MS-PVD Nb thin films 20 Marco Sebastiani, Legnaro National Laboratories October 4 2010

34. Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 21 Marco Sebastiani, Legnaro National Laboratories October 4 2010

35. Lime Lab: DualBeam (FEI) Helios 600 Nanolab early 2008 - Omnprobe - GIS (4 gases) - STEM - EDS (Oxford) Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 22 Marco Sebastiani, Legnaro National Laboratories October 4 2010

36. Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 23 Marco Sebastiani, Legnaro National Laboratories October 4 2010

37. DualBeam (FEI) SEM Column: spatial resolution 0,69 nm @ 15 kVFIB Column: spatial resolution5 nm @ 30 kV Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 24 Marco Sebastiani, Legnaro National Laboratories October 4 2010

38. High resolution electron column 0,69 nmresolution, up to more that 1 million X Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 25 Marco Sebastiani, Legnaro National Laboratories October 4 2010

39. Ion-Sample interaction Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 26 Marco Sebastiani, Legnaro National Laboratories October 4 2010

40. Principal FIB modes: Deposition Imaging Milling Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 27 Marco Sebastiani, Legnaro National Laboratories October 4 2010

42. Principal FIB modes: Deposition Imaging Milling Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 29 Marco Sebastiani, Legnaro National Laboratories October 4 2010

43. FIB modes: Ion Milling Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 30 Marco Sebastiani, Legnaro National Laboratories October 4 2010

44. Principal FIB modes: Deposition Imaging Milling Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 31 Marco Sebastiani, Legnaro National Laboratories October 4 2010

45. FIB modes: Deposition (Pt) Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 32 Marco Sebastiani, Legnaro National Laboratories October 4 2010

46. Possibility to reveal microstructure beneath the surface with very little or practically no artifacts Accurately localize the region of interest(within few nanometers) Excellent technique even on soft materials Time to produce a TEM lamella (couple of hours) EDS and Scanning Transmission Imaging within the DualBeam also available Using a DualBeam to characterize samples; what is new with respect to conventional characterization route? Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 33 Marco Sebastiani, Legnaro National Laboratories October 4 2010

47. Cross sections Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 34 Marco Sebastiani, Legnaro National Laboratories October 4 2010

48. TEM lamella preparation Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 35 Marco Sebastiani, Legnaro National Laboratories October 4 2010

49. 36 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 Resultsofexperimentalactivities and discussion

50. Depositionofcoatings 37 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 Fourdifferentcoatings are produced, byvarying the appliedbiasvoltage and the nature of the substrate.

51. Nanoindentation: howtoget the right numbers? 38 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 1000 nm 500 nm 200 nm 100 nm

52. Nanoindentation: howtoget the right numbers? 39 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 1000 nm 500 nm 200 nm 100 nm

54. Nanoindentation: surfaceeffects 41 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 Inaccuracies on hardness and modulus at indentation depths lower than 30 nm, due to roughness and tip rounding effects Elastic modulus at 25 nm: 93,2 ± 35,82GPa Elastic modulus at 100 nm: 106,21 ± 13,35GPa

56. Relative sliding of columnar grain

57. Direct measurement of piling-up

59. No more evidence of columns sliding and bending

63. Lateral plastic flow, with strong column bending

66. FIB-SEM microstructural analysis An improper preparation of the copper substrate (usually electro-polishing + chemical polishing) dramatically decrease the interface and film quality, and the surface smoothness Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 53 Marco Sebastiani, Legnaro National Laboratories October 4 2010

67. Correlationtosuperconductingperformances 54 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 A difference in terms of Tc is observed between biased and unbiased films on quartz; This suggest a microstructural and morphological difference (density, grain size, lattice distortion, residual stress) Should we expect the same differences in terms of Tcalso for coatings on Copper?

68. Discussion: whataboutresidual stress? 55 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010 A difference in residual stress is expected both in case of different substrate and in case of applied bias voltage: Residual stress can reduce the elastic strain to failure, thus enhancing the possibility of defect formation (dislocations, delamination, micro-cracking and debonding of grains) Residual stress arises from a lattice distortion, thus influencing the electron-phonon coupling constant and the superconducting performance.

69. Conclusions Nano-mechanical testing can be a valuable procedure for the assessment of basic quality indicators for Nb films on either Copper or Quartz; Careful analysis of the raw data coming from nanoindentation allows to find out several important information: Actual elastic modulus and hardness of the coatings, leading to more reliable comparisons between samples; Presence of a thin oxide layer Surface and substrate effects Indirect evaluation of coating density A correlation is found between surface micro-roughness, coating density, mechanical properties and superconducting performance; The coupled use of HR microscopy techniques and hardness testing has been the key point of all research activities Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 56 Marco Sebastiani, Legnaro National Laboratories October 4 2010

71. Depth resolution: <100 nm

72. Semi-automated procedure

73. This is an example of a PVD Gold film on Silicon…

75. 59 Nano-mechanical and microstructural characterization of MS-PVD Nb thin films Marco Sebastiani, Legnaro National Laboratories October 4 2010

76. The starting point of the work here presented are the results published in a recent paper on Supercond. Sci. Technol. (21 (2008) 125026): Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 60 Marco Sebastiani, Legnaro National Laboratories October 4 2010

77. Dynamic characterization of polymers in the time and frequency domains using a flat punch Highly plasticized polyvinylchloride, the complex modulus at 22 oC

78. Supeconducting properties of MS-PVD Nb films are significantly affected by Grain size surface roughness coating density interfaces integrity A completely different behaviour is expected for the BIASED Nb coating deposited on Quartz substrate Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 62 Marco Sebastiani, Legnaro National Laboratories October 4 2010

79. MTS Nano Event Roma TRE, April 3/04/2008 M. Sebastiani: Applications of nanoindentation techniques to non-homogeneous materials and structures 63 Film Hsubstrate Hfilm Hardness Hardness Substrate Hfilm Hsubstrate 1 2 1 Indentation Depth/Film Thickness Indentation Depth/Film Thickness Surface Layers - Thin Film Effects Soft film on hard substrate Hard film on soft substrate Film Substrate

80. Ap h Apand Tip Calibration Perfect pyramid or cone: (C0 constant, 24.5 for Vickers) Perfect tip shape do not exist in the reality (tip blunting) Perfect tip Real tip Tip Calibration Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 64 Marco Sebastiani, Legnaro National Laboratories October 4 2010

81. Hard inclusion in a soft metal Nano-mechanical and microstructural characterization of MS-PVD Nb thin films 65 Marco Sebastiani, Legnaro National Laboratories October 4 2010