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SRF Niobium Characterization Using SIMS and FIB-TEM

SRF Niobium Characterization Using SIMS and FIB-TEM. Fred A. Stevie Analytical Instrumentation Facility North Carolina State University Raleigh, North Carolina. Outline. SIMS analysis for interstitials (H, C, N, O) in niobium SIMS detection of significant H

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SRF Niobium Characterization Using SIMS and FIB-TEM

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  1. SRF Niobium Characterization Using SIMS and FIB-TEM Fred A. Stevie Analytical Instrumentation Facility North Carolina State University Raleigh, North Carolina aif.ncsu.edu jlab.org

  2. Outline • SIMS analysis for interstitials (H, C, N, O) in niobium • SIMS detection of significant H • H decreased after heat treatment • Quantification issues: Mobility of H and D in niobium • SIMS measurements in niobium oxide • FIB sample preparation • TEM analyses of FIB prepared samples • Performance improvement related to Ti contamination • Summary ncsu.edu/aif jlab.org

  3. Analytical Techniques • Secondary ion mass spectrometry (SIMS) • Depth profiles • High sensitivity • Able to detect H • Focused ion beam (FIB) • Site specific material removal and deposition • Can prepare soft metals for TEM analysis • Transmission electron microscopy (TEM) • High resolution images • Elemental identification Aif.ncsu.edu jlab.org

  4. Initial SIMS Mass spectra • Very high H level • Factor of 100 decrease after heat treatment (800C/3hr, 140C/12h) • Intense NbHx- peaks removed with heat treatment Cs+ primary beam NbH2- NbH- NbH3- Nb- NbH4- Nb- NbH5- NbH- NbH2- Before heat treatment After heat treatment A. D. Batchelor et al., Proceedings of Single Crystal Niobium Technology Workshop, Brazil, AIP Conference Proceedings, Melville, NY (2007) 72-83 ncsu.edu/aif jlab.org

  5. Optical Image of Fine Grain Nb Surface Polycrystalline sample Sample W3 (Nomarski image) • Surface is rough • Poor depth resolution • SIMS craters not measurable Mechanical polish + 10min BCP 1:1:1 + 180C 12 hr in air BCP is buffered chemical polish using several acids ncsu.edu/aif jlab.org

  6. Optical Image of Nanopolished Nb Surface Single crystal sample • Large Grain BCP nanopolished • Relatively smooth surface • Ion implanted sample used to quantify C, N, and O Pit SIMS craters ncsu.edu/aif jlab.org

  7. SIMS Quantification Using Ion Implantation Energy SIMS depth profile of ion implanted sample Dose • All elements and isotopes possible • Implantation into any substrate or structure • Vary peak concentration by varying dose • Vary depth of peak with implant energy

  8. SIMS Quantification of C, N, O (Problem with H) Implant D because H level in Nb too high to quantify using ion implantation D, C, N, O implanted – no implant peak found for D in Nb H D H D Depth profile for H, D, C, N, O in Nb Depth profile for H, D, C, N, O in Si P. Maheshwari et al., Surf. Int. Analysis 43, 151-153 (2011) ncsu.edu/aif jlab.org

  9. Carbon Concentration for Control and Heat Treated Samples Aif.ncsu.edu jlab.org

  10. Nitrogen Concentration for Control and Heat Treated Samples Aif.ncsu.edu jlab.org

  11. Oxygen Concentration for Control and Heat Treated Samples ncsu.edu/aif jlab.org

  12. Hydrogen / Niobium Ratio for Control and Heat Treated Samples ncsu.edu/aif jlab.org

  13. H SIMS results correlate with improved performance • Performance of Nb cavities at high fields (>90mT) • characterized by exponential increase of rf field losses • Performance improvement typically improved • with heat treatment • Heat treatment sequence of 800C 3hr, 120C 12hr • provided improved performance and SIMS analyses • indicated very large reduction in H G. Ciovati, G. Myneni, F. Stevie, P. Maheshwari, D. Griffis Physical Review Special Topics Accelerators and Beams 13, 022002 (2010) ncsu.edu/aif jlab.org

  14. Diffusion of H and D in Nb Implantd D does not show any peak in Nb. Possible causes: • High diffusion coefficient for H in Nb • H moves due to ion beam Table showing diffusion coefficients in Nb, steel and Si Diffusion rate for H in Nb = 5.7E4 nm/s = 57µm/s J. Volkl, H. Wipf, Hyperfine Interactions 8 ( 1981) 631 E. Hörnlund et.al., Int. J. Electrochem. Sci., 2 (2007) 82 ncsu.edu/aif jlab.org

  15. >25µm analysis shows no decrease in H • Cs+ 8.5 nm /sec compared with 5.7E4 nm/sec for H in Nb • Cannot sputter faster than diffusion rate of H

  16. SIMS Analysis in 120nm Anodized Nb Oxide Layeer Ion implanted: 1H, 2H, 18O Nb oxide Nb Peak shape if D not mobile in Nb • Cs+ 6keV impact • Peaks were observed for H, O • D has peak at • interface • H not mobile in oxide P. Maheshwari et al., Symposium on the Superconducting Science and Technology of Ingot Niobium, AIF Conference Proceedings G. R. Mynenei, G. Ciovati, M. Stuart, eds. 1352, 151 (2011) ncsu.edu/aif jlab.org

  17. Estimate of H in Nb Based on H in Nb2O5 • Calculate RSF for H in Nb2O5 • Note that Nb matrix signal very similar in Nb2O5 and Nb • Assume same RSF for H in Nb • Result • 2E22 atoms/cm3 or 40% atomic for non-heat treated • ~2E20 or 0.4% atomic for heat treated • Nb density is 5.44E22 atoms/cm3

  18. Nuclear Reaction Analysis (NRA) • Not affected by H mobility • Results show H at high concentration just below surface • Significant decrease in H after heat treatment • Average non-heat treated ~40% atomic H at peak 2.4 Control Nb samples prepared with BCP 1.6 H Conc. (1022 atoms/cm3) 0.8 120ºC 48hr bake 20 50 Depth (nm) G. Ciovati, J. Appl. Phys. 96, 1591 (2004) ncsu.edu/aif jlab.org

  19. Explanation of H SIMS Depth Profiles Cs+ Nb oxide Hydrogen H H • SIMS analysis penetrates niobium oxide • H present at high concentration just below oxide • H free to move at > 50µm/s • H continues to arrive at SIMS sputtered surface • Cs may attract H ncsu.edu/aif jlab.org

  20. FIB Lift-Out with Micromanipulator Micromanipulator tip Attached with Pt deposition Analytical Instrumentation Facility, North Carolina State University

  21. TEM analysis of Oxide layer on fine grain Nb sample W Au-Pd Nb2O5 Nb • FIB sample preparation of sample W3 (polycrystalline Nb) • W3: mechanical polish + 10min BCP + 180°C 12hr in air • Surface protected with sputtered 60nm Au-Pd and 2µm FIB W • Analysis with HD2300 STEM (bright field)

  22. TEM Analysis of Oxide layer on fine grain Nb sample Sample W3 Au-Pd Nb2O5 Nb Oxide is uniform with no apparent oxygen region below oxide HD2300 STEM (bright field)

  23. TEM Analysis of Niobium Oxide Thickness • Surface oxide is diffusion barrier for H • Niobium oxide < 10nm thick for single crystal Nb ~7.5nm Nb A D. Batchelor, D. N. Leonard, P. E. Russell, F. A. Stevie, D. P. Griffis, G. R. Myneni, Proceedings of Single Crystal Niobium Technology Workshop, Brazil, AIP Conference Proceedings, Melville, NY (2007) 72-83 ncsu.edu/aif jlab.org

  24. Oxide Layer Grain Boundary Au-Pd Pt TEM Micrograph of Bi-Crystal Control • No discontinuity across grain boundary or in surface oxide • No oxide layer at grain boundary

  25. Oxide Layer Au-Pd Pt TEM Micrograph of Bi-Crystal Heat Treated Grain Boundary Results similar to Control sample

  26. TOF-SIMS Images of Bi-Crystal Non heat treated (control) Heat treated sample • ION TOF • TOF-SIMSV • Clean with • 20nA 10keV Cs+ • 180 x 180µm raster • Analyze with • 0.7pA 25keV Bi3+ • 100 x100µm raster C- C- Grain boundary O- O- • C segregates to • grain boundary • O does not

  27. TOF-SIMS Images of Bi-Crystal Non heat treated (control) Heat treated sample H- H- No discontinuity at interface Nb- Nb-

  28. Ti+ High Ti levels found in TOF-SIMS analysis of 1400oC HT sample Dynamic SIMS depth profile of Ti implanted Nb sample Metallic Impurity in 1400oC/3hrs: Ti • 200% increase in cavity efficiency after high temperature anneal • Mass spectra showed high levels of Ti on the surface of the 1400oC heat treated sample. • Ti implanted into one of the control Nb samples to quantify Ti

  29. SIMS Analysis of O and Ti after 1400oC/3hrs 14.5 keV Cs+ for O 5.5keV O2+ for Ti • High Ti concentration • Ti tracks O profile When source of Ti removed, performance decreased

  30. Summary SIMS analyses show high H concentration in Nb control samples and low H after heat treatment C, N, O at much lower levels and no significant change with heat treatment H very mobile in Nb but not mobile in Nb oxide (diffusion barrier for H) FIB provides sample preparation for TEM analysis TEM results show continuous oxide on Nb and no other features Bi-crystals show C movement to interface after heat treatment Ti contamination appears related to performance improvement ncsu.edu/aif jlab.org

  31. Acknowledgments Support of Jefferson Laboratory CBMM JLAB CRADA JSA 2004S002 under U. S. DOE Contract No. DE-AC05-05OR23177. Contributions of JLAB researchers and NCSU analysts JLAB: G. Mynenei, G, Ciovati, A.-M. Valente Feliciano, P. Dhakal, H. Yian, C. E. Reece, M. Kelley NCSU: P. Maheshwari, C. Zhou, R. Garcia, D. Batchelor, P. E. Russell, D. P. Griffis, J. M. Rigsbee ncsu.edu/aif jlab.org

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