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 1

ncsu.edu/aif 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

Aif.ncsu.edu 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

ncsu.edu/aif jlab.org 4 Before heat treatment Initial SIMS Mass spectra Nb - NbH 2 - Very high H level Factor of 100 decrease after heat treatment (800C/3hr, 140C/12h) Intense NbH x - peaks removed with heat treatment Nb - NbH 2 - After heat treatment NbH - NbH 5 - NbH 4 - NbH 3 - NbH - Cs + primary beam 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 Mechanical polish + 10min BCP 1:1: C 12 hr in air BCP is buffered chemical polish using several acids Polycrystalline sample Optical Image of Fine Grain Nb Surface Sample W3 (Nomarski image) Surface is rough Poor depth resolution SIMS craters not measurable

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

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

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

Aif.ncsu.edu 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

ncsu.edu/aif 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 G. Ciovati, G. Myneni, F. Stevie, P. Maheshwari, D. Griffis Physical Review Special Topics Accelerators and Beams 13, (2010) 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

ncsu.edu/aif jlab.org 14 Implantd D does not show any peak in Nb. Possible causes: High diffusion coefficient for H in Nb H moves due to ion beam Diffusion of H and D in Nb ElementNb 300K (m 2 /s) 520MC Steel 300K (m 2 /s) Si 300K (m 2 /s) H8.06 x x x D5 x x J. Volkl, H. Wipf, Hyperfine Interactions 8 ( 1981) 631 E. Hörnlund et.al., Int. J. Electrochem. Sci., 2 (2007) 82 Table showing diffusion coefficients in Nb, steel and Si Diffusion rate for H in Nb = 5.7E4 nm/s = 57µm/s

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

ncsu.edu/aif jlab.org 16 SIMS Analysis in 120nm Anodized Nb Oxide Layeer Ion implanted: 1 H, 2 H, 18 O Cs + 6keV impact Peaks were observed for H, O D has peak at interface H not mobile in oxide Peak shape if D not mobile in Nb 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) Nb oxide Nb

Estimate of H in Nb Based on H in Nb 2 O 5 Calculate RSF for H in Nb 2 O 5 Note that Nb matrix signal very similar in Nb 2 O 5 and Nb Assume same RSF for H in Nb Result 2E22 atoms/cm 3 or 40% atomic for non-heat treated ~2E20 or 0.4% atomic for heat treated Nb density is 5.44E22 atoms/cm 3

ncsu.edu/aif jlab.org 18 G. Ciovati, J. Appl. Phys. 96, 1591 (2004) Nb samples prepared with BCP 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 120ºC 48hr bake Control Depth (nm) H Conc. (10 22 atoms/cm 3 )

ncsu.edu/aif jlab.org 19 Explanation of H SIMS Depth Profiles 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 Cs + Nb oxide HH Hydrogen

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

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) TEM analysis of Oxide layer on fine grain Nb sample W Au-Pd Nb 2 O 5 Nb

Sample W3 Nb Au-Pd Nb 2 O 5 Oxide is uniform with no apparent oxygen region below oxide TEM Analysis of Oxide layer on fine grain Nb sample HD2300 STEM (bright field)

ncsu.edu/aif jlab.org 23 TEM Analysis of Niobium Oxide Thickness Surface oxide is diffusion barrier for H Niobium oxide < 10nm thick for single crystal 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) ~7.5nm Nb

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

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

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

Heat treated sampleNon heat treated (control) H-H- Nb - H-H- TOF-SIMS Images of Bi-Crystal No discontinuity at interface

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

keV Cs + for O 5.5keV O 2 + for Ti SIMS Analysis of O and Ti after 1400 o C/3hrs High Ti concentration Ti tracks O profile When source of Ti removed, performance decreased

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 30 Summary

ncsu.edu/aif jlab.org 31 Acknowledgments Support of Jefferson Laboratory CBMM JLAB CRADA JSA 2004S002 under U. S. DOE Contract No. DE-AC05-05OR 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