1. 1 SIMS Characterization of Impurity Elements in Nb: The Effect of Heat Treatments ncsu.edu/aif jlab.org 1 P. Maheshwari, F. Stevie, D. Griffis, M. Rigsbee North Carolina State University G. Myneni, G. Ciovati Jefferson Laboratory

2. 2 ncsu.edu/aif jlab.org 2 Large grain sample Secondary Ion Mass Spectrometry (SIMS) Escape depth of sputtered species only few Angstroms All elements and isotopes measurable (including H) ppm to ppb detection limit 10-20 nm depth resolution typical, 1-2nm at low energies R. G. Wilson, F. A. Stevie, C. W. Magee: Secondary Ion Mass Spectrometry, Wiley-Interscience (1989)

3. 3 ncsu.edu/aif jlab.org 3 1. Effect of High Temperature HT 800 o C/3hrs, 120 o C/12hrs 1200 o C/6hrs 800 o C/3hrs, 400 o C/20min Control H levels decrease up to a factor of 100, believed to be responsible for improvement in Q o (~45% at 90 mT, 2.0 K, 1.5 GHz); Insignificant changes in O, C and N levels

4. 4 ncsu.edu/aif jlab.org 4 No heat treatment (Control) Intense hydride peaks observed in the mass spectra of a non heat treated sample; greatly reduced after high temperature heat treatment After 800 o C/3hrs, 400 o C /20min heat treatment SIMS measurements at 14.5 keV impact energy with Cs + NbH 5 - NbH 4 - NbH 3 - NbH 2 - Nb - NbH - Nb - NbH 2 - NbH - 1. Effect of High Temperature on Nb Hydrides

5. 5 ncsu.edu/aif jlab.org 5 2. Effect of Baking Some change in H, Some diffusion of O in Nb (first 60nm), Reduction in Q-drop might be related to impurities? SIMS Analysis 6 keV Cs + (Low Energy) 120 o C/48hrs Control O Concentration after 120 o C/48 hrs HT H Levels after 120 o C/48 hrs HT H - /Nb - Ratio 120 o C/48hrs Control

6. 6 ncsu.edu/aif jlab.org 6 3. O and Ti diffusion in Nb for HT >1000 o C O diffuses to the surface of Nb above 1000 o C in the presence of Ti Conc. (atoms/cm 3 ) 1400 o C/3hrs 1200 o C/2hrs,120 o C/12hrs O concentration in Nb Conc. (atoms/cm 3 ) 1400 o C/3hrs 1200 o C/2hrs,120 o C/12hrs Ti concentration in Nb SIMS measurements: 14.5 keV impact energy with Cs + SIMS measurements: 5.5 keV impact energy with O 2 +

7. 7 4. Impurities in Grain Boundaries of LG Nb No segregation of H at grain boundary Segregation of C at grain boundary Grain Boundary of Bicrystal TOF SIMS Analysis using 25 keV Bi 3+ after sputtering for 3 min with 10 keV Cs + 800 o C/3hrs, 120 o C/24hrs Control

8. 8 ncsu.edu/aif jlab.org 8 5. Quantification of H in Nb using Nb 2 O 5 Oxide acts as a passivation layer on the Nb surface; anodization of a control sample used to create a layer, about 150nm in thickness D and H implanted into the oxide to create SIMS standards Peaks observed for H, D in Niobium Oxide H increases as Oxide-Nb interface reached D implanted to place peak at Nb oxide/Nb interface SIMS measurements at 6 keV impact energy with Cs + Oxide Substrate

9. 9 ncsu.edu/aif jlab.org 9 5. Estimate of H in Nb SIMS measurements at 6 keV impact energy with Cs + Matrix signal similar in oxide and in Nb, can use RSF from oxide to quantify H in Nb H concentration estimated to be 2e22 atoms/cm 3 ≈ 37 % atomic H on a control Nb sample surface 2E22 at./cm 3 Since the Nb secondary ion intensity shows little change between oxide and substrate, an estimate of H in Nb can be made using the RSF derived by taking Niobium Oxide implant as standard

10. 10 Low Energy SIMS Analysis 6 keV Cs + ; Ti : 1.25 keV O 2 + 6. 1400 o C/3hrs,120 o C/12hrs SRF cavity had record Q 0 of 4.6  10 10 at 90 mT, 2.0 K, 1.5 GHz ( Yesterday’s talk by P. Dhakal ) H - /Nb - Ratio Control 1400 o C/3hrs,120 o C/12hrs H - /Nb - Ratio Conc. (at/cm 3 )

11. Corroborated by NRA Analysis Control 800 o C/2hrs, 120 o C/24hrs Courtesy : Pacific Northwest National Lab., USA

12. Conclusions H seen to influence cavity performance, cavity efficiency increases by an average of 40% after heat treatment, possibly due to the reduction of H. H concentrations of ~40 at.% in BCP-treated Nb samples have been estimated by SIMS. H depth profiling is limited by matrix effect and high diffusion rate C segregation at grain boundaries was found by TOF-SIMS after heat treatment at 800 °C Significant concentrations (> 1 at.%) of Ti,O and low H levels were found in sample heat treated at 1400 °C (cavity with record Q 0 )