Advanced Nb oxide surface modification by cluster ion beams

1. Advanced Nb oxide surface modification by cluster ion beams Zeke Insepov, Jim Norem (ANL), David Swenson (TEL Epion Inc)

2. Outline • Motivation • Gas Cluster Ion Beam (GCIB) treatment for Q-cure • Grand Challenge: GCIB + Multi-scale simulation of Nb oxides: Ab-initio – MD - Continuum scale • Preliminary results of NbO electronic structure • Preliminary results of MD simulation of Oxygen cluster infusion into Nb • Summary

3. Motivation • Quality degradation of SC FE-free cavities at high E gradients (> 30 MV/m) and peak B ~ 100 mT is the main challenge for materials science • Q-drop prevents of achieving gradients 35 MV/m needed for future accelerators, such as the ILC • Baking cures Q-drop but not Q-slope and there is no understanding of its causes. Many models exist but none was thoroughly proven. Many experiments need to be explained by one model.

4. Gas Cluster Ion Beam processing Epion Corporation

5. GCIB smoothing of electrodes • Smoothing • Suppressing field emission from asperities and surface roughness • Remove nanoscale tips that can be ripped off by high RF fields contributing to RF breakdown • Hardening the surface – density is higher • Cleaning/Etching • Chemically altering the surface -- Oxidizing, Nitriding -- Infusing -- Deposition

6. Cluster impact on a surface target Cluster Impact Shock waves Lateral sputtering Arn+ target Lateral sputtering causes surface smoothing Ra = 4Å Surface smoothing effect GCIB removes sharp tips - the most Field- Emitting tips, and which most probably lead to the breakdown. Ra =12Å Epion AFM image of Ta film surface

7. Experimental work on GCIB Removing sharp tips on steel Epion Corp.

8. GCIB smoothing of electrodes • Smoothing • Suppressing field emission from asperities and surface roughness • Remove nanoscale tips that can be ripped off by high RF fields contributing to RF breakdown • Hardening the surface – density is higher • Cleaning/Etching • Chemically altering the surface • Oxidizing, Nitriding • Infusing • Deposition

9. DC field emission experiments DC Field emission measurements of 116 cm2 Stainless Steel electrodes GCIB makes dense surface layers that may cause the effect The mechanical polish of unprocessed substrate was much better that that of the GCIB polished substrate. Currents below 1e-12 were not measurable. [Cornell (Sinclair) & Jlab measurements]

10. Removing sharp tips (O2)n+ Nb Local m is defined by the surface curvature and it is higher for sharp tips.

11. Grand Challenge • GCIB - a clean solution for oxides: NbO, NbO2, Nb2O5; removes FE; reduces the surface roughness up to atomically low level; makes more dense surface layers; modifies grains. • GCIB can be a reference method: it can create Oxygen saturated areas to test cluster formation/diffusion models • We need electronically aware materials science of the Nb oxides under extremely high electric and magnetic fields • Theory can calculate the diffusivity and precipitation of Oxygen in Nb, - this helps understanding baking

12. NbO Structure • Electronic and structural properties of NbO were not yet studied • O diffusion characteristics were not studied theoretically • DFT & MD calculations were not performed for electronic, structural and thermal properties of the Nb oxides NbO has a FCC cubic cell and space group Pm_3m (#221). Three lattice parameters: a = b = c = 4.212Å,V = 504.44 bohr3. The unit cell contains two nonequivalent atoms: Nb at {0.5;0.5;0.0}, O at {0.5;0.0;0.0}. Nb O

13. The Full Potential LAPW DFT method • The LAPW method: The Linearized Augmented Plane Wave (LAPW) method is among the most accurate methods for performing electronic structure calculations for crystals. • Forms of LSDA potentials exist in the literature, but recent improvements using the Generalized Gradient Approximation (GGA) are available too. • For valence states relativistic effects can be included either in a scalar relativistic treatment or with the second variational method including spin-orbit coupling • Core states are treated fully relativistically

14. Preliminary result #2: Total Energy of NbO Equation of state Murnaghan EOS: a=-23398.4 Ry; b=874.4 c=-9477.8; d=31920.0 Pressure: V0=515.1 b3 (d = 2%) B = 242.8 GPa BP = 4.4 E0 = -23374.8 Ry [Teter et al, PRB 1995]

15. Preliminary result #3: Sticking probability Sticking probability was calculated as the Number of trapped versus the total number of impact ones: (O2)n+ reflection trapped Nb 13 135 1055 2171 3043

16. Preliminary result #4: Number of infused molecules MD simulation predicts infusion as a feasible process (O2)n+ Nb

17. 30kV (O2)n (n=13-3000) cluster Infusion in Nb (100) N=13 N=135 N=2171 N=1055

18. Summary • Better understanding of the NbO is needed. Theoretical & experimental data on NbxOy are very limited – GCIB can be a reference method as it is clean • Ab-initio (DFT) simulations of NbO gives preliminary Etot, P(V), electronic states  build a multi-scale approach to oxygen diffusion. • Our MD shows that infusion of Oxygen cluster is most probable for large (~ 2000) molecular clusters. • Smaller cluster significantly damage the surface.