# Presentation: 561148

## Advanced Nb oxide surface modification by cluster ion beams 

***Zeke Insepov, Jim Norem (ANL),  ***
***David Swenson (TEL Epion Inc)***

## 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**

## Motivation

** ****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.**

***Motivation***

## Gas Cluster Ion Beam processing

**Epion Corporation**

## 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

## Cluster impact on a surface

**Ar****n****+**

**Shock waves**

**target**

- Surface smoothing effect
- GCIB removes sharp tips - the most Field- Emitting tips, and which most probably lead to the breakdown.

**Lateral sputtering**

**Cluster Impact**

**Lateral sputtering causes surface smoothing**

- Epion AFM image of Ta film surface

**Ra =12Å**

**Ra = 4Å**

## Experimental work on GCIB

**Epion Corp.**

***Removing sharp tips on steel***

## 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**

## DC field emission experiments

**DC Field emission measurements of 116 cm****2**** Stainless Steel electrodes**

**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]**

- GCIB makes dense
- surface layers that
- may cause the
- effect

## Removing sharp tips

***Removing sharp tips***

**(O****2****)****n****+**

**Nb**

**Local ********** is defined**
**by the surface **
**curvature and it is**
**higher for sharp**
**tips. **

## 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 **

## NbO Structure

- 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**

-   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

## The Full Potential LAPW DFT method

-  **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

***The Full Potential LAPW DFT method***

## Preliminary result #2: Total Energy of NbO 

**Murnaghan EOS:**
- a=-23398.4 Ry; b=874.4
- c=-9477.8; d=31920.0

- [Teter et al, PRB 1995]

**Pressure:**
**V****0****=515.1** b3 (= 2%)
- B = 242.8 GPa
- BP = 4.4
- E0 = -23374.8 Ry

**Equation of state**

## Preliminary result #3: Sticking probability 

## Preliminary result #4: Number of infused molecules

## 30kV (O2)n (n=13-3000) cluster Infusion in Nb (100)

- N=13

- N=135

- N=1055

- N=2171

## 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.