Title: Plasma Etching of Niobium surfaces: Studies on samples and Single-Cell Cavities
Author: AnneMarie Valente
Institution: Jefferson Lab – Newport News (VA) USA
Abstract: Plasma based surface modification provides an excellent opportunity to eliminate impurities and defects in the penetration depth region of Nb SRF cavity surfaces. It also allows a better control of the final SRF surface as final surface modifications like oxidation or nitridation can be done in the same process cycle.
In the framework of a collaboration between ODU and Jefferson Lab, we are pursuing the use of environmentally friendly dry etching of SRF cavity in an Ar/Cl2 discharge. The experimental conditions in the microwave glow discharge system with a barrel-type reactor have been optimized. The viability of plasma etching as an alternative surface preparation method for bulk Nb surfaces has been demonstrated on flat samples by achieving etching rates comparable to wet processes, such as BCP or EP.
The optimized experimental conditions are now being applied to the preparation of single cell cavities. The geometry of SRF cavities made of bulk Nb defines the use of asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of a driven and grounded electrode creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the plasma sheath attached to the cavity surface. A specially designed single cell cavity with sample holders is used to study these asymmetric discharges. The sample holder ports can be used for both diagnostics and sample etching purposes. The approach is to combine radially and spectrally resolved profiles of optical intensity of the discharge with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions and sequences.