PLASMA CLEANING OF BEAMLINE OPTICAL-COMPONENTS - CONTAMINATION AND GAS-COMPOSITION EFFECTS

We have initiated a program to study the impact of gas composition on the carbon removal rate during plasma cleaning of optical components, and of possible contamination due to the plasma processing. The measurements were performed in a test chamber designed to simulate the geometry of the grating/Codling mirror section of a Grasshopper monochromator. Removal rates were determined for a direct-current (dc) (Al electrode) discharge using a quartz crystal microbalance coated with polymethylmethacrylate, located at the position of the grating. Auger electron spectroscopy analysis of strategically located, gold-coated stainless steel samples was employed to determine contamination. The relative removal rates of the gases studied were 3% C2F6/O2>> O2 + H2O > O2 approximately N2O > H2 > N2. Although the C2F6/O2 gas mixture showed a 20 times greater removal rate than its nearest competitor, it also caused significant contamination to occur. Contamination studies were performed for both dc and radio-frequency (rf) discharges. For the dc discharge we found that great care must be taken in order to avoid A1 contamination; for the rf discharge, significant Fe contamination was observed.