Oxygen plasma asher contamination: An analysis of sources and remedies

The low Earth orbit (LEO) environment is commonly simulated using oxygen plasma ashers to determine the effects of LEO on spacecraft materials. However, plasma ashers can also contaminate samples during plasma exposure, making them less than ideal for space simulation. This study results from attempts to minimize or eliminate contamination. Optical methods of variable angle spectroscopic ellipsometry and reflectance spectrophotometry were used to quantify contaminant stoichiometry and deposition rate. Auger electron spectroscopy identified deposited contaminants and their surface coverage. Contamination results from etching of the rubber chamber seals by the plasma. The deposited contaminant was nearly indistinguishable from fully stoichiometric SiO₂. Contaminant deposition rates up to 0.27 nm/min have been observed and these layers effectively passivate the surface by depositing an overcoat of SiO_x. Placing metal into the path of the plasma before it can reach the chamber seals greatly reduces contamination. A newly designed chamber confines the plasma to a small volume away from the chamber seals. For fluences as high as 3.5 × 10²² atoms/cm², equivalent to 7.5 years of space exposure for the International Space Station, the redesigned asher showed less than one monolayer of deposited contaminant.