Contamination in Ultra-High Vacuum Plant

Abstract

Using an electron bombarded target the presence of silicone vapor was detected in the vessel of a bakeable ultra‐high vacuum system made of stainless steel exhausted by a silicone oil diffusion pump. Silicone molecules temporarily adsorbed on the target were desaturated by the impinging electrons and polymerized to form a deposit. The deposit thickness was measured by multiple‐beam interferometry. The diffusion pump was charged with silicone 704 and fitted with a chevron (trap II) and a container trap (trap I) next to the vessel. Liquid nitrogen cooling both the chevron and container‐type trap next to the vessel after baking (p_ultimate≃10⁻⁹ Torr) gave a deposit growth rate of −9 Torr gave rise to a deposit growth rate of 160 Å/h on the next baking cycle. The contamination rate was reduced to its lowest amount during baking (35 Å/h) when the chevron baffle was cooled throughout the pumping cycle. Using a new silicone fluid (705) gave a deposit growth rate of about one‐half to one‐quarter of fluid 704.