Contamination in Ultra-High Vacuum Plant

Publisher Website
Google Scholar
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 (pultimate≃10−9 Torr) gave a deposit growth rate of <5 Å/h. When the chevron trap was not operated during ultra-high vacuum pumping the container-type trap condensed sufficient oil vapor for the vessel to be contaminated when the vessel and trap were subsequently baked. The contamination rate measured during baking was dependent on the time the system had been previously under ultra-high vacuum without the chevron trap being cooled. Ultra-high vacuum pumping for 38 h at 10−9 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.

This publication has 6 references indexed in Scilit: