Vacuum properties of palladium thin film coatings

A recent development, carried out at CERN for particle accelerator applications, showed that a vacuum chamber coated with a thin getter film and then exposed to ambient air may be transformed into a pump by "in situ" heating at temperatures as low as 180degreesC. Heating activates the diffusion into the film of the oxygen present in the surface passivation layer. Repeated air exposure-activation cycles progressively enrich the film with oxygen, reducing its performance and shortening its operating life. To overcome this inconvenience, noble metal coatings were considered. At distinction with getters, noble metals may release all the pumped gases by heating, resulting in a practically unlimited life. Thin film coatings of palladium were studied by surface analysis, electron stimulated desorption and pumping speed measurements. These coatings were found to pump H-2 and CO, even without activation by heating, but not N-2 or CO2. Thin Pd and Pd-Ag films were also used as overlayers for protecting a getter film from oxidation while not impairing its H2 pumping. The result of these studies are presented and discussed. (C) 2003 Elsevier Ltd. All rights reserved.