Surface Contamination by Ion Bombardment
In electron and ion beam systems, a surface bombarded with the beam is often contaminated by growth of a thin organic layer. This phenomenon is due to the polymerization of organic molecules adsorbed on the surface by electron or ion bombardment, and the sources of the organic molecules are vapors of diffusion pump oil, grease and other chemicals adsorbed in the inner wall of the vacuum chamber. In this paper, the contamination growth by ion bombardment as compared with electron bombardment is described. The experimental data are compared with a simple theory which takes sputtering into account and good qualitative agreement is obtained. According to this theory, the higher contamination growth by ion bombardment as compared with electron bombardment can be explained by the larger cross section for polymerization of organic molecules adsorbed on the surface. Furthermore, it is ascertained that heating the substrate or using a cold trap surrounding the substrate can effectively reduce the contamination growth.
KeywordsOrganic Molecule Surface Contamination Large Cross Section Cold Trap Electron Bombardment
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