Direct Observation of Surface Contamination formed in Demountable Vacuum Systems

Abstract

THE visible contamination of specimens examined in the electron microscope is the result of a reaction between the electron beam and matter adsorbed on the irradiated surface. This adsorbed material is known to consist of hydrocarbon molecules1, though it may also contain some water molecules; it exists on surfaces in all demountable vacuum systems. The layer is troublesome in many experiments and processes involving such systems, but evidence of its presence is only indirect and little is known about its thickness or composition. It is particularly noticeable in high-resolution shadow-casting for the electron microscope (unpublished work), since it limits the resolution by causing a fine background structure. In the course of an investigation of this structure, a method was found for studying the adsorbed layer directly, and also for estimating its thickness. It was found that the layer was visible in the electron microscope when viewed end-on and sandwiched between two carbon films (see Fig. 1). The procedure is carried out as follows.