Coating Techniques for SEM and Microanalysis

Nearly all nonconductive specimens examined in the scanning electron microscope or analyzed in an electron probe microanalyzer need to be coated with a thin film of conducting material. This coating is necessary to eliminate or reduce the electric charge which builds up rapidly in a nonconducting specimen when scanned by a beam of high-energy electrons. Figures 10.1a and 10.1b show examples of pronounced and minor charging as observed in the SEM. In the absence of a coating layer, nonconductive specimens examined at optimal instrumental parameters invariably exhibit charging phenomena which result in image distortion and thermal and radiation damage which can lead to a significant loss of material from the specimen. In extreme situations the specimen may acquire a sufficiently high charge to decelerate the primary beam and the specimen may act as an electron mirror. Numerous alternatives to coating have been proposed and some of these will be discussed in this chapter. Much of what will be discussed is directed towards biological material and organic samples simply because these types of specimens are invariably poor conductors and more readily damaged by the electron beam than most inorganic materials. However, it is safe to assume that the methods which will be described for organic samples will be equally effective for nonconducting inorganic specimens.