Specimen Coating

Abstract

It is the purpose of this chapter to review the advances in coating techniques for nonconducting samples which are necessary to eliminate problems of surface charging. The ways by which one may hope to improve the bulk conductivity of such samples are discussed elsewhere in this book. It is not our intent to discuss the optimal parameters of standard coating procedures such as thermal evaporation and diode sputtering, which are designed to increase the secondary electron emission and surface conductivity of samples. Such details may be found in SEMXM. There is now considerable overlap between the high-resolution coating techniques which were once used exclusively for transmission electron microscopy and the current refinements of the methods routinely used for scanning electron microscopy. This overlap is an inevitable consequence of the improving resolution of the conventional SEM, the increasing use of STEMs, and the appearance of hybrid machines, which allow a conventional TEM to be used as an SEM. It is generally accepted that the increased resolution available on most electron beam instruments necessitates much more attention being paid to the ways by which the coating layers are applied to samples. It should be remembered that the two main reasons for coating nonconductive samples are to eliminate the artifacts brought about by electrical charging and by thermal loading. Whereas a thin (1–3 nm) continuous layer of metal is a reasonably good electrical conductor, at least double that thickness of metal is needed to conduct away the heat which can readily build up in the specimen.