Scanning microscopy with electrons and x-rays.

The principle of sequential imaging, by means of a signal generated with an electron probe scanning the specimen in a regular raster, has been successfully developed into the form of practical microscopes during the past few years. The scanning electron microscope has been constructed first, but it was slow to find its proper fields of application. Now it is proving valuable primarily in solid state physics and its technological aspects, such as integrated circuits. The scanning microscope which forms an image with characteristic X-rays, usually known as the electron probe microanalyser, found more immediate practical uses because of its power of element discrimination. It is now an important analytical tool in metallurgy , and mineralogy, but its value in biological and medical research is also under active investigation. An outline is given of the physical ' principles of these two types of instruments, of their design and construction , and of the scope and limitations of their uses. 1.1. Introduction OOME early attempts to build a scanning type of electron microscope came to nothing, mainly because the recording systems then available were not sensitive enough to produce an image in a reasonably short time (von Ardenne, 1938; Zworykin, Hillier and Snyder, 1942); in modern terminology, the signal-to-noise ratio was much too low. The effective development of the instrument began with the work of McMullan (1953) in the Engineering Laboratory of Cambridge University, and was continued there by a succession of workers under the direction of Professor Oatley, resulting finally in the prototype of a production scanning microscope. The working out of the practical problems involved in its design and operation has thus been almost entirely the work of that Laboratory, so no apology is necessary for the fact that this survey will describe mainly the research done at Cambridge. Other laboratories have begun to take up the technique in the past few years, and some of them reported their contributions in other sessions of the Kyoto Congress. An extensive review of the subject has recently been published by Oatley, Nixon and Pease (1965), in which further details may be found of aspects for which space does not permit full discussion here. The scanning microscope, in its usual form, forms an image in terms of the electrons scattered back from the surface of a specimen on which a fine probe of electrons is incident. The probe is typically a few hundred …