ABSTRACT
References ...................................................................................................................................... 685
Knoll and Van Ardenne in Germany constructed the rst SEM in the late 1930s. Figure 38.1 illustrates a modern SEM. This instrument differs fundamentally from the conventional TEM, and all other common optical instruments, in that no lens acts on the image-forming radiation after it strikes the specimen. Instead, the electron beam is focused on a very small diameter by
two or three electromagnetic lenses, which are functionally analogous to the condenser lenses of a TEM, and then caused to scan the specimen in a raster pattern by a system of beam de§ecting coils. The image is then formed in a point-by-point manner using various types of signals generated by interaction of the electron beam with the specimen. The image is displayed on a cathode ray tube (CRT) whose electron beam is driven by the same scanning drive generator that scans the beam over the specimen. There is thus a one-to-one correspondence between points on the viewing screen of the CRT and points on the specimen. Magni cation is changed simply by an ampli er, which changes the size of the area scanned on the specimen relative to that of the CRT.
