Introduction

The scanning electron microscope consists essentially of the following (as described in Chapter 3): a source of electrons; lenses for focussing them to a fine beam; facilities for sweeping the beam in a raster; arrangements for detecting electrons (and possibly other signals) emitted by the specimen; and an image-display system. Secondary-electron (SE) images, which show topographic features of the specimen, are the most commonly used type. Backscattered-electron (BSE) images are principally used to reveal compositional variations. An X-ray spectrometer (Chapter 5) is an optional extra enabling the SEM to be used for element mapping and analysis (Chapters 6–8). Other types of image can also be produced, as described at the end of this chapter.

Magnification and resolution

The magnification of a scanning image is equal to the ratio of the size of the image as viewed by the user to that of the raster scanned by the beam on the specimen. The minimum magnification is determined by the maximum angle through which the beam can be deflected, and depends on the working distance, being least when this is greatest. Typical minimum magnification is about 10, with a scanned area of the order of 1 cm2. Magnification can be increased by reducing the amplitude of the scanning waveform. There is no advantage in increasing magnification beyond the point at which the image starts to appear unsharp.