Image Formation in the Scanning Electron Microscope

One of the most surprising aspects of scanning electron microscopy is the apparent ease with which images of rough objects can be interpreted by newcomers to the field, or even by laymen unfamiliar with the instrument. However, there is frequently more than meets the eye in SEM images, even images of simple objects, and to gain the maximum amount of information, it is necessary to develop skills in image interpretation. Moreover, to ensure that the image has been properly constructed and recorded in the first place, a good working knowledge of the entire image formation process is necessary. In this chapter, we will consider the major aspects of the SEM imaging process: (1) the basic scanning action used for the construction of an image; (2) the origin of the commonly encountered contrast mechanisms which arise from the electron-specimen interaction; (3) the characteristics of detectors for the various signals and their influence on the image; (4) signal quality and its effect on image quality; and (5) signal processing for the final display.