Electron Beam–Specimen Interactions

In the introductory survey presented in Chapter 1, we learned that the SEM image is constructed by scanning a finely focused probe in a regular pattern (the scan raster) across the specimen surface. The spatial resolution achieved in this imaging process is ultimately limited by the size and shape of the focused probe that strikes the specimen. In Chapter 2 we learned how to control the critical parameters of the electron beam, energy, diameter, current, and divergence, through the use of electrical fields in the gun, magnetic fields in the lenses and stigmators, and beam-defining apertures. We saw how, depending on the type of electron source (tungsten hairpin, lanthanum hexaboride, thermal field emission, or cold field emission) and its inherent brightness (a constant dependent upon the beam energy that has been selected), it is possible to create focused beams with sizes ranging from nanometers to micrometers (three orders of magnitude) carrying currents ranging from picoamperes to microamperes (six orders of magnitude). This great flexibility in operational conditions permits the SEM microscopist/microanalyst to successfully attack a wide range of problems, provided that the proper strategy is employed. The strategy needed for selecting the proper operating conditions depends critically upon understanding (1) what happens when the beam reaches the specimen and (2) how the signals produced by the electron beam–specimen interactions are converted into images and/or spectra that convey useful information.