References

Figure 2. Schematic illustration of SEM electron

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The basic components of a SEM are the vacuum system, electron gun, lens system, electron detector, imaging system, and the electronics associated with these components. The vacuum system is necessary to minimize the interference of air particles with the electron beam and to prevent rapid oxidation of the tungsten filament. The vacuum system consists of roughing pumps, an oil diffusion pump, and various vacuum fittings, valves and seals that provide a working pressure in the SEM of 10-6 to 10-4 Torr (10-4 to 10-2 Pa). Our SEM has a vacuum system that may be switched to low vacuum mode (close to atmospheric pressure). The electron gun in our JEOL SEM contains a tungsten filament that is heated with a filament power supply and maintained at a high negative voltage (typically 10-20 kV) during operation. When the tungsten filament is heated, electrons are emitted from the tip and accelerated to ground by the 10-20 kV potential between the filament

and the anode. Figure 2 shows the typical configuration of an electron gun in a SEM. After electrons are emitted from the gun and accelerated down the SEM column in an electron beam, they are controlled and directed to the specimen by a series of electromagnetic lenses and apertures.

When the electrons in the electron beam hit the specimen, a number of electron- specimen interactions may occur. Some of these interactions include elastic scattering of electrons, secondary electron emission (emission of loosely bound electrons of the conduction band), ionization of inner shell electrons (produces x-rays and Auger electrons),

and excitation of phonons (causes heating of the specimen). If a sample is thin enough, some electrons will be transmitted all the way through the sample. Different materials and sample geometries will produce different amounts or different types of secondary electrons, backscattered electrons, Auger electrons, transmitted electrons, and x-rays, and all of these interactions may be used for imaging or analysis of the sample. The most common type of imaging in a SEM is secondary electron imaging (SEI), which involves the use of a secondary electron detector. The secondary electron detector collects secondary electrons and some backscattered electrons that are emitted from the specimen surface, amplifies the detected signal, and converts the electron signal into a video signal that is sent to the monitor.

•JEOL JSM-35CF Scanning Microscope Instruction Manual

•J. I. Goldstein, et al., Scanning Electron Microscopy and X-Ray Microanalysis, Plenum Press, New York (1981).