ABSTRACT
The electron gun operates typically over a voltage range 0 to 30 keV sometimes extending up to 60 keV depending upon the type of instrument and application where the specimen is maintained at earth potential. In the scanning electron microscope (SEM), as with electron optical instruments, an incident electron beam is focused on to the specimen, but in this particular case, it is also scanned over it. The resolution is set by the diameter of the incident beam and the mode of operation. An environmental control stage with a limiting aperture was described by Lane for an SEM together with the appropriate scattering cross sections, mean free paths for electrons in various gases. To maximise SEM imaging use is made of the different signals produced when the electron beam interacts with the bulk specimen. Electron yield varies with the accelerating voltage and therefore it is desirable to control the selected accelerating voltage to produce image which contains the required information.
