The electron column consists of an electron gun, one object lens and two condenser lenses. The electron gun produces a source of electrons and accelerates these electrons to an energy in the range 1-30 keV. Electron lenses are used to reduce the diameter of this source of electrons and place a small, focused electron beam on the specimen. Electron probe refers to the focused electron beam at the specimen. In order to eventually construct an image, the beam must be moved from place to place by means of a scanning system. The beam electrons interact both elastically and inelastically with the specimen, forming the limiting interaction volume from which the various types of radiation emerge, including backscattered, secondary, absorbed electrons and x rays. By using suitable detectors, we make image signals. Image of SEM is dependent upon the beam parameters: electron-probe size, electron-probe-current and electron probe convergence angle. These three parameters cannot be selected independently because they are interrelated.
