Electron Optics of a Scanning Electron Microscope

Abstract

Electrons emitted from thermionic, Schottky or field-emission cathodes are accelerated by a voltage of 0.1–50 keV between cathode and anode. The purpose of the electron optics of a SEM is to produce a small electron probe at the specimen by demagnifying the smallest virtual cross-section of the electron beam near the cathode. For the practical operation of a SEM, it must be possible to vary the electron-probe size, aperture and current; these cannot, however, be varied independently because they are related via the gun brightness. A geometric optical theory of electron-probe formation can be employed when using a thermionic cathode but for a field-emission gun a wave-optical theory is necessary. Electron-beam deflection by transverse electrostatic and magnetic fields is incorporated for scanning the electron probe across the specimen, for tilting the direction of the incident electron beam for stereoviewing and for recording electron channelling patterns. Deflection systems are further used for blanking and chopping the electron beam up to gigahertz frequencies for stroboscopic modes and for generating time-resolved signals. Owing to the large depth of focus, focusing of SEM images raises no problems but the resolution is limited by the electron-probe size, which decreases with decreasing spherical (C s) and chromatic (C c) aberration coefficients of the final probe-forming lens. Though a large working distance between specimen and lower polepiece has the advantage that free space is available for the SE and BSE detectors, a stronger lens excitation and an in-lens position of the specimen decreases the aberration coefficients by one order of magnitude.