SPECIMEN-ELECTRON BEAM INTERACTIONS

When a beam of primary electrons emitted by a filament in an electron gun hits the surface of a specimen, excitation by the beam causes the emission of secondary electrons, backscattered (reflected primary) electrons, Auger electrons, photons, characteristic X-rays, continuum X-rays, and heat (Fig. 2.1). These signals are produced from a specific emission volume within the specimen, the size of which depends upon the primary electron beam energy (Eo) and the average atomic number (Z) of the specimen. The incident primary electrons undergo two types of scattering upon entering the specimen: elastic scattering and inelastic scattering. During elastic scattering, the incident electrons interact with the nuclei of atoms in the specimen, causing a significant deflation of the trajectories, but they lose little energy. Two types of elastic scattering can occur. Rutherford scattering, which occurs in the Coulomb field of the nucleus, is a single scattering event that results in a large change in trajectory direction, in some cases exceeding 90°. Multiple scattering comprises several small scattering events. A proportion of the elastically scattered electrons is eventually directed back out of the specimen as backscattered electrons. The remainder diffuse through the sample in a random manner and are eventually absorbed. During inelastic scattering, incident electrons may either (1) interact with the nuclei of atoms in the specimen, losing energy in the Coulomb field and emitting white or continuum X-ray radiation or (2) collide with loosely bound electrons in the specimen, causing the latter to be ejected as secondary electrons.