Secondary electrons (SEs) are electrons that are weakly bound
to the specimen and are ejected from the specimen by primary electrons and may or may not subsequently leave the specimen. The emission of electrons, by the surface of a solid object when it is bombarded by electrons, was discovered in 1902 by the German physicists L. Austin and H. Starke. As shown in Figure 4822, three groups of electrons are present in the electron flow. The electron beam bombarding the object are called incident beam, and the emitted electrons are called SEs. These electrons’ energy and momentum prove to be sufficiently great to surmount the potential barrier on the surface of the object and leave the surface. Since such SEs have energies less than 50 eV, they can only travel short distances through materials (< ~10 nm), and thus originate from the near-surface region. The reflected electrons without energy loss are called elastical backscattering electrons and the reflected electrons with energy loss are called in-elastical backscattering electrons.
Figure 4822a. Distribution of electrons according to energies: (I) elastically reflected backscattering electrons at an energy level
of incident electron beam (εp), (II) inelastically reflected backscattering electrons, (III) secondary electrons.
In bulk materials, SEs are emitted in the backward direction as shown in Figure 4822b, while SEs are emitted in both backward and forward directions. One of the reasons why the SEM detectors are inserted above the sample stage is because SEM systems are mostly used for observing bulk materials.
Figure 4822b. Secondary electron emission: (a) backward-SEs, (b) forward-SEs
Secondary electrons (SEs) are produced by inelastic interactions of high energy electrons with valence electrons of atoms in the specimen, causing the ejection of the electrons from the atoms in the specimen. These ejected valence electrons are called as secondary electrons. This generation process of SEs causes a slight energy loss and path change in the incident electron. Each incident electron can produce several secondary electrons, especially depending on thickness of the specimen.
Figure 4822c shows the generation of secondary electrons (SEs) and kinetic energies of the emitted SEs. EKE1 and EKE2 represent the kinetic energies of the two generated SEs. The kinetic energy of the generated SEs is normally in the range of 0 to 50 eV. ΔE1 and ΔE2 represent the energy losses of the incident electrons after the incident electrons interact with the electrons in the K and L3 subshells, respectively. E1 and E2 are the binding energies of the two electrons. E0 is the energy of the incident electrons in the EMs.
Figure 4822c. The generation of secondary electrons (SEs) and kinetic energies of the emitted SEs.
The main inelastic scattering mechanisms are:
i) Phonon excitation (heat).
ii) Plasmon excitation (valence electrons).
iii) Single electron excitation (inner and outer shell scattering).
iv) Direct radiation losses (Bremsstrahlung radiation due to deceleration of the electron beam in the Coulomb field of an atom).
v) Excitation of conducting electrons leading to secondary electron emissions.