secondary electron, SE

“Secondary electron(s)” mean the excited electrons having an energy being defined to be less than 50 eV among all of the excited electrons, which are generated from the constituent atoms of a specimen by inelastic scattering of the incident electrons (primary electrons) in a specimen.
Although the secondary electrons are excited irrespective of the depth of the specimen, the electrons which can reach the specimen surface are limited only to those excited near the surface of the specimen because the secondary electrons have small energies and are scattered in the specimen. That is, the escape depth of secondary electrons from the specimen is as small as 5 to 10 nm for most metals. Thus, the secondary electrons are emitted more at an oblique incidence of the incident probe onto the specimen surface than at a perpendicular incidence of the probe.
Fig. (a) shows an energy spectrum of the emitted electrons from a specimen surface for a primary electron beam of 20 keV. A large peak at the left is due to the secondary electrons, extending to about 50 eV. The electrons with more than 50 eV are backscattered electrons (reflected electrons) existing over a wide energy range. Small peaks seen in the range of backscattered electrons are attributed to the Auger electrons.
The major part of secondary electrons is excited directly by the incident electrons along the incident electron path and emitted only near the specimen surface. These secondary electrons are called “SE1(s).” SE1 is used to form a secondary electron image, which possesses information on the specimen shape (angle of the specimen surface against the incident electron beam) at the incident electron position and on the work function of the specimen.
It should be noted that there is another type of secondary electrons. When the incident electron energy is large (accelerating voltage being high), the diffusion region of the incident electrons becomes large. The backscattered electrons (reflected electrons) into a region distant from the incident electron path excite secondary electrons. These secondary electrons are called “SE2(s)”, which form the background of the secondary electron image (formed by SE1), degrading the image contrast. In addition, if the incident electron energy is as large as a few 10 eV, the backscattered electrons coming out from the specimen hit the SEM components (objective lens, detector, etc.) and can excite secondary electrons from the components. Those secondary electrons are called “SE3(s)” and form the background of the secondary electron image or decrease the contrast of the secondary electron image. The generation process of SE1, SE2 and SE3 is shown in Fig. (b).

Fig.（a） Energy spectrum of electrons emitted from a specimen at a primary electron probe of 20 keV.

Fig.(b) Generation processes of SE1, SE2 and SE3.