backscattered electron yield, backscattered electron coefficient, backscatter yield, backscatter coefficient

The ratio of the number of backscattered electrons emitted from a specimen to the number of incident electrons (primary electrons) onto the specimen. The emission ratio (coefficient) η is defined as I_b/I_p, where I_b is the electric current of the backscattered electrons and I_p is the electric current of the primary electrons.
As shown in Fig. (a), the emission coefficient η is dependent on the atomic number of a material. Thus, the compositional difference of a specimen is revealed using a backscattered electron image.
Furthermore, the emission coefficient η is large in the direction of specular reflection of the incident electron beam with respect to the specimen surface. Fig. (b) shows the characteristic feature of the specular reflection. That is, when the primary electron beam is incident on an inclined specimen surface with an angle α, backscattered electrons are strongly emitted in the direction of angle α (shown by a thick blue line) with respect to the specimen surface. Those emitted electrons travel straight because of their high energy (similar to that of the primary electrons). When a detector is placed in the travel direction of the backscattered electrons, they are detected with a high efficiency. As a result, strong topographic contrast, which depends on the position of the detector, is obtained as if the specimen is illuminated from a definite direction (illumination effect).

Fig. (a) Dependence of backscattered electron emission coefficient η on the atomic number

Fig. (b) Dependence of backscattered electron emission on the angle of the specimen surface against the incident electron probe.
Elastically-scattered backscattered electrons have a strong intensity in the direction of the specular reflection of the incident electron beam with respect to the specimen surface. Thus, the detected intensity of the backscattered electrons largely changes with the position of the detector. The backscattered electron image shows a strong illumination effect.