secondary yield, secondary electron emission coefficient

The ratio (δ) of the number of secondary electrons emitted from a specimen to the number of incident electrons (primary electrons) onto the specimen. The emission ratio (coefficient) δ is defined as I_s/I_p, where I_s is the electric current of the secondary electrons and I_p is the electric current of the primary electrons.

• Dependence of δ on the angles of the specimen surface against the incident electron probe
  Since the secondary electron energy is very small to be a few 10 eV even at maximum, the electrons excited only near the surface (approximately 10 nm thick from the surface) are emitted beyond the surface. This indicates that δ increases when the tilt angle of the specimen is large against the specimen surface as is shown in Fig. (a). In Fig. (a), let θ be the angle between the primary electron beam and the normal to the specimen surface at the incident point of the primary electrons: θ＝0 for the perpendicular incidence of the primary electrons onto the specimen surface. For the perpendicular incidence, the number of secondary electrons emitted from the specimen becomes the largest at the minimum escape depth Z. When the specimen is tilted by θ, the minimum escape depth becomes small to Z・cosθ. Then, the secondary electron emission increases with increasing θ, compared to the case of θ＝0. This means that the secondary electron image provides contrast dependent on tilt of the specimen against the incident beam.
• Dependence of δ on accelerating voltages
  Fig. (b) shows the dependence of δ on the accelerating voltage of the primary electrons. δ increases with the accelerating voltage until about several 100 eV. That is, δ takes maximum at voltages between 300 to 800 eV (different for constituent materials) and then decreases gradually with increasing the accelerating voltage.

Fig. (a) Dependence of secondary electron emission on specimen tilts

Fig. (b) Dependence of secondary electron emission coefficient on accelerating voltages (δ_max is less than 1 for some materials)