The use of backscattered electron (BSE) imaging in low voltage scanning electron microscopy (SEM) has increased over the past few years. This appears to be due to several factors including improved performance of SEMs at low voltages, reduced beam penetration, more reliable metrology, improved atomic number (Z) contrast information (for low Z) and reduced charging artefacts over secondary electron (SE) imaging. Understanding the factors involved in low voltage BSE detection may assist in improving the information attainable.
It has been shown that the signal Sdet from a BSE detector, for EB ≫ Ew is given by
where η is the BSE yield, Ω is the solid angle subtended by the detector to the specimen, D is the internal conversion efficiency of the detector, EB is beam accelerating voltage, Ew is the energy barrier of the dead layer on the detector's surface, IB is the beam current, F(Z) and F(Ω) are functions which take into account the variation of BSE energy with atomic number Z and collection angle respectively.