Target material dependence of secondary electron images induced by focused ion beams

Monte Carlo calculation of ion-induced kinetic secondary electron emission was performed to study the target material dependence of secondary electron images in scanning ion microscopy (SIM) by using focused ion beams, which is different from that in scanning electron microscopy (SEM). In the calculation, the electron excitation by projectile ions is treated using the partial wave scattering cross-section of conduction electrons by the ions, and the cascade multiplication process of the excited electrons is simulated as well as the elastic collision cascade of recoiling target atoms. The calculated secondary electron yields of Al (Z2=13), Cu (Z2=29) and Au (Z2=79) for 30 and 10 keV Ga+ ion impacts decrease with increasing atomic number, Z2, of the materials, whereas those for 1 keV H+ ions and 10 keV electrons increase with Z2. This is consistent with the observed opposite trend between the SIM and SEM in the Z2 dependence of secondary electron images. Simultaneous calculation of individual elastic and inelastic collisions of the projectile ion, the recoiling target atoms and the excited electrons suggests that the Z2 dependence of the secondary electron yield for heavy-ion impact is related to large elastic energy loss of the projectile ion and low energy of the excited electrons in the solid.