Monte Carlo study of secondary electron emission from SiO2 induced by focused gallium ion beams

In order to compare secondary electron images of insulating materials obtained using scanning ion and scanning electron microscopes, secondary electron (SE) emission from SiO2 under Ga ion and electron bombardment was simulated by a Monte Carlo model. The model is characterized by SE generation by excitation of valence electrons across a large energy gap and the energy loss of low-energy SEs via interaction with phonons. The calculated SE yield under electron bombardment was much larger for SiO2 than for Si, as in many experiments, while showing a decrease with increasing material temperature. Due to a threshold effect originating from a large energy gap, the SE yield under Ga ion bombardment was smaller for SiO2 than for Si at energies of a few tens of keV. The calculated depth distributions of SEs that escaped from the surface showed the electron escape depth is larger for SiO2 than for Si, whereas the lateral distributions are much wider for electron bombardment than for Ga ion bombardment.