Modeling of charging effects in scanning ion microscopes

Abstract

Unwilling deformations of secondary electron (SE) images due to charging of an insulating layer on materials is one of important issues for semiconductor industry applications of scanning ion microscopes (SIM). This paper presents a Monte Carlo model of SE emission from SiO 2 in which the charging induced by ion bombardment at the energy range of tens of keV is taken into account. A self-consistent calculation is carried out for the transport of a projectile ion, recoiled material atoms and SEs, the creation of space charges trapped in the material and the resultant electric field in/out the material. Drift motion of trapped charges is calculated as well, where the recombination with a charge of opposite sign is taken into account. Therefore, the evolution of the charging is simulated with successive arrivals of ions. Since the surface voltage is positive due to ejection of SEs and injection of positive ions, some of ejected SEs are drawn back to the surface and can rebound on it; these SEs are unable to produce a net emission. Dynamic changes in the SE yield and surface voltage are compared among He ions, Ga ions and low-energy (<1 keV) electrons, along with the space charge distributions and the in/out electric fields. The net SE yield is decreased during ion bombardment and finally it vanishes, which is different from the case of electron bombardment where the net SE yield (including BSEs) is kept to one due to a balance between coming and outgoing electrons. Even if there is not net emission of SEs, the surface voltage does not reach any steady-state condition but progressively increases due to successive injection of positive ions. The growth rate of the surface voltage depends on both the SE yield with no charging and the spatial distribution of the ions penetrating into the material.