Space charge characteristics of an insulating thin film negatively charged by a low-energy electron beam.

In this study, based on a comprehensive numerical simulation of self-consistent charging, we investigate the formation, evolution and influencing factors of space charge distributions for a grounded insulating thin film of SiO(2) negatively charged by a keV non-penetrating focused electron beam. The simulated space charge presents first positive distributions and then negative ones along both the radial and depth directions because of the difference between electron and hole transports. The variations in distribution occur within a range of the minimum potential acting as a potential barrier for carrier transport. The negative space charge is distributed more widely and deeply, though its peak value in density is usually lower than that of the positive one. Electrons trapped outside the minimum potential range dominate the strength of negative charging. With the increase in potential barrier and the occurrence of leakage current, the space charge eventually reaches equilibrium and exhibits an approximately one-dimensional axial distribution outside the minimum potential range. Distribution features of the space charge density in the equilibrium state correlate with the film and beam parameters via transients of the leakage current. These results and analyses provide new insights into the negative charging effects involved in various electron-beam-based surface microscopic methods, analyses and fabrication techniques.