Second crossover energy of insulating materials using stationary electron beam under normal incidence

The purpose of this paper is to give some aspects of charging effects on dielectric materials submitted to continuous electron beam irradiation in a scanning electron microscope (SEM). When the dielectric is irradiated continuously, the so-called total yield approach (TYA) used to predict the sign of the charge appeared on electron irradiated insulators fails because the charge accumulated in the dielectric interferes with the electrons emission processes. Based on previous experimental and theoretical works found in the literature, an analysis of the evolution of the electron yield curves σ = f(E0) of insulators during irradiation is given. The aim of this work is firstly to determine experimentally the second crossover energy E2C under continuous electron irradiation (charging conditions) and secondly to demonstrate that the charge balance occurs at this beam energy and not at E2 the energy deduced from non-charging conditions (pulse primary electron beam experiments) as commonly asserted. It is however possible to apply the TYA by substituting the critical energy E2 for E2C. The experimental procedure is based on simultaneous time dependent measurements of surface potential, leakage current and displacement current. The study underlines the difference between the landing energy of primary electrons EL at the steady state and the second crossover energy, E2C, for charged samples. Some preliminary results are also obtained concerning the influence of the incident beam density on the energy E2C. The samples used for this study are PMMA, polycrystalline silicone dioxide (p-SiO2), polycrystalline alumina (p-Al2O3) and soda lime glass (SLG).