Scenario for time evolution of insulator charging under various focused electron irradiations

Abstract

The expected time evolution of the secondary electron emission (SEE) yield, δ, and charging of insulators irradiated with keV electron probes of various sizes is deduced from the use of basic laws of electrostatics. Simple models of trapped charge distributions permit, next, quantitative estimates. With respect to the linear increase of δ from 0 towards its nominal value, ${\delta }^0,$ the initial phase is characterized by a deficit of δ for incident spots in the submicron range and incident charge of a few ${10}^2$ primary electrons (PEs). This deficit occurs even when charging is as a whole negative and it results from a partial mirror effect for the less energetic ${\delta }_P$ secondary electrons (SEs) (directly excited by the PEs) combined to attraction of ${\delta }_B\mathrm{SE}$ (excited by the backscattered electrons) towards the central spot while the more energetic ${\delta }_p$ SEs are successively focused and next defocused. The next phase starts for incident charges in the pC range and it concerns the evolution of the total yield, $\mathrm{\delta +\eta }$ from its nominal value up to the unity. Besides the increase of the SEE yield, the external slowing down of the PEs plays the main role in the compression of the distribution of newly trapped electrons. The main dynamical aspects of the internal field are also established and its influence on the trapped charge distribution is easily deduced. The present analysis is supported by some published data and the consequences concern any type of insulating material irradiated with defocused probes and “short” pulse excitation (for ${\delta }^0$ measurements) or stationary fine probes (for the investigation of the space charge effect by the mirror method) or scanning beams (in scanning electron microscopy).