SCANNING ELECTRON MICROSCOPE INVESTIGATION OF CHARGING AND DISCHARGING BEHAVIOR IN INSULATIONS

Abstract

In the thesis, experimental work on the charging and discharging phenomena in dielectrics is carried out by the use of a scanning electron microscope (SEM). The work is divided into two parts. The first part deals with the experimental methods measuring charge trapped in an insulator and the charge distribution volume. Charging and discharging processes under high energy electron beam irradiation are studied in the second part of the thesis. The trajectory of an electron traveling in an electrostatic field is studied in the mirror image technique. The classical Kepler's equation for the calculation of satellite trajectory is modified to describe such a process. A scattering model is proposed to determine the charge implanted in an insulator from experimentally measured quantities. On the basis of the scattering model, we propose another fitting model that includes a dipole distribution. Both models are used to calculate the value of the charge in our experiments. The results are found consistent and these two models establish the mirror image method (MIM). The MIM method is further developed to measure the charge distribution volume. An expression of the electrostatic potential is derived by assuming an hemispheroid distribution. and a dipolar approximation. Dielectric samples with different relative permittivity are used in the charging experiments to justify our approach. The proposed method is employed to measure the radius of the charge distribution volume in Polymethylmethacrylate (PMMA) samples. Experimental results obtained are in good agreement with those obtained through the use of other experimental techniques and Monte Carlo simulation. With the MIM method, the charging behavior of PMMA is investigated. Charges trapped in the PMMA specimens are measured under different conditions of radiation time, accelerating voltage and beam current. The influence of the surface potential and the internal electric field in the PMMA specimen is studied and the latter is found to play a more important role in the charging process. There exists a saturation value for the charge that can be trapped in the specimen under a fixed charging voltage. Beyond the saturation, the trapped charge destabilizes and surface discharge is observed. A Gaussian model of charge distribution is used for the calculation of the internal electric field. The strength of the electric field initiating surface discharge is estimated. A slow discharge process from an implanted charge in PMMA is studied. A grounded metal aperture is applied on the specimen surface, and the discharge is confined to the small vacuum-insulator interface. An oscillation phenomenon is observed in which concentric bright rings along with a disk of the mirror image appear. Traces of flashover from the rings are found. The mechanism of this oscillation phenomenon is explained as the result of hot electron propagation, collision and retrapping in the insulator.