S. Fakhfakh

Dynamic investigation of electron trapping and charge decay in electron-irradiated Al2O3 in a scanning electron microscope: Methodology and mechanisms

Abstract The charging and discharging of polycristalline Al 2 O 3 submitted to electron-irradiation in a scanning electron microscope (SEM) are investigated by means of the displacement current method. To circumvent experimental shortcomings inherent to the use of the basic sample holder, a redesign of the latter is proposed and tests are carried out to verify its operation. The effects of the primary beam accelerating voltage on charging, flashover and discharging phenomena during and after electron-irradiation are studied. The experimental results are then analyzed. In particular, the divergence between the experimental data and those predicted by the total electron emission yield approach (TEEYA) is discussed. A partial discharge was observed immediately after the end of the electron-irradiation exposure. The experimental data suggests, that the discharge is due to the evacuation to the ground, along the insulator surface, of released electrons from shallow traps at (or in the close vicinity of) the insulator/vacuum interface.

A new experimental approach for characterizing the internal trapped charge and electric field build up in ground-coated insulators during their e− irradiation

Abstract An original method is proposed to investigate the dynamical trapping properties of bulk insulators during their irradiation by keV electrons when they are coated with a grounded metallic film. This method is based on the measurement of the displacement current and it allows to evaluate time constants for charging and discharging the dielectric as well as to evaluate the electric field build up and trapped charge density below the coating. This method is illustrated by the estimate of the charging and discharging time constants in e − irradiated PMMA and the estimate of the magnitude of the electric field which drives the migration of the mobile ions in e − irradiated glasses.

Charge Implantation Measurement on Electron-Irradiated Insulating Materials by Means of a SEM Technique

The goal of this article is first to review the charging effects occurring when an insulating material is subjected to electron irradiation in a scanning electron microscope (SEM) and next their consequences from both scanning electron microscopy and electron probe microanalysis (EPMA) points of view. When bare insulators are observed, the so-called pseudo mirror effect leads to an anomalous contrast and also to an erroneous surface potential, V(S), measurement when a Duane-Hunt limit (DHL) method is used. An alternative possibility is to use an electron toroidal spectrometer (ETS), specially adapted to a SEM, which directly gives the V(S) value. In the case of a bulk specimen coated with a grounded layer, although the layer prevents external effects of the trapped charge, the electric field beneath the coating is reinforced and leads to loss of ionizations that reduces the number of generated X-ray photons. To take into account both effects mentioned above, whether the studied insulator is coated or not, a method is proposed to deduce the trapped charge inside the insulator and the corresponding internal or external electric field.

Charging effects of PET under electron beam irradiation in a SEM

This paper deals with charge trapping and charge transport of polyethylene terephthalate (PET) polymer subjected to electron irradiation in a scanning electron microscope (SEM). Measurement of displacement current and leakage current using an arrangement adapted to the SEM allows the amount of trapped charge during and after electron irradiation to be determined and the charge mechanisms regulation to be studied. These mechanisms involve several parameters related to the electronic injection, the characteristics of insulator and the effects of the trapped charge itself. The dynamic trapping properties of PET samples are investigated and the time constants of charging are evaluated for various conditions of irradiation. The determination of the trapping cross section for electrons is possible by using the trapping rate at the onset of irradiation. Many physical processes are involved in the charging and discharging mechanisms; among them surface conduction is outlined. Through the control of irradiation conditions, various types of surface discharging (flashover phenomenon) behaviour are also observed. The strength of the electric field initiating surface discharge is estimated.

A new method for charge trapping measurement during electron beam irradiation: application to glass containing alkali ions and single-crystalline quartz

The aim of this work is to study the electron irradiation behaviour of an insulating material surface using a scanning electron microscope (SEM). The charging phenomena caused in two kinds of insulating materials (quartz and glass) by continuous electron irradiation have been observed. The discharging phenomena following switching off of irradiation have also been studied. The trapped charge density is determined by using the so-called electrostatic influence method based on the measurement, during and after the irradiation, of the influence and leakage currents using an arrangement adapted to the SEM. The experimental results reveal that the behaviour under irradiation of glass is entirely different from that of quartz. The trapped charges are found to be different, and the dependence of charging on the primary beam energy is discussed.The charging and discharging time constants have been determined accurately, and their evolution versus the mean electron penetration depth is qualitatively explained. Moreover, the role of secondary electron emission in the regulation mechanism of charging is underlined.

An experimental approach for measuring surface potential and second crossover energy in insulators

The goal of this work is to first measure the second crossover energy E2S under stationary electron irradiation (charging conditions) and then to show that the charge balance occurs at this beam energy and not at E2C, the energy deduced from non-charging conditions (short pulse irradiation) as commonly assumed. The experimental procedure is based on simultaneous time dependent measurements of surface potential, leakage and displacement currents. The study is illustrated by the estimate of the real landing energy of primary electrons EL and the second crossover energy E2S for soda-lime glass.

EPMA analysis of insulating materials: Monte Carlo simulations and experiments

Using a realistic model for the electric field build-up that takes into account detrapping processes in insulating materials irradiated by electrons, a Monte Carlo approach has been applied to ground-coated binary oxides such as Al2O3 and Nb2O5. Changes entailed by the internal electric field build-up on the generation of the characteristic x-ray quanta and also on backscattered electron emission are investigated. The results clearly show that the depth distribution of characteristic x-ray production is modified and the Φ(ρz) function for both metal and oxygen Kα lines is compressed towards the surface while the backscattering electron emission is roughly unchanged. The change of the x-ray intensities as a function of the electric field is clearly established. The outcome is checked experimentally by measuring simultaneously the trapped charge and the emitted x-ray spectra during electron irradiation.

Quantitative voltage contrast method for electron irradiated insulators in SEM

A surface potential mapping method for electron irradiated insulators in the scanning electron microscope (SEM) is proposed. This method, based on the use of a highly compact electrostatic toroidal spectrometer specially adapted to SEM applications, is able to monitor the spatial variation of surface potentials of strongly negatively charged materials. The capabilities of this method are tested on a made-up heterogeneous sample. First results prove that the method is particularly appropriate for the reconstitution of the surface potential distribution.

Dynamic investigation of the trapping properties of electron irradiated dielectrics in a SEM

A method is described that allows the amount of trapped charge in insulating materials during their electron irradiation in a scanning electron microscope (SEM) to be provided and the charge mechanisms regulation to be understood. The trapped charge regulation mechanisms involve several parameters related to the electronic injection, the characteristics of insulator and the effects of the trapped charge itself. For that purpose we propose an arrangement adapted to the SEM in order to measure, one among these parameters, the leakage current and also the displacement current. The dynamic trapping properties of glass-ceramic are investigated and the time constants for charging and discharging processes are evaluated. By correlating the leakage and displacement currents, the total electron yield /spl sigma/ during irradiation is also determined.