O. Jbara

Time-dependent measurement of the trapped charge in electron irradiated insulators: Application to Al2O3–sapphire

A method is described which uses a scanning electron microscope for the investigation of charge trapping in insulators under electron bombardment. The technique commonly used to deduce the amount of trapped charge and its spatial extent is based on the mirror effect, while in the present approach the electron-beam deflections are measured during the primary irradiation. We have performed measurements of the trapped charge during time in an Al2O3–sapphire sample under electron irradiation. Furthermore, the effects of the electron-beam energy and current on charging are also examined and the errors concerning the method are discussed in detail.

Analysis of two methods of measurement of surface potential of insulators in SEM: electron spectroscopy and X-ray spectroscopy methods

Abstract We have measured the induced surface potential on single crystal of Al2O3 under electron irradiation in scanning electron microscope (SEM), using X-ray spectroscopy method in conjunction with the electron spectroscopy method. A big disagreement between the two methods is observed. The reason of this disagreement is analysed. We show that the position of the experimentally measured high energy cut-off limit of X-ray continuous radiation (bremsstrahlung) detected from the charged sample is not consistent with the effective landing energy of the primary electrons and hence its position is not directly connected with the surface potential of the charged sample.

Surface potential measurements of electron-irradiated insulators using backscattered and secondary electron spectra from an electrostatic toroidal spectrometer adapted for scanning electron microscope applications

A technique for the accurate determination of the surface potential US and its evolution during irradiation, is proposed. The technique is based on detecting both backscattered (BSE) and secondary electrons (SE) in a scanning electron microscope (SEM). The (BSE+SE) spectra are measured using a compact, highly sensitive electrostatic toroidal spectrometer (ETS), specially adapted for SEM applications. The use of an ETS analyzer set in a SEM for deducing the surface potential from (SE+BSE) spectra of electron irradiated insulators is introduced here. The surface potential is determined, either from the measured maximum energy of the secondary electron peak, or from its beginning. Various dielectric materials such as MgO, Al2O3, Y2O3, mica (potassium silicate aluminum), and Teflon were studied by this technique. Experimental investigations of the beam energy and current effects on the surface potential of bare insulators are reported. The change, due to this surface potential, in some physical quantities suc...

Analysis of electrical charging and discharging kinetics of different glasses under electron irradiation in a scanning electron microscope

This paper presents a comparative study of electrical charging and discharging behavior of different glasses submitted to electron beam irradiation in scanning electron microscope. Charge storage and charge spreading in these glasses have been examined with help of a time resolved current method. Our interest concerns more particularly the dynamic behavior and the amount of the space charge build-up during and after electron irradiation under different experimental conditions of primary beam energy and current density. The precise contributions of different possible self regulation processes (leakage current and secondary electron emission) for charge accumulation are analyzed. Moreover, to characterize the ability of glasses to store charges in a stable way we introduce a relevant parameter that expresses quantitatively the variation in the released charge. The primary beam energy and the current density effects on the evolution of secondary electron emission yield during irradiation are also examined. As expected, the charge storage and spreading processes appear to be extremely dependent on the incident beam energy, current density and on the chemical composition of the studied glasses.

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.

Study of electrical properties of silica glasses, intended for FED spacers, under electron irradiation

The charge properties, under electron irradiation, of three types of glasses are studied by employing scanning electron microscope (SEM) associated with the technique called the electrostatic influence method. The experimental conditions are closed to those of typical field emission display (FED) operation. To determine the amount of trapped charges during and after electron irradiation, a special arrangement adapted to the SEM was used. This arrangement allows displacement and leakage currents to be simultaneously measured. The secondary electron emission yield during electron irradiation is also deduced. The trapping ability of each glass is analyzed taking into account the regulation mechanisms involved under electron irradiation. Finally useful indications permitting an adequate selection of glasses that may be used as FED spacers are deduced.

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.