S Rondot

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.

Device intended for measurement of induced trapped charge in insulating materials under electron irradiation in a scanning electron microscope

A device for simultaneously measuring two currents (i.e. leakage and displacement currents) induced in insulating materials under electron irradiation has been built. The device, suitably mounted on the sample holder of a scanning electron microscope (SEM), allows a wider investigation of charging and discharging phenomena that take place in any type of insulator during its electron irradiation and to determine accurately the corresponding time constants. The measurement of displacement current is based on the principle of the image charge due to the electrostatic influence phenomena. We are reporting the basic concept and test results of the device that we have built using, among others, the finite element method for its calibration. This last method takes into account the specimen chamber geometry, the geometry of the device and the physical properties of the sample. In order to show the possibilities of the designed device, various applications under different experimental conditions are explored.

X-ray radiography applied to an investigation of thermal diffusion

A laboratory-built x-ray radiography device (combining an x-ray tube as source and a CCD camera as detector) that is simple and easy to use is presented as a complementary analysis tool for the analysis of chemical processes. It allows the in situ acquisition of x-ray absorption images of solid or liquid samples with a lateral resolution of 50 µm and a sensitivity of 10−3 atom per atom. Its main application is in the analysis of liquid ionic solutions such as those met in electrochemical reactions where heavy ions move in a less absorbing medium. Despite a low resolution, a good absorption contrast in images of moving species in a liquid is obtained within a few seconds and the digital format of these images allows quantitative measurements of local concentration changes in solutions. For instance, the system has here been applied to investigate thermal diffusion in a binary solution of potassium iodide KI. X-ray imaging of the solution leads to the determination of the Soret coefficient and to its evolution as a function of the muzzle concentration. These measurements complement existing data on this thermodynamic effect in a scarcely explored concentration field.

Effect of nanoclay concentration level on the electrical properties of polypropylene under electron irradiation in a SEM

For studying the electrical properties (charge trapping, transport and secondary electron emission) of the polypropylene‐based nanocomposites with different contents of natural clay, the specimens were submitted to electron irradiation of a scanning electron microscope. A device, suitably mounted on the sample holder of the scanning electron microscope, was used to measure two currents (i.e. leakage and displacement currents) induced in the polypropylene‐based nanocomposites (polymer nanocomposites) under electron irradiation. The evolution of trapped charge during irradiation for each type of studied polymer nanocomposites is deduced. The amount of trapped charge at the steady state is also determined by measuring the change of secondary electron image size associated to the electron trajectory simulation. It is found, surprisingly, that not only the leakage current increases as a function of clay loading level but also trapped charge. However, this could be related to the increase of conductivity in one hand and to proliferation of interfaces between nanoparticles and neighbouring materials on the other hand. These two processes play crucial role in controlling the carrier transport (through polymer nanocomposites or/and along its surface) closely related to the charge storage and leakage current.

Polypropylene-nanoclay composites under electron irradiation in SEM: Structure, charge trapping and electron emission properties

In this work, polypropylene (PP) and its polymer nanocomposites (PNCs) films containing very low contents (2–6 wt %) of Cloisite 20A natural montmorillonite clay platelets were investigated. In a first, they were characterized by means of several analytical techniques such as X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), Differential Scanning Calorimetry (DSC) and UV-visible spectroscopy. Secondly, the nanocomposites are submitted to an electron beam in a Scanning Electron Microscope (SEM) and the induced currents are measured using a time-resolved current method. These currents are used to determine the trapped charge and the electron emission yield. The aim is to contribute to the understanding of the effect of different concentrations of nanoclay platelets on the studied properties by correlating the results obtained by both techniques. The analytical characterization showed that upon the increase of nanoclays concentration an intercalated structure and an increase of crystallinity are induced. On the other hand, the optical band gap energy is modified because the increasing of the density of localized states in the forbidden energy band. As regards the measurement of the trapped charge, it was found, surprisingly, that not only the leakage current increases as a function of clay loading level but also the trapped charge (or the surface potential). This could be related to the increase of conductivity in one hand and to proliferation of interfaces between nanoparticles and neighboring materials on the other hand.

Study of temperature effects on the electrical behavior of polypropylene-clay nanocomposites submitted to electron beam irradiation in a SEM

Charge transport and electron emission properties in polypropylene and its nanocomposites filled with nanoclay particles submitted to an electron irradiation, in a Scanning Electron Microscope (SEM), are investigated using induced displacement and leakage currents. The measurements have been performed at various temperatures ranging from 20°C to 75°C at a primary beam energy of 20keV and a primary beam current of 1nA with the aim to highlight the effect of temperature and nanoclay content on these properties. The results show, at a given temperature, that the incorporation of clay in polypropylene (PP) matrix paradoxically leads to a concomitant increase in the electrical conductivity and the charge accumulated. In contrast, if the clay content is fixed, there is an increase in conductivity and a reduction of the charge accumulated when the temperature increases. The mobility of charge carriers and the corresponding activation energy are deduced from the measured leakage current during discharging step. The mobility was found to be an order of magnitude higher for the nanocomposites. The study of the influence of the temperature and nanoclay concentration on electron emission yield is also addressed.