Conference Paper

Electron Beam Trajectory in Sense of Mirror Effects Induced . From a Polyester Sample in Scanning Electron Microscope

  • University of Baghdad/ College of Education for Pure Science-Ibn Al Haitham
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This paper is devoted first, to introduce a theoretical expression that describes the scanning electron motion upon a charged sample and producing minor image. Absolutely, such a process depends on several parameters like the electrical properties for investigated material, concentration of trapped charge on the sample, potential and direction of incident electrons and the operation condition of the experiment (such as working distance, pressure, and temperature). Secondly, a previous experimental condition for certain insulator (polyester) is employed to simulate the scanning electron motion .with the introduced expression. It is found that the simulation results are in good agreement with those obtained through that experimental study.

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Mirror effects occur when a primary electron beam scans an insulating sample and the charges on its surface accumulate to a high density. When the energy of the electrical field becomes higher than the primary beam one it prevents the charged particles from reaching the sample surface, reflecting them somewhere else in the vacuum chamber whose walls act as a mirror. The inner part of the specimen chamber can be therefore imaged. The phenomenon was explained in terms of something very close to what happens to photons interacting with an optical mirror (Mirror Effect-ME or Pseudo Mirror Effect-PME). Much effort has been devoted to make sure that this effect does not occur. On the other hand electron mirror effects could be quite useful: one can create a controllable electron mirror and use it as an analytical tool for imaging or for obtaining information about the sample dielectric properties. We will tackle with major issues of the electron transport inside the chamber together with the role of the electron beam and material parameters as well as sample geometrical features and orientation, with different detectors operational regimes.
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Despite progress in the study of dielectric properties and the formation of secondary electrons images and especially in the understanding of their mirror curve shape, since the first models for mirror equation were developed, an exact quantitative prediction of the mirror curve for most materials has remained an unsolved problem. In this paper, recent development in the characterization of charge trapping ability of insulators using the scanning electron microscope mirror method (SEMMM) is reviewed. All this work has resulted in unprecedented insights into the early stage of dielectric study and it is also relevant for a deeper understanding of this anomalous effect (mirror effect) as well as for discussion of the factors affecting it. So the dependency of elliptic mirror and the anisotropic effect in the trapping phenomena of charge is highlighted.
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The electron beam induced selfconsistent charge transport in layered insulators is described by means of an electron-hole fight-drift model FDM and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field-enhanced detrapping are described by the Poole-Frenkel effect. Furthermore, an additional surface layer with a modified electric surface conductivity is included which describes the surface leakage currents and will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate.
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The mechanism of image drift in the observation of a boundary between a metal and an insulator by scanning electron microscope (SEM) is clarified by electron-trajectory simulation and experiment. In the region involving a straight boundary between a large-area metal layer and an insulating substrate, the largest image drift is expected to be observed owing to an asymmetric charging on the sample surface. The simulation result shows that a metal--insulator boundary in the SEM image shifts toward the metal part over several seconds, which is induced by a positively charged area outside of the irradiation region in the insulator part. This simulation result is confirmed to qualitatively coincide with the experimental one. In addition, we demonstrate that the direction and magnitude of the image drift can be controlled by changing the charging voltage of the insulating substrate by applying a bias voltage to the anode facing the sample surface.
The fundamental aspects of charge trapping in electron irradiated insulators have been investigated by means of a scanning electron microscope (SEM). The technique commonly used to perform charge injection, their transport and trapping ability is based on the absorbed current method and the SEM mirror effect. In this work, we have studied the influence of temperature on the charge trapping/diffusion during injection time at industrial porcelain samples given by S.T.E.G (Tunisian Society of Electricity and Gas).
Several experiments have shown that dielectric improvement of ceramics could be partially explained by the decrease of the trapped charge density. To carry out the study of charged dielectrics through to a successful conclusion, preliminary analyses are necessary to establish standard norms of utilisation of ceramics. Through recent studies, it was admitted that a trapping phenomenon is taking place on specific sites: point defects (impurities, vacancies…), lattice distortion and extended defects (grain boundaries, dislocations…) etc. Model of impurities states and Anderson states can analyse respectively the trapping and the diffusion of charges in ceramics. The aim of this paper is to outline the polishing effect on the charge trapping phenomena in a single crystal of magnesium oxide (MgO). In this way, using a scanning electron microscope (SEM), the charging phenomena was characterised for MgO as polished or post-annealed. In addition the role of the crystallographic orientation was studied. The amount of trapped charge is determined using the mirror method associated with the absorbed current method.
The use of ceramic components in electrical engineering and mechanical applications is rapidly increasing. Most of these applications require the use of ceramics bonded with metal. In this paper, we have studied the role of residual stresses occurring after joining between an industrial alumina ceramic (Al2O3) and Ni-based super-alloy, on the dielectric behaviour of ceramics. The electric charging phenomenon i.e. trapping–detrapping or diffusion of electric charges is studied by Scanning Electron Microscope Mirror Effect (SEMME) coupled with the Induced Current Method (ICM). Knowing that localized trapped charges in ceramics is a source of damage, the correlation between residual stress intensity, apparent-toughness of ceramics and ability to trap charges near the interface was demonstrated: the SEMME and ICM measurements of the quantities of trapped charges near the interface, highlighted the changes in the ceramic properties related to residual stresses due to both thermo-mechanical effect and diffusion of metallic species in the ceramics, during the bonding process.
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.
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Dynamic Investigation of the Trapping Properties of Electron Irradiated Dielectrics in a SEM
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  • S Jbara
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S. Fakhfakh. O. Jbara, and S. Rondot, "Dynamic Investigation of the Trapping Properties of Electron Irradiated Dielectrics in a SEM", 2004 International Conference on Solid Dielectric^, Toulouse, France (2004).
Charge Transport and Behaviour Analysis with Electron Irradiated (PMMA)
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S. Fakhfakh, O. Jbara, "Charge Transport and Behaviour Analysis with Electron Irradiated (PMMA)", 2009 Annual Report Conference on Electric Insulation and Dielectric Phenmena. IEEE, pp. 441-445 (2009).
PET Mirror Image Characterizations
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FE N. Al-Obaidi, F. A. Al-Saymary, A. A. Ali. "PET Mirror Image Characterizations". Proc. Summ. Scho., Sept. 8 th Oct. 8 th, Milano-Itaiy, (2008).
Formal Investigation of the Mirror Effect in SEM
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T. H. Abbood, "Formal Investigation of the Mirror Effect in SEM", Ph.D., thesis, Physics Department, University of Al-Mustansiriyah, Baghdad, Iraq (2011).