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Scanning Microscopy

Abstract

In the environmental scanning electron microscope (ESEM), the gas flow around the main pressure limiting aperture establishes a density gradient through which the electron beam passes. Electron beam losses occur in this transition region and in the uniform gas layer above the specimen surface. In the oligo-scattering regime, the electron distribution consists of a widely scattered fraction of electrons surrounding an intact focussed probe. Secondary electrons are multiplied by means of gaseous ionization and detected both by the ionization current and the accompanying gaseous scintillation. The distribution of secondary electrons is governed by the applied external electric and magnetic fields and by electron diffusion in the gas. Backscattered electrons are detected both by means of the gaseous detection device and by solid scintillating detectors. Uncoated solid detectors offer the lowest signal-to-noise ratio especially under low beam accelerating voltages. The lowest pressure of operation with uncoated detectors has been expanded by the deliberate introduction of a gaseous discharge near the detector. Gaseous scintillation also offers the possibility of low noise detection and signal discrimination. The "absorbed specimen current" mode is re-examined in the conditions of ESEM. It is found that the current flowing through the specimen is not the contrast forming mechanism: it is all the electric carriers in motion that induce signals on the surrounding electrodes. The electric conductivity of the specimen may affect the contrast only indirectly, i.e., as a secondary, not a primary process. The ESEM can operate under any environment including high and low pressure, low or rough vacuum and high vacuum; it operates at both high and low beam accelerating voltage so that it may be considered as the universal instrument for virtually any application previously accessible or not to the conventional SEM.

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