Gaseous detection device

The gaseous detection device-GDD is a method and apparatus for the detection of signals in the gaseous environment of an environmental scanning electron microscope (ESEM) and all scanned beam type of instruments that allow a minimum gas pressure for the detector to operate.

In the course of development of the ESEM, the detectors previously employed in the vacuum of a scanning electron microscope (SEM) had to be adapted for operation in gaseous conditions. The backscattered electron (BSE) detector was adapted by an appropriate geometry in accordance with the requirements for optimum electron beam transmission, BSE distribution and light guide transmission. However, the corresponding secondary electron (SE) detector (Everhart-Thornley detector) could not be adapted, because the high potential required would cause a catastrophic breakdown even with moderate increase of pressure, such as low vacuum. Danilatos (1983) overcame this problem by using the environmental gas itself as the detector, by virtue of the ionizing action of various signals. With appropriate control of electrode configuration and bias, detection of SE was achieved. A comprehensive survey dealing with the theory and operation of GDD has been published, from which the majority of the material presented below has been used.

The GDD is in principle an adaptation of techniques for particle detection used in nuclear physics and astronomy. The adaptation involves the parameters required for the formation of images in the conditions of an electron microscope and in the presence of gas inside the specimen chamber. The signals emanating from the beam specimen-interaction, in turn, interact with the surrounding gas in the form of gaseous ionization and excitation. The type, intensity and distribution of signal-gas interactions vary. It is fortunate that generally the time-constant of these interactions is compatible with the time-constant required for the formation of images in the ESEM. The establishment of this compatibility constitutes the basis of the invention of GDD and the leap from particle physics to electron microscopy. The dominant signal-gas interactions are those by the BSE and SE, as they are outlined below.

In its simplest form, the GDD involves one or more electrodes biased with a generally low voltage (e.g. up to 20 V), which is sufficient to collect the ionization current created by whatever sources. This is much the same as an ionization chamber in particle physics. The size and location of these electrodes determine the detection volume in the gas and hence the type of signal detected. The energetic BSE traverse a long distance, whereas the SE travel a much shorter lateral distance mainly by way of diffusion in the gas. Correspondingly, an electrode placed further away from the beam axis will have a predominantly BSE component in comparison to the predominant SE component collected by an electrode placed close to the axis. The precise proportion of signal mix and intensity depends on the additional parameters of gas nature and pressure in conjunction with electrode configurations and bias, bearing in mind that there is no abrupt physical distinction between SE and BSE, apart from the conventional definition of the 50 eV boundary between them.

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