G. D. Danilatos

Foundations of Environmental Scanning Electron Microscopy

Publisher Summary The purposes of this chapter are (1) to present new theoretical derivations and computations, (2) to explain and review experimental findings, (3) to survey existing literature and collect data and information relevant to environmental scanning electron microscope or microscopy (ESEM), and (4) to create the foundations for further use and development of ESEM. The chapter emphasizes on the understanding and development of the theory of the ESEM. Doing this requires not only knowledge within the electron microscopy field in its present form, but also from other disciplines such as fluid mechanics, ionization of gases and plasma physics. The electron beam penetration of gases and the overall performance of the instrument have been analyzed in the chapter. Mathematical derivations for the stationary gas in the specimen chamber and the gas flowing through apertures, tubes and pumps have been given in the chapter. Theoretical calculations can be made for some cases, while for others the predictions may be poor and the employment of experimental methods more practical. The chapter outlines the general reactions in the broadest terms and some of these are analyzed. The study of the electron physics of the beam-gas system in relation to the most elementary and immediate needs of ESEM has been mentioned.

Review and outline of environmental SEM at present

The environmental scanning electron microscope (ESEM) allows the examination of specimens in a gaseous environment. It is based on an integration of efficient differential pumping with a new design of electron optics and detection systems. Backscattered, cathodoluminescence and X‐ray detectors can be designed to fit and to perform optimally in the ESEM. The secondary electron signal can be detected with the gaseous detector device, which is a new multipurpose detector. Insulating, uncoated, wet and generally both treated or untreated specimens can be studied.

Theory of the Gaseous Detector Device in the Environmental Scanning Electron Microscope

Publisher Summary This chapter presents a survey of previous works in related fields, coupled with current experience in the environmental scanning electron microscope (ESEM). This microscope allows the examination of specimens in the presence of a gaseous environment. The scintillation produced by various signals can also be used for making images, and a generalized gaseous detector device (GDD) was proposed. GDD is based on the principle of classical gaseous particle detectors adapted to the specific requirements of the ESEM. The chapter presents an account of the new terminology associated with ESEM. Physical principles for the analysis of ESEM are electron and ion temperature, imaging parameters, theory of induced signal, and induced signals in the ESEM. Whereas, the physical parameters involved in the performance of the GDD are electron and ion mobilities, diffusion, recombination, electron attachment, and effective ionization energy. The chapter outlines the discharge characteristics and accounts for amplification, parallel plates; the first Townsend coefficient; the Paschen law; and secondary processes. The ESEM is a radiation source immersed inside a gas, and, therefore, the methods of nuclear and particle physics for spectroscopic analysis can be applied. Principles of spectroscopy including spectroscopy, statistics, and energy resolution and environmental scanning transmission electron microscopy are discussed. The chapter focusses on mechanisms of a high gaseous gain.

Mechanisms of detection and imaging in the ESEM

For proper understanding of image formation using charge carriers, it is shown that signal detection by means of induction must be considered. This explains the possibility of imaging insulators as well as other phenomena especially in the conditions of the environmental scanning electron microscope. In addition, a basic principle and method to separate the secondary and backscattered electrons is demonstrated.

A gaseous detector device for an environmental SEM

Abstract A new detection system has been used in the environmental SEM. The presence of gaseous and liquid phases in the microscope has made possible the formation of images of both insulators and conductors using the current mode. In addition, the ionizing radiations create in the gas negative and positive charge carriers which contain information from the beam-specimen interaction. These carriers, when collected by suitable means, such as a biased wire away from the specimen, can modulate the display signal to form images. Thus, the gas itself constitutes the basic component of a signal detector device, apart from its use as a conditioning medium.

The examination of fresh or living plant material in an environmental scanning electron microscope

An environmental scanning electron microscope (ESEM) has been developed which allows the examination of specimens at pressures up to 6 kPa. This new technique has been applied to the examination of live and wet plant specimens. The results are compared with those obtained using similar material that has been dehydrated or prepared by conventional techniques. The plant materials can survive the hypobaric pressure and beam irradiation, especially if the latter is carefully controlled.

Environmental scanning electron microscopy and microanalysis

It is shown that the environmental scanning electron microscope is the natural extension of the scanning electron microscope. The former incorporates all of the conventional functions of the latter and, in addition, it opens many new ways of looking at virtually any specimen, wet or dry, insulating or conducting. The environmental scanning electron microscope is characterised by the possibility of maintaining a gaseous pressure in the specimen chamber. All operational parameters can be varied within a range which is a function of pressure. It can be used with all types of gun and all basic modes of detection and, hence, it can be applied both to morphological and to microanalytical studies. It has opened many novel ways of looking at specimens and phenomena not previously accessible with scanning electron microscopy.

Environmental Scanning Electron Microscopy

Following early works on in-situ transmission electron microscopy by using environmental cells, the environmental scanning electron microscope (ESEM) has formed the counterpart for the examination of specimen surfaces in a gaseous environment at pressures up to one atmosphere. As accelerating voltages are relatively low in ESEM, it has been necessary to establish the optimum electron beam transfer conditions from a high vacuum to a high pressure region by using windowless apertures. Studies on the gas and electron dynamics of the system have determined that it is possible to use tungsten, LaB6 and field emission guns without compromising the useful probe size in the presence of gas. The backscattered electron, cathodoluminescence and x-ray detection modes are preserved with proper modification of the detectors. A new method for detection of the secondary and backscattered electron signal has been introduced by the use of the ionisation and scintillation of the environmental gas by corresponding signals. Further, the ionised gaseous environment substitutes the conventional conductive coating or treatment techniques necessary for insulators in vacuum SEM. The high pressure also allows a fully or partially moist environment for the examination of biological or wet specimens, or of chemical reactions in the gas/liquid/solid phases. The possibility of examining the natural or true surface of practically any specimen has added a new dimension to electron microscopy. New contrast mechanisms reveal information not previously possible to see. It has greatly facilitated the examination of specimens by eliminating or reducing the specimen preparation procedures and the specimen exchange time. Based on the success of an experimental ESEM, new commercial instruments are now available making this technology accessible to all. Published scientific literature demonstrates that ESEM has been applied to the most diverse disciplines. A future prospect is to integrate and jointly develop the scanning transmission electron microscope towards a universal kind of environmental EM.

Design and construction of an atmospheric or environmental SEM—2

Abstract This paper is a continuation of a series of reports on the design and construction of an atmospheric or environmental SEM. The work described is an extended study of the gas jet developed above the pressure limiting aperture (Bell, 1974; Lacaze et al., 1977). Experiments specifically aimed to establish how the vacuum in the electron optics system was affected by the relative positioning of the objective and pressure limited aperture, as well as the pumping speeds employed, specimen chamber pressure, geometry and size of apertures, and by other means. Further, the nfluence of the jet deflectors, to control the effects of this jet on the microscope system were studied quantitatively using a specifically designed apparatus. In addition, the study of the pressure gradients below the pressure limiting aperture revealed that specimens can be placed as close as radius from the aperture and still experience an almost saturated vapour pressure environment. The results of the present study are currently being used in the design of an optimum detection configuration. A preliminary result has allowed the use of 140 μm pressure limiting aperture to observe specimens at atmospheric pressures as well as the use of low accelerating voltages (e.g. 7 kV) at TV scanning rates to record on video cassette dynamic phenomena, including wetting or recrystallizing salt solutions, etc.

Environmental scanning electron microscopy in colour

The environmental SEM has been developed to a stage where colour can be introduced during imaging. A new approach to production of colour micrographs is demonstrated together with some typical uses of the environmental mode of SEM. A combination of the outputs of two backscattered electron (BSE) detectors alone or in combination with the outputs from a gaseous detector device can form images corresponding to particular aspects of a given specimen. Two or more of these images can be superimposed on to the same colour print to produce a colour micrograph.