The scanning electron microscope (SEM) is usually operated with a beam voltage, V0, in the range of 10-30 kV, even though many early workers had suggested the use of lower voltages to increase topographic contrast and to reduce specimen charging and beam damage. The chief reason for this contradiction is poor instrumental performance when V0 = 1-3 kV, The problems include low source brightness, greater defocusing due to chromatic aberration greater sensitivity to stray fields, and difficulty in collecting the secondary electron signal. Responding to the needs of the semiconductor industry, which uses low V0 to reduce beam damage, considerable efforts have been made to overcome these problems. The resulting equipment has greatly improved performance at low kV and substantially removes the practical deterrents to operation in this mode. This paper reviews the advantages of low voltage operation, recent progress in instrumentation and describes a prototype instrument designed and built for optimum performance at 1 kV. Other limitations to high resolution topographic imaging such as surface contamination, the de-localized nature of the inelastic scattering event and radiation damage are also discussed.

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Low voltage scanning electron microscopy.

The nickel-based alloy Inconel 625 was thermally sprayed by two different variants of the high velocity oxy-fuel process. In this study, coatings deposited by a liquid-fuelled gun were compared with those produced by a gas-fuelled system; in general, the former generates higher particle velocities but lower particle temperatures. Investigations into the microstructural evolution of the coatings, using scanning electron microscopy and X-ray diffraction, are presented along with results on their aqueous corrosion behaviour, obtained from salt spray and potentiodynamic tests. It is inferred from coating microstructures that, during spraying, powder particles generally comprised three separate zones as follows: fully melted regions; partially melted zones; and an unmelted core. However, the relative proportions formed in an individual powder particle depended on its size, trajectory through the gun, the gas dynamics (velocity/temperature) of the thermal spray gun and the type of gun employed. Cr2O3 was the principal oxide phase formed during spraying and the quantity appeared to be directly related to the degree to which particles were melted. The salt spray test provides a sensitive means of determining the presence of interconnected porosity in coatings and those produced with the liquid-fuelled gun exhibited reduced interconnected porosity and increased corrosion resistance compared with deposits obtained from the gas-fuelled system. In addition, potentiodynamic tests revealed that passive current densities are 10–20 times lower in liquid-fuel coatings than in those sprayed with the gas-fuelled gun.

Microstructure formation and corrosion behaviour in HVOF-sprayed Inconel 625 coatings

Transport in materials via random individual atomic migration steps (jumps) is called ‘diffusion’. General understanding and detailed theory of diffusional transport represent a classical field of material science. While in liquids and gases, diffusion rate ranges up to millimeters or even centimeters per second, transport in solid materials is rather slow. In densely packed metals near to the melting point, one can expect about a micrometer per second and this rate drops down to about a nanometer per second at half the melting temperature. At room temperature atomic migration is practically frozen, except for few exceptional cases of small interstitial impurities. Measurement of diffusion in solids needs therefore sensitive techniques and well-developed microscopy. Not surprising that a scientific proof of diffusion in solid metals came rather late in history of science (Roberts-Austen, 1896).

Diffusion in Metals and Alloys

A grey pyritic mudstone from Central Wales (Red Vein unit of the Dicellograptus anceps zone, Upper Ordovician Ashgill) has been examined in thin section by scanning electron microscopy using backscattered electrons. Using backscatter it is possible to identify individual mineral constituents of the mudstone by virtue of their atomic number (Z) contrast and differential hardness (relief). The amount of detail observable is far greater than that possible with optical microscopy. Valuable information can be obtained relating to particle form, orientation, texture and internal structure which aids in interpretation of the deformational and diagenetic history of the rock. The adoption of electron microscopical methods in the study of thin sections and polished rock chip surfaces promises to revolutionize the field of shale petrology.

Application of backscattered electron microscopy in shale petrology

Publisher Summary The chapter discusses some of the contrast mechanisms that contribute to the image in these instruments. The most recent generation of low-voltage scanning electron microscopes yields resolutions well below one nanometer thanks to the use of field-emission guns. This chapter also considers aspects of scanning electron microscopes (SEM) design and operation that affect the contrast and resolution of the secondary emission (SE) and the backscattering (BSE) image. The SEMs developed and used world-wide since that time owe most of their important features to early developments by this group. An SE image of a rough microscopic specimen can be easily and accurately interpreted in terms of its topographic shape. The total BSE signal is a strong function of the density of the specimen under the beam, and BSE images are therefore primarily two dimensional maps of material density versus position. In addition, as the image information is carried as a time-varying electronic signal, a variety of analog and digital signal processing procedures can easily be applied to this signal to emphasize the particular aspects of it that are of interest to the viewer.

LVSEM for High Resolution Topographic and Density Contrast Imaging

SUMMARY

The atomic number dependence of electron backscattering can be used as the basis of a microanalysis technique. The operating procedures and condition for quantitative measurements of specimen atomic number are outlined and an expression relating the accuracy of composition to the atomic number sensitivity has been derived. Some measurements of the spatial resolution of backscattered electron microanalysis are also presented and compared with the resolution of X-ray microanalysis.



Although the range of application of this technique is limited, where it can be applied it has the following advantages: (i) higher spatial resolution than X-ray microanalysis for bulk specimens; (ii) very rapid measurement; (iii) can be applied to compounds of low atomic number elements, (e.g. borides, carbides, nitrides, etc.); (iv) specimen preparation is often relatively straightforward.

D. G. McCartney

Papers

The measurement of atomic number and composition in an SEM using backscattered detectors