Scanning Electron Microscopy


The scanning electron microscope (SEM) usually operates with a beam voltage, V0, in the range of 10-30 kV, even though many early workers 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 low instrumental performance when V0 = 1-3 kV. The problems include low source brightness, greater defocussing due to chromatic aberration, greater sensitivity to internal and external stray fields and difficulty in collecting the secondary electron signal without defocussing the probe. Recently considerable efforts have been made to overcome these problems because the semi-conductor industry, which is now the major user of the SEM, has found that low V0 is necessary to reduce beam damage. The resulting equipment has greatly improved performance at low kV and substantially removes the practical deterrents to operation in this mode on other types of samples. This paper reviews the advantages of low voltage operation for topographic imaging, 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 in-elastic scattering event and radiation damage are also discussed.

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