Scanning Electron Microscopy


Field emission occurs at the sharp apex of a metal needle, so that it can be considered as providing a point electron source, which is a desirable feature for a fine focused electron beam. The disadvantage is its low stability. It is shown that the current fluctuations occur mainly due to the interaction with the residual gas in the vacuum; thus reduction of the interaction with the residual gas is essential for a stable field electron source. The stringency of the vacuum requirement has prevented wide application of the field emission electron source. New materials which have low work functions and high melting points are examined in the view of a stable field electron emitter, and it is shown that carbides of transition metals have potential as a stable field emitter. Very stable field emission has been reported for TiC single crystals. Operation in the thermal-field emission mode is examined and it is shown that ZrO/W(100) emitter gives stable emission, whose fluctuation is less than 0.23% in the frequency interval 1 to 5000 Hz. The only disadvantage of the ZrO/W thermal field emitter is its rather high level of instability at very low frequencies. This paper reviews the development of field electron emission as it is applied to electron sources.

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