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Investigations relating to the applications of field emission cathodes

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Swann, David John 


This dissertation describes the author's research at the Cambridge University Engineering Laboratories between October 1966 and January 1970, under the supervision of ,Dr. K.C.A. Smith, to whom I am indebted not only for suggesting the subject of this dissertation, but also f or his continued help and encouragement throughout the whole of the research period. The work was directed towards the practical application of field emission cathodes in electron optical instruments in particular, the scanning electron microscope . Although it has been realized f or many years that the field emission cathode offers the possibility of an extremely bright electron emitter, its exploitation has been prevented both by practical difficulties of operation t which normally requires an ultra high vacuum environment, and by electron optical problems arising from the extremely small size of the source. A field emission microscope was constructed with ultra high vacuum capability, in which the behaviour of field emission cathodes was investigated over a "wide range of operating conditions. The results give better understanding of the physical processes involved, and provide design data suitable for predicting emitter lifetimes and emission noise, both of which are necessary for the future development of electron optical applications.

To investigate means by which the stringent vacuum requirements for field emission might be reduced, a study was made of the operation of cathodes at elevated temperatures and under pulsed conditions. In both cases it proved possible to obtain stable emission is considerably worse vacuum levels. A scanning electron optical column was also designed and constructed, on the basis of conventional vacuum techniques , with a differentially pumped field emission gun built to ultra high vacuum standards. A single magnetic electron lens was used to form a probe, and facilities were similar to those found in the final stages of the scanning electron microscope . Using the apparatus, measurements were made of probe diameters, currents and brightnesses from field emission sources under particular imaging conditions. Brightnesses were obtained which were significantly greater than may be achieved with conventional thermionic emitters.





Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge