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The Schottky electron source is predominant in today’s focusing electron beam equipment. It is operated at high temperature (1800 K) and in an electric field of ~1 V/nm. The source is made of an etched single crystalline tungsten wire with the (100) planes perpendicular to the wire axis. To reduce the work function of the bare W(100) ZrO2 is added at the base of the wire. The zirconium-oxygen complexes diffuse along the wire to reach the emitter end and enhance the electron emission.
The beam current used in an electron microscope originates from the central part of the flat (100) end facet of the emitter. The local electric field, and therefore the beam properties, can be affected by changes of the facet size & shape. Any changes should be prevented to provide a stable beam. It has been known that the shape of a Schottky electron source gradually changes over its lifetime of 1-2 years: the overall radius of the tip end grows. For low fields, the tip growth has been ascribed to a mechanism called "ring collapse": the planes at the tip end peel off sequentially to be redistributed on the shanks. Less is known however about the short term effects on the facet size & shape induced by changes in temperature and extraction voltage. This becomes particularly important when a larger area of the end facet is of interest, e.g. for multibeam applications.
We have investigated the size and shape of the end facet of a commercial Schottky electron source by recording the emission pattern and current of the end facet as function of temperature, extraction voltage and time.
It was found that the size of the end facet shrinks upon reduction of the extraction voltage and grows, but more rapidly, upon restoration of the original voltage. Furthermore, by tuning temperature and extraction voltage, the shape of the end facet could be made to change from round to octagonal and square.
The effect of the changes on the beam properties itself can be investigated by looking at the central part of the emission pattern. The current from the central part can be calculated from its emission intensity and the total facet current, assuming intensity is proportional to current.
Better knowledge of the in-situ shape of the emitter allows for a better prediction of its performance and stability.
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