Field emission guns for electron probe instruments

In conventional electron probe instruments with thermionic cathodes, the probe current is limited by the brightness of the electron beam. Field emission cathodes can provide significantly higher brightness, but their properties necessitate substantial changes to the form of the electron gun and of the probe-forming system. These changes are illustrated in this paper by analysis of two types of field emission electron gun, each with an emission control electrode in close proximity to the cathode ; the triode gun has a single stage of acceleration, whilst the tetrode gun has two stages. The characteristics of complete systems incorporating such guns are evaluated, for comparison with one another and with thermionic systems. It is shown that field emission cathodes can provide larger currents into electron probes with diameter less than about 100 nm, but that thermionic cathodes are to be preferred for the production of probes with greater diameters. The application of a variable voltage to the emission control electrode, for probe current stabilization, is shown to cause significant defocusing of the electron probe.