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Brightness, energy spread and emission area are key parameters of electron sources for instruments such aselectron microscopes and electron beam lithography tools. In developing transmission-mode NEAphotocathodes as sources for these applications, these characteristics have been measured in specializedsealed tubes. Average lateral energies were measured at 63 meV for a 1 .5 micron thick photocathode, and83meV for a 0.5 micron thick photocathode, which was known to be emitting "hot" electrons. A currentdensity of 841A/cm2 was obtained from a 1 .7 micron diameter emission area. This high cunent density canbe explained in terms of lateral drift and diffusion of surface trapped electrons. Combined angular and current density data indicate a brightness of —108A/cm2-sr at 3kV. Keywords: negative electron affinity, GaAs, Cs, brightness, energy spread, emission area, angulardistribution I . INTRODUCTIONThe emission of photostimulated electrons from a negative electron affinity surface was first demonstratedby Scheer and Van Laar 1969.l Since then, many applications have developed which take advantage ofunique characteristics of negative electron affinity photocathodes — |
