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The modular cathode described in this invention consists of an array of field emitter elements, each of which comprises a nanoscale field emitter tip supported by a high-aspect ratio silicon column. An annular gate electrode is located around each emitter tip. When a voltage is applied between gate electrodes and emitter tips, the resulting concentrated electric fields cause electrons to be emitted from the tips.

An electron-transparent, gas-impermeable membrane covers the aperture of each gate electrode, allowing emission of electrons while protecting the emitter tip from erosion by gaseous ions created when electrons collide with gaseous molecules downstream. In addition, the membrane also forms a hermetic seal around the emitter tip. This allows the high vacuum necessary for optimal emitter operation to be restricted to a small region around the tip. More generally, it allows the cathode to be packaged as a well-encapsulated module that can be optimized without affecting downstream processes making use of the electron beam.