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Electron sources are ubiquitous in vacuum based electronics and are used for a variety of purposes. These applications range from x-ray production used for imaging in both the medical and security applications, to travelling wave tubes to enable radar, to source initiators for free electron lasers. There are two methods for producing electrons: thermionic emission, where electrons are produced through heating a filament, and field emission, where electrons are extracted through an electric field. The aim of this project is to develop appropriate metrology for characterization of nanostructured field emitters.
Currently, most electron sources are thermionic, where heating of a metallic filament results in electrons being “sprayed off” and extracted through a biasing grid. These thermal electron sources have limitations due to the required high operating temperature, power consumption, and lack of compactness. Further, as the electrons are boiled off in all directions, the emittance, or spatial kinetic energy distribution, of the source can be quite high and require complex electron focusing optics. In contrast, field emitters extract electrons through a large electric field without using high temperature. The emission process is quantum-mechanical tunneling through vacuum, and is directional. The resulting electron source requires no heating and has reduced emittance. While the most critical parameters for a thermionic source are the temperature and work function of the emitter, for a field emitter source, there are other experimental variables, most notably the geometric shape of the emitter, which impacts the tunneling probability. These properties are shown in the tunneling diagrams below.
Fig. 1. Tunneling diagrams showing quantum mechanical electron tunneling in thermionic and field emitter sources. For thermionic emission, the temperature and work function are most important. For field emission, the emitter shape is most important.
Fig. 2. The emission areas of these nanofabricated structures is quite small, but they are capable of producing significant emission at low electric field gradients due to their high aspect ratio.