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.
The primary focus of this effort is to develop metrology for improving the characterization of the emission and emittance properties of nanostructured field emitters. Using nanofabrication techniques, these emitters are being fabricated from carbon nanotubes and compound semiconductors that have been patterned into one-dimensional structures as shown below.
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.
We are developing measurement techniques using a custom-built apparatus designed to characterize these nanostructured emitters. These include design and fabrication of arrays of micro-patterned anode collectors that can simultaneously determine the global emission from the array structure, and the local emission from individual emission sites. These collectors will also be individually addressed electronically to ascertain emitter-emitter interactions and their impact on device emittance. The goal is to develop these measurement techniques into appropriate metrology tools for this enabling nanotechnology.