High-Current Field Emission Cold Cathodes with Temporal and Spatial Emission Uniformity

Field emission arrays (FEAs) are an attractive alternative to mainstream thermionic cathodes, which require high vacuum and high temperature to operate. Field emission of electrons consists of the following two processes: first, the transmission of electrons (tunneling) through the potential barrier that holds electrons within the material (workfunction phi) when the barrier is deformed by a high electrostatic field and second, the supply of electrons from the bulk of the material to the emitting surface. Either the transmission process or the supply process could be the limiting step that determines the emission current of the field emitter. Due to the exponential dependence on the field factor, the emission current from the tips is extremely sensitive to tip radii variation. We have a process to achieve uniform emission from nanosharp FEAs by both fabricating highly uniform tip arrays and controlling the supply of electrons to the emitting surface.

We have designed and fabricated FEAs in which each field emitter is individually ballasted using a vertical ungated field effect transistor (FET) made from a high aspect ratio (40:1) n-type silicon pillar. Each emitter has a proximal extractor gate that is self-aligned for maximum electron transmission to the anode (collector). The modeling suggests that these cathodes can emit as much as 30 A.cm-2 uniformly with no degradation of the emitters due to Joule heating; also, these cathodes can be switched at microsecond-level speeds. The design process flow, mask set, and pillar arrays have been completed with the self-aligned extractor gate. An ultra-high vacuum chamber has been built to test the devices. The chamber can test full 150mm wafers with six high voltage feed through and a step-down anode at 2x10-10 torr pressure while also imaging the electron emission on a phosphorus screen.