Field emission arrays (FEAs) are an attractive alternative to mainstream thermionic cathodes, which are power hungry and require high vacuum and high temperature to operate. Field emission of electrons consists of the following two processes: 1) transmission of electrons (tunneling) through the potential barrier that holds electrons within the material (workfunction φ) when the barrier is deformed by the application of a high electrostatic field and 2) 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 (FE). Control of the transmission process (Fowler Nordheim) to produce high uniform current from FEAs is very challenging due to the physics of the field emission process. Due to the exponential dependence on the field factor and, hence, the tip radius, emission currents are extremely sensitive to tip radii variation; unfortunately, nanometer-sized tip radii in FEAs have a distribution with long tails that causes severe FEA underutilization. A better approach for achieving uniform emission from nanosharp FEAs is controlling the supply of electrons to the emitting surface. In a metal, the supply of electrons is very high, making the control of the supply challenging. However, in a semiconductor, where the local doping level and the local potential determine the concentration of electrons, it is possible to configure the emitter such that either the supply process determines the emission current.

We have designed and fabricated FEAs where 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). Our 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 (Figure 1) with the self-aligned extractor gate to be completed by the spring of 2013. An ultra-high vacuum chamber has been designed and built to test the devices (Figure 2). The chamber can test full 150mm wafers with four high voltage probes at 10^-10 torr pressure.

A collaboration of RLE and MTL investigators is creating the scientific and engineering knowledge for a compact coherent X-ray source for phase contrast medical imaging based on inverse Compton scattering of relativistic electron bunches. The X-ray system requires a low emittance electron source that can be switched at timescales of tens of femtoseconds or faster; the focus of our work has been the design, fabrication and characterization of massive arrays of a nanostructured high aspect-ratio silicon (Si) structures to implement low-emittance and high-brightness cathodes that can be triggered very fast using laser pulses to produce spatially uniform electron bunches. Si nanostructure arrays with highly uniform sub-10 nm tip radii have been fabricated via a combined optical lithography and diffusion limited oxidation technique. The fabrication process allows nanometer-level control over the dimensions of the electron emitter structures. Figure 1 shows an array of Si tips with 1.25 µm hexagonal pitch have an average radius of curvature of 6.2 nm and standard deviation of 1.1 nm (n=29); when the radius of curvature is changed to 21.6nm, the standard deviation remains approximately the same, i.e., 1.25 nm (n=69).

The tips are illuminated at a grazing incidence of roughly 84 degrees with a 1 kHz titanium sapphire laser (800 nm wavelength) with a pulse duration of 35 fs; the high electric field of the laser pulse is amplified by the silicon tips so the electrons can quantum tunnel from the tips into the vacuum. Experimental results using a time of flight spectrometer show electron beamlet array emission with 3-photon absorption. Work is ongoing to optimize the tip geometry for both low emittance and high current. We are also designing and building a new vacuum chamber to test the devices (Figure 2). The chamber will pump down to 10^-7 torr in ~15min with an anode bias up to 1100V.