Field Emission Cathode for X-Ray Generation

This work focuses on the design and fabrication of a cathode for a portable x-ray source. The cathode is made of an array of individually addressable electron guns, each containing double-gated field emitters. Compared to thermionic cathodes, field emission arrays operate at lower vacuum and lower temperatures, use less power and are more portable. The electron beam from each gun is extracted by a proximal gate and collimated using a distal gate before it hits an anode in a micron-sized spot that generates Bremsstrahlung x-rays. Each field emitter is fabricated on top of a vertical ungated field-effect transistor (FET) that acts as a current source due to the velocity saturation of electrons in silicon when the voltage across the FET is above a saturation voltage. Current source-like behavior provides spatial and temporal uniformity of the output current across the emitter array; it also protects against emitter burnout and current surges. Individual addressability is achieved by fabricating the structure on SOI wafers to create electrically isolated strips of silicon. The extractor and focus gates are monolithically integrated with the cathode chip. They are patterned in strips that are orthogonal to the silicon strips, so that a single electron gun can be turned on at once. sity of 139 µA/cm2.