Cathode for X-ray Generation with Arrays of Individually Addressable Field Emitters Controlled by Vertical Ungated FETs
- Category: Medical Electronics
- Tags: frances hill, luis velasquez-garcia
Print This Page
Download as PDFThis 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)[1][2] 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. The extractor and focus gates are monolithically integrated with the cathode chip, and they are patterned in orthogonal strips so that each electron gun can be turned on individually. The first step to make the double-gated field emitters with integrated FETs is to fabricate the FETs and characterize their electrical properties. The fabricated chip is shown in Figure 1a. The FETs are 25 μm tall silicon columns that are 500 nm in diameter and etched in 11 by 11 arrays (Figure 1b and c). The gaps around the columns are filled in with TEOS oxide, vias are etched to the tops of the columns, and then aluminum is deposited to make electrical contact to the tops of the columns (Figure 1d). Electrical characterization tests show that the current saturates across the columns and demonstrate that the current scales with the number of FETs in an array (Figure 2).
-
- Figure 1: (a) Fabricated cathode chip to characterize the electrical properties of the FETs; (b) scanning electron microscope image of a fabricated array of vertical FETs without dielectric filler; (c) magnified image of the top of a vertical FET without dielectric filler; (d) magnified image of the top of a vertical FET with dielectric filler and deposited metal.
-
- Figure 2: Per-FET current-voltage characteristics showing current saturation and demonstrating that the total current scales with the number of FETs in an array.
- L. F. Velásquez-García, S. Guerrera, Y. Niu, and A. I. Akinwande, “Uniform high-current cathodes using massive arrays of Si field emitters individually controlled by vertical Si ungated FETs—Part 1: Device fabrication and characterization,” IEEE Transactions in Electron Devices, vol. 58, pp. 1783-1791, June 2011. [↩]
- L. F. Velásquez-García, S. Guerrera, Y. Niu, and A. I. Akinwande, “Uniform high-current cathodes using massive arrays of Si field emitters individually controlled by vertical Si ungated FETs—Part 2: Device design and simulation,” IEEE Transactions in Electron Devices, vol. 58, pp. 1775-1782, June 2011. [↩]