Double-Gated Singly-Addressable Polysilicon Tip Array Fabrication and Characterization
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DOUBLE-GATED SINGLY-ADDRESSABLE POLYSILICON TIP ARRAY FABRICATION AND CHARACTERIZATION N.N. Chubun, A.G. Chakhovskoi, M. Hajra, and C.E. Hunt, Electrical and Computer Engineering Department, University of California, Davis, CA, 95616 Phone 530-752-2735, fax 530-754-9217; e-mail: [email protected] Abstract Polysilicon-on-insulator singly-addressable arrays, consisting of double-gated field emission cells, were fabricated and tested. The field-emission tips were formed by a subtractive technique, using 2.5 µm thick polysilicon stripes on an insulating substrate. The tip structure was oxidized for dielectric isolation and coated with a 0.4 µm polysilicon layer as a first gate electrode. The polysilicon layer was then subsequently oxidized to provide a second isolation layer for separation from a 0.1 µm gold film, deposited as a second gate electrode. Finally, the 1.5 µm aperture was formed, combining wet etching of the silicon dioxide and dry etching of the polysilicon layers. The matrix allows addressing electrically any emission cell at the intersection of a cathode column and an extracting gate line. An independent voltage can be applied to the second gate during operation to focus the electron beam of an operating tip. Introduction A conventional field-emission cell, which usually consists of an emission tip surrounded by a small aperture in an extracting electrode (grid), has very complicated electron-optical characteristics. Electron beam trajectories are influenced not only by the electric field formed between the emission tip and the gate, but also by the field in the proximity of the emission site. In a multiple-cell array, the electrons emitted from a single tip at a large emission angle interfere with the electrical field generated by the neighboring cells. This problem can be effectively resolved by controlling the shape of the individual electron beams. It is possible to generate a focused or collimated electron beam by using the fringe electric field formed along the edge of the extracting grid electrode if an adjacent electrode – a focusing lens having a lower potential is placed in the vicinity of the extraction grid. The nearly uniform accelerating field provided by an anode positioned at some macroscopic distance above the device will be distorted by the difference of the potentials between the extracting grid and the lens electrodes, producing a focusing electric field in the extended volume above the field emitters, while not significantly affecting the emission current. It has been proposed in [1] that by fabricating the lens electrodes coaxial with each tip to form an electrostatic lens system, one could force all the electrons emitted from every tip of the array to travel in the same direction. Several researchers [2, 3] have calculated the design parameters (spacing, aperture radii and voltages) to meet these conditions. Focusing effect can be achieved by two approaches of integrating the focusing lens to the conventional gated field emission arrays. The first approach is a “coplanar type”
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