Advanced CVD Diamond Microtip Devices for Extreme Applications
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electron sources for microwave tubes, and large area electron guns for flat panel displays.
The most critical component in VME devices is the field emission cathode. These cathode structures are microfabricated by several methods. The original Spindt technique involves vapor deposition of the tip through a hole of decreasing diameter [3, 4, 5]. This and similar techniques are the predominant methods used to produce cold cathodes for vacuum microelectronic devices. Typically, the tips are formed using molybdenum because it is compatible with other procedures involved in the microfabrication process. Other techniques of field-emitter array fabrication involve etching to form emitter tips from single-crystal silicon, mold casting, and directional Field emission cathocies developed using tnese techniques still suffer from a variety 01 problems. There are emission uniformity and reproducibility problems due to differing emission characteristics of individual tips. There are problems with contamination by background gas and back sputtering of ionized particles. Additionally, the relatively high work function of the emissive material, capacitance of the device, and space charge effects also have adverse effects on the performance of these devices. Extreme Devices has recently demonstrated that diamond microtip cathodes can be fabricated and used as a replacement for the molybdenum or silicon based microtip field emission cathodes. Diamond microtip cathodes provide the solution to the inherent material short-comings of the traditional microtip cathodes. One of the drawbacks to the use of diamond as a cold cathode emitter has been that it was difficult to process CVD diamond films using common IC fabrication techniques. A key development is that Extreme Devices has perfected a fabrication technique resulting in self-aligned gated diamond microtip cathodes. This advance will allow diamond emitters to be integrated into true VLSI-type devices using equipment and processes that are common to the semiconductor industry. The inherent superiority of gated diamond microtip 65
Mat. Res. Soc. Symp. Proc. Vol. 509 ©1998 Materials Research Society
cathodes will enable VME devices to fulfill the promise of these devices to meet the demands for a variety of applications. EXPERIMENT Diamond microtips are grown using standard microwave chemical vapor deposition (CVD) techniques onto a silicon "mold" containing anisotropically etched square pyramidal pits [6, 71. A schematic of this process is presented as Figure 1 and is thoroughly discussed in the cited references. In a major departure from the cited references, Extreme Devices is using a 5 kW ASTeX high pressure microwave CVD reactor to grow thick (200 - 500 jgm), mechanically strong, free-standing 50 mm diameter diamond wafers. The free-standing wafers can contain thousands of devices depending on the final device dimensions and are fully capable of withstanding all of the gate fabrication processes. This advance allows us to fabricate devices using true IC fabrication techniques.
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