Micropatterned Diamond Vacuum Field Emission Devices
Chemical vapor deposited (CVD) diamond or related carbon materials are excellent materials for electron field emitters. However, the key factor to successful exploitation of CVD diamond for vacuum microelectronic applications is closely related to further understanding and control of the physics, materials, and microfabrication technology. To achieve this goal, this research has been focused on two main issues: 1) Design, fabricate, characterize, and investigate the physics of diamond field emissions utilizing shape-designed diamond microtip structures. 2) Develop fabrication processes for achieving monolithic diamond vacuum diodes and triodes, evaluate and model their field emission performance. In the first part of this research, a uniquely engineered mold transfer process has been developed for the fabrication of micro-patterned pyramidal diamond emitters with uniform microtip structures. In addition, methods to improve the diamond field emission behavior have been systematically studied. These include the incorporation of sp2 into diamond tips, vacuum-thermal-electric (VTE) treatment, p-type doping, and tip sharpening. Measurements from field emission of the fabricated diamond microtips have achieved low turn-on field of ~1 V/mm. Hypotheses to explain the resulting diamond field emission enhancement have been proposed and modeled. The second part of this research focused on the development of diamond field emission devices: monolithic diamond vacuum diodes and triodes. For monolithic diamond vacuum diodes, fabrication methods, namely, (1) electrostatic bonding, (2) self-align volcano anode, and (3) self-align-anode molding (utilizing standard, epitaxial, and SOI wafers) have been developed. Monolithic diamond vacuum diodes have low turn-on voltage achieving the lowest turn-on voltage of 0.7 V and high emission current. Also, the emission current was observed to be temperature insensitive up to 200 °C. For the monolithic diamond vacuum triode, two fabrication methods were developed, integrated anode by electrostatic bonding and integrated anode utilizing SOI substrate. Triode emission characteristics were studied as a function of the device parameters of anode-cathode spacing, diamond doping, and array sizes. The anode emission current of the diamond vacuum triode was modeled based on the modified Fowler-Nordhiem (F-N) triode equation and a new empirical model for the emission transport factor. Diamond vacuum triodes with good emission performance, low gate turn-on voltage (10 V), high dc voltage gain (800), and high transconductance (100 mS) have been achieved. Moreover, a diamond triode amplifier with characteristics of high ac voltage gain (~65) and large ac output voltage (~90 Vpeak-peak) was demonstrated. The performance of these devices demonstrates their potential use for vacuum microelectronic applications.