Diamond vacuum field emission devices

By W.P. Kang, J.L. Davidson, A. Wisitsora, Y.M. Wong, R. Takalkara, K. Holmes, D.V. Kerns

Diamond & Related Materials 13 (2004) 1944– 1948

Purpose of the study:

This article reports the development of (a) vertical and (b) lateral diamond vacuum field emission devices and testing their field emission characteristics.

Methodes:

These diamond field emission devices, diode and triode, were fabricated using a self-aligning gate formation technique from silicon-on-insulator wafers. The SOI wafer is comprised of a 15um thick Si active layer, 1um thick SiO2 layer (BOX) and 525um thick Si handle.

For vertical devises, 0.2um thick SiO2 layer was grown on the wafer surfaces. Inverted pyramidal cavities were then formed on the silicon active layer by photolithographic patterning and anisotropic etching of Si using KOH solution. The square patterns are sized such that complete inverted pyramidal cavities are formed within the Si active layer. Next, a SiO2 layer was grown on the active Si layer to form the gate dielectric, which also produces a well-sharpened apex on the inverted pyramidal SiO2 layer. Diamond was then deposited in the mold by plasma enhanced chemical vapor deposition technique (PECVD). Next, the backside of the silicon was etched away and stopped at the embedded SiO2 layer. Finally, the SiO2 layer was etched and the sharpened diamond pyramidal apexes exposed. The remaining SiO2 and Si form the dielectric spacer and the gate, respectively. For the diode configuration the SiO2 spacer and the remaining spacer were also etched to completely expose the diamond pyramids.

For lateral devices, 1um thick SiO2 layer was first grown onto the SOI wafer. Conventional photolithography was then performed to pattern the anode and cathode structures onto the SiO2 layer. The exposed SiO2 was etched away using BOE exposing the Si below. Next, electrically conductive diamond was preferentially grown on Si using biasenhanced PECVD, by introducing trimethyl boron (TMB) gas in the plasma mixture for boron doping. The unwanted diamond that grew on SiO2 was lifted-off by etching the SiO2 using an HF.The patterned diamond layer was then used as a masking layer to etch Si to get the required final structure.

The fabricated diamond emission diodes and triodes were tested for electron emission under high vacuum. The emission current was recorded as a function of applied voltages. Fowler–Nordheim (F–N) equation was used to analyze the diamond field emission data.

Key findings:

1. A diamond field emission diode operable at high emission current over 0.1 A in an indented anode vertical configuration has been achieved.

2. A diamond field emission triode with excellent transistor characteristics of high DC voltage gain and large AC voltage amplification is achieved with high DC gain of ~800 and large AC output voltage of ~100 V p–p.

3. A lateral diamond field emitter (cathode–anode spacing less than 2 um) with the lowest turn-on voltage (~5 V) and high emission current (6 uA) has been realized.

4. The low turn-on voltage (field ~3 V/um) and high emission characteristics are the best of reported lateral field emitter structures.

Keywords: Diamond; Field emission; High current; DC gain; Lateral field emitter