High Aspect Ratio 0.1microns Tungsten Gates for InGaAs/InAlAs Heterojunction Transistors.

High aspect ratio sub-micron W gate structures as short as 0.1microns have been made by e-beam lithography and reactive ion etching. An SF sub 6/CHF sub 3 gas mixture has been studied to limit the undercutting observed in SF sub 6 alone. Etch profiles in 1microns high tungsten have been systematically studied as a function gas composition and an optimum ratio found. Data will also be presented on etch selectivity, bias voltage effects, and overetching. High performance InAlAs/InGaAs heterojunction insulated gate FETs (HIGFETs) incorporating W airbridge gates as short as 0.3microns have been made using this technology. These devices exhibit a unity current gain, f sub t of 80 GHz as measured and an estimated 118 GHz when corrected for pad capacitance. Both pinchoff characteristics and threshold voltage data have been measured for devices with gate lengths from 0.4 to 1.2microns and are found to be suitable for enhancement mode operation.