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

High aspect ratio sub-micron W gate structures as short as 0.1micron 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 1micron 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 0.3micron have been made using this technology. These devices exhibit a unity current gain, f sub t of 80 GHz as measured and an estimated 115 GHz when corrected for pad capacitance. Pinchoff characteristics and threshold voltage data for devices with gate lengths from 0.4 to 1.2micron will also be presented.