High aspect ratio 0.1 μm tungsten gates for InGaAs/InAlAs heterojunction transistors

Abstract

High aspect ratio submicron W gate structures as short as 0.1 μm have been made by e-beam lithography and reactive ion etching. An SF6/CHF3 gas mixture has been studied to limit the undercutting observed in SF6 alone. Etch profiles in 1-μm-high tungsten have been systematically studied as a function of 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 air-bridge gates as short as 0.3 μm have been made using this technology. These devices exhibit a unity current gain frequency ft 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 1.2 μm and are found to be suitable for enhancement mode operation.