Fabrication of Short-Gate GaAs MESFETs by Electron Beam Lithography
We have used an ISI-100B scanning electron microscope to fabricate GaAs MESFETs with gate lengths between 30 and 100 nm. Direct write was achieved with 40 kV electron exposure of 950 000 molecular weight PMMA and a liftoff process. All fabrication levels were performed using EBL with a desktop computer used to control all timing and vector scan settings. Output was via two 12-bit D/As. The gate metal was 40 nm thick gold and fit in 2.3 to 3 micron source-drain spacings. Maximum gm achieved was 85 ms/mm (with calculated intrinsic gm of 110 ms/mm) for a 45 nm gate device. This is consistent with modeling performed by W. CURTICE at RCA in Princeton, New Jersey, for parameters similar to ours. It was found that pinchoff was achieved for gate voltages of −3 volts and drain voltages up to 2.5 volts. It is believed that charge injection into the substrate caused lack of pinchoff after that point.