Electron beam lithography process for T- and Γ-shaped gate fabrication using chemically amplified DUV resists and PMMA

A new process has been developed to fabricate T-shaped gates and $\Gamma$ -shaped gates for high performance metal–semiconductor field effect transistors and high electron mobility transistors using a bilayer of Shipley UVIII DUV resist and poly(methylmethacrylate). The two resists are separated by a 20–30 nm thick layer of aluminum and after patterning by electron beam lithography a two-stage development technique is used to remove the aluminum and to produce well-defined resist profiles. The process can be used to fabricate T-shaped and $\Gamma$ -shaped gates with footwidth sizes as small as 50 nm and headwidth to footwidth ratios in excess of 40:1 for T gates and 35:1 for $\Gamma$ gates. The ability to fabricate gates with these dimensions arises from the fact that the UVIII resist is considerably more sensitive to electron beam exposure than PMMA. Further benefits derived from using a UVIII: PMMA bilayer are better control of footwidth dimensions and shorter electron beam patterning times compared to bilayers of PMMA with copolymers of PMMA. This article describes process optimization and the relationship between feature size and exposure dose.