Four-layer resist process for asymmetric gate recess

In this paper, we present an optimized four-layer resist (PMMA and its copolymers) process for the fabrication of T-shaped gates used in compound semiconductor field effect transistors (FETs). The process is capable of producing a profile which acts as both the etch mask for the wide, asymmetric recess trench as well as the liftoff mask for a T-shaped gate metal. The resist profile is achieved in a single step using electron beam lithography, eliminating the need for two separate lithography steps and the crucial alignment between them. Gate lengths of 100 mm are achieved using this process. Recess widths on the drain side of the gate range from 50 to 300 nn, and recess widths on the source side of the gate are 50 nm.