Process for 20nm T gate on Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epilayer using two-step lithography and zigzag foot

Abstract

After metallization, a 20nm T gate with a straight foot is not mechanically stable because the support given by the foot is too weak. We have proposed a zigzag gate foot to enhance mechanical support and developed a process using two-step electron beam lithography and zigzag foot shape to fabricate 20nm T gates for high performance Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs modulation-doped field-effect transistors. Two-step lithography reduces electron forward scattering by defining the foot on a thin (40nm) bottom layer of polymethyl methacrylate at the second step, the T-gate head having been developed at the first step. Adopting a low temperature development technique for the second step reduces the detrimental effect of head exposure on foot definition. With this process, stand-alone 20nm zigzag T gates have been successfully fabricated on an Al0.25Ga0.75As∕In0.2Ga0.8As∕GaAs epitaxial wafer using a 20keV electron beam. With a higher-voltage electron beam, this process can be used to fabricate sub-20-nm T gates.