Imprint lithography issues in the fabrication of high electron mobility transistors

Abstract

In earlier work we described procedures for the fabrication of High Electron Mobility Transistors (HEMTs) using nanoimprint lithography to produce T-shaped resist profiles for gate metallization. For reliable liftoff the T-gate process requires a clear undercut in resist, and this is intrinsically impossible with imprint lithography. In addition, the imprinting process resulted in fracture defects in the resist unless release was carried out at elevated temperatures. Both these problems required improvement. In this article we describe an improved method for obtaining metallized imprinted T-gates by creating resist undercut using a double angled evaporation. HEMTs with self-aligned gate structures were fabricated using this procedure. Multilayer resist stacks were used to investigate resist flow during imprinting and we show that such stacks are not suitable for liftoff enhancement of T-gates. We further show how resist fracture defects can be minimized by increasing the pattern density on the die.