Self-aligned high electron mobility transistor gate fabrication using focused ion beams

Abstract

A new gate fabrication technique has been developed based on focused ion beam exposure and reactive ion etching of a polymethylmethacrylate (PMMA)/Ge/PMMA multilevel resist structure. The focused ion beam exposes the thin PMMA imaging layer that is transferred directly to the germanium layer using reactive ion etching (RIE). The underlying resist is etched first in oxygen at high pressure, providing an undercut, bowl-shaped profile, followed by an extremely low pressure, anisotropic oxygen process, which etches a narrow stem down to the base of the resist, resulting in a chalice-shaped profile. Removing the germanium layer allows the profile to be used to lift off the Ti/Pt/Au gate. This process is compatible with our self-aligned gate high electron mobility transistor (HEMT) process and provides an alternative technique for gate fabrication below 75 nm. Focused ion beam gate structures have been fabricated with 60 nm gate length and 0.15 μm2 cross section using this technique and shown that the resulting gate profile can be used to shadow a 200 nm thick Ohmic contact evaporation to form the self-aligned structure with a source/drain spacing of 225 nm. It has also been determined that ion bombardment damage to the HEMT channel during RIE can be eliminated by limiting the plasma potential to less than 45 V.