Electron beam lithography of HSQ/PMMA bilayer resists for negative tone lift-off process

A HSQ/PMMA bilayer resist system, in which HSQ as negative tone electron beam resist top layer and PMMA as bottom layer, has been investigated for negative tone lift-off process. Patterns are first defined on the HSQ resist using electron beam lithography, and then transferred into the bottom PMMA layer using oxygen reactive ion etching. Electron beam exposure of HSQ on top of PMMA layer has been characterised, showing the PMMA underlayer has no effect on the exposure of HSQ. Optimum conditions for reactive ion etching of PMMA underlayer have been established. The undercut length in the PMMA layer is found near linearly dependence on etching time. Well defined undercut profile has been achieved in the HSQ/PMMA bilayer resist system, and good negative tone metal lift-off structures have been successfully produced.