Comparison of charge dissipation layers and dose sensitivity of PMMA electron beam lithography on transparent insulating substrates such as GaN

We compare the influence of electron-beam and thermal evaporation of a charge dissipation layer on the edge roughness and dose-sensitivity of polymethyl methacrylate (PMMA) in an electron-beam lithography (EBL) process on semi-insulating GaN. It is shown that the deposition of the Aluminum film with an electron-beam process leads to an increased sensitivity of the PMMA to small dose variations and a more than three times higher edge roughness compared to a thermally deposited Aluminum layer. The study has direct bearing on the manufacturability of mm-wave devices on GaN.