Solvent-free polymer lithography via the Kirkendall effect

An alternative route to form micron scale features in a PMMA resist that does not rely on solvent-based developers is reported. Instead of dissolving out irradiated regions of the resist, annealing is used to diffuse the irradiated material into the surrounding areas. Radiation-induced chain scission results in regions of reduced molecular weight adjacent to the virgin material, which have a much greater diffusion coefficient than the surrounding material. When annealed above the glass transition temperature, the fast-diffusing irradiated material swells the relatively immobile virgin PMMA. The extent of swelling is analysed to estimate the effective molecular weight of the irradiated material, and potential applications for solvent-free lithography based on proton beam writing are discussed.