Control of negative poly(methyl methacrylate) for the elaboration of planar electrodes separated by a sub-10 nm gap

The influence of the irradiation parameters of 200 keV electron beam lithography on poly(methyl methacrylate) (PMMA) was investigated to study its transformation from a positive resist to a negative resist, in order to create sub-10nm nanogap electrodes. At doses lower than 900 nC/cm (for line exposures) and 40 000 _μC/cm² (for area exposures), the scission process dominates, allowing PMMA to be used as a positive resist. At higher doses, however, the polymerization process becomes dominant and PMMA can be used as a negative resist. Specific features like sensitivity, contrast, resolution, and proximity effects were analyzed for both positive and negative processes on two kinds of isolated and nonisolated patterns: the study of the dose variation showed that it is more convenient to use a 2000 nC/cm dose to diminish the sensitivity to proximity effects of line exposure. As a result of our study, sub-10nm nanogap electrodes were then fabricated with negative PMMA using optimized conditions. We show that negative PMMA can be used as a high-resolution resist similarly to the positive resist in an simple process.