Sidewall quality in proton beam writing

Proton beam writing has been shown to allow the fabrication of high aspect ratio nanostructures at sub-100 nm dimension and with smooth and vertical sidewalls. For applications such as the fabrication of waveguides, sidewall smoothness is an important issue. We report results from investigations into side wall roughness measured directly with Atomic Force Microscopy. Structures were written in bulk poly(methylmethacrylate) (PMMA) with 2 MeV protons specifically to allow side access. We studied the effects of different scanning algorithms and also the variation of wall roughness with development time and ion penetration depth. Our results indicate that sidewall rms roughness of less than 7 nm is readily achievable. Multi-loop scanning and optimization of the scanning algorithm can lead to significant improvements in sidewall smoothness.