Negative tone photolithography with photo-sensitised polymethyl methacrylate (PMMA)

Abstract

We describe the science and technology of negative tone photolithography using polymethyl methacrylate (PMMA), photo-sensitised with the addition of the commercial photoinitiator – Irgacure 651. This versatile two-active-ingredient resist system is shown to enable negative tone photolithography which can be customised to application requirements by adjusting the ratio of the two ingredients. This study describes the complete lithographic process – from resist preparation to pattern transfer – in considerable detail. Furthermore, we also elucidate the mechanism of resist action through exploration of the effects of such parameters as the molecular weight of PMMA, exposure wavelength, post-exposure bake conditions and resist development temperature, among others. This work also demonstrates that photo-cross-linkable resists can be made resistant to dry etching by a post-development blanket UV exposure step. Finally, we also show example structures fabricated using both additive and subtractive techniques, with the use of this resist system.