A review of polymethyl methacrylate (PMMA) as a versatile lithographic resist – With emphasis on UV exposure

This review describes the utility of polymethyl methacrylate (PMMA) as a resist for several types of lithographic processes. Ordinarily, PMMA is only considered an electron beam resist but in fact it can be used as a resist material with exposure systems based on both particle beams and electromagnetic radiations. In each case, both positive tone and negative tone resist action can be realized with appropriate process design. We first describe and review the mechanisms that bring about lithographic pattern formation in this acrylic polymer. This is followed by descriptions of processes for using PMMA in different ways as a lithographic resist. Use of PMMA at both long and short UV wavelengths is treated in more detail as this aspect is less well-known for lithography with PMMA. We also describe several new findings from own work here, such as cross-linking PMMA with Michler's ketone (MK), use of resist reflow for three-dimensional structure fabrication and lithographic tone switching with change in UV exposure dose. The aim of this exposition is to emphasize the great versatility of PMMA as a resist for micro- and nano-electronic device fabrication.