Synthesis and Characterization of Eco-Friendly Water-Processable Dual-Tone Photoresists Containing Photoreversible Coumarin Groups

In this study, we successfully synthesized and assessed eco-friendly water-soluble polymers containing photoreversible coumarin groups as dual-tone photoresists. The synthesized polymers do not require any photoinitiator or photoactive compound and can be used as both negative- and positive-type photoresists by simply controlling the exposure conditions. The monomer, 7-(4-vinyl-benzyloxyl)-coumarin (VBCC), incorporating a coumarin group, was synthesized through etherification between 4-vinyl benzyl chloride and 7-hydroxycoumarin, with its chemical structure confirmed by NMR and FT-IR spectroscopies. Subsequently, VBCC underwent polymerization with sodium p-styrenesulfonate (NaSS) to impart water solubility. The chemical structure of P(VBCC-co-NaSS) at various monomer ratios was also verified using NMR and FT-IR spectroscopies, while its photoreversible chemical reaction was confirmed through UV–vis and FT-IR spectroscopic techniques. Under 365 nm UV exposure, a cyclobutane ring formed via the photodimerization of coumarin groups, which was then cleaved and reverted to its original coumarin structure under DUV exposure. The water solubility of P(VBCC-co-NaSS) increased with a higher NaSS amount, albeit at the expense of photospeed. Considering the water solubility and photospeed, P(VBCC-co-NaSS) with a VBCC to NaSS ratio of 6:4 was chosen as the optimal photoresist. Well-defined negative- and positive-type patterns could be achieved depending on photolithographic processes. This study underscores the potential of P(VBCC-co-NaSS) as an environmentally friendly, water-processable, and photoreversible photoresist.