Photoreactions in polymers containing benzil units: a comparative study under excimer laser and Hg-lamp irradiation

Abstract

Polymers bearing benzil units in the main chain (polymer I, poly(oxyhexyleneoxy-4,4′-benzilylene) and polymer II, poly(oxy-4,4′-benzilyleneoxysebacoyl) or as pendant side groups (polymer III, poly(phenyl methacrylate)- co-4-methacryloyloxy-4′-methoxybenzil) were synthesized via polycondensation and free radical polymerization. The polymers became crosslinked during u.v. irradiation and were assessed as potential negative resist materials. The lithographic sensitivity (i.e. the onset of gel formation) of polymers I– III depended on the irradiation source (Hg-lamp, 311 nm; excimer laser, 308 nm) and on the pulse energy of the excimer laser (50–400 J m −2). For example, for polymer I the onset of gel formation dropped from E′ g =2000 J m −2 to E′ g =400 J m −2 when the laser pulse energy was raised from 100 to 400 J m −2. The sensitivity under continuous Hg-lamp irradiation was similar to that under low-energy laser irradiation. A Charlesby–Pinner analysis of the crosslinking data revealed that both crosslinking and chain fragmentation occurred during u.v. irradiation (0.24< Φ CS· Φ CL −1<0.54). During u.v. irradiation, the benzil chromophores and—in the case of polymer II—also the ester units were transformed during u.v. irradiation. FT i.r. data showed that the photoreactions and their efficiency varied depending on the excitation source. These laser-specific reactions are probably due to two-photon processes.