Photodegradation mechanism of phenyl isopropenyl ketone – methyl methacrylate copolymers in the solid phase and their performance as a deep UV resist

Phenyl isopropenyl ketone (PIPK) copolymer films were irradiated with a low-pressure mercury lamp, and benzoyl side-chain cleavage and main-chain scission at various temperatures were followed by UV spectral changes and gel permeation chromatography, respectively. Quantum yields of both reactions and the ratio of the latter to the former reaction increased markedly above their glass transition temperatures. The main-chain scission continued to proceed by irradiation even after the side chain ceased to split. These experimental results suggest a new reaction mechanism of photosplitting of the methyl methacrylate ester side chain on the carbon adjacent to the C=C bond followed by β-scission, in addition to the known benzoyl side-chain cleavage followed by β-scission or C=C bond formation. The enhanced quantum yield for the main-chain scission of the PIPK copolymer at 130 °C realized sensitivity of 100 mJ/cm2 as a DUV resist, which was 1200 times as high as that of poly(methyl methacrylate) exposed at room temperature and developed under similar conditions. Keywords: Norrish type I cleavage, β-scission of main chain, temperature dependence, photosplitting of ester side chain, conjugated polymer radical.