Toward Environmentally Friendly Photolithographic Materials: A New Class of Water-Soluble Photoresists

New water-soluble styrenic polymers bearing two functional groups, pendant ammonium salts of half-esters of malonic acids and acid-labile alkyl esters, were synthesized and evaluated for water-soluble positive-tone photoresist application. These polymers feature two solubility switches: insolubilization of the entire film by baking and selective solubilization upon exposure to UV light. Time-resolved FT-IR measurement of the baked films showed sequential evaporation of ammonia from the films and the decarboxylation of the malonate half-esters. The rates of decarboxylation depend on the structure of substituents at the 2-position of the malonates. The choice of acid-labile esters, the structure of the half-esters, and the polymer compositions were carefully optimized, and high-resolution positive tone images were obtained that were fully processed in aqueous media.