Possibilities for photoimaging using onium salts

The recent discovery that the photolysis of certain onium salts produces strong Brønsted acids presents a unique opportunity to design novel photoimaging systems based on these materials. For example, when used as photoinitiators, they can induce the negative image wise cationic polymerization of multifunctional epoxides, vinyl ethers, and lactones. Alternatively, they may be used to catalyze the degradation of hydrolytically sensitive polymers to give positive photoresists. The ability of still other onium salts to generate radical species as well as strong acids has been exploited to develop photoimaging processes which have no parallel in present photoresist technology. The processes consist of simultaneous radical polymerization and acid‐induced cationic polymerization. Finally, the recent observation that the onium salts can be dye‐sensitized allows them to be responsive to both the ultraviolet and visible regions of the spectrum and to be used with many commercial optical imaging systems.