Approaches to Polymer Curing and Imaging Via the In Situ Generation of a Catalyst

The in situ photogeneration of catalytic species such as acids, bases, or free radicals from soluble organic precursors can be used in polymer curing or in photoresist processes. Convenient photoprecursors of acids include iodonium and sulfonium salts as well as a large number of halogenated aromatic compounds, sulfonate esters, etc. The photogeneration of amine is possible from a variety of active carbamates, while hydroxide or alkoxide bases can be obtained by irradiation of hydrox or alkoxy derivatives of triphenylmethane. Specially designed polymers that include acid or base sensitive groups have been prepared, mainly for use as imaging and resist materials for microlithography. For example a photosensitive polyimide can be formulated from a polyamic acid derivative and a photoprecursor of base. Of particular interest are systems that incorporate chemical amplification. In these systems the initial radiation induced process, photogeneration of the acid or bass catalyst within the polymer film, is followed by a cascade of reactions affecting the polymer. Overall this leads to highly sensitive photopolymer formulations in which the effective quantum yield is many times higher than that of the original photoevent. Practical examples of this concept in resists that operate on the basis of multiple main-chain cleavage processes, side-chain modification reactions, and electrophilic crosslinking are described.