Chemically Amplified Imaging Materials Based on Electrophilic Aromatic Substitution: Poly[4-(acetoxymethyl)styrene-co-4-hydroxystyrene]

A new family of versatile negative resist materials based on phenolic resins has been prepared. The resist design is based on radiation-induced cross-linking via an electrophilic aromatic substitution process in a series of copolymers consisting of vinylphenol and vinylbenzyl acetate units. Copolymers of varying compositions are prepared by random copolymerization of 4-(acetoxymethyl) styrene with 4-(tert-butyl-oxycarbonyloxy)styrene, a monomer pair with suitable reactivity ratios, followed by removal of the t-BOC protecting groups using hot acetic acid. Compositions rich in hydroxystyrene have high T_g values and are suitable for imaging experiments. When mixed with 5–10% of a compound such as a triarylsulfonium salt that generates acid upon radiolysis, the copolymers can be used as resist materials for microlithography. Exposure results in the generation of a latent image of acid dispersed into the copolymer; a postexposure heating step provides the activation energy necessary for the generation of benzylic carbocationic species that react in an electrophilic aromatic substitution process. Since the initiating proton is regenerated at each step, the process is catalytic or “chemically amplified”. The new resists have been patterned by exposure to deep-UV, electron beam, or synchrotron X-ray radiation and show extremely high sensitivies and resolutions.