Cationic chemistry and chemically amplified resist materials for microlithography: synthesis and applications of copolymers of 4-(1-hydroxy-1-methylethyl)styrene and styrene or 4-hydroxystyrene

Two new copolymers based on 4-(1-hydroxy-1-methylethyl)styrene and styrene or 4-hydroxystyrene have been designed and prepared for use as ultrahigh resolution resist materials in microlithography. The tertiary benzylic alcohol moieties contained in these copolymers undergo acid-catalysed dehydration in a process that proceeds via a stabilized carbenium intermediate, leading to the eventual crosslinking of the copolymers. Resist formulations incorporating these copolymers are readily obtained through the use of a photoacid generator such as triphenylsulfonium hexafluoroantimonate. Irradiation of a film of the resist with deep-u.v. light or an electron beam results in the liberation of protons within the exposed areas of the polymer film. Subsequent heating of the exposed film provides the activation energy that is necessary for the dehydration and crosslinking steps to proceed. The design of the new copolymers is such that no swelling of the crosslinked polymers is seen upon solvent development of the resist images as the polarity of the crosslinked areas is changed drastically as a result of the dehydration step. The resists show extremely high sensitivities to both deep-u.v. and electron-beam radiation and are useful as imaging materials.