Radiation-induced chemistry of poly(4-[(tert-butoxycarbonyl)oxy]styrene-co-sulfur dioxide)

Copolymers of 4-[(tert-butoxycarbonyl)oxy]styrene (TBS) and sulfur dioxide (SO[sub 2]) have been found to act as sensitive X-ray ([lambda] = 14 [angstrom]) single-component, chemically amplified, aqueous-base-soluble positive-acting resists. The X-ray response of these materials was a function of copolymer composition and independent of molecular weight. It was observed that increasing the SO[sub 2] content enhanced the resist sensitivity. Initial investigation into the radiation-induced reaction mechanism provided evidence that acid formation occurs via polymer main-chain scission. It is proposed that at the scission sites radical species are produced which in turn are responsible for the formation of the acid moieties. Heat treatment of resist films after exposure converted copolymers to poly(4-hydroxystyrene sulfone) and permitted exposed film areas to be developed in an aqueous base solution. Preliminary lithographic evaluation has resolved 0.5-[mu]m line and space patterns in 0.65-[mu]m-thick 1.75/1 TBS/SO[sub 2] resist films using an X-ray dose of 10 mJ/cm[sup 2]. No change in X-ray dose was required to resolve the same width feature in the same resist having a film thickness of 1.0 [mu]m.