Terpolymerization of Styrenic Photoresist Polymers: Effect of RAFT Polymerization on the Compositional Heterogeneity

Abstract

The heterogeneity of the chemical structure of photoresist polymers, both within and between the polymer chains, is believed to contribute to the phenomenon of line edge roughness in high-resolution lithography. The copolymerization and terpolymerization of three important photoresist monomers—4-acetoxystyrene (AOST), styrene (Sty), and tert-butyl acrylate (tBA)—are examined in detail in this work. In particular, the effect of the use of a reversible addition–fragmentation chain transfer (RAFT) agent in these reactions is reported. The pairwise copolymerization reactivity ratios for the RAFT polymerizations are close to but different from those measured under the same conditions but in the absence of the RAFT agent. The differences are suggested to be related to the effect of the RAFT agent on the environment of the locus of polymerization. The course of the terpolymerization reactions can be well predicted from the copolymer reactivity ratios and the classical Alfrey–Goldfinger expression, indicating that the reactions in this system conform to terminal model of reactivity. Calculations of the expected sequence distributions as a function of composition and conversion clearly demonstrate that the polymers prepared under RAFT conditions are significantly more homogeneous than their counterparts prepared using conventional free radical polymerization. The significance of these results for the performance of the RAFT terpolymers in photoresist formulations is highlighted.