Excited-state charge-transfer interactions in electron-beam resists based on styrene copolymers

Halogenated derivatives of polystyrene that are known to perform as single-stage crosslinking electron-beam resists, such as copolymers of styrene or a methylstyrene with a chlorostyrene or chloromethylstyrene, and similar structures formed by chloromethylation of polystyrene or chlorination of a polymethylstyrene, are shown in general to function through parallel non-interacting radiation-induced crosslinking mechanisms. One of these is the same radical mechanism that would operate within the homopolymers of the constituent comonomers. The other mechanism is characteristic of the systems being copolymers and arises from intramolecular excited-state charge-transfer interactions between adjacent chlorine-containing and non-chlorine-containing units of the polymer chains. Pulse radiolysis studies demonstrate the formation of a transient intermediate in poly(p-methylstyrene-stat-p-chlorostyrene), the spectrum of which is consistent with that of a methylstyryl radical-cation. The radiation chemical yields for crosslinking derived from the lithographic sensitivities of a range of such systems, are modelled in terms of these mechanisms and the variation with copolymer composition is shown to be in excellent accord with experimental observation.