Radiation chemical yields and lithographic performance of electron-beam resists based on poly(methylstyrene-co-chlorostyrene)

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Abstract
Copolymers of methylstyrene and chlorostyrene cross-link when irradiated with 20 keV electrons and hence act as negative-working electron-beam resists. o-Methylstyrene/p-chlorostyrene and p-methylstyrene/p-chlorostyrene copolymers have been prepared by a free-radical mechanism over the entire composition range and the lithographic performance of the materials has been evaluated. Radiation chemical yields for cross-linking and chain scission have also been estimated. None of the materials undergoes significant chain scission upon irradiation. In contrast to the corresponding methylstyrene/chloromethylstyrene copolymer systems, the resist sensitivities maximize at compositions containing ca. 30% chlorostyrene. A cross-linking mechanism involving an excited-state charge-transfer interaction of adjacent methylstyrene and chlorostyrene chain units is proposed. The copolymers of optimal composition display sufficiently high lithographic sensitivities and contrasts to commend their application as electron-beam resists.

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