Comparison of the Electron-Beam Lithographic Properties of a Range of Poly (Methacrylates) and Their Silicon-Containing Analogs

In order to understand and rationalise the difficulties encountered in achieving high lithographic sensitivity combined with dry-etch durability in electron-beam resists based on methacrylate polymers, the homologous series of n-alkyl methacrylates from methyl to butyl together with phenyl and benzyl methacrylates and the corresponding omega-trimethylsilylalkyl or p-trimethylsilylaryl methacrylates, where necessary, have been synthesized and polymerized using free-radical initiation. The polymer structures have been characterized using NMR spectroscopy and size-exclusion chromatography, and their thermal properties determined using differential scanning calorimetry. The polymers have been evaluated for their performance as electron-beam resists, both lithographically and in terms of their erosion rates in oxygen plasma processing; for these purposes, the performance of poly(methyl methacrylate) was used as a reference. Positive-working behaviour was observed for all the polymers with non-silylated ester functions, the lithographic sensitivities varying in the order butyl > ethyl > methyl approximately propyl > phenyl >> benzyl, and oxygen plasma etch rates being of the order of 2000 angstrom min-1. With the possible exception of poly(trimethylsilylmethyl methacrylate), the incorporation of trimethylsilyl groups in the alkyl ester functions is found to reduce significantly the lithographic sensitivities of the resists, and for the trimethylsilyl, ethyl, propyl and butyl polymers, a dominant negative-working behaviour pattern is revealed through judicious choice of developing solvents. In contrast, positive-working behaviour is maintained for the trimethylsilylaryl methacrylate polymers. The sensitivities of these are greater than those of the parent non-silylated polyesters, markedly so in the case of the benzyl polymer. With the exception of poly(trimethylsilylbenzyl methacrylate), the etch rate of which is still half that of the parent methacrylate polymer, oxygen plasma etch rates of all the polymers are reduced significantly by the incorporation of silicon. These effects are discussed in terms of the radiation chemistries of the various structures.