Radiation chemistry and the lithographic performance of chemical amplification resists formulated from poly(4-epoxystyrene-stat-styrene) and a photoacid generator

Copolymers of styrene and 4-epoxystyrene in formulations with triphenylsulfonium hexafluoroantimonate as a photoacid generator undergo a crosslinking reaction by a chain mechanism when irradiated with 20 keV electrons and hence act as negative-working electron-beam resists. The copolymers have been prepared by a free radical mechanism over the entire composition range and the lithographic performance of the materials has been evaluated. The sensitivities of the resist formulations are shown to correlate with the epoxystyrene content of the copolymers in accordance with a simple model of the radiation chemistry of the system. Contrast variations are explained in terms of the statistical structure of the copolymer and it is demonstrated that at low epoxystyrene contents the systems behave in accordance with an unsensitised single-stage crosslinking mechanism. The copolymers with epoxystyrene contents greater than ca. 8% have such high lithographic sensitivities and show such a good tolerance of processing variations as to commend the optimisation of their performance with a view to their subsequent application as electron-beam resists.