Radiolysis of resist polymers. III. Copolymers of methyl-α-chloroacrylate and trihaloethylmethacrylates

Homopolymers of 2,2,2-trifluoroethyl(methacrylate) (TFEMA) and 2,2,2-trichloroethyl-(methacrylate) (TCEMA) and copolymers with methyl-α-chloroacrylate (MCA) in a range of compositions were synthesized. The reactivity ratios were obtained; the two copolymerizations were close to ideal. Poly(MCA) showed G_s = 7.4 and G_s = 0.9 by γ-radiolysis. On the other hand, poly(TFEMA) and poly(TCEMA) and G_s values of 2.0 and 2.4, respectively, and G_x = 0. Radiolysis of copolymers was initiated to a large degree by dissociative electron capture by the halogen atoms in both comonomers, as revealed by the ESR spectra of radicals derived from them. Germinal recombinations in irradiated poly(TFEMA) suggested the presence of radicals in proximity. This process was absent in the copolymers. GC-MS analysis of volatile products and other supporting evidence showed that TFEMA monomers tended to depolymerize; the TCEMA monomers did not. The radiolysis yields varied monotonically with the comonomer composition for the MCA–TFEMA system but the yield–composition relationship was irregular in MCA–TCEMA copolymers. Four noncrosslinking systems are potential radiation resists arranged in increasing order of promise: poly(TFEMA) (G_s = 2.0, T_g = 70°); poly(TCEMA) (G_s = 2.7, T_g = 142°); poly(94MCA-co-6TCEMA) (G_s = 2.7, T_g = 142°); and poly(68MCA-co-32TFEMA) (G_s = 3.0, T_g = 112°). These materials merit further investigation for E-beam or x-ray lithographic applications. Mechanisms of radiolysis for these materials, based on ESR, GC-MS, and radiolysis yield data, were discussed.