Copolymers of t-butyl methacrylate and 3-(t-butoxycarbonyl)-1-vinylcaprolactam (BCVC) were synthesized with various monomer feed ratios. Diffusion of photogenerated acid in the copolymer films and the acid trapping capability of the basic BCVC units were investigated. The evaporation of acid was also studied by a spectral change of an indicator film that consisted of poly(vinyl alcohol) and tetrabromophenol blue sodium salt. It is found that only acid surviving deactivation by the BCVC units diffuses into unexposed areas and evaporates from the copolymer film. The evaporation of a low molecular weight basic additive is also examined. Whereas the low molecular weight basic additive is evaporated during baking, the basic monomer units in the copolymer are not evaporated at all. Thus, the copolymer with the basic monomer can control the acid diffusion and evaporation effectively. The new resist system enables us to form fine patterns even after a 2 h postexposure delay without any additional treatment.