Generation and relaxation of free volume during the post exposure bake

Upon ultraviolet exposure chemically amplified resists generate small, catalytic amounts of acid. During the post exposure bake, this acid is used to catalyze a solubility switching reaction that allows for the eventual development of latent image. The potential exists for acid catalyst to migrate from exposed regions into unexposed regions during the post exposure baking step and, thus cause feature width spread, or loss of critical dimension control. As features decrease in size, the spread caused by acid migration becomes a proportionately larger problem. This paper describes the results of experimental investigations into the fundamental mechanisms of acid transport. A fuller understanding of these mechanisms can serve both as a guide for optimizing current resist systems and for designing next generation resists.