Using the critical ionization model for resist development to estimate contrast curves and roughening

The dissolution of exposed regions of polymeric resists in aqueous base to form a pattern is a complex reactive process, and is usually described only empirically. It has recently been proposed that a critical level of ionization is required for a polymer chain to move from the film into solution, and we use this model as a framework for a simple reaction scheme that describes the transformation of a polymer chain from an unsolvated form into a solvated one. Simulations of the dissolution process are used to predict the dose dependence of thickness loss, resist contrast and line shapes as a function of local extent of polymer deprotection in p-tert-butyloxycarbonyloxystyrene (PTBOCST) chemically amplified positive tone photoresist, and examine local composition and kinetics factors that lead to roughening. The results show that nonlinearities inherent in the dissolution kinetics are responsible for resist imaging. The simulations can be used to define a simple functional form for thickness loss as a function of dose, providing a physically-based alternative to phenomenological dissolution models for prediction of developed profiles from calculated latent images. The extendability of the approach to copolymers and to systems that undergo significant gellation is discussed.