Kinetic model for positive tone resist dissolution and roughening

The dissolution of exposed regions of polymeric resists in aqueous base to form a pattern is a complex reactive process. It has recently been proposed that a critical level of ionization is required for a polymer chain to move from the film into solution. That model successfully predicts many of the features of polymer dissolution such as dependence on chain length and solution pH but has not been used to describe the detailed kinetics of the dissolution process. In this work we use the critical ionization model as a framework for a simple reaction scheme that describes the coupled reversible ionization-relaxation steps that transform a polymer chain from an unsolvated form into a solvated one. Simulations of the dissolution process are used to predict line shapes as a function of local extent of polymer deprotection in chemically amplified positive tone photoresists and examine chemical factors that lead to roughening. The results show that nonlinearities inherent in the dissolution kinetics are responsible for resist imaging. The kinetics also lead to increased roughening as the aerial image contrast is decreased. Implications of these results for current models of resist development and roughening are discussed. A kinetic criterion connecting critical ionization to the state of the materials in solid and solvated phases is proposed.