Mechanism of phenolic polymer dissolution: Importance of acid-base equilibria

The dissolution phenomena that are the basis of microlithography are largely dependent on the acid-base equilibrium of phenolic polymers in aqueous base. Fundamental equations are derived to relate the probabilistic quantities of the critical-ionization model to experimentally measurable acid-base properties in such polymer systems: solution pH, polymer pK(a), degree of polymerization, and average degree of ionization. Model predictions for the dependence of the dissolution rate on these properties support previous experimental observations. A method for estimating the pK(a) of phenolic polymers as a function of the average degree of ionization is developed, and the results of this approach for novolac and poly(hydroxystyrene) agree with the observed differences in the dissolution rates of these two species. These results also corroborate the hydrogen-bonding dissolution inhibition model previously reported. The change in dissolution rate accompanying the substitution of deuterium for hydrogen in the phenol group is interpreted in terms of the deuterium isotope effect on pK(a).