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

The dissolution phenomena that are the basis of microlithography are largel y dependent on the acid-base equilibrium of phenolic polymers in aqueous ba se. Fundamental equations are derived to relate the probabilistic quantitie s of the critical-ionization model to experimentally measurable acid-base p roperties in such polymer systems: solution pH, polymer pK(a), degree of po lymerization, and average degree of ionization. Model predictions for the d ependence of the dissolution rate on these properties support previous expe rimental observations. A method for estimating the pK(a) of phenolic polyme rs as a function of the average degree of ionization is developed, and the results of this approach for novolac and poly(hydroxystyrene) agree with th e 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 subst itution of deuterium for hydrogen in the phenol group is interpreted in ter ms of the deuterium isotope effect on pK(a).