No abstract

The function of common, positive tone photoresist materials is based on radiation-induced modulation of the dissolution rate of phenolic polymer films in aqueous base. The process through which novolac and other low molecular weight phenolic polymers undergo dissolution is examined from a new perspective in which the “average degree of ionization” of the polymer is regarded as the principal factor that determines the rate of dissolution rather than a diffusive, transport process. This perspective has been coupled with a probabilistic model that provides an explanation for the dependence of the dissolution rate on molecular weight, base concentration, added salts, residual casting solvent, and the addition of “dissolution inhibitors”. It predicts the observed minimum base concentration below which dissolution is no longer observed, and it predicts a molecular weight dependence of that phenomenon. A series of experiments was designed to test this predicted molecular weight response. The results of these experiments are in good agreement with the predicted response.

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.