Mechanism of Phenolic Polymer Dissolution:  Importance of Acid−Base Equilibria

Abstract

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.

Understanding nonlinear dissolution rates in photoresists

“…However, α is very dependent upon the structure and acidity of a given polymer. Specifically, the equilibrium of the deprotonation reaction dictates that α is related to the pH of the developer near the surface of the film (pH o ) and the pK a of the polymer by: The acidity function of the polymer dictates that α is related to the pKa of the film 24 :…”

“…This result was used to explain why PHOST dissolved much quicker than novolac, all relevant conditions being equal. 24 Here, this result is used to provide insight into surface inhibition.…”

“…In a previous work, the CI model was shown to correctly predict that PHOST dissolves faster than novolac (all relevant parameters being equal). 24 In this work, the same argument is used to demonstrate that the model also predicts surface inhibition in materials with a low fraction of ionized surface sites (such as novolac). The model also predicts an increase in surface inhibition with decreasing void fraction.…”

“…The chemical structure (pKa) of the polymer has a strong effect on the fraction of surface sites that may ionize. 24 For example, PHOST has a much lower pKa than novolac (for pH conditions above 7), and the fraction of surface sites that may ionize on a PHOST film is thus much higher than novolac. In a previous work, the CI model was shown to correctly predict that PHOST dissolves faster than novolac (all relevant parameters being equal).…”

Polymer dissolution model: an energy adaptation of the critical ionization theory

“…(2) -(5) can be solved simultaneously to get the fractional surface ionization as a function of developer concentration. Throughout the simulation, the value of pKa used is 10.25, same as that for homogenous solution [13]. Fig.…”