Molecular-scale spectroscopic insights on temperature-induced dissolution inhibition in DNQ/novolac photoresists

Novolac-based photoresists, widely utilized in i-line and broadband photolithography, rely on diazonaphthoquinone (DNQ) photoactive compounds to enable UV-patterned dissolution via photochemically generated carboxylic acids. While the dissolution mechanism is typically governed by enhanced base solubility following DNQ photolysis and phenolic deprotonation, a counterintuitive decrease in dissolution rate with increasing temperature has been repeatedly observed in an intermediate thermal window—manifesting as a negative activation energy. Previous models have attributed this phenomenon to enhanced photoactive compound–novolac complexation or temperature-induced disruption of phenolate–tetramethylammonium ion pairs. However, these explanations fall short of accounting for additional observations such as surface densification and anion-specific effects. In this study, we uncover a complementary mechanism in which rapid formation of phenolate and carboxylate species leads to hydration-shell collapse and a subsequent densification of the novolac matrix near its glass transition temperature. Through systematic spectroscopic analysis, we propose a unified model that incorporates molecular interactions, ion-pairing dynamics, and polymer network reorganization, providing a comprehensive framework for understanding temperature-dependent dissolution behavior in DNQ/novolac photoresists.