Mechanistic Insights into Acid Generation from Nonionic Photoacid Generators for Extreme Ultraviolet and Electron Beam Lithography.

Abstract

Nonionic photoacid generators (PAGs) have emerged as key components in advanced extreme ultraviolet (EUV) and electron beam (EB) photoresists, offering advantages such as low dark loss, reduced outgassing, and suppressed phase separation. However, the lack of molecular-level understanding of their acid generation mechanisms hinders rational design and leads to reliance on trial-and-error synthesis. In this study, we perform a comprehensive density functional theory (DFT) investigation on 22 representative nonionic PAGs to elucidate their postexposure reaction pathways, encompassing bond dissociation, byproduct formation, and proton transfer mechanisms. Our findings reveal four distinct electron-triggered dissociation modes, including productive N-O/C-O bond cleavage and competing, nonproductive S-O bond cleavage. We identify the relative energy barrier between productive and unproductive pathways as a critical descriptor for photoacid generation efficiency and, by extension, photoresist sensitivity. Moreover, we demonstrate that molecular conformation (bent vs extended) and electron-withdrawing or electron-donating substituents profoundly impact the selectivity of bond dissociation. Importantly, this study also clarifies the roles of various proton sources (phenolic -OH⁺, t-BOC⁺ protecting groups, and intermediates during byproduct formation) in facilitating acid formation. Our analysis quantifies the energy barriers associated with each route, highlighting structure-dependent modulation of acid generation efficiency. These insights collectively establish a structure-mechanism-function relationship for nonionic PAGs and offer a predictive framework for designing next-generation high-sensitivity PAGs tailored for advanced lithographic applications.