Atomistic Modeling Approach for Predicting Association of Photoacid Generators in Extreme Ultraviolet Polymeric Photoresists

This work advances a computational framework to probe the molecular inhomogeneities that occur in chemically amplified photoresists intended for ultraviolet (EUV) lithography. Atomistic molecular dynamics simulations were used to probe the effect of ionic and steric interactions on the dispersibility of photoacid generators (PAGs) in the polymer medium before EUV exposure, as poor dispersibility is a potential source of postexposure feature roughness. The PAGs studied include ionic salts containing a triphenyl sulfonium (TPS) cation with trifluoromethane (TFMeS), nonafluorobutane (NFBuS), perfluorophenyl (PFPS), and adamantyltetrafluoroethyl (ATFEtS) sulfonate anions, and a nonionic naphthalimide sulfonate PAG. The model photoresists studied are poly(tert-butyl methacrylate) (PtBMA) and poly(4-hydroxy styrene) (PHS) homopolymers and alternating copolymers thereof. The dissociation free-energy of isolated single and dimer PAGs in the different polymers indicates that PHS segments provide a better solvation environment for ionic PAGs than PtBMA segments and that the nonionic naphthalimide PAG has the highest dispersibility, while TPS-TFMeS solvates better than the bulkier TPS-NFBuS and TPS-ATFEtS salts. The dissociation free-energies were consistent with a trend of increasing PAG clustering in less polar media at high PAG concentrations (7 to 36 wt %), and of increasing dipole moments for PAGs in less polar implicit solvation media as estimated from density functional theory. Both simulations and infrared spectroscopy of the copolymer provide evidence of hydrogen bonded carbonyl groups whose prevalence diminishes upon the addition of ionic PAGs. For TPS-TFMeS loadings between 25 and 29 wt %, our analysis revealed short string-like salt clusters in PHS homopolymer, large globular clusters in PtBMA homopolymer, and a heterogeneous distribution of string-like and globular PAG aggregates in the alternating copolymer. For all PAG loadings, TPS-ATFEtS and TPS-NFBuS PAGs exhibited the highest extent of aggregation. Altogether, the advocated methodology is a potentially effective tool to rank and identify polymer and PAG chemistries with improved compatibility and mixing homogeneity.