Atomistic modeling approach for predicting association of ionic salts in extreme ultraviolet polymeric photoresists

Extreme Ultraviolet (EUV) lithography has proven to be a promising tool to achieve sub-10nm device features. EUV photoresists are a complex blend of components involving ionic photoacid generator (PAG) and quencher salts in an acid-sensitive polymer. We employed atomistic molecular dynamics simulations to probe the effect of ionic and steric interactions on the dispersibility of PAGs in the polymer medium before EUV exposure, as poor dispersibility is a potential source of post-exposure feature roughness. We chose as model PAGs Triphenyl sulfonium (TPS) cation with triflate and adamantlytetrafluoroethyl sulfonate anions, and as photoresists poly(tert-butyl methacrylate) (PtBMA) and poly(4-hydroxy styrene) (PHS) homopolymers and copolymers. We calculated the potential of mean force (PMF) for the dissociation of PAGs at infinite dilution in different polymers. Our results indicate that PHS segments provide a better solvation environment for the PAGs than the PtBMA segments and the triflate PAG solvates better than the bulky TPS-adamantlytetrafluoroethyl sulfonate salt. Local nanostructural signatures (like the extent of hydrogen bonding) and high PAG concentration (up to 28% wt.) simulations confirm that a higher tendency of PAG association correlates with a higher PMF dissociation barrier at infinite dilution, underlining the latter's predictive potential.