EUV-induced activation mechanism of photoacid generators: key factors affecting EUV sensitivity

Theoretical lithography performance prediction of photoresist material has important role to design better material but the exact prediction was still difficult because there are too many conditions to be considered together. We investigated the EUV-induced photochemical reactions of conventional triphenylsulfonium (Ph3S+; TPS) PAG-cation in both “electron-trapping” and “internal excitation” mechanisms using atomic-scale materials modeling. By obtaining full energy profiles of protonation process of TPS molecule, we could find that the acid generation yield strongly depends on two main factors: the LUMO of PAG-cation in which the lower LUMO of PAG-cation, the reduction step of PAG-cation is easier and the proton (H+) dissociation ability (pKa) at the ortho-positions of thiol ether fragment cation(Ph2S+), in which lower pKa will give high acid generation. By matching computational analysis with experimental results, we developed a two-parameter model to predict the EUV exposure Dose from the target PAG–cation’s LUMO and pKa of thiol ether-derivatives. We applied our new model to other three sets of TPS samples and they also shows good correlation with experimental data. Finally, we proposed a strategy to design new PAG molecules for sensitivity improvement by functionalization of TSP-cation with electron donating group. Our new strategy can be a powerful tool to design novel PAG cation for EUV photoresist for improving Resolution-LER-Sensitivity trade-off.