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

Abstract

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.

The importance of less damaged and surface sensitive metrology to identify true EUV process monitoring

“…The PAG activation by EUV light uniquely proceeds with electron trapping event, where the liberated electrons (that have lost majority of their energy during subsequent scattering events, generally with energy under 15eV) captured by PAG cation via a reduction event (figure 1B). Symmetry of equally distributed electric charge is broken just after electron trapping [11][12][13] as electron charge is localized to a cation. Dissociation is likely to happen as this charge redistribution weakens ionic bond of PAG.…”

“…Dissociation is likely to happen as this charge redistribution weakens ionic bond of PAG. As a result, neutralized cation release anion, which in combination with the radical polymer group generated in the initial ionization of resist matrix proceeds to cleave the acid labile functional groups of the polymer [1,5,6,[11][12][13][14][15][16][17]. Comparing exposure to EUV photons and 80eV electrons, similar chemical reactions expected to occur, given the equivalence of outgassing species [18][19].…”

Acid generation efficiency prediction by bond cleavage calculation of EUV photoacid generators

“…Park et al designed to predict EUV dose of new tri-substituted symmetric sulfonium cations. [4] They showed the pKa and LUMO values were well-correlated to Hammett para-position parameters. They constructed multiple-linear prediction models with 9 PAG molecules to predict new PAG properties.…”

“…Photoacid generation of the sulfonium-type cations can come from two different pathways; one is internal excitation and the another is electron trapping. [4] We focused on one main mechanistic pathway, which was electron trapping. In the previous study by H. Conner et al [3] , the electron trapping mechanism of PAGs was discussed with the structure-fixed electron affinity values, but in this study, we aim to extend it to include the actual mechanism of C-S bond dissociation.…”

“…Figure 5. (A) Primary dissociation of Alkyl R'' in PAG cation 11 (B) Proposed mechanism after dissociation of aryl or alkyl radical species from the sulfonium cations[4] …”