Ab Initio Study of the Photochemical Decomposition Mechanism of Sulfonium Compounds, a Photoacid Generator

摘要

The photochemical decomposition mechanism of sulfonium compounds, an important photoacid generator (PAG) in lithography, was extensively studied by ab initio quantum chemical research. The homolytic bond cleavage takes place in the lowest triplet state T_1 as well as the lowest excited singlet state S_1 at the initial stage of the decomposition reaction, but the final products from S_1 are those generated from the heterolytic bond cleavage. The homolytic reaction is maintained from the initial stage to the final products only in the lowest triplet state T_1. In the ground state S_0, the heterolytic bond cleavage occurs at the initial stage, but the final products are those from the homolytic bond cleavage. SDTQ-CI calculations revealed that this mysterious phenomenon is explained as the results generated from the mixing of the lowest excited singlet state S_1 and the ground state S_0. It was concluded that homolytic bond cleavage is the predominant mechanism in the resist films of lithography. The theoretically obtained results were compared with experiment.