Complex triarylsulfonium salts as photoacid generators for deep UV microlithography: Synthesis, identification and lithographic characterization of key individual components

Triarylsulfonium salts are among the most commonly used photoinitiators in the fields of radiation sensitive coatings and microlithography. In this paper, assorted sulfonium salts are evaluated as photoacid generators for DUV microlithography. The preparation of assorted triarylsulfonium salts from commercially available triarylsulfonium chloride is described. Analysis of this class of photoacid generators revealed that it comprises a mixture of triarylsulfonium cations. These materials are essentially complex mixtures derived from the various sulfonium cationic species which are present in the starting triarylsulfonium chloride. In order to better understand the unique properties of these photoacid generators, we focused on identifying the major triarylsulfonium cations present in the mixture. This paper describes the synthesis, identification and lithographic characterization of each of the components of this class of photoacid generators. The identity of each component was verified spectroscopically (H-1 and C-13 NMR, IR and UV) and the compounds were also characterized by thermogravimetric analysis. The acid generating efficiency of each component was determined using tetrabromophenol blue as a spectrophotometric indicator dye. Lastly, full lithographic characterization of each component was performed and the results compared and contrasted with the triarylsulfonium mixtures.