Photochemical fate of sulfonium photoacid generator cations under photolithography relevant UV irradiation

Abstract

Onium salts, widely used as photoacid generators (PAGs) in the semiconductor industry, are receiving increased regulatory scrutiny due to their potential hazardous properties. Evaluating the environmental impacts of these chemicals requires knowledge of the fate of the onium species during photolithography steps, which remains unclear for many PAGs. The current study investigated the fate of four representative sulfonium PAGs using an irradiation setup simulating industrial photolithography. The photolytic fate for three of the four selected onium cations is reported for the first time. All onium compounds reacted rapidly under 254 nm irradiation, and their half-lives ranged from 51 to 214 s with high quantum yields (0.23−0.85). Multiple highly aromatic and hydrophobic photoproducts were characterized using HPLC-DAD and GC-MS; eight PAG photoproducts were identified for the first time. Authentic commercial standards of some of the photoproducts were available, enabling quantitative estimates of recoveries. The molar recovery of aromatic rings based on these quantified photoproducts was in the range of 4.9–71.4 %. A homolytic reaction mechanisms is proposed and discussed. The results provide insight on the potential environmental impact pertinent to the use of PAGs, e.g., prediction of residual onium levels and the identity of photoproducts in effluents from semiconductor fabrication facilities.