Sulfonium Salts of Alicyclic Group Functionalized Semifluorinated Alkyl Ether Sulfonates As Photoacid Generators

We describe new sulfonium salts incorporating semifluorinated sulfonate anions and their successful application as photoacid generators (PAGs) intended to solve the environmental problems caused by perfluorooctyl sulfonate (PFOS). By utilizing simple and unique chemistries based on 5-iodooctafluoro-3-oxapentanesulfonyl fluoride, a series of alicyclic-group functionalized octafluoro-3-oxapentane-sulfonate anions were prepared in high purity and high yield. Triphenylsulfonium (TPS) salts of norbornyl and γ-butyrolactone groups functionalized octafluoro-3-oxapentanesulfonates were extensively studied. They have excellent thermal stability and good solubility in various polar solvents. Their optical and thermal properties were thoroughly investigated. Angle resolved X-ray photoelectron spectroscopy (XPS) analysis confirmed that the new TPS salts are uniformly distributed in polymer films, whereas TPS PFOS is heavily segregated to a polymer film surface. Lithographic performance of the new salts together with TPS PFOS and triphenylsulfonium perfluorobutyl sulfonate (TPS PFBS) in model positive tone resists was evaluated and compared at 193 nm (both dry and immersion) and extreme ultraviolet (EUV) wavelengths. Resist compositions containing the new salts are capable of resolving sub-100 nm dense lines and spaces at both tested wavelengths. Especially, a pattern of 42 nm dense lines was successfully demonstrated in 193 nm immersion exposures. A specific TPS salt of monosaccharide-functionalized octafluoro-3-oxapentanesulfonate, a “sweet” PAG, was also synthesized and studied. This PAG is capable of resolving 90 nm lines in 193 nm lithography. These new PAGs show comparable performance with TPS PFBS and better performance than TPS PFOS, which makes them very promising environmentally friendly candidates for high resolution lithography applications.