Synthesis of some onium salts and their comparison as cationic photoinitiators in an epoxy resist

Several onium salts have been synthesized and compared as cationic photoinitiators in an epoxy resist. 1-(4-Hydroxy-3-methylphenyl)tetrahydrothiophenium hexafluoroantimonate exposed at 365 nm gave the best results, due to its superior absorption at this wavelength. A series of bis-(triphenyl phosphoranylidene) ammonium salts performed poorly with 254 nm, 365 nm and E-beam irradiation even with the addition of photosensitizer. Triphenyl sulfonium hexafluoroantimonate and triphenyl sulfonium triflate formulated the most sensitive epoxy resist materials to both 254 nm and E-beam radiation sources (0.32 mJ cm^-2 and 0.3 μC cm^-2, 59.2 mJ cm^-2 and 8.0 μC cm^-2 at 20 kV, respectively).