Investigation of onium salt type photoacid generators in positive DUV resist systems

The effect of onium salt structure in partially blocked poly(p- vinyl)phenol/photoacid generator (PAG) resist matrices on resolution, inhibition, and postexposure delay stability is reported. The PAG structure, M⁺X^-, was varied such that M⁺ was either triphenyl sulfonium (TPS⁺) or diphenyl iodonium (DPI⁺), and X^- represented trifluoromethanesulfonate (TFA^-) toluenesulfonate (TSA^-), camphorsulfonate (CSA^-), and hexadecylsulfonate (HDSA^-). The relative photospeed of these resists corresponded to the relative pK_a of the acid generated from the anoin, TFA>TSA>HDSA>CSA. The resolution of the resists using TPS⁺ were better than that of the DPI⁺-based resists. The best PAG from a resolution standpoint was triphenylsulfonium tosylate. The TPS⁺TSA_- also showed the lowest measured diffusion coefficient, D equals 1.1 X 10$=-4) micrometers ²/s. However, variation in PAG structure did not show any advantage in postexposure delay stability, with all the resists studied showing either 't-top' or scumming as failure modes. The larger acids, CSA and HDSA, showed a greater tendency to scum over a one hour delay period, which may indicate less acid evaporation, and more lateral diffusion to unexposed areas. The smaller acids, TSA and TFA, both showed strong 't-top' formation which is due to their increased acid volatility.