Onium salt compound, resist composition, and pattern forming process

A resist composition comprising

• (A) an onium salt compound,
• (B) a polymer,
• (C) a photoacid generator, and
• (D) an organic solvent,
• wherein the onium salt compound has the general formula (1):
• wherein R1 to R4 are each independently hydrogen, -L-CO2−, or a straight C1-C20 or branched or cyclic C3-C20 monovalent hydrocarbon group which may be substituted with or separated by a heteroatom, or a pair of R1 and R2, R2 and R3, or R3 and R4 may bond together to form a ring with the carbon atoms to which they are attached, L is a single bond or a straight C1-C20 or branched or cyclic C3-C20 divalent hydrocarbon group which may be substituted with or separated by a heteroatom,
• R5 is hydrogen or a straight C1-C20 or branched or cyclic C3-C20 monovalent hydrocarbon group which may be substituted with or separated by a heteroatom,
• Z+ is a sulfonium cation of the general formula (a) or iodonium cation of the general formula (b):
• wherein R100, R200 and R300 are each independently a straight C1-C20 or branched or cyclic C3-C20 monovalent hydrocarbon group which may be substituted with or separated by a heteroatom, or any two or more of R100, R200 and R300 may bond together to form a ring with the sulfur atom, R400 and R500 are each independently a straight C1-C20 or branched or cyclic C3-C20 monovalent hydrocarbon group which may be substituted with or separated by a heteroatom,
• the partial moiety of the formula:
• designates a cyclic structure of 2 to 5 carbon atoms separated by nitrogen, wherein a hydrogen atom attached to any carbon atom of the cyclic structure may be substituted by a straight C1-C20 or branched or cyclic C3-C20 monovalent hydrocarbon group or -L-CO2−, and any carbon atom of the cyclic structure may be replaced by a sulfur, oxygen or nitrogen atom,
• with the proviso that at least one substituent -L-CO2− is included in formula (1), and
• wherein said polymer comprises recurring units having the general formulae (2) and (3):
• wherein R1a is hydrogen, fluorine, methyl or trifluoromethyl, Za is a single bond, phenylene, naphthylene or (backbone)-C(═O)—O—Z′—, Z′ is a straight C1-C10 or branched or cyclic C3-C10 alkylene group which may contain a hydroxyl radical, ether bond, ester bond or lactone ring, or phenylene or naphthylene group, XA is an acid labile group, and YL is hydrogen or a polar group having at least one structure selected from the group consisting of hydroxyl, cyano, carbonyl, carboxyl, ether bond, ester bond, sulfonate bond, carbonate bond, lactone ring, sultone ring, and carboxylic anhydride.