Novel onium salt compound, resist composition, and pattern forming process

An onium salt compound having 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 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 each independently as straight C1C20 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-CO231, 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).