Onium salt and photoacid generator

A chemically amplifiable resist composition comprising:

• an onium salt represented by formula (1),
• [(R2)n+1-E]+[(R1)3(F)3P]− (1),
• wherein:
• R1 is a halogen-substituted C1-C18 alkyl group or a C6-C18 (excluding the carbon number of the following substituents) aryl group, with at least one hydrogen in the aryl group being optionally replaced by a C1-C18 alkyl group, a halogen atom, a halogen-substituted C1-C8 alkyl group, a C2-C18 alkenyl group, or a C2-C18 alkynyl group;
• E is S or I with a valence of n;
• n is an integer of 1 to 3;
• and
• R2 is an organic group attached to the E, with the number of R2s being n+1, these R2s being the same as or different from each other, and two or more R2s being optionally attached to each other directly or with —O—, —S—, —SO—, —SO2—, —NH—, —CO—, —COO—, —CONH—, an alkylene group, or a phenylene group in between to form a ring structure containing the E, and
• wherein the onium salt represented by formula (1) comprises an anion structure with a proportion of a facial isomer in a total of the facial isomer and a meridional isomer of 15.0% by weight or lower.