Onium salt and photoacid generator

1 . A chemically amplifiable resist composition comprising:

an onium salt represented by formula (1),

[(R 2 ) n+1 -E] + [(R 1 ) 3 (F) 3 P] −   (1),

wherein:

R 1 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

R 2 is an organic group attached to the E, with the number of R 2 s being n+1, these R 2 s being the same as or different from each other, and two or more R 2 s being optionally attached to each other directly or with —O—, —S—, —SO—, —SO 2 —, —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.

2 . The chemically amplifiable resist composition according to claim 1 ,

wherein the proportion of the facial isomer is 0.1 to 15.0% by weight.

3 . The chemically amplifiable resist composition according to claim 1 ,

wherein R 1 in the formula (1) is a perfluoroalkyl group or a fluorine-substituted phenyl group.