Onium salt, chemically amplified resist composition, and pattern forming process

An onium salt comprising a cation having the following formula (1A) and an anion having the following formula (1B):

• wherein p is 1, 2, or 3, n1 is 0 or 1, n2 is 1 or 2, n3 is 0, 1, 2, or 3, provided that, when n1 is 0, 1≤n2+n3≤5, and when n1 is 1, 1≤n2+n3≤7,
• R1 is a halogen atom, a nitro group, a hydroxy group, a carboxy group, a C1-C20 hydrocarbyl group which may contain a heteroatom, a C1-C20 hydrocarbyloxy group which may contain a heteroatom, a C1-C20 hydrocarbylthio group which may contain a heteroatom, or a C2-C20 hydrocarbyloxycarbonyl group which may contain a heteroatom, when n3 is 2 or 3, respective R1 may be the same as or different from each other, and two R1's may bond together to form a ring with the carbon atom to which they are attached,
• R2 is a halogen atom, or a C1-C30 hydrocarbyl group which may contain a heteroatom, when p is 1, two R2's may be the same as or different from each other, two of three substituents bonded to S+ may bond together to form a ring with the sulfur atom to which they are bonded, and
• wherein n11 is 0 or 1, n12 is 1, 2, 3, or 4, n13 is 0, 1, or 2, n14 is 0, 1, 2, 3, or 4, provided that, when n11 is 0, 2≤n12+n13+n14≤5, and when n1 is 1, 2≤n12+n13+n14≤7,
• LA is a single bond, an oxygen atom, a sulfur atom, an ester bond, or a carbonate bond,
• R3 is a hydrogen atom, or a C1-C20 hydrocarbyl group which may contain a heteroatom when LA is a single bond, and is a hydrogen atom, a C1-C20 hydrocarbyl group which may contain a heteroatom (provided that an acid labile group is excluded), or an acid labile group when LA is an oxygen atom, a sulfur atom, an ester bond, or a carbonate bond, and
• R4 is a halogen atom other than an iodine atom, a nitro group, a cyano group, a C1-C20 hydrocarbyl group which may contain a heteroatom, a C1-C20 hydrocarbyloxy group which may contain a heteroatom, or a C1-C20 hydrocarbylthio group which may contain a heteroatom.