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

An onium salt containing an anion having the formula (A1) and a cation selected from the formulae (A1-a) to (A1-c):

• wherein k1 is an integer of 0 to 2, k2 is an integer meeting 0≤k2≤4+4k1, m1 is an integer of 1 to 4, m2 is each independently an integer of 0 to 4, n is each independently an integer of 0 to 10, p is an integer of 0 to 3, q is an integer of 0 to 2, r is an integer of 0 to 4,
• Ra1 and Ra2 are each independently hydrogen or a C1-C10 hydrocarbyl group in which some or all hydrogen atoms may be substituted by halogen and a constituent —CH2— may be replaced by —O— or —C(═O)—, both Ra1 and Ra2 are not hydrogen at the same time, Ra1 and Ra2 may bond together to form an aliphatic ring with the carbon atom to which they are attached,
• Ra3 is each independently a C1-C20 hydrocarbyl group which may contain a heteroatom, when n is 2 or more, Ra3 may be the same or different and two or more Ra3 may bond together to form a ring with the atom on W2 to which they are attached,
• Ra4 is a C1-C10 hydrocarbyl group which may contain a heteroatom,
• W1 is a C3-C20 (n+2)-valent group having a mono- or polycyclic structure, W2 is each independently a C3-C20 (m2+n+1)-valent group having a mono- or polycyclic structure, in which a constituent —CH2— may be replaced by —O—, —S— or —C(═O)—,
• La1 and La2 are each independently a single bond, ether bond, ester bond, sulfonate bond, carbonate bond or carbamate bond,
• XL is a single bond or a C1-C40 hydrocarbylene group which may contain a heteroatom,
• R1, R2 and R3 are each independently a C1-C20 hydrocarbyl group which may contain a heteroatom, any two of two R1 and W2 may bond together to form a ring with the sulfur atom in the formula, any two of three R3 and W2 may bond together to form a ring with the nitrogen atom in the formula.