Chemically amplified negative resist composition and resist pattern forming process

A chemically amplified negative resist composition comprising (A) a sulfurane or selenurane compound having the following formula (A1) and (B) a base polymer containing a polymer comprising repeat units having the following formula (B1),

• wherein R1 to R4 are each independently a C1-C20 hydrocarbyl group which may contain a heteroatom, R1 and R2 may bond together to form a ring with L1 and M to which they are attached, R1 and R2, and R3 and R4 may bond together to form spiro rings with M as the spiro atom,
• L1 and L2 are each independently —O— or —N(R)—, R is hydrogen or a C1-C20 hydrocarbyl group which may contain a heteroatom, and
• M is sulfur or selenium,
• wherein RA is hydrogen, fluorine, methyl or trifluoromethyl,
• R11 is halogen, an optionally halogenated C2-C8 saturated hydrocarbylcarbonyloxy group, optionally halogenated C1-C6 saturated hydrocarbyl group, or optionally halogenated C1-C6 saturated hydrocarbyloxy group,
• A1 is a single bond or a C1-C10 saturated hydrocarbylene group in which any constituent —CH2— may be replaced by —O—,
• s is 0 or 1, w is an integer of 0 to 2, a is an integer satisfying 0≤a≤5+2w−b, and b is an integer of 1 to 3.