Monomer, polymer, negative resist composition, photomask blank, and resist pattern forming process

A polymer consisting of

• recurring units having the formula (a),
• recurring units having the formula (b1′),
• recurring units of at least one type selected from recurring units having the formula (c), recurring units having the formula (d), and recurring units having the formula (e), and
• optionally recurring units of at least one type selected from recurring units having the formula (f1), recurring units having the formula (f2), and recurring units having the formula (f3):
• wherein RA is hydrogen or methyl, R1 and R2 are each independently a C1-C15 primary or secondary alkyl group in which some hydrogen may be substituted by a hydroxyl or C1-C6 alkoxy moiety, or R1 and R2 may bond together to form a ring with the carbon atom to which they are attached, k1 is 0 or 1, and k2 is an integer of 2 to 4,
• wherein RB is hydrogen, fluorine, methyl or trifluoromethyl,
• A1 is a single bond or a C1-C10 alkanediyl group which may contain an ether bond,
• R13 is a halogen atom, optionally halogenated C2-C8 acyloxy group, optionally halogenated C1-C6 alkyl group, or optionally halogenated C1-C6 alkoxy group,
• t1 is 0 or 1, x1 is an integer of 0 to 2, c1 is an integer satisfying 0≤c1≤5+2(x1)−c2, c2 is an integer of 1 to 3,
• wherein RB is hydrogen, fluorine, methyl or trifluoromethyl,
• R15 and R16 are each independently a halogen atom, optionally halogenated C2-C8 acyloxy group, optionally halogenated C1-C6 alkyl group, or optionally halogenated C1-C6 alkoxy group,
• R17 is an acetyl, acetoxy, C1-C20 alkyl, C1-C20 primary alkoxy, C2-C20 secondary alkoxy, C2-C20 acyloxy, C2-C20 alkoxyalkyl, C2-C20 alkylthioalkyl, halogen, nitro or cyano group,
• A4 is a single bond or a C1-C10 alkanediyl group which may contain an ether bond,
• e is an integer of 0 to 5, f is an integer of 0 to 3, g is an integer of 0 to 5, t3 is 0 or 1, and x3 is an integer of 0 to 2,
• wherein RA is hydrogen or methyl,
• Z1 is a single bond, phenylene group, —O—Z11—, —C(═O)—O—Z11—, or —C(═O)—NH—Z″—, Z11 is a C1-C6 alkanediyl, C2-C6 alkenediyl or phenylene group which may contain a carbonyl, ester bond, ether bond or hydroxyl moiety,
• Z2 is a single bond or —Z21—C(═O)—O—, Z21 is a C1-C20 divalent hydrocarbon group which may contain a heteroatom,
• Z3 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—Z31—, —C(═O)—O—Z31— or —C(═O)—NH—Z31—, Z31 is a C1-C6 alkanediyl, C2-C6 alkenediyl or phenylene group which may contain a carbonyl, ester bond, ether bond or hydroxyl moiety,
• R21 to R28 are each independently a C1-C20 monovalent hydrocarbon group which may contain a heteroatom, R21 and R22 may bond together to form a ring with the sulfur atom to which they are attached, any two of R23, R24 and R25 may bond together to form a ring with the sulfur atom to which they are attached, any two of R26, R27 and R28 may bond together to form a ring with the sulfur atom to which they are attached, and
• M− is a non-nucleophilic counter ion.