Sulfonium salt monomer, polymer, chemically amplified resist composition, and pattern forming method

A sulfonium salt monomer having the following formula (A):

• wherein n1 is 0 or 1, n2 is 0, 1, or 2, n3 is 0, 1, or 2, n4 is 0, 1, or 2, provided that 0≤n2+n3+n4≤5 when n1 is 0, and 0≤n2+n3+n4≤7 when n1 is 1, n5 is 0 or 1, n6 is 0, 1, or 2, n7 is 0, 1, or 2, n8 is 0, 1, or 2, provided that 0≤n6+n7+n8≤5 when n5 is 0, and 0≤n6+n7+n8≤7 when n5 is 1, n9 is 0 or 1, n10 is 0, 1, or 2, n11 is 0, 1, or 2, n12 is 0, 1, or 2, provided that 0≤n10+n11+n12≤5 when n9 is 0, and 0≤n10+n11+n12≤7 when n9 is 1, 1≤n2+n6+n10≤6, 1≤n3+n7+n11≤6,
• R1, R2, and R3 are each independently a halogen atom, a nitro group, a cyano 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, or a C1-C20 hydrocarbylthio group which may contain a heteroatom, when n4 is 2, R1s may be the same as or different from each other, and two R1s may be bonded to each other to form a ring together with carbon atoms to which they are bonded, when n8 is 2, R2s may be the same as or different from each other, and two R2s may be bonded to each other to form a ring together with carbon atoms to which they are bonded, when n12 is 2, R3s may be the same as or different from each other, and two R3s may be bonded to each other to form a ring together with carbon atoms to which they are bonded,
• RAL1, RAL2, and RAL3 are each independently an acid-unstable group, when n3 is 2, RAL1s may be the same as or different from each other, when n7 is 2, RAL2s may be the same as or different from each other, when n11 is 2, RAL3s may be the same as or different from each other,
• at least one —SF5 group is bonded to a carbon atom adjacent to a carbon atom to which —O—RAL1, —O—RAL2, or —O—RAL3 is bonded,
• two of the three aromatic rings bonded to S+ may be bonded to each other to form a ring together with the sulfur atom to which they are bonded, and
• Z− is an aromatic sulfonic acid anion having a polymerizable group.