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

An onium salt monomer having the formula (A):

• wherein n1 is 0, 1, 2, 3 or 4, n2 is 0 or 1, n3 is 0, 1, 2, 3 or 4, n4 is 0, 1, 2, 3 or 4, meeting 0≤n3+n4≤4 when n2=0 and 0≤n3+n4≤6 when n2=1,
• RA is each independently hydrogen, fluorine, methyl or trifluoromethyl,
• R1 is halogen, hydroxy, nitro, cyano, 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 n1 is 2, 3 or 4, a plurality of R1 may be identical or different and may bond together to form a ring with the carbon atom to which they are attached,
• R2 is halogen exclusive of fluorine, nitro, hydroxy, a C1-C20 hydrocarbyl group which may contain a heteroatom exclusive of fluorine, a C1-C20 hydrocarbyloxy group which may contain a heteroatom exclusive of fluorine, or a C1-C20 hydrocarbylthio group which may contain a heteroatom exclusive of fluorine;
• when n4 is 2, 3 or 4, a plurality of R2 may be identical or different and may bond together to form a ring with the carbon atoms to which they are attached,
• RF is fluorine, a C1-C6 fluorinated saturated hydrocarbyl group, a C1-C6 fluorinated saturated hydrocarbyloxy group, or a C1-C6 fluorinated saturated hydrocarbylthio group, when n3 is 2, 3 or 4, a plurality of RF may be identical or different,
• LA and LB are each independently a single bond, ether bond, ester bond, amide bond, sulfonate ester bond, sulfonamide bond, carbonate bond or carbamate bond,
• XL1 is a single bond or a C1-C40 hydrocarbylene group which may contain a heteroatom, and
• Z+ is an onium cation.