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

An onium salt monomer having the formula (a):

• wherein n1 is 0 or 1, n2 is 0 or 1, n3 is an integer of 0 to 4, n4 is an integer of 0 to 4, n5 is an integer of 0 to 4, n4+n5 is from 0 to 4 when n2=0, n4+n5 is from 0 to 6 when n2=1, excluding that all n1 to n5 are 0,
• RA is hydrogen, fluorine, methyl or trifluoromethyl,
• R1 is halogen or a C1-C20 hydrocarbyl group which may contain a heteroatom, a plurality of R1 may bond together to form a ring with the carbon atoms on aromatic ring to which they are attached when n3 is 2, 3 or 4,
• R2 is a C1-C20 hydrocarbyl group which may contain a heteroatom, a plurality of R2 may bond together to form a ring with the carbon atoms to which they are attached when n4 is 2, 3 or 4,
• RF is fluorine, C1-C6 fluorinated alkyl group, C1-C6 fluorinated alkoxy group, or C1-C6 fluorinated alkylthio group,
• LA and LB are each independently a single bond, ether bond, ester bond, sulfonate ester bond, sulfonic amide bond, carbonate bond or carbamate bond,
• XL is a single bond or C1-C40 hydrocarbylene group which may contain a heteroatom, and
• Z+ is an onium cation.