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

An onium salt monomer having the following formula (a):

• wherein m1 is 0 or 1, m2 is 0, 1, 2, 3 or 4 in case of m1=0, and 0, 1, 2, 3, 4, 5 or 6 in case of m1=1, m3 is 0, 1, 2, 3 or 4 in case of m1=0, and 0, 1, 2, 3, 4, 5 or 6 in case of m1=1, m2+m3 is from 0 to 4 when m1=0 and m2+m3 is from 0 to 6 when m1=1, m4 is 0 or 1, m5 is 1, 2, 3 or 4 when m4=0, and 1, 2, 3, 4, 5 or 6 when m4=1,
• RA is hydrogen atom or methyl,
• R1 is halogen exclusive of iodine, nitro group, hydroxy group, cyano 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, a plurality of R1 may be identical or different and a plurality of R1 may bond together to form a ring with the carbon atoms to which they are attached, when m3 is 2 or more,
• R2 is halogen exclusive of iodine, or an electron withdrawing group, a plurality of R2 may be identical or different and a plurality of R2 may bond together to form a ring with the carbon atom to which they are attached, when m5 is 2 or more, except that all R2 is fluorine,
• LA, LB, LC and LD are each independently a single bond, ester bond, ether bond, amide bond, sulfonic ester bond, sulfonic amide bond, carbonate bond or carbamate bond,
• X1 and X2 are each independently a single bond, or a C1-C40 hydrocarbylene group which may contain a heteroatom,
• Z+ is a sulfonium cation having the following formula (Z-1) or an iodonium cation having the following formula (Z-2);
• wherein Rct1 to Rct5 are each independently halogen, or a C1-C30 hydrocarbyl group which may contain a heteroatom, and Rct1 and Rct2 may bond together to form a ring with the sulfur atom to which they are attached.