Polymer, chemically amplified resist composition, and pattern forming process

A polymer comprising

• repeat units having the formula (a), and
• repeat units capable of generating an acid upon light exposure, derived from an onium salt compound containing a fluorosulfonic acid anion having a polymerizable group and an iodized aromatic ring structure,
• wherein m1 is 0 or 1, m2 is an integer of 0 to 3 when m1=0 and an integer of 0 to 5 when m1=1,
• RA is hydrogen, fluorine, methyl or trifluoromethyl,
• XA is a single bond, *—C(═O)—O— or *—C(═O)—NH—, * designates a point of attachment to the carbon atom in the backbone,
• R1 and R2 are each independently hydrogen or a C1-C20 hydrocarbyl group which may contain a heteroatom, R1 and R2 may bond together to form a ring with the carbon atom to which they are attached,
• R3 is halogen, hydroxy, cyano, nitro, a C1-C20 hydrocarbyl group which may contain a heteroatom, C1-C20 hydrocarbyloxy group which may contain a heteroatom, C2-C20 hydrocarbyloxycarbonyl group which may contain a heteroatom, C1-C20 hydrocarbylthio group which may contain a heteroatom, or —N(R3A)(R3B), R3A and R3B are each independently hydrogen or a C1-C6 hydrocarbyl group, a plurality of R3 may bond together to form a ring with carbon atoms in the aromatic ring to which they are attached when m2 is 2 or more,
• LA is a single bond, a C1-C4 aliphatic hydrocarbylene group, carbonyl, sulfonyl, or a combination thereof,
• LB and LC are each independently oxygen or sulfur, LA and LC must bond to vicinal carbon atoms on the aromatic ring.