Positive resist composition and pattern forming process

A positive resist composition comprising a base polymer comprising repeat units (a) having the structure of a sulfonium salt of a fluorinated phenol compound;

• wherein the repeat units (a) have the formula (a):
• wherein RA is hydrogen or methyl,
• X1 is, an ester bond, ether bond, phenylene group or naphthylene group,
• X2 is a single bond, a phenylene group, or a C1-C12 saturated hydrocarbylene group which may contain an ether bond, ester bond, amide bond, lactone ring or sultone ring,
• X3 is a single bond, ester bond or ether bond,
• Rf is a fluorine atom, trifluoromethyl, trifluoromethoxy, or trifluoromethylthio group,
• R1 is a C1-C4 alkyl group,
• R2 to R4 are each independently a halogen or a C1-C20 hydrocarbyl group which may contain a heteroatom, R2 and R3 may bond together to form a ring with the sulfur atom to which they are attached,
• m is an integer of 1 to 4, n is an integer of 0 to 3, and m+n is from 1 to 4.