Positive resist composition and patterning process

A positive resist composition comprising a base polymer comprising recurring units (a) having the structure of an ammonium salt of fluorosulfonic acid having an iodine or bromine-substituted aromatic ring, and recurring units of at least one type selected from recurring units (b1) having a carboxyl group substituted with an acid labile group and recurring units (b2) having a phenolic hydroxyl group substituted with an acid labile group, wherein the recurring units (a) have the formula (a):

• wherein RA is hydrogen or methyl,
• X1A is a single bond, ester bond or amide bond,
• X1B is a single bond or a C1-C20 di- or trivalent hydrocarbon group which may contain an ether bond, carbonyl moiety, ester bond, amide bond, sultone moiety, lactam moiety, carbonate moiety, halogen, hydroxyl moiety or carboxyl moiety,
• R1, R2 and R3 are each independently hydrogen, a C1-C12 straight or branched alkyl group, C2-C12 straight or branched alkenyl group, C6-C12 aryl group, or C7-C12 aralkyl group, R1 and R2, or R1 and X1B may bond together to form a ring with the nitrogen atom to which they are attached, the ring may contain oxygen, sulfur, nitrogen or a double bond,
• R4 is a hydroxyl group, carboxyl group, fluorine, chlorine, bromine, amino group, or a C1-C20 alkyl group, C1-C20 alkoxy group, C2-C20 alkoxycarbonyl group, C2-C20 acyloxy group, or C1-C20 alkylsulfonyloxy group, which may contain fluorine, chlorine, bromine, hydroxyl, amino, or C1-C10 alkoxy moiety, or —NR4A—C(═O)—R4B, or —NR4A—C(═O)—O—R4B, R4A is hydrogen or a C1-C6 alkyl group which may contain halogen, hydroxyl, C1-C6 alkoxy moiety, C2-C6 acyl moiety or C2-C6 acyloxy moiety, R4B is a C1-C16alkyl group, C2-C16 alkenyl group or C6-C12 aryl group, which may contain halogen, hydroxyl, C1-C6 alkoxy moiety, C2-C6 acyl moiety or C2-C6 acyloxy moiety, R4 may be the same or different when n and/or q is 2 or 3,
• Rf1 to Rf4 are each independently hydrogen, fluorine or trifluoromethyl, at least one of Rf1 to Rf4 being fluorine or trifluoromethyl, Rf1 and Rf2, taken together, may form a carbonyl group,
• XBI is iodine or bromine, XBI may be the same or different when m and/or q is 2 or more,
• L1 is a single bond, ether bond, ester bond, or a C1-C6 alkanediyl group which may contain an ether bond or ester bond,
• L2 is a single bond or a C1-C20 divalent linking group when q=1, or a C1-C20 (q+1)-valent linking linking group when q=2 or 3, the linking group may contain oxygen, sulfur or nitrogen,
• m is an integer of 1 to 5, n is an integer of 0 to 3, 1≤m+n≤5, p is 1 or 2, and q is an integer of 1 to 3.