Positive resist composition and patterning process

1. A positive resist composition comprising a base polymer comprising recurring units (a) having the structure of an ammonium salt of a carboxylic acid having an iodine or bromine-substituted aromatic ring,

wherein the recurring units (a) have the formula (a):

wherein R A is hydrogen or methyl,

X 1A is a single bond, ester bond or amide bond,

X 1B is a single bond or a C 1 -C 20 di- to tetravalent hydrocarbon group which may contain an ether bond, carbonyl moiety, ester bond, amide bond, sultone ring, lactam ring, carbonate bond, halogen, hydroxyl moiety or carboxyl moiety,

R 1 , R 2 and R 3 are each independently hydrogen, a C 1 -C 10 straight or branched alkyl group, C 2 -C 10 straight or branched alkenyl group, or C 6 -C 10 aryl group, R 1 and R 2 , or R 1 and X 1B 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,

R 4 is hydrogen, hydroxyl group, an optionally halo-substituted C 1 -C 6 alkyl group, optionally halo-substituted C 1 -C 6 alkoxy group, optionally halo-substituted C 2 -C 6 acyloxy group, optionally halo-substituted C 1 -C 4 alkylsulfonyloxy group, fluorine, chlorine, bromine, amino, nitro, cyano, —NR 4A —C(═O)—R 4B or —NR 4A —C(═O)—O—R 4B , R 4A is hydrogen or a C 1 -C 6 alkyl group, R 4B is a C 1 -C 6 alkyl group or C 2 -C 8 alkenyl group,

X bi is bromine or iodine,

L 1 is a single bond or a C 1 -C 20 divalent linking group which may contain an ether bond, carbonyl moiety, ester bond, amide bond, sultone ring, lactam ring, carbonate bond, halogen, hydroxyl moiety or carboxyl moiety,

m is an integer of 1 to 5, n is an integer of 0 to 3, and p is 1 or 2.

2. The resist composition of claim 1 wherein the base polymer further comprises recurring units (b) having a carboxyl or phenolic hydroxyl group substituted with an acid labile group.

3. The resist composition of claim 2 wherein the recurring units (b) are selected from recurring units having the formula (b1) and recurring units having the formula (b2),

wherein R A is each independently hydrogen or methyl, Y 1 is a single bond, phenylene, naphthylene, or a C 1 -C 12 linking group containing an ester bond, ether bond or lactone ring, Y 2 is a single bond, ester bond or amide bond, R 11 and R 12 each are an acid labile group, R 13 is fluorine, trifluoromethyl, cyano, or a C 1 -C 6 alkyl group, R 14 is a single bond or C 1 -C 6 straight or branched alkanediyl group in which some carbon may be replaced by an ether or ester bond, a is 1 or 2, and b is an integer of 0 to 4.

4. The resist composition of claim 1 wherein the base polymer further comprises recurring units (c) containing an adhesive group selected from the group consisting of hydroxyl, carboxyl, lactone ring, carbonate, thiocarbonate, carbonyl, cyclic acetal, ether bond, ester bond, sulfonic acid ester bond, cyano, amide, —O—C(═O)—S—, and —O—C(═O)—NH—.

5. The resist composition of claim 1 wherein the base polymer further comprises recurring units of at least one type selected from recurring units having the formulae (d1) to (d3):

wherein R A is each independently hydrogen or methyl,

Z 1 is a single bond, phenylene, —O—Z 11 —, —C(═O)—O—Z 11 — or —C(═O)—NH—Z 11 —, Z 11 is a C 1 -C 6 alkanediyl group, C 2 -C 6 alkenediyl group or phenylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety,

Z 2A is a single bond or ester bond, Z 2B is a single bond or C 1 -C 12 divalent group which may contain an ester bond, ether bond, lactone ring, bromine or iodine,

Z 3 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—Z 31 —, —C(═O)—O—Z 31 — or —C(═O)—NH—Z 31 —, Z 31 is a C 1 -C 6 alkanediyl group, C 2 -C 6 alkenediyl group or phenylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety,

Rf 1 to Rf 4 are each independently hydrogen, fluorine or trifluoromethyl, at least one thereof being fluorine,

R 21 to R 28 are each independently a C 1 -C 20 monovalent hydrocarbon group which may contain a heteroatom, any two of R 23 , R 24 and R 25 or any two of R 26 , R 27 and R 28 may bond together to form a ring with the sulfur atom to which they are attached, and

M − is a non-nucleophilic counter ion.

6. The resist composition of claim 1 , further comprising an acid generator.

7. The resist composition of claim 1 , further comprising an organic solvent.

8. The resist composition of claim 1 , further comprising a quencher.

9. The resist composition of claim 1 , further comprising a surfactant.

10. A pattern forming process comprising the steps of applying the positive resist composition of claim 1 to form a resist film on a substrate, exposing the resist film to high-energy radiation, and developing the exposed resist film in a developer.

11. The process of claim 10 wherein the high-energy radiation is i-line, KrF excimer laser, ArF excimer laser, EB or EUV of wavelength 3 to 15 nm.