Resist composition and patterning process

1. A resist composition comprising a base polymer and a quencher, the quencher being an ammonium salt consisting of an ammonium cation having an iodine-substituted aromatic ring bonded to the nitrogen atom via a C 1 -C 20 divalent hydrocarbon group which may contain at least one moiety selected from ester bond and ether bond and a carboxylate anion, fluorine-free sulfonimide anion, sulfonamide anion or halide anion,

wherein the ammonium salt has the formula (A):

wherein R 1 is hydroxyl, C 1 -C 6 alkyl group, C 1 -C 6 alkoxy group, C 2 -C 6 acyloxy group, fluorine, chlorine, bromine, amino group, —NR 1A —C(═O)—R 1B , or —NR 1A —C(═O)—O—R 1B , R 1A is hydrogen or a C 1 -C 6 alkyl group, R 1B is a C 1 -C 6 alkyl, C 2 -C 8 alkenyl, C 6 -C 12 aryl or C 7 -C 13 aralkyl group,

R 2 is hydrogen, nitro, or a C 1 -C 20 monovalent hydrocarbon group which may contain at least one moiety selected from hydroxyl, carboxyl, thiol, ether bond, ester bond, nitro, cyano, halogen and amino moiety, in case of p=1 or 2, two R 2 may bond together to form a ring with the nitrogen atom to which they are attached, the ring optionally containing a double bond, oxygen, sulfur or nitrogen, or R 2 and X may bond together to form a ring with the nitrogen atom to which they are attached, the ring optionally containing a double bond, oxygen, sulfur or nitrogen,

X is a C 1 -C 20 divalent hydrocarbon group which may contain at least moiety selected from ester bond and ether bond,

A q− is a carboxylate anion, fluorine-free sulfonimide anion, sulfonamide anion or halide ion,

m and n are independently an integer meeting 1≤m≤K 5, 0≤n≤4 and 1≤m+n≤5, p is 1, 2 or 3, and q is 1 or 2.

2. The resist composition of claim 1 , further comprising an acid generator capable of generating a sulfonic acid, imide acid or methide acid.

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

4. The resist composition of claim 1 wherein the base polymer comprises recurring units having the formula (a1) or recurring units having the formula (a2):

wherein R A is each independently hydrogen or methyl, R 11 and R 12 each are an acid labile group, Y 1 is a single bond, phenylene group, naphthylene group, or C 1 -C 12 linking group containing at least one moiety selected from ester bond and lactone ring, and Y 2 is a single bond or ester bond.

5. The resist composition of claim 4 which is a chemically amplified positive resist composition.

6. The resist composition of claim 1 wherein the base polymer is free of an acid labile group.

7. The resist composition of claim 6 which is a chemically amplified negative resist composition.

8. 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 (f1) to (f3):

wherein R A is each independently hydrogen or methyl,

Z 1 is a single bond, phenylene group, —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, ester bond, ether bond or hydroxyl moiety,

Z 2 is a single bond, —Z 21 —C(═O)—O—, —Z 21 —O— or —Z 21 —O—C(═O)—, Z 21 is a C 1 -C 12 alkanediyl group which may contain a carbonyl moiety, ester bond or ether bond,

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, phenylene group, fluorinated phenylene group, or trifluoromethyl-substituted phenylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxy moiety,

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,

G is hydrogen or trifluoromethyl, and

M − is a non-nucleophilic counter ion.

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

10. The resist composition of claim 1 , further comprising a quencher other than the ammonium salt.

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

12. The process of claim 11 wherein the high-energy radiation is ArF excimer laser radiation of wavelength 193 nm or KrF excimer laser radiation of wavelength 248 nm.

13. The process of claim 11 wherein the high-energy radiation is EB or EUV of wavelength 3 to 15 nm.

14. The resist composition of claim 1 wherein m is an integer of 2 to 4.