Resist composition and patterning process

1. A resist composition comprising a base polymer and a sulfonium salt having the formula (A-1) and/or an iodonium salt having the formula (A-2):

wherein R 1 is hydroxyl, carboxyl, C 1 -C 6 straight, branched or cyclic alkyl, alkoxy or alkoxycarbonyl group, C 2 -C 6 alkenyloxy or alkynyloxy group, C 2 -C 6 straight, branched or cyclic acyloxy group, fluorine, chlorine, amino group, —NR 7 —C(═O)—R 8 or —NR 7 —C(═O)—O—R 8 wherein R 7 is hydrogen or a C 1 -C 6 straight, branched or cyclic alkyl group, R 8 is a C 1 -C 8 straight, branched or cyclic alkyl group or C 2 -C 8 straight, branched or cyclic alkenyl group,

R 2 , R 3 and R 4 are each independently fluorine, chlorine, bromine, iodine, or a C 1 -C 12 straight, branched or cyclic alkyl group, C 2 -C 12 straight, branched or cyclic alkenyl group, C 6 -C 20 aryl group, C 7 -C 12 aralkyl group or C 7 -C 12 aryloxoalkyl group, at least one hydrogen in the foregoing groups being optionally substituted by a hydroxyl, carboxyl, halogen, oxo, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or at least one carbon atom in the foregoing groups being optionally substituted by an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety, or R 2 and R 3 may bond together to form a ring with the sulfur atom to which they are attached,

R 5 and R 6 are each independently a C 6 -C 10 aryl group, C 2 -C 6 straight, branched or cyclic alkenyl group, C 2 -C 6 straight, branched or cyclic alkynyl group, or trifluoromethyl group, in which at least one hydrogen may be substituted by halogen, trifluoromethyl, C 1 -C 10 straight, branched or cyclic alkyl or alkoxy moiety, hydroxyl, carboxyl, C 2 -C 10 straight, branched or cyclic alkoxycarbonyl moiety, nitro or cyano moiety,

X is a single bond, or a (p+1)-valent C 1 -C 20 inking group which may contain an ether, carbonyl, ester, amide, sultone, lactam, carbonate, halogen, hydroxyl or carboxyl moiety,

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

2. The resist composition of claim 1 wherein the sulfonium salt and/or iodonium salt functions as a quencher.

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

4. The resist composition of claim 1 wherein the sulfonium salt and/or iodonium salt functions as an acid generator.

5. The resist composition of claim 4 , further comprising a quencher.

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

7. 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, Y 1 is a single bond, phenylene group, naphthylene group, or C 1 -C 12 linking group containing an ester moiety and/or lactone ring, Y 2 is a single bond or ester group, R 11 and R 12 each are an acid labile group, R 13 is halogen, trifluoromethyl, cyano, a C 1 -C 6 straight, branched or cyclic alkyl or alkoxy group, or a C 2 -C 7 straight, branched or cyclic acyl, acyloxy or alkoxycarbonyl group, R 14 is a single bond or C 1 -C 6 straight or branched alkylene group in which at least one carbon atom may be substituted by an ether or ester moiety, q1 is 1 or 2, and q2 is an integer of 0 to 4.

8. The resist composition of claim 7 , further comprising a dissolution inhibitor.

9. The resist composition of claim 7 which is a chemically amplified positive resist composition.

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

11. The resist composition of claim 10 , further comprising a crosslinker.

12. The resist composition of claim 10 which is a chemically amplified negative resist composition.

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

14. The resist composition of claim 1 wherein the base polymer further comprises recurring units of at least one type selected from the formulae (f1) to (f3):

wherein R A is each independently hydrogen or methyl,

Z 1 is a single bond, phenylene group, —O—Z 12 —, or —C(═O)—Z 11 —Z 12 —, Z 11 is —O— or —NH—, Z 12 is a C 1 -C 6 straight, branched or cyclic alkylene group, C 2 -C 6 straight, branched or cyclic alkenylene group, or phenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety,

R 31 to R 38 are each independently a C 1 -C 12 straight, branched or cyclic alkyl group which may contain a carbonyl, ester or ether moiety, or a C 6 -C 12 aryl group or C 7 -C 20 aralkyl group, in which at least one hydrogen may be substituted by a C 1 -C 10 straight, branched or cyclic alkyl moiety, halogen, trifluoromethyl, cyano, nitro, hydroxyl, mercapto, C 1 -C 10 straight, branched or cyclic alkoxy moiety, C 2 -C 10 straight, branched or cyclic alkoxycarbonyl moiety, or C 2 -C 10 straight, branched or cyclic acyloxy moiety, any two of R 33 , R 34 and R 35 or any two of R 36 , R 37 and R 38 may bond together to form a ring with the sulfur atom to which they are attached,

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 straight, branched or cyclic alkylene group which may contain a carbonyl, ester or ether moiety,

A is hydrogen or trifluoromethyl,

Z 3 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—Z 32 —, or —C(═O)—Z 31 —Z 32 —, Z 31 is —O— or —NH—, Z 32 is a C 1 -C 6 straight, branched or cyclic alkylene group, phenylene group, fluorinated phenylene group, trifluoromethyl-substituted phenylene group, or C 2 -C 6 straight, branched or cyclic alkenylene group, which may contain a carbonyl, ester, ether or hydroxyl moiety, and

M − is a non-nucleophilic counter ion.

15. 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 film in a developer.

16. The process of claim 15 wherein the high-energy radiation is ArF excimer laser radiation of wavelength 193 nm or KrF excimer laser radiation of wavelength 248 nm.

17. The process of claim 15 wherein the high-energy radiation is electron beam or extreme ultraviolet radiation of wavelength 3 to 15 nm.