Resist composition and patterning process

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

wherein R 1 is hydrogen, hydroxy, carboxy, nitro, cyano, fluorine, chlorine, bromine, amino group, or a straight, branched or cyclic, C 1 -C 20 alkyl, C 1 -C 20 alkoxy, C 2 -C 20 alkoxycarbonyl, C 2 -C 20 acyloxy or C 1 -C 4 alkylsulfonyloxy group, which may contain fluorine, chlorine, bromine, hydroxy, amino or alkoxy moiety, or —NR′—C(═O)—R 8 or —NR 7 —C(═O)—O—R 8 , R 7 is hydrogen, or a straight, branched or cyclic C 1 -C 6 alkyl group which may contain halogen, hydroxy, alkoxy, acyl or acyloxy moiety, R 8 is a straight, branched or cyclic, C 1 -C 16 alkyl or C 2 -C 16 alkenyl group, or C 6 -C 12 aryl group, which may contain halogen, hydroxy, alkoxy, acyl or acyloxy moiety,

X 1 is a single bond or a C 1 -C 20 divalent linking group when p=1, or a C 1 -C 20 tri- or tetravalent linking group when p=2 or 3, the linking group optionally containing an oxygen, sulfur or nitrogen atom,

Rf 1 to Rf 4 are each independently hydrogen, fluorine or trifluoromethyl, at least one of Rf 1 to Rf 4 being fluorine or trifluoromethyl, or Rf 1 and Rf 2 , taken together, may form a carbonyl group,

R 2 , R 3 , R 4 , R 5 and R 6 are each independently a C 1 -C 12 straight, branched or cyclic alkyl group which may contain an oxo moiety, C 2 -C 12 straight, branched or cyclic alkenyl group which may contain an oxo moiety, C 2 -C 12 straight, branched or cyclic alkynyl group, C 6 -C 20 aryl group, C 7 -C 12 aralkyl group or C 7 -C 12 aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, or R 2 and R 3 may bond together to form a ring with the sulfur atom to which they are attached,

m is an integer of 1 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 , further comprising an organic solvent.

3. 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, C 1 -C 12 linking group containing ester group or lactone ring, phenylene group or naphthylene group, Y 2 is a single bond or ester group, and R 12 each are an acid labile group, R 13 is fluorine, 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, q is 1 or 2, and r is an integer of 0 to 4.

4. The resist composition of claim 3 , further comprising a dissolution inhibitor.

5. The resist composition of claim 3 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 , further comprising a crosslinker.

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

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

10. 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,

R 31 is a single bond, phenylene group, —O—R 41 —, or —C(═O)-Q 1 -R 41 —, Q 1 is —O— or —NH—, R 41 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 hydroxy moiety,

R 32 to R 39 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, C 7 -C 20 aralkyl group or mercaptophenyl group,

Z 1 is a single bond, —Z 11 —C(═O)—O—, —Z 11 —O— or —Z 11 —O—C(═O)—, Z 11 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 2 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R 42 —, or —C(═O)—Z 12 —R 42 —Z 12 is —O— or —NH—, R 42 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 hydroxy moiety,

M − is a non-nucleophilic counter ion.

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 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 electron beam or extreme ultraviolet radiation of wavelength 3 to 15 nm.

14. The resist composition of claim 1 wherein m is an integer of 2 to 5.

15. A sulfonium salt having the formula (A-1-1):

wherein X 2 is a single bond or a C 1 -C 6 straight or branched alkylene group, R 2 , R 3 , and R 4 are each independently a C 1 -C 12 straight, branched or cyclic alkyl group which may contain an oxo moiety, a C 2 -C 12 straight, branched or cyclic alkenyl group which may contain an oxo moiety, C 2 -C 12 straight, branched or cyclic alkynyl group, C 6 -C 20 aryl group, C 7 -C 12 aralkyl group or C 7 -C 12 aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, or R 2 and R 3 may bond together to form a ring with the sulfur atom to which they are attached, and X is an iodine atom or hydroxy group.

16. An iodonium salt having the formula (A-2-1):

wherein X 2 is a single bond or a C 1 -C 6 straight or branched alkylene group, R 5 and R 6 are each independently a C 1 -C 12 straight, branched or cyclic alkyl group which may contain an oxo moiety, a C 2 -C 12 straight, branched or cyclic alkenyl group which may contain an oxo moiety, C 2 -C 12 straight, branched or cyclic alkynyl group, C 6 -C 20 aryl group, C 7 -C 12 aralkyl group or C 7 -C 12 aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxy, carboxy, halogen, cyano, amide, nitro, sultone, sulfone or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene between carbon atoms, and X is an iodine atom or hydroxy group.