Resist composition and patterning process

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

wherein R 1 is each independently a hydroxyl, C 1 -C 20 straight, branched or cyclic alkyl or alkoxy group, C 2 -C 20 straight, branched or cyclic acyl or acyloxy group, fluorine, chlorine, bromine, amino, or alkoxycarbonyl-substituted amino group,

R 2 is each independently a single bond or C 1 -C 4 alkylene group,

R 3 is a single bond or 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 4 , R 5 , R 6 , R 7 and R 8 are each independently 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 or C 7 -C 12 aralkyl or aryloxyalkyl group, in which at least one hydrogen may be substituted by a hydroxyl, carboxyl, halogen, cyano, oxo, amide, nitro, sultone, sulfone, or sulfonium salt-containing moiety, or in which an ether, ester, carbonyl, carbonate or sulfonic acid ester moiety may intervene in a carbon-carbon bond, or R 4 and R 5 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, X 1 is a single bond, phenylene group or naphthylene group, or C 1 -C 12 linking group containing an ester moiety or lactone ring, X 2 is a single bond or ester group, R 11 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, r is 1 or 2, and q 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)—Y 1 —R 41 , Y 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 hydroxyl 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.