Resist composition and patterning process

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

wherein R 1 , R 2 and R 3 are each independently hydrogen, hydroxyl, C 1 -C 6 alkyl, C 1 -C 6 alkoxy, C 2 -C 6 acyl, C 2 -C 6 alkoxycarbonyl, C 6 -C 10 aryl, halogen, nitro, amino, or cyano group, in which at least one hydrogen may be substituted by C 1 -C 6 alkyl, hydroxyl, halogen, nitro or amino moiety, or at least one carbon may be substituted by an ether bond or thioether bond, R 1 and R 2 may bond together to form an alicyclic or aromatic ring with the carbon atom to which they are attached, at least one hydrogen on the ring may be substituted by C 1 -C 6 alkyl, hydroxyl, halogen, nitro or amino moiety, all of R 1 ,R 2 and R 3 are not hydrogen at the same time,

X is a single bond,

R 4 , R 5 and R 6 are each independently halogen or a C 1 -C 20 monovalent hydrocarbon group which may contain a heteroatom, any two of R 4 , R 5 and R 6 may bond together to form a ring with the sulfur atom to which they are attached, and

wherein the anion in the sulfonium salt having formula (A) is selected from the group consisting of the following formulae:

2. The resist composition of claim 1 , further comprising an acid generator capable of generating 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, Y 1 is a single bond, phenylene group, naphthylene group, or C 1 -C 12 linking group containing an ester bond or lactone ring, Y 2 is a single bond or ester bond, R 11 and R 12 each are an acid labile group.

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

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

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

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

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

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

11. 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 alkylene group, C 2 -C 6 alkenylene 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 alkylene group which may contain a carbonyl, ester bond or ether bond,

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 alkylene group, phenylene group, fluorinated phenylene group, trifluoromethyl-substituted phenylene group, or C 2 -C 6 alkenylene group, which may contain a carbonyl, ester bond, ether bond or hydroxyl moiety, and

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,

A is hydrogen or trifluoromethyl, and

M − is a non-nucleophilic counter ion.

12. 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.

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

14. The process of claim 12 wherein the high-energy radiation is EB or EUV of wavelength 3 to 15 nm.

15. A resist composition comprising a base polymer and a sulfonium salt having the formula (A):

wherein R 1 , R 2 and R 3 are each independently hydrogen, hydroxyl, C 1 -C 6 alkyl, C 1 -C 6 alkoxy, C 2 -C 6 acyl, C 2 -C 6 alkoxycarbonyl, C 6 -C 10 aryl, halogen, nitro, amino, or cyano group, in which at least one hydrogen may be substituted by C 1 -C 6 alkyl, hydroxyl, halogen, nitro or amino moiety, or at least one carbon may be substituted by an ether bond or thioether bond, R 1 and R 2 may bond together to form an alicyclic or aromatic ring with the carbon atom to which they are attached, at least one hydrogen on the ring may be substituted by C 1 -C 6 alkyl, hydroxyl, halogen, nitro or amino moiety,

X is a C 1 -C 10 divalent aliphatic hydrocarbon group in which at least one hydrogen may be substituted by halogen, or at least one carbon may be substituted by an ether bond, ester bond or carbonyl moiety,

R 4 , R 5 and R 6 are each independently halogen or a C 1 -C 20 monovalent hydrocarbon group which may contain a heteroatom, any two of R 4 , R 5 and R 6 may bond together to form a ring with the sulfur atom to which they are attached.

16. A process for forming a pattern comprising the steps of applying the resist composition of claim 15 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.

17. The resist composition of claim 1 wherein the anion in the sulfonium salt having formula (A) is selected from the group consisting of the following formulae: