Resist composition and pattern forming process

1. A resist composition comprising as base resin a polymer comprising recurring units represented by the formula (a) and recurring units containing a group capable of polarity switch with the aid of acid, the polymer having a weight average molecular weight of 1,000 to 500,000,

wherein R 1 is hydrogen or methyl, R 2 is hydrogen, a C 1 -C 8 straight, branched or cyclic alkyl group, C 2 -C 6 straight, branched or cyclic acyl group, or C 2 -C 6 straight, branched or cyclic alkoxycarbonyl group, X 1 is a single bond, phenylene group, naphthylene group, or C 1 -C 12 linking group containing an ester moiety, ether moiety or lactone ring, and a represents a molar fraction of the recurring units of formula (a) in the polymer and is a positive number in the range: 0<a<1.0,

wherein the recurring unit containing a group capable of polarity switch with the aid of acid is a recurring unit which switches from hydrophilic to hydrophobic due to dehydration reaction with the aid of acid.

2. The resist composition of claim 1 wherein the recurring unit containing a group capable of polarity switch with the aid of acid is derived from a monomer selected from the following monomers:

wherein R 9 is hydrogen or methyl.

3. The resist composition of claim 1 wherein the polymer further comprises recurring units having an adhesive group selected from the group consisting of hydroxyl, carboxyl, lactone ring, carbonate, thiocarbonate, carbonyl, cyclic acetal, ether, ester, sulfonic acid ester, cyano, amide, and —O—C(═O )-G- wherein G is —S— or —NH—.

4. The resist composition of claim 1 wherein the polymer further comprises recurring units of at least one selected from recurring units represented by the formulae (d1) to (d3):

wherein R 20 , R 24 ,and R 28 are each independently hydrogen or methyl, R 21 is a single bond, phenylenc, —O—R A —, or —C(═O )—Y 0 —R A —, Y 0 is —O— or —NH—, R A is a C 1 -C 6 straight, branched or cyclic alkylene or C 2 -C 6 straight, branched or cyclic alkenylene group which may contain a carbonyl, ester, ether or hydroxyl moiety, or phenylene group, R 22 , R 23 , R 25 , R 26 , R 27 , R 29 , R 30 , and R 31 are are each independently a C 1 -C 12 straight, branched or cyclic alkyl group which may contain a carbonyl, ester, ether or hydroxyl moiety, or a C 6 -C 12 aryl, C 7 -C 20 aralkyl, or mercaptophenyl group, Z 1 is a single bond, a C 1 -C 12 straight, branched or cyclic alkylene or C 2 -C 12 straight, branched or cyclic alkenylene group which may contain an ether moiety, ester moiety or lactone ring, or C 6 -C 10 arylene group, Z 2 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, —O—R 32 —, or —C(═O )—Z 3 —R 32 —, Z 3 is —O— or —NH—, R 32 is a C 1 -C 12 straight, branched or cyclic alkylene or C 2 -C 12 alkenylene group which may contain a carbonyl, ester, ether or hydroxyl moiety, or phenylene group, M − is a non-nucleophilic counter ion, d1, d2 and d3 represent molar fractions of recurring units of formulas (d1), (d2) and (d3), respectively, in the polymer and are numbers in the range: 0≤d1≤0.5, 0≤d2≤0.5, 0≤d3≤0.5, and 0<d1+d2+d3≤0.5.

5. The resist composition of claim 1 , further comprising an acid generator and an organic solvent.

6. The resist composition of claim 1 , further comprising a basic compound and/or a surfactant.

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

8. The process of claim 7 wherein the high-energy radiation is i-line, KrF excimer laser, ArF excimer laser, EB or EUV of wavelength 3 to 15 nm.