Positive resist composition and pattern forming process

1 . A positive resist composition comprising a base polymer comprising repeat units having the formula (a):

wherein R A is hydrogen or methyl,

X 1 is a single bond, phenylene group, naphthylene group or a C 1 -C 12 linking group which contains at least one moiety selected from an ester bond, ether bond and lactone ring,

R is a group having the formula (a1):

wherein R 1 is a C 1 -C 6 straight or branched aliphatic hydrocarbyl group which may contain oxygen, R 2 is a C 2 -C 6 straight or branched, unsaturated aliphatic hydrocarbyl group which may contain oxygen, R 1 and R 2 bond together to form a C 5 -C 12 ring with the carbon atom to which they are attached,

R 3 is a halogen atom, cyano group, C 1 -C 4 fluorinated alkyl group, C 1 -C 4 fluorinated alkoxy group, or C 1 -C 4 fluorinated alkylthio group,

R 4 is a C 1 -C 4 alkyl group,

m is an integer of 1 to 5, n is an integer of 0 to 4, and m+n is from 1 to 5, and

the broken line designates a valence bond.

2 . The resist composition of claim 1 wherein the base polymer further comprises repeat units of at least one type selected from repeat units having a carboxy group whose hydrogen is substituted by an acid labile group other than the group of formula (a1), and repeat units having a phenolic hydroxy group whose hydrogen is substituted by an acid labile group.

3 . The resist composition of claim 2 wherein the repeat units having a carboxy group whose hydrogen is substituted by an acid labile group other than the group of formula (a1) are represented by the formula (b1), and the repeat units having a phenolic hydroxy group whose hydrogen is substituted by an acid labile group are represented by the formula (b2):

wherein R A is each independently hydrogen or methyl,

Y 1 is a single bond, phenylene group, naphthylene group or a C 1 -C 12 linking group which contains at least one moiety selected from an ester bond, ether bond and lactone ring,

Y 2 is a single bond, ester bond or amide bond,

Y 3 is a single bond, ether bond or ester bond,

R 11 is an acid labile group other than the group of formula (a1),

R 12 is an acid labile group,

R 13 is fluorine, trifluoromethyl, cyano, or a C 1 -C 6 saturated hydrocarbyl group,

R 14 is a single bond or a C 1 -C 6 alkanediyl group in which some —CH 2 — may be replaced by an ether bond or ester bond,

a is 1 or 2, b is an integer of 0 to 4, and a+b is from 1 to 5.

4 . The resist composition of claim 1 wherein the base polymer further comprises repeat units having an adhesive group selected from among hydroxy, carboxy, lactone ring, carbonate bond, thiocarbonate bond, carbonyl, cyclic acetal, ether bond, ester bond, sulfonic ester bond, cyano, amide bond, —O—C(═O)—S—, and —O—C(═O)—NH—.

5 . The resist composition of claim 1 wherein the base polymer further comprises repeat units of at least one type selected from repeat units having the formulae (d1) to (d3):

wherein R A is each independently hydrogen or methyl,

Z 1 is a single bond, or a C 1 -C 6 aliphatic hydrocarbylene group, phenylene group, naphthylene group, or C 7 -C 18 group obtained by combining the foregoing, or —O—Z 11 —, —C(═O)—O—Z 11 — or —C(═O)—NH—Z 11 —, Z 11 is a C 1 -C 6 aliphatic hydrocarbylene group, phenylene group, naphthylene group, or C 7 -C 18 group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond or hydroxy moiety,

Z 2 is a single bond or ester bond,

Z 3 is a single bond, —Z 31 —C(═O)—O—, —Z 31 —O— or —Z 31 —O—C(═O)—, Z 31 is a C 1 -C 12 aliphatic hydrocarbylene group, phenylene group, or C 7 -C 18 group obtained by combining the foregoing, which may contain a carbonyl moiety, ester bond, ether bond, iodine or bromine,

Z 4 is methylene, 2,2,2-trifluoro-1,1-ethanediyl or carbonyl,

Z 5 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, trifluoromethyl-substituted phenylene group, —O—Z 51 —, —C(═O)—O—Z 51 —, or —C(═O)—NH—Z 51 —, Z 51 is a C 1 -C 6 aliphatic hydrocarbylene group, phenylene group, fluorinated phenylene group, or trifluoromethyl-substituted phenylene group, which may contain a carbonyl moiety, ester bond, ether bond, halogen or hydroxy moiety,

R 21 to R 28 are each independently halogen or a C 1 -C 20 hydrocarbyl group which may contain a heteroatom, a pair of R 23 and R 24 or R 26 and R 27 may bond together to form a ring with the sulfur atom to which they are attached, and

M − is a non-nucleophilic counter ion.

6 . The resist composition of claim 1 , further comprising an acid generator.

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

8 . The resist composition of claim 1 , further comprising a quencher.

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

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

11 . The process of claim 10 wherein the high-energy radiation is i-line, KrF excimer laser, ArF excimer laser, EB or EUV of wavelength 3 to 15 nm.