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[0102] Recurring units (dl) to (d3) have the function of acid generator. The attachment of an acid generator to the polymer main chain is effective in restraining acid diffusion, thereby preventing a reduction of resolution due to blur by acid diffusion. Also LWR is improved since the acid gen› erator is uniformly distributed. When a base polymer com› prising recurring units ( d) is used, an acid generator of addition type (to be described later) may be omitted. 

[0103] Besides the recurring units described above, further recurring units ( e) may be incorporated in the base polymer, examples of which include styrene, acenaphthylene, indene, coumarin, and coumarone. 

[0104] In the base polymer comprising recurring units (al), (a2), (bl), (b2), (c), (dl), (d2), (d3), and (e), a fraction of these units is: preferably 0<a<l.0, 0sbls0.9, 0sb2s0.9, 0sbl+b2s0.9, 0scs0.9, 0sdls0.5, 0sd2s0.5, 0sd3s0.5, 0sdl+d2+d3s0.5, and 0ses0.5; more preferably 0.Olsas0.8, 0sbls0.8, 0sb2s0.8, 0sbl+ b2s0.8, 0scs0.8, 0sdl s0.4, 0sd2s0.4, 0sd3s0.4, 0sdl + d2+d3s0.4, and 0ses0.4; and even more preferably 0sal s0. 7, 0sa2s0.7, 0.02sal+a2s0.7, 0sbls0.7, 0sb2s0.7, 0sbl+ b2s0.7, 0scs0.7, 0sdls0.3, 0sd2s0.3, 0sd3s0.3, 0sdl+ d2+d3s0.3, and 0ses0.3. Notably, al+a2+bl+b2+c+dl+d2+ d3+e=l.0. 

[0105] The base polymer may be synthesized by any desired methods, for example, by dissolving one or more monomers selected from the monomers corresponding to the foregoing recurring units in an organic solvent, adding a radical polymerization initiator thereto, and heating for polymerization. Examples of the organic solvent which can be used for polymerization include toluene, benzene, tetra› hydrofuran (THF), diethyl ether, and dioxane. Examples of the polymerization initiator used herein include 2,2’-azobi› sisobutyronitrile (AIBN), 2,2’-azobis(2,4-dimethylvaleroni› trile ), dimethyl 2,2-azobis(2-methylpropionate ), benzoyl peroxide, and lauroyl peroxide. Preferably the reaction tem› perature is 50 to 80° C., and the reaction time is 2 to 100 hours, more preferably 5 to 20 hours. 

[0106] In the case of a monomer having a hydroxyl group, the hydroxyl group may be replaced by an acetal group susceptible to deprotection with acid, typically ethoxy› ethoxy, prior to polymerization, and the polymerization be 

followed by deprotection with weak acid and water. Alter› natively, the hydroxyl group may be replaced by an acetyl, formyl, pivaloyl or similar group prior to polymerization, and the polymerization be followed by alkaline hydrolysis. [0107] When hydroxystyrene or hydroxyvinylnaphthalene is copolymerized, an alternative method is possible. Spe› cifically, acetoxystyrene or acetoxyvinylnaphthalene is used instead of hydroxystyrene or hydroxyvinylnaphthalene, and after polymerization, the acetoxy group is deprotected by alkaline hydrolysis, for thereby converting the polymer product to hydroxystyrene or hydroxyvinylnaphthalene. For alkaline hydrolysis, a base such as aqueous ammonia or triethylamine may be used. Preferably the reaction tempera› ture is -20° C. to 100° C., more preferably 0° C. to 60° C., and the reaction time is 0.2 to 100 hours, more preferably 0.5 to 20 hours. 

[0108] The base polymer should preferably have a weight average molecular weight (Mw) in the range of 1,000 to 500,000, and more preferably 2,000 to 30,000, as measured by GPC versus polystyrene standards using tetrahydrofuran (THF) solvent. With too low a Mw, the resist composition may become less heat resistant. A polymer with too high a Mw may lose alkaline solubility and give rise to a footing phenomenon after pattern formation. 

[0109] If a base polymer has a wide molecular weight distribution or dispersity (Mw/Mn), which indicates the presence of lower and higher molecular weight polymer fractions, there is a possibility that foreign matter is left on the pattern or the pattern profile is degraded. The influences of Mw and Mw/Mn become stronger as the pattern rule becomes finer. Therefore, the base polymer should prefer› ably have a narrow dispersity (Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in order to provide a resist composition suitable for micropatterning to a small feature size. 

[0110] The base polymer may be a blend of two or more polymers which differ in compositional ratio, Mw or Mw/Mn. It may also be a blend of a polymer comprising recurring units (a) and a polymer comprising recurring units (bl) and/or (b2), but not recurring units (a). 

[0111] Acid Generator 

[0112] The positive resist composition may contain an acid generator capable of generating a strong acid, also referred to as acid generator of addition type. As used herein, the "strong acid" is a compound having a sufficient acidity to induce deprotection reaction of acid labile groups on the base polymer. The acid generator is typically a compound (PAG) capable of generating an acid upon exposure to actinic ray or radiation. Although the PAG used herein may be any compound capable of generating an acid upon exposure to high-energy radiation, those compounds capable of generating sulfonic acid, imidic acid (imide acid) or methide acid are preferred. Suitable PAGs include sulfonium salts, iodonium salts, sulfonyldiazomethane, N-sulfonyloxy› imide, and oxime-O-sulfonate acid generators. Suitable PAGs are as exemplified in U.S. Pat. No. 7,537,880 (JP-A 2008-111103, paragraphs [0122]-[0142]). 

[0113] Also sulfonium salts having the formula (1-1) and iodonium salts having the formula (1-2) are useful PAGs. 

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