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[0088] 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 generator is uniformly distributed. Where a base polymer containing recurring units (f) is used, the blending of an acid generator of addition type may be omitted. 

[0089] The base polymer for formulating the positive resist composition comprises recurring units (al) or (a2) having an acid labile group as essential component and additional recurring units (b ), ( c ), ( d), ( e ), and (0 as optional components. A fraction of units (al), (a2), (b), (c), (d), (e), and (f) is: preferably 0sal <1.0, 0sa2<1.0, 0<al +a2<1.0, 0sbs0.9, 0scs0.9, 0sds0.8, 0ses0.8, and 0sfs0.5; more preferably 0sals0.9, 0sa2s0.9, 0.lsal+a2s0.9, 0sbs0.8, 0scs0.8, 0sds0.7, 0se 0.7, and 0sfs0.4; and even more preferably 0sals0.8, 0sa2s0.8, 0.lsal+a2s0.8, 0sbs0.75, 0scs0.75, 0sds0.6, 0ses0.6, and 0sfs0.3. Notably, f=fl+ f2+f3, meaning that unit (f) is at least one of units (fl) to (f3), and al+a2+b+c+d+e+f=l.0. 

[0090] For the base polymer for formulating the negative resist composition, an acid labile group is not necessarily essential. The base polymer comprises recurring units (b ), and optionally recurring units (c), (d), (e), and/or (f). A fraction of these units is: preferably 0<bsl.0, 0scs0.9, 0sds0.8, 0ses0.8, and 0sfs0.5; more preferably 0.2sbsl .0, 0scs0.8, 0sds0.7, 0ses0.7, and 0sfs0.4; and even more preferably 0.3sbsl.0, 0scs0.75, 0sds0.6, 0ses0.6, and 0sfs0.3. Notably, f=fl+f2+f3, meaning that unit (f) is at least one of units (fl) to (f3), and b+c+d+e+f=l.0. 

[0091] 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, 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 system is 

heated at 50 to 80° C. for polymerization to take place. The reaction time is preferably 2 to 100 hours, more preferably 5 to 20 hours. 

[0092] Where a monomer having a hydroxyl group is copolymerized, the hydroxyl group may be replaced by an acetal group susceptible to deprotection with acid, typically ethoxyethoxy, prior to polymerization, and the polymeriza› tion be followed by deprotection with weak acid and water. Alternatively, the hydroxyl group may be replaced by an acetyl, formyl, pivaloyl or similar group prior to polymer› ization, and the polymerization be followed by alkaline hydrolysis. 

[0093] 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. 

[0094] 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. 

[0095] 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 molecular weight and dispersity become stronger as the pattern rule becomes finer. Therefore, the base polymer should preferably 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. 

[0096] It is understood that a blend of two or more polymers which differ in compositional ratio, Mw or Mw/Mn is acceptable. 

[0097] Acid Generator 

[0098] The resist composition may comprise an acid gen› erator capable of generating a strong acid (referred to as acid generator of addition type, hereinafter). As used herein, the term "strong acid" refers to a compound having a sufficient acidity to induce deprotection reaction of an acid labile group on the base polymer in the case of a chemically amplified positive resist composition, or a compound having a sufficient acidity to induce acid-catalyzed polarity switch reaction or crosslinking reaction in the case of a chemically amplified negative resist composition. The inclusion of such an acid generator ensures that the iodized aromatic ring› containing amine compound functions as a quencher and the inventive resist composition functions as a chemically amplified positive or negative resist composition. 

[0099] 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 

