42  Jun.  18,2020  US 2020/0192221 Al  -continued  Q_  +s--0  6  R"  Q  _  ~so,  s--0  6  [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.  (1-1)  x-