US 2020/0050105 Al  40  Feb. 13, 2020  -continued  Q_  +s--0  6  Q_  +s--0  6  [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 