US 10,606,172 B2  55  56  -continued  -continued  Rh  Y› o  The attachment of an acid generator to the polymer main  chain is effective in restraining acid diffusion, thereby pre› venting a reduction of resolution due to blur by acid diffu› sion. Also edge roughness is improved since the acid gen› erator is uniformly distributed. Where a base polymer  containing recurring units  (f)  is used, the addition of a  separate PAG may be omitted.  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  (f)  as optional compo- nents. A fraction of units (al), (a2), (b), (c), (d), (e), and  (f)  is: preferably 0sal<l.0, 0sa2<1.0, 0<al+a2<1.0, 0sbs0.9,  to 0scs0.9, 0sds0.8, 0ses0.8, and0sfs0.5; more preferably  0sal s0.9, 0sa2s0.9, 0.1 <al +a2s0.9, 0sbs0.8, 0scs0.8,  0sds0.7, 0ses0.7, 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, unit  (f)  is at least  one of units (fl), (f2) and (f3), i.e., f=fl+f2+f3, and al+a2+  b+c+d+e+f=l .0.  For the base polymer for formulating the negative resist  composition, an acid labile group is not necessarily essen› tial. 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. Nota› bly, unit  (f)  is at least one of units (fl), (f2) and (f3), i.e.,  f=fl+f2+f3, and b+c+d+e+f=l.0.  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 effecting heat polym- erization. Examples of the organic solvent which can be  used for polymerization include toluene, benzene, tetrahy› drofuran, 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 2 to 100 hours, preferably 5 to 20 hours.  When hydroxystyrene or hydroxyvinylnaphthalene is  copolymerized, an alternative method is possible. Specifi› cally, 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-