81  US 9,989,847 B2  82  -continued  The polymer used herein generally has a weight average  molecular weight (Mw) of 1,000 to 500,000, and preferably  3,000 to 100,000, as measured by GPC using polystyrene  standards. Outside the range, there may result an extreme  drop of etch resistance, and a drop of resolution due to  difficulty to gain a dissolution rate difference before and  after exposure.  The polymer may be synthesized by any desired method,  for example, by dissolving an unsaturated bond-containing  monomer or monomers in an organic solvent, adding a  radical initiator, and effecting heat polymerization. Suitable  organic solvents used herein include toluene, benzene, tet› rahydrofuran, diethyl ether, and dioxane. Examples of the  polymerization initiator used herein include 2,2’-azobisobu- tyronitrile 2,2’-azobis(2,4-dimethylvaleronitrile  ), dimethyl  2,2-azobis(2-methylpropionate ), benzoyl peroxide, and lau› royl 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. The acid labile group  that has been incorporated in the monomer may be kept as  such, or the polymer may be protected or partially protected  therewith at the end of polymerization.  In the polymer (B), appropriate molar fractions (mo!%)  of the respective recurring units derived from the monomers 25  are given below although the invention is not limited thereto.  The polymer may comprise:  I) constituent units of at least one type having formula (2)  in a fraction of 1 to 60 mo! %, preferably 5 to 50 mo! %, and  more preferably 10 to 50 mo!%,  II) constituent units of at least one type having formula (3)  in a fraction of 40 to 99 mo! %, preferably 50 to 95 mo! %,  and more preferably 50 to 90 mo! %, and optionally,  III) constituent units of at least one type having formula  ( dl) or ( d2) in a fraction of O to 30 mo! %, preferably O to  20 mo! %, and more preferably  O  to 10 mo! %, and  optionally,  IV) constituent units of at least one type derived from  another monomer in a fraction of  O  to 80 mo! %, preferably 40  0 to 70 mo! %, and more preferably  O  to 50 mo! %.  (C) PAG of Formula ( 4)  While the resist composition of the invention essentially  comprises the acid diffusion controlling agent (onium salt)  having formula (1), it preferably further comprises a pho› toacid generator (PAG) having the general formula ( 4).  Herein  R 100 , R 200 ,  and  R $^{300 }$are as defined above. x- is an  anion of the general formula (5), (6), (7) or (8).  Herein R$^{77 }$is hydrogen or trifluoromethyl; R$^{88 }$is a straight  C$_{1}$-C$_{30 }$or branched or cyclic C$_{3}$ -C$_{30 }$monovalent hydrocar› bon group which may be substituted with or separated by a  heteroatom; R 100 , R $^{200 }$and R$^{300 }$are as defined above.  In formula (5’), R $^{88 }$is a straight C$_{1 }$-C$_{30 }$ or branched or  cyclic C$_{3}$-C$_{30 }$monovalent hydrocarbon group which may be  substituted with or separated by a heteroatom. Of the het› eroatoms contained in R 88 , oxygen, nitrogen, sulfur and  halogen atoms are preferred, with oxygen being most pre› ferred. Of the monovalent hydrocarbon groups represented  by R 88 , those groups of 6 to 30 carbon atoms are preferred  from the aspect of achieving a high resolution in forming  patterns of fine feature size. Suitable monovalent hydrocar- bon groups represented by R  $^{88 }$include, but are not limited  to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec› butyl, tert-butyl, pentyl, neopentyl, cyclopentyl, hexyl,  cyclohexyl, 3-cyclohexenyl, heptyl, 2-ethylhexyl, nonyl,  undecyl, tridecyl, pentadecyl, heptadecyl, 1-adamantyl,  2-adamantyl, 1-adamantylmethyl, norbomyl, norbornylm› ethyl, tricyclodecanyl, tetracyclododecanyl, tetracyclodode- canylmethyl, dicyclohexylmethyl, eicosanyl, ally!, benzyl,  diphenylmethyl, tetrahydrofuryl, methoxymethyl, ethoxym› ethyl, methylthiomethyl, acetamidomethyl, trifluoromethyl,  (2-methoxyethoxy)methyl, acetoxymethyl, 2-carboxy-1-cy› clohexyl, 2-oxopropyl, 4-oxo-1-adamantyl, and 3-oxocyclo› hexyl.  Herein  R’a,  Rfbv  W 2 , R’c1, R’c 2 , and R’c3 are each indepen› dently fluorine or a straight C$_{1 }$-C$_{40 }$ or branched or cyclic  C$_{3}$-C$_{40 }$ monovalent hydrocarbon group which may be sub› stituted with or separated by a heteroatom. A pair ofW$^{1 }$and  W 2 ,  or R’c1 and R’c 2  bond together to form a ring with the  carbon atoms to which they are attached.  R’d  is a straight  C$_{1}$-C$_{40 }$or branched or cyclic C$_{3}$ -C$_{40 }$monovalent hydrocar- bon group which may be substituted with or separated by a  heteroatom.  In formula (5),  R’a  is fluorine or a straight C$_{1}$-C$_{40 }$ or  branched or cyclic C$_{3}$-C$_{40}$, monovalent hydrocarbon group  which may be substituted with or separated by a heteroatom.  Of the structures of formula (5), a structure having the  general formula (5’) is preferred.  RIOO  I  x- s+-R3oo  I  R200  ( 4 (5) (6)  (5’)  (7)  (8) 