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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- 15 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). 

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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). 

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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. 

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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. 

( 4 

(5) 

(6) 

-continued 

(7) 

(8) 

(5’) 

