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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. 

The base polymer should preferably have a weight aver› age molecular weight (Mw) in the range of 1,000 to 500,000, 5 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 10 phenomenon after pattern formation. 

If a base polymer has a wide molecular weight distribu› tion 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 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. 

It is understood that a blend of two or more polymers which differ in compositional ratio, Mw or Mw/Mn is acceptable. 

## Acid Generator 

To the resist composition comprising the base polymer and the iodonium salt having formula (A), an acid generator may be added so that the composition may function as a chemically amplified positive or negative resist composi- tion. 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 gener- ating sulfonic acid, imide acid (imidic acid) or methide acid are preferred. Suitable PAGs include sulfonium salts, iodo› nium salts, sulfonyldiazomethane, N-sulfonyloxyimide, and oxime-O-sulfonate acid generators. Exemplary PAGs are described in JP-A 2008-111103, paragraphs [0122]-[0142] (U.S. Pat. No. 7,537,880). 

As the PAG used herein, those having the formula (1) are preferred. 

In formula (1 ), R 101 , R 102 and R 103 are each indepen› dently a C 1 -C$_{20 }$ straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. Any two ofR 101 , R 102 and R 103 may bond together to form a ring with the sulfur atom to which they are attached. 

In formula (1 ), x- is an anion of the following formula (IA), (18), (IC) or (ID). 

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In formula (IA), wa is fluorine or a cl -C40 straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. 

Of the anions of formula (IA), an anion having the formula (IA’) is preferred. 

In formula (IA’), R 104 is hydrogen or trifluoromethyl, preferably trifluoromethyl. R 105 is a C 1 -C 38 straight, branched or cyclic monovalent hydrocarbon group which may contain a heteroatom. As the heteroatom, oxygen, nitrogen, sulfur and halogen atoms are preferred, with oxygen being most preferred. Of the monovalent hydrocar› bon groups represented by R 105 , 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 hydrocarbon groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobu› tyl, s-butyl, t-butyl, pentyl, neopentyl, cyclopentyl, hexyl, cyclohexyl, 3-cyclohexenyl, heptyl, 2-ethylhexyl, nonyl, undecyl, tridecyl, pentadecyl, heptadecyl, 1-adamantyl, 2-adamantyl, 1-adamantylmethyl, norbornyl, norbornylm› ethyl, tricyclodecanyl, tetracyclododecanyl, tetracyclodode› canylmethyl, dicyclohexylmethyl, eicosanyl, ally!, benzyl, diphenylmethyl, tetrahydrofuryl, methoxymethyl, ethoxym› ethyl, methylthiomethyl, acetamidomethyl, trifluoroethyl, (2-methoxyethoxy)methyl, acetoxymethyl, 2-carboxy-1-cy› clohexyl, 2-oxopropyl, 4-oxo-1-adamantyl, and 3-oxocyclo› hexyl. In these groups, one or more hydrogen atoms may be substituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, or one or more carbon atoms may be substituted by a moiety containing a heteroa› tom such as oxygen, sulfur or nitrogen, so that the group may contain a hydroxyl, cyano, carbonyl, ether, ester, sul- fonic acid ester, carbonate, lactone ring, sultone ring, car› boxylic anhydride or haloalkyl moiety. 

With respect to the synthesis of the sulfonium salt having an anion of formula (IA’), reference may be made to JP-A 2007-145797, JP-A 2008-106045, JP-A 2009-007327, and JP-A 2009-258695. Also useful are the sulfonium salts described in JP-A 2010-215608, JP-A 2012-041320, JP-A 2012-106986, and JP-A 2012-153644. 

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