57  US 10,606,172 B2  58  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.  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.  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).  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.  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).  As the PAG used herein, those having the formula (1) are  preferred.  Acid Generator  (1)  x- (lA)  (lB)  (lA’)  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.  -continued  (lC)  (lD) 