Sep. 10,2020  59  US 2020/0285149 Al  -continued  (lD)  0  H  R106  11x  so- RI07.............._Q  C/ 3  /\  F  F  (lB)  (lC)  (lA)  [0117]  In formula (IA), Wais fluorine or a C$_{1}$-C$_{40 }$mon› ovalent hydrocarbon group which may contain a heteroa› tom. The monovalent hydrocarbon group may be straight,  branched or cyclic, and examples thereof include those  exemplified later for R  107 .  [0118]  Of the anions of formula (IA), an anion having the  formula (IA’) is preferred.  [0119]  In formula (IA’), R$^{106 }$is hydrogen or trifluorom› ethyl, preferably trifluoromethyl.  [0116]  In formulae (1-1) and (1-2), x-is an anion of the  following formula (IA), (18), (IC) or (ID).  [0120]  R  107  is a C$_{1 }$-C$_{38 }$ monovalent hydrocarbon group  which may contain a heteroatom. As the heteroatom, oxy› gen, nitrogen, sulfur and halogen atoms are preferred, with  oxygen being most preferred. Of the monovalent hydrocar› bon groups represented by R 107 , 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. The  monovalent hydrocarbon groups may be straight, branched  or cyclic. Examples thereof include, but are not limited to,  straight or branched alkyl groups such as methyl, ethyl,  propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pen› tyl, neopentyl, hexyl, heptyl, 2-ethylhexyl, nonyl, undecyl,  tridecyl, pentadecyl, heptadecyl, and eicosanyl; monovalent  saturated alicyclic hydrocarbon groups such as cyclopentyl,  cyclohexyl, 1-adamantyl, 2-adamantyl, 1-adamantylmethyl,  norbornyl, norbomylmethyl, tricyclodecanyl, tetracyclodo› decanyl, tetracyclododecanylmethyl, and dicyclohexylm› ethyl; monovalent unsaturated aliphatic hydrocarbon groups  such as ally! and 3-cyclohexenyl; aryl groups such as  phenyl, 1-naphthyl and 2-naphthyl; and aralkyl groups such  as benzyl and diphenylmethyl. Examples of the monovalent  hydrocarbon group having a heteroatom include tetrahydro› furyl, methoxymethyl, ethoxymethyl, methylthiomethyl,  acetamidomethyl, trifluoroethyl, (2-methoxyethoxy)methyl,  acetoxymethyl,  2-carboxy-1-cyclohexyl,  2-oxopropyl,  4-oxo-1-adamantyl, and 3-oxocyclohexyl. In these groups,  some hydrogen may be substituted by a moiety containing a  heteroatom such as oxygen, sulfur, nitrogen or halogen, or  some carbon may be replaced by a moiety containing a  heteroatom such as oxygen, sulfur or nitrogen, so that the  group may contain a hydroxyl, cyano, carbonyl, ether bond,  ester bond, sulfonic acid ester bond, carbonate moiety,  lactone ring, sultone ring, carboxylic anhydride or haloalkyl  moiety.  [0121]  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 