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to 40 carbon atoms optionally containing a heteroatom, preferably a fluorine atom or a linear fluorinated alkyl group having 1 to 4 carbon atoms. R’c1 and R’c $^{2 }$may be bonded to form a ring together with a carbon atom bonded thereto and an atom (-CF $_{2}$-S0$_{2}$ ----e--S0$_{2}$----CF $_{2}$-), and preferably forms a ring structure by a fluorinated ethylene group or a fluorinated propylene group. 

R’d in the general formula (SD) represents a linear, a branched, or a cyclic monovalent hydrocarbon group having 1 to 40 carbon atoms optionally containing a heteroatom. These anions are described in detail in JP-A-2010-21560S and JP-A-2014-133723. Illustrative example thereof includes the anions in these patent documents and an anion represented by the formula ( 4b ). The photo acid generator having an anion represented by the general formula (SD) has no fluorine atom at a position of a sulfa group, but has two trifluoromethyl groups at ~ position. Consequently, it has an acidity strong enough to cut an acid labile group in a resist polymer, which can thus be used as a photo acid generator. 

Illustrative example of the anion represented by any of the general formula (SA), (SB), (SC), or (SD) includes the following anions, but the present invention is not restricted thereto. A$^{1 }$represents a hydrogen atom or a trifluoromethyl group, 

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