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The onium salts represented by the foregoing formulae (PAG3) and (PAG4) are known compounds and can be synthesized using the methods as described in J. W. Knapc› zyk et al., J. Am. Chem. Soc., 91, 145 (1969), A. L. Maycok et al., J. Org. Chem., 35, 2532 (1970), E. Goethas et al., Bull Soc. Chem. Belg., 73, 546 (1964), H. M. Leicester, J. Am. Chem. Soc., 51, 3587 (1929), J. V. Crivello et al., J. Polym. Chem. Ed., 18, 2677 (1980), U.S. Pat. No. 2,807,648 and 4,247,473, and JP-A-53-101331. 

(3) Disulfone derivatives represented by the following for› mula (PAG5) or iminosulfonate derivatives represented by the following formula (PAG6): C 

In the above formulae, Ar 3 and Ar 4 each represents a substituted or unsubstituted aryl group independently, R 206 represents a substituted or unsubstituted alkyl or aryl group, and A represents a substituted or unsubstituted alkylene, alkenylene or arylene group. 

The compounds illustrated below are examples of those derivatives, but these examples should not be construed as limiting photo-acid generators usable in the invention. 

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