36  US 7,232,640 Bl  35  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.  (PAG5-7)  (PAG5-8)  (PAG5-9)  (PAG5-10)  ~S02-S02-o-OCH3  Cl~  (PAG5-11)  Cl--o-S02-S02-o-OCH3  (PAG5-12)  (PAG5-13)  (PAG5-14)  0-0-S02-S02-o  (PAG5-15)  (PAG6-1)  (PAG5)  (PAG6)  Cl--o-S02-S02-o-Cl  (PAG5-1)  (PAG5-2)  H3C--o-S02-S02-o-CH3  (PAG5-3)  H3CO--o-S02-S02-o-OCH3  (PAG5-4)  H3C--o-S02-S02-o-Cl  F3C--o-S02-S02-o-CF3  (PAG5-5)  (PAG5-6)  0-so,-so,--p  0  Cl  -continued 