134  US 9,703,193 B2  133  -continued  (Z2)  Herein R 251 , R 252  and R 253  are each independently a  straight, branched or cyclic  C$_{1 }$-C$_{20 }$monovalent hydrocarbon  group which may contain a heteroatom, any two or more of  R 251 , R $^{252 }$and R$^{253 }$may bond together to form a ring with  the atom to which they are attached and intervening atoms.  R$^{254 }$is a straight, branched or cyclic C$_{1}$-C$_{40 }$ monovalent  hydrocarbon group which may contain a heteroatom. R 255  and R 256  are each independently hydrogen or trifluorom› ethyl. R 257  and R 258  are each independently hydrogen,  fluorine or trifluoromethyl. R 259  is hydrogen, hydroxyl, a  straight, branched or cyclic C$_{1 }$-C$_{35 }$monovalent hydrocarbon  group which may contain a heteroatom, or optionally sub› stituted CrC$_{30 }$aryl group. The subscript r is an integer of 1  to 3, z 1 , z and z 3  are each independently an integer of  O  to  5.  Illustrative, non-limiting examples of the onium salts of  a-position non-fluorinated sulfonic acid and carboxylic acid  are given below.  (Zl)  acid which is not fluorinated at a-position are exemplified in  JP-A 2010-155824, paragraphs [0019] to [0036] and JP-A  2010-215608, paragraphs [0047] to [0082]. The onium salts  of carboxylic acid are described in JP 3991462.  The anion in formula (6) or (7) is a conjugated base of  weak acid. As used herein, the weak acid indicates an acidity  insufficient to deprotect an acid labile group from an acid  labile group-containing unit in the base resin. The onium salt  having formula ( 6) or (7) functions as a quencher when used  in combination with an onium salt type photoacid generator  having a conjugated base of a strong acid, typically a  sulfonic acid which is fluorinated at a-position as the  counter anion.  In a system using a mixture of an onium salt capable of  generating a strong acid ( e.g., a-position fluorinated sulfonic  acid) and an onium salt capable of generating a weak acid  (e.g., a-position non-fluorinated sulfonic acid or carboxylic  acid), if the strong acid generated from the photoacid  generator upon exposure to high-energy radiation collides  with the unreacted onium salt having a weak acid anion, then  a salt exchange occurs whereby the weak acid is released  and an onium salt having a strong acid anion is formed. In  this course, the strong acid is exchanged into the weak acid  having a low catalysis, incurring apparent deactivation of  the acid for enabling to control acid diffusion.  In particular, since sulfonium salts and iodonium salts of  an a-position non-fluorinated sulfonic acid and a carboxylic  acid are photo-decomposable, those portions receiving a  high light intensity are reduced in quenching capability and  increased in the concentration of an a-position fluorinated  sulfonic acid, imide acid, or methide acid. This enables to  form a pattern having an improved contrast in exposed area,  further improved DOF and satisfactory dimensional control.  If a photoacid generator capable of generating a strong  acid is an onium salt, an exchange from the strong acid  generated upon exposure to high-energy radiation to a weak  acid as above can take place, but it never happens that the  weak acid generated upon exposure to high-energy radiation  collides with the unreacted onium salt capable of generating  a strong acid to induce a salt exchange. This is because of a  likelihood of an onium cation forming an ion pair with a  stronger acid anion.  In case the acid labile group is an acetal group which is  very sensitive to acid, the acid for eliminating the protective 40  group need not necessarily be an a-fluorinated sulfonic acid,  imide acid or methide acid. Sometimes, deprotection reac› tion may take place even with a-position non-fluorinated  sulfonic acid. In this case, since an onium salt of sulfonic  acid carmot be used as the quencher, an onium salt of $_{45 }$ carboxylic acid is preferably used alone as the quencher.  Of the onium salts of a-position non-fluorinated sulfonic  acid and carboxylic acid, sulfonium salts of sulfonic acid  having the following formula (Zl) and sulfonium salts of  carboxylic acid having the following formula (Z2) are  preferred. 