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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. 

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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. 

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