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[0082] The sulfonium compound of the invention is char› acterized by having a betaine structure and a fluoroalcohol unit. Because of the betaine structure, a bulky structure is retained even after acid generation, indicating that acid diffusion is controllable at a high level. In general, when the generated acid is of bulky structure, acid diffusion is reduced. In addition, owing to the fluoroalcohol unit which is highly compatible, the sulfonium compound has a higher solubility in resist solvent and developer, leading to fewer defects. 

[0083] The resist composition comprising the betaine structure sulfonium compound of the invention is superior in sensitivity, CDU and LWR to prior art resist compositions comprising an acid generator of betaine structure, as described in Patent Documents 5 and 6, for example. A reduction in coating defects and post-development defects is another advantage. Although the reason is not well under› stood, the following two reasons are probable. 

[0084] The sulfonium compound of the invention has a fluorinated alcohol unit, i.e., partial structure having formula (1-1 ), which exerts an effect of improving solvent solubility. In general, because of their high polarity, betaine com› pounds have poor solubility in solvents, for example, in propylene glycol monomethyl ether acetate (PGMEA) com- 

manly used as the casting solvent for resist compositions. In contrast, the inventive sulfonium compound has a fluori› nated alcohol unit which contributes to an improvement in solvent solubility. As a result, the sulfonium compound is uniformly dispersed in the resist composition, probably leading to improvements in CDU and LWR. The improved compatibility leads to a reduction of defects originating from the PAG. 

[0085] Another reason which can be contemplated herein is that the sulfonium compound of the invention has a fluorinated alcohol unit having formula (1-1) whereby the hydroxyl group in that structure forms a hydrogen bond with the acid generated upon exposure, leading to acid diffusion suppression. As a result, various lithography performance factors are improved. 

[0086] Further, when the compound has a partial structure of formula (1-2), which is preferred among the structures of formula (1-1), some protons generated upon exposure are trapped to form a 5-membered ring, as shown by the following formula with the oxygen or sulfur atom in L 2 a moiety and the oxygen atom in the terminal hydroxyl group, leading to suppression of acid diffusion and improvements in various lithography performance factors. 

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Herein L$^{2}$a, Ra 1 , Ra 2 , Rf1a and Rf2a are as defined above. 

[0087] Described below is the synthesis of the inventive sulfonium compound. Reference is made to the synthesis of the compound of formula (1) wherein Z is a group of formula (IA) and X is an ether bond as a typical example. While there are several synthesis routes, one typical route is by reacting a sulfoalkyloxybenzene or sulfoaryloxybenzene with a diary! sulfoxide in the presence of an acid catalyst. The reaction is outlined below as Scheme A. 

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Scheme A 

