Feb.28,2019  10  US 2019/0064665 Al  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.  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.  [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- [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.  Q  (’YS~  CF3  o~s~o  1•  ,s03•  II  /  X  CF 3  O  F  F  CF3  OH  Scheme A  (R1c)m3  (}-l-S0;-0  acid 