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profile, and minimal defects. The inventive resist composi› tions are also useful in the EB lithography. 

[0356] It is evident from the data in Tables 3 to 5 that the resist compositions containing sulfonium compounds within the scope of the invention as PAG exhibit a good balance of sensitivity, LWR, LER, and CDU. Satisfactory results are also obtained with respect to defects. The inventive PAG has the advantages of suppressed acid diffusion owing to its betaine structure and improved compatibility and hence more uniform dispersion owing to its hexafluoroalcohol unit, which lead to improvements in LWR, LER and CDU. Further the improved compatibility leads to a reduction of defects. 

[0357] Japanese Patent Application No. 2017-161031 is incorporated herein by reference. 

[0358] Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A sulfonium compound having the formula (1): 

wherein Lis a straight, branched or cyclic C$_{1 }$-C$_{20 }$divalent hydrocarbon group which may contain a heteroatom› containing moiety, X is a divalent linking group, and Z is a group of sulfonium cation structure having the formula (IA), (18) or (IC): 

<img src='content_image/907553.jpg'>

<img src='content_image/907554.jpg'>

<img src='content_image/907555.jpg'>

wherein the broken line denotes a valence bond to X, A is a single bond, methylene, carbonyl, sulfinyl, sulfonyl, amino, ether bond, thioether bond, ester bond, carbonate bond, carbamate bond or sulfonic acid ester bond, 

R 1$_{a }$ to R$^{1}$c are each independently a straight, branched or cyclic C$_{1 }$-C$_{20 }$ monovalent hydrocarbon group which may contain a heteroatom-containing moiety, at least one ofR$^{1$_{a }$}$to R$^{1}$cbeing a group of the formula (1-1) shown below, with the proviso that where at least two R la are included, two of them may bond together to form a ring with carbon atoms on the benzene ring to which they are attached, where at least two R $^{16 }$are included, two of them may bond together to form a ring with carbon atoms on the benzene ring to which they are attached, and where at least two R$^{1}$c are included, two of them may bond together to form a ring with carbon atoms on the benzene ring to which they are attached, 

ml, m2 and m3 are integers in the range: 0smls5, 0sm2s5, 0sm3s4, and ml+m2+m3;d, 

nl, n2 and n3 are integers in the range: 0snls4, 0sn2s4, 0sn3s4, and nl+n2+n3;d, 

kl, k2 and k3 are integers in the range: 0sks5, 0sk2s4, 0sk3s3, and kl+k2+k3;;,;l, 

<img src='content_image/907552.jpg'>

wherein L 2 is a straight, branched or cyclic C$_{1 }$-C$_{20 }$diva› lent hydrocarbon group which may contain a heteroa› tom-containing moiety, R:’1 and w2 are each indepen› dently hydrogen, a straight, branched or cyclic C$_{1 }$-C$_{20 }$ alkyl, fluorine, or straight, branched or cyclic C$_{1 }$-C$_{20 }$ fluoroalkyl having at least one fluorine atom, at least one of R:’1 and w2 being fluorine or fluoroalkyl, the broken line denotes a valence bond. 

2. The sulfonium compound of claim 1 wherein R:’1 and w2 are trifluoromethyl. 

3. The sulfonium compound of claim 1 which has the formula (2): 

<img src='content_image/907556.jpg'>

wherein X and Z are as defined above, R:13, R’ 4 , w5 and R:15 are each independently hydrogen, fluorine or trif› luoromethyl. 

4. The sulfonium compound of claim 3 wherein both w5 and R’ 6 are fluorine. 

5. A photoacid generator comprising the sulfonium com› pound of claim 1. 

6. A resist composition comprising the photoacid genera› tor of claim 5. 

7. The resist composition of claim 6, further comprising a base resin containing a polymer comprising recurring units having the formula (a) and recurring units having the formula (b ): 

