Feb.28,2019  73  US 2019/0064665 Al  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):  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):  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 ):  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,  wherein X and Z are as defined above, R:13, R’ 4 ,  w5  and  R:15 are each independently hydrogen, fluorine or trif› luoromethyl.  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.  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):  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,  (1-1)  (lA)  (lB)  (lC)  W3  W5  Z-X++S03- Rf21  Wo  (2) 