Feb.28,2019  6  US 2019/0064665 Al  [0075]  In formula (1-2), L 2$_{a }$ is an ester, ether or thioether  bond, with the ether bond being preferred.  Ra 1  and  Ra 2  are  each independently hydrogen or methyl, preferably hydro› gen.  R:’1-a  and  R:12a  are each independently fluorine or a  straight, branched or cyclic C$_{1 }$-C$_{20 }$fluoroalkyl group (hav› ing at least one fluorine atom), preferably fluorine or trif› luoromethyl, with trifluoromethyl being most preferred.  [0076]  In formulae (IA) to (IC), R 1 a to R 1 c are monova› lent hydrocarbon groups. Suitable monovalent hydrocarbon  groups include alkyl groups such as methyl, ethyl, propyl,  isopropyl, n-butyl,  s-butyl, t-butyl, t-pentyl, n-pentyl,  n-hexyl, n-octyl, n-nonyl, n-decyl, cyclopentyl, cyclohexyl,  2-ethylhexyl, cyclopentylmethyl, cyclopentylethyl, cyclo› pentylbutyl, cyclohexylmethyl, cyclohexylethyl, cyclohex› ylbutyl, norbomyl, tricyclo[5.2.l.0 2 • $^{6}$]decanyl, adamantyl  and adamantylmethyl; and aryl groups such as phenyl,  naphthyl and anthracenyl. Also included are substituted  forms of the foregoing in which at least one ( one or more)  hydrogen is replaced by a moiety containing a heteroatom  such as oxygen, sulfur, nitrogen or halogen, or in which at  least one carbon atom is replaced by a moiety containing a  heteroatom such as oxygen, sulfur, or nitrogen; so that the  group may contain a halogen atom, hydroxyl, cyano, car› bonyl, ether bond, ester bond, sulfonic acid ester bond,  carbonate bond, lactone ring, sultone ring, carboxylic acid  anhydride, or haloalkyl moiety.  [0077]  Of the sulfonium compounds having formula (1),  compounds having the formula (2) are preferred.  Herein X and Z are as defined above, R13,  W 4 ,  w5  and  R1°  are each independently hydrogen, fluorine or trifluorom› ethyl.  [0078]  Preferably at least one of  R13,  W 4 ,  w5  and  R1°  is  fluorine or trifluoromethyl because the acid generated by the  corresponding sulfonium compound has a higher acidity  sufficient for effective cleavage of acid labile groups on the  base resin. More preferably both  w5  and  R1°  are fluorine.  That is, a structure having fluorine at a-position of the sulfa  group as represented by the following formula (3) is pre› ferred because the acid generated therefrom has a higher  acidity.  Herein X,  Z,  R13 and  W 4  are as defined above.  [0079]  Examples of the sulfonium compound having for› mula (1) wherein Z is a group of formula (IA) are shown  below, but not limited thereto.  (2)  W3  F  z-x+t-s0 3- wi  F  (3)  Q  ~S~  CF3  OA/  lAO~S03- CF3  CF3  CF3  CF3  ¢ 0~:::  OH  #  ~S~  CF3  oA/ lA))( 0 ’  l  .,,CF3  F  F  ’f--CF3  CF3 