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[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. 

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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. 

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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. 

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