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Exemplary structures for the onium salt include arbitrary combinations of anions with cations, both as exemplified above, but are not limited thereto. 

The monomer of formula (1) is characterized by a high freedom of structural design because the L 3 and A $^{1 }$moieties of the anion and the cation M+ may be freely designed. An optimum structural design may be made so as to be com› pliant with variable factors in a particular application includ- 

ing exposure wavelength, other copolymerized units in the base resin, and a casting solvent in a resist composition. 

The inventive monomer may be synthesized according to the following scheme, for example, although the synthesis route is not limited thereto. 

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Herein Z, L1, L 2 , L 3 , A1, xa, X 6 , m, and M+ are as defined above, Ma+ and Mb+ each are a cation, and x- is an anion, 

First, amide compound (la) is reacted with sulfuryl chlo› ride under basic conditions to synthesize a sulfuryl chloride derivative (1 b) having imide acid structure. At this point, the sulfuryl chloride derivative (1 b) may be isolated or passed as such in one-pot to subsequent reaction without isolation. 

Examples of the base which can be used herein include amines such as ammonia, triethylamine, pyridine, 4-dimeth› ylaminopyridine, lutidine, collidine, and N,N-dimethylani› line; hydroxides such as sodium hydroxide, potassium hydroxide, and tetramethylammonium hydroxide; carbon› ates such as potassium carbonate and sodium hydrogencar› bonate; metals such as sodium; metal hydrides such as sodium hydride; metal alkoxides such as sodium methoxide and potassium t-butoxide; organometallic compounds such as butyl lithium and ethylmagnesium bromide; and metal amides such as lithium diisopropylamide, which may be used alone or in admixture. Inter alia, pyridine is preferred from the standpoint of base stability of the intermediate. 

An appropriate amount of the base used is 0.5 to 10 moles, more preferably 1.0 to 4.0 moles per mole of amide com› pound (la). An appropriate amount of sulfuryl chloride used is 0.5 to 3.0 moles, more preferably 0.8 to 1.5 moles per mole of amide compound (la). Outside the range, a less amount of the base or sulfuryl chloride may be insufficient to promote reaction whereas an excessive amount may induce side reactions and increase the reactant cost. 

A solvent may be used for the reaction. Suitable solvents include hydrocarbons such as toluene, xylene, hexane and heptane; chlorinated solvents such as methylene chloride, chloroform, and dichloroethane; ethers such as diethyl ether, tetrahydrofuran and dibutyl ether; ketones such as acetone and 2-butanone; esters such as ethyl acetate and butyl 

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