US 10,180,626 B2  64  63  Examples of the solvent used in Scheme A include  hydrocarbons such as toluene, xylene, hexane and heptane;  chlorinated solvents such as methylene chloride, chloroform  and dichloroethane; ethers such as diethyl ether, tetrahydro› furan and dibutyl ether, ketones such as acetone and 2-bu- tanone; esters such as ethyl acetate and butyl acetate; nitriles  such as acetonitrile; and aprotic polar solvents such as  N,N-dimethylformamide  and  N,N-dimethylacetamide,  which may be used alone or in admixture. The reaction may  be performed even in a solventless system. The reaction  temperature is preferably from - 70°  C. to about the boiling  point of the solvent if any. While the reaction temperature  may be selected as appropriate depending on other reaction  conditions, it is typically from -10°  C. to about the boiling  point of the solvent.  By further effecting ion exchange with sulfonium salt  (le)  using a salt having a predetermined anion  (Z-),  the desired  sulfonium salt (1) may be synthesized. Ion exchange is  readily performed by any well-known methods, for example,  with reference to Patent Document 4.  Another exemplary method for synthesizing the inventive  sulfonium salt is illustrated by the following Scheme B.  A sulfonium salt (le) is synthesized by acting an orga› nometallic reagent or metal on a triarylamine (ld) to form a  nucleophile, and reacting it with a sulfoxide (1 b ). Suitable  organometallic reagents include trityl lithium, trityl sodium,  trityl potassium, methyl lithium, phenyl lithium, sec-butyl  lithium, tert-butyl lithium, and ethylmagnesium bromide.  Suitable metals include lithium, sodium, potassium and  magnesium. The amount of the organometallic reagent or  metal used is preferably 0.5 to 5.0 moles, more preferably  0.8 to 2.0 moles per mole oftriarylamine (ld).  If  the amount  is too short, formation of the nucleophile may be retarded,  resulting in a lowering of yield.  If  the amount is excessive,  there may occur side-reactions such as reaction of unreacted  organometallic reagent with sulfoxide (1 b ), and it may be  difficult to reduce the metal content in the final product,  sulfonium salt (1 ), which is detrimental from the standpoint  of quality control.  The amount oftriarylamine (ld) used is preferably 0.5 to  10.0 moles, more preferably 0.8 to 5.0 moles per mole of  sulfoxide (1 b ).  If  the amount is too short, the reaction may  be retarded, resulting in a lowering of yield.  If  the amount  is excessive, there may arise economic disadvantages due to  increase of reactant cost and lowering of pot yield, and there  may be formed more impurities originating from triarylam› ine (ld), leading to difficulty of purification.  Examples of the solvent used in Scheme B include aprotic  solvents, for example, hydrocarbons such as toluene, xylene,  hexane and heptane; chlorinated solvents such as methylene  chloride, chloroform and dichloroethane; and ethers such as  diethyl ether, tetrahydrofuran and dibutyl ether, which may  be used alone or in admixture. The reaction temperature is  preferably from - 70°  C. to about the boiling point of the  solvent. While the reaction temperature may be selected as  appropriate depending on other reaction conditions, it is  typically from -10°  C. to about the boiling point of the  solvent.  Where little reaction takes place with the nucleophile  alone, an activator for sulfoxide (1 b) may be added. The  activator used herein may be selected from the aforemen- tioned esterifying agents and chlorosilanes such as trimeth› ylsilyl chloride and triethylsilyl chloride. In case an esteri› fying agent which is able to react with the nucleophile is  used as the activator, preferably the sulfoxide (1 b) is first  reacted with the activator to form an active intermediate  before it is reacted with the nucleophile.  By further effecting ion exchange with sulfonium salt (1 e)  using a salt having a predetermined anion  (z-),  the desired  sulfonium salt (1) may be synthesized. Ion exchange is  readily performed by any well-known methods, for example,  with reference to Patent Document 4.  Resist Composition  Another embodiment of the invention is a chemically  amplified resist composition comprising (A) a photoacid  generator in the form of a sulfonium salt having formula (1)  as an essential component. The resist composition may  further comprise:  having formula (1) (also referred to as second photoacid  generator),  insoluble in water and soluble in alkaline developer, and/or  a surfactant which is insoluble or substantially insoluble in  water and alkaline developer, and  Components (D),(E),(F) and (G) are optional, that is, may be  added if necessary.  In the resist composition, an appropriate amount of the  PAG as component (A) is 0.1 to 40 parts by weight, more  preferably 0.2 to 20 parts by weight per 100 parts by weight  (B) a base resin,  (C) an organic solvent,  (D) a photoacid generator other than the sulfonium salt  (E) a quencher,  (F) a surfactant which is insoluble or substantially  (G) another component.  Herein R$^{1 }$to R 5 , p, q, r, Z, and M $_{1 }$+areas defined above,  Hal is halogen, X$_{2 }$- is an anion, and M$_{2 }$ is a metal.  Scheme B  Organometallic  reagent  orM$_{2 }$ (lb)  (1) 