JP2008120700A - Sulfonium salt - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sulfonium salt hardly migrating to a water phase in a resist for liquid immersion exposure. <P>SOLUTION: The sulfonium salt is represented by formula (1) [wherein, R<SB>1</SB>and R<SB>2</SB>are the same or different, and are each a 1-18C alkyl, a 1-18C alkoxy or a 1-8C perfluoroalkyl; m and n are the same or different and are each 1-5; m R<SB>1</SB>s and n R<SB>2</SB>s are the same or different; R<SB>3</SB>is a 1-18C alkyl, a 1-18C alkoxy, a 1-8C perfluoroalkyl or a group of formula (2); when R<SB>3</SB>is a 1-18C alkyl, a 1-18C alkoxy or a 1-8C perfluoroalkyl, k is 0-5; k R<SB>3</SB>s are the same or different, and when R<SB>3</SB>is a group of formula (2), k is 1; and X<SP>-</SP>is an anion]. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a sulfonium salt, and more particularly to a photoacid generator for a chemically amplified resist used for semiconductor pattern formation, particularly as a photoacid generator for immersion exposure using water as an immersion medium. It is related with the sulfonium salt suitable for.

  Semiconductors, especially LSIs, are becoming increasingly finer and denser, and the resist used for semiconductor pattern formation is exposed by conventional g-line (438 nm) and i-line (365 nm), so KrF excimer lasers are used. (248 nm) or an ArF excimer laser (193 nm). Here, the resist for KrF excimer laser or ArF excimer laser is called a so-called chemical amplification resist, and its principle is as follows. Organic solvent solutions of resins such as those in which the hydroxyl group of polyhydroxystyrene is protected with a tert-butoxycarbonyl group, or the carboxyl group of poly (meth) acrylic acid is protected with an alicyclic hydrocarbon group (for example, an adamantyl group) When a coating film formed by applying this solution on a silicon substrate and removing the solvent is exposed with a KrF excimer laser or an ArF excimer laser, the photoacid generator is used. An acid or proton (H +) is generated, which removes the protecting group of the resin. As a result, the phenolic hydroxyl group and carboxyl group protected by the protective group are regenerated, and the exposed portion of the resist film can be alkali-developed.

  Particularly recently, as an exposure technique for further miniaturization, a method of increasing the resolution by filling the space between the resist film and the exposure lens with a liquid such as water and changing the refractive index, called immersion exposure, has been studied. Yes. In this regard, a pattern formation method based on an immersion process (see, for example, Patent Document 1), a resist composition used in a resist pattern formation method that includes an immersion exposure step (see, for example, Patent Document 2), and a time lapse after exposure. A positive resist composition (see, for example, Patent Document 3) in which deterioration of the profile such as resist pattern collapse is improved has been studied.

  In immersion exposure technology that uses water as the immersion liquid, water is filled between the resist film and the exposure lens, so components such as photoacid generators migrate from the resist film to the water and contaminate the optical system. There is a problem of fear. In particular, when the photoacid generator moves to water, the amount of the photoacid generator substantially present in the resist film is reduced, which affects the pattern formation, and does not cause these problems. A generator is desired (see Non-Patent Documents 1 and 2).

Japanese Examined Patent Publication No. 63-49893 JP 2005-208509 A JP 2005-266799 A Latest resist material handbook Information organization p. 138-145 (2005) Latest Trends in UV and EB Curing Technology CMC Publishing p. 73-74 (2006)

  In light of the above background, the present invention substantially prevents the transition to the aqueous phase in the immersion exposure resist using water as an immersion liquid, compared to the conventionally used sulfonium salts. An object of the present invention is to provide a sulfonium salt capable of preventing the above.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a sulfonium salt having a specific structure exhibits excellent properties suitable for the above-mentioned purpose, and in a resist for immersion exposure using water, from a resist composition. It was found that elution of the sulfonium salt in water was substantially avoided, and the present invention was completed.

That is, the present invention provides the following.
1. General formula (1)

(In the formula (1), R ₁ and R ₂ are the same or different from each other, alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms M and n are the same as or different from each other and represent an integer of 1 to 5, m R _{1 s} may be the same or different from each other, and n R _{2 s} are the same as each other R ₃ may be an alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms, a perfluoroalkyl group having 1 to 8 carbon atoms, or the formula (2)

When R ₃ represents an alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms or a perfluoroalkyl group having 1 to 8 carbon atoms, k is an integer of 0 to 5. And k R _{3 s} may be the same as or different from each other. When R ₃ represents a group represented by the formula (2), k is 1, and X ⁻ is an anion paired with sulfonium. In formula (2), R ₁ , R ₂ , m, n and X ⁻ are the same as defined above. The sulfonium salt shown by this.
2. The sulfonium of 1 above, wherein X ⁻ is an anion moiety as a conjugate base of at least one compound selected from fluorinated alkanesulfonic acid, fluorinated arenesulfonic acid, fluorinated alkanesulfonylimide and fluorinated alkanesulfonylmethide salt.
3. 1 or 2 above, wherein at least one of m R ₁ is an alkyl group having 4 to 18 carbon atoms, and at least one of n R ₂ is an alkyl group having 4 to 18 carbon atoms. The sulfonium salt.
4). The sulfonium salt according to any one of 1 to 3 above, wherein the solubility in water (20 ° C.) is 100 ppm or less.
5. A photoacid generator for a chemically amplified photoresist, comprising the sulfonium salt according to any one of 1 to 4 above.

  According to the present invention, a sulfonium salt that is substantially insoluble in water is obtained. Therefore, the sulfonium salt obtained by the present invention is a photo-acid generator for a water phase when used as a photoacid generator for a resist for immersion exposure using water as an immersion liquid in a chemically amplified resist for semiconductor microfabrication. Since migration of the agent is substantially prevented, it is advantageous in preventing contamination of the optical system and forming a good pattern.

  The substituted sulfonium salt of the present invention is represented by the following formula (1).

In formula (1), R ₁ and R ₂ are each an alkyl group having 1 to 18 carbon atoms (preferably 1 to 12 carbon atoms, more preferably 1 to 10 carbon atoms, particularly preferably 1 to 8 carbon atoms), carbon An alkoxy group having 1 to 18 carbon atoms (preferably 1 to 10 carbon atoms, more preferably 1 to 4 carbon atoms, particularly preferably 1 to 2 carbon atoms), or 1 to 8 carbon atoms (preferably 1 to 4 carbon atoms, Particularly preferably, it represents a perfluoroalkyl group having 1 to 2 carbon atoms.

  Specific examples of the alkyl group having 1 to 18 carbon atoms include methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-octyl, n-decyl, n-dodecyl, n-tetradecyl and n-hexadecyl. Linear alkyl groups such as n-octadecyl, isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, neopentyl, tert-pentyl, isohexyl, 2-ethylhexyl, 1,1,3,3-tetramethylbutyl, etc. Examples include branched alkyl groups, cyclic alkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl, and bridged cyclic alkyl groups such as norbornyl, adamantyl, and pinanyl.

  Specific examples of the alkoxy group having 1 to 18 carbon atoms include methoxy, ethoxy, methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, tert-butoxy and the like.

  Specific examples of the perfluoroalkyl group having 1 to 8 carbon atoms include trifluoromethyl, pentafluoroethyl, nonafluorobutyl, perfluorooctyl and the like.

  Of these specific examples, preferred are linear alkyl groups such as methyl, n-butyl, n-octyl, n-decyl, n-dodecyl, n-tetradecyl, n-hexadecyl, n-octadecyl; tert-butyl, Branched alkyl groups such as neopentyl, tert-pentyl, 1,1,3,3-tetramethylbutyl; cyclic alkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl; bridged cyclic alkyl groups such as norbornyl, adamantyl and pinanyl Alkoxy groups such as methoxy, ethoxy and tert-butoxy; perfluoroalkyl groups such as trifluoromethyl, nonafluorobutyl and perfluorooctyl, more preferably tert-butyl, tert-pentyl, 1,1, 3,3-tetramethylbutyl, 1-adamantyl, etc. Alkyl group having grade carbons; and trifluoromethyl, nonafluorobutyl, a fluorinated alkyl group, such as perfluorooctyl.

Further, m and n represents the number of substituents R ₁ and R ₂ in the formula (1) is an integer of 1 to 5, respectively. R ₁ and R ₂ may be the same as or different from each other. Further, m R _{1 s} may be the same as or different from each other, and n R _{2 s} may be the same as or different from each other.

The substituent R ₃ in the formula (1) is an alkyl group having 1 to 18 carbon atoms (preferably 1 to 12 carbon atoms, more preferably 1 to 10 carbon atoms, particularly preferably 1 to 8 carbon atoms), 1 to 18 (preferably 1 to 10 carbon atoms, more preferably 1 to 4 carbon atoms, particularly preferably 1 to 2 carbon atoms) alkoxy group, 1 to 8 carbon atoms (preferably 1 to 4 carbon atoms, particularly preferably Represents a perfluoroalkyl group having 1 to 2 carbon atoms or a group represented by the following formula (2).

In formula (2), R ₁ , R ₂ , m and n are the same as defined in formula (1). X ⁻ represents an anion paired with sulfonium.

When the substituent R ₃ is an alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms, k is an integer of 0 to 5, and specific examples thereof Is as described herein. When the substituent R ₃ is represented by the formula (2), k is 1.

X ^{− in} formula (1) and formula (2) represents an anion paired with sulfonium, and examples thereof include fluorinated alkanesulfonic acid, fluorinated arenesulfonic acid, fluorinated alkanesulfonylimide, and fluorinated alkanesulfonyl. An anion portion which is a conjugate base of a compound such as methide.

  Specific examples of the fluorinated alkanesulfonic acid include, for example, sulfonic acids having a perfluoroalkyl group having 1 to 20 carbon atoms, and those having 1 to 10 carbon atoms that are economically available are preferable. The perfluoroalkyl group having 1 to 10 carbon atoms is linear, branched or cyclic, or a perfluoroalkylalkoxy group in which one or more fluorine atoms of these perfluoroalkyl groups are 1 to 4 carbon atoms. And further substituted. Specific examples include trifluoromethanesulfonic acid, pentafluoroethanesulfonic acid, nonafluorobutanesulfonic acid, perfluorooctanesulfonic acid, perfluorocyclohexanesulfonic acid, perfluoroethoxyethanesulfonic acid, and the like. Particularly preferred are trifluoromethanesulfonic acid, pentafluoroethanesulfonic acid, and nonafluorobutanesulfonic acid.

  Examples of the fluorinated arene sulfonic acid include sulfonic acids having 6 to 20 carbon atoms and having an aromatic group substituted with at least one fluorine atom or trifluoromethyl group, and of these, 6 to 10 carbon atoms. Those are preferred. Specific examples include 2-fluorobenzenesulfonic acid, 4-fluorobenzenesulfonic acid, 2,4-difluorobenzenesulfonic acid, 3,5-difluorobenzenesulfonic acid, 2,6-difluorobenzenesulfonic acid, 2,4, 6-trifluorobenzenesulfonic acid, perfluorobenzenesulfonic acid, perfluoronaphthalenesulfonic acid, 2- (trifluoromethyl) benzenesulfonic acid, 4- (trifluoromethyl) benzenesulfonic acid, 3,5-bis (trifluoro) And methyl) benzenesulfonic acid. Particularly preferred are 4-fluorobenzenesulfonic acid, 2,4,6-trifluorobenzenesulfonic acid, perfluorobenzenesulfonic acid, and 4- (trifluoromethyl) benzenesulfonic acid.

  The fluorinated alkanesulfonylimide and the fluorinated alkanesulfonylmethide are represented by the following formulas (3) and (4), respectively.

(C _f SO ₂ ) ₂ NH (3)
(C _f SO ₂ ) ₃ CH (4)

C _f in the formulas (3) and (4) represents a perfluoroalkyl group having 1 to 10 carbon atoms, which may be linear or branched, and a plurality of perfluoroalkyl groups may be mutually linked. May be formed.

  Specific examples of the fluorinated alkanesulfonylimide and the fluorinated alkanesulfonylmethide include bisperfluorobutanesulfonylimide, bistrifluoromethanesulfonylimide, trifluoromethylpentafluoroethylsulfonylimide, hexafluorotrimethylenesulfonylimide, tristrifluoride. Examples include lomethanesulfonylmethide, trispentafluoroethylsulfonylmethide, and the like. Bisperfluorobutanesulfonylimide, hexafluorotrimethylenesulfonylimide, and tristrifluoromethanesulfonylmethide are preferable.

  The synthesis of sulfonium salts from diaryl sulfoxides and diaryl sulfides and the method of anion substitution are well known to those skilled in the art (for example, JP-A-61-100557, JP-A-5-4996, Chem. Pharm. Bull. 29 (12) 3753 (1981)). Accordingly, the production of the sulfonium salt of the present invention can be carried out in accordance with, for example, diaryl sulfoxide having a substituent corresponding to the target sulfonium salt and diaryl sulfide using a strong acid such as sulfuric acid in the presence of a dehydrating agent. This can be done by condensation. The strong acid to be used may have the same structure as the counter anion constituting the sulfonium salt or may be different. When acids having different structures are used, a sulfonium salt having the desired structure can be obtained by an anion exchange reaction as appropriate.

  In the production of the sulfonium salt, diaryl sulfoxide is, for example, a compound given by the following formula (5).

In the formula (5), R ₁ , R ₂ , m and n are the same as described above.

  In the production of the sulfonium salt, the diaryl sulfide is, for example, a compound given by the following formula (6).

In the formula (6), R ₃ and k are the same as defined above.

  In the production of the sulfonium salt, the acid used is sulfuric acid, methanesulfonic acid, trifluoromethanesulfonic acid, the above-mentioned fluorinated alkanesulfonic acid, fluorinated arenesulfonic acid, fluorinated alkanesulfonylimide, fluorinated alkanesulfonylmethide. Etc.

  In the production of the sulfonium salt, as the dehydrating agent, carboxylic acid anhydrides such as acetic anhydride, propionic anhydride, trifluoroacetic anhydride, trifluoromethanesulfonic anhydride, polyphosphoric acid, diphosphorus pentoxide, etc. should be used. Can do. These dehydrating agents may be used alone or in combination of two or more.

  The production of the sulfonium salt can be carried out in the presence of a solvent if necessary. Examples of the solvent used in this case include ethers such as diethyl ether and dipropyl ether, chlorinated organic solvents such as chloroform and dichloromethane, alcohols such as methanol and ethanol, ketones such as acetone and methyl ethyl ketone, acetic acid, propion, and the like. Examples thereof include organic acids such as acids, acetic anhydride, organic acid anhydrides such as propionic anhydride, nitriles such as acetonitrile, and other polar organic solvents. These solvents may be used alone or in combination of two or more.

  When anion exchange is performed by the salt exchange method, a salt solution having a target anion structure, for example, an inorganic salt such as sodium salt, potassium salt or ammonium salt as an aqueous solution, and a solvent solution containing the sulfonium salt obtained by the above reaction It is obtained by mixing with. The sulfonium salt before anion exchange may be isolated and purified in advance. In this case, the sulfonium salt before anion exchange may be dissolved again in the above solvent and mixed with an aqueous solution of a salt having the target anion structure.

  In the production of the sulfonium salt, the obtained sulfonium salt may be purified as necessary. This purification can be carried out by directly or directly concentrating the solution containing the sulfonium salt to a desired concentration and then adding a poor solvent to precipitate the sulfonium salt. Examples of the poor solvent used herein include ethers such as diethyl ether and dipropyl ether, esters such as ethyl acetate and butyl acetate, aliphatic hydrocarbons such as hexane and cyclohexane, and aromatic hydrocarbons such as toluene and xylene. And the like. When the precipitate is an oily substance, the oil layer is separated, and the target sulfonium salt can be obtained by removing the solvent under reduced pressure. When the deposit is a solid, the precipitated solid can be filtered and dried to obtain the target product. Further, when the target sulfonium salt is a solid, it can be purified by recrystallization. That is, the obtained solid is dissolved in a solvent, for example, a chlorinated organic solvent such as chloroform or dichloromethane, an alcohol such as methanol or ethanol, a ketone such as acetone or methyl ethyl ketone, or a nitrile such as acetonitrile, and is cooled. The crystal can be purified by utilizing the difference in solubility, or can be purified by gradually adding a poor solvent as exemplified above to the solvent solution containing the sulfonium salt to precipitate the crystal.

  The sulfonium salt of the present invention is suitably used as a photoacid generator for a chemically amplified resist used for semiconductor pattern formation. That is, when pattern formation is performed using the resist composition comprising the sulfonium salt obtained in the present invention as the basic resin, additive, solvent, and photoacid generator of a photoresist, a good resist pattern is obtained.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not intended to be limited thereby.

Example 1: Synthesis of (4-phenylthiophenyl) -di [2-methoxy-5- (1,1,3,3-tetramethyl) butyl] phenylsulfonium nonafluorobutanesulfonate

In a 100 ml reaction vessel, 30.0 g of dichloromethane, 10.19 g (20.9 mmol) of di [2-methoxy-5- (1,1,3,3-tetramethyl) butyl] phenyl sulfoxide, 4.02 g of diphenyl sulfide (21 .6 mmol) and 6.60 g (64.8 mmol) of acetic anhydride were added and mixed uniformly. Then, 7.77 g (25.9 mmol) of nonafluorobutanesulfonic acid was added dropwise at room temperature over 40 minutes. During the dropping, the liquid temperature rose to 45 ° C. due to heat generation. Further, after stirring at 45 ° C. for 4 hours, the reaction solution was washed 4 times with 30.0 g of ion-exchanged water, and the organic layer and the aqueous layer were separated. When the organic layer was concentrated and diethyl ether was gradually added thereto, the solution became cloudy and a white powder precipitated. This was filtered off and dried at 55 ° C. under reduced pressure to obtain 11.4 g (yield 57%) of white powder. This is designated as sulfonium salt A-1.
As a result of analysis by ¹ H-NMR, ¹³ C-NMR, IR, and HPLC (using high-performance liquid chromatograph L-7000, manufactured by Hitachi, trade name, hereinafter the same), this is the above-mentioned formula which is the target product (4-phenylthiophenyl) -di [2-methoxy-5- (1,1,3,3-tetramethyl) butyl] phenylsulfonium nonafluorobutanesulfonate having a purity of 99.2%. ¹ H-NMR (300 MHz, DMSOd-6) δppm: 0.59 (s, 18H), 1.15 (d, 12H), 1.55 (t, 4H), 3.85 (s, 3H), 6.75 (d, 2H), 7.40 (d, 2H), 7.44-7.58 (m, 7H), 7.62 (d, 2H), 7.89 (dd, 2H)

Example 2: Synthesis of (4-phenylthiophenyl) -di (3-tert-butyl-4-methoxy-5-methyl) phenylsulfonium nonafluorobutanesulfonate

Instead of di [2-methoxy-5- (1,1,3,3-tetramethyl) butyl] phenyl sulfoxide, 8.41 g of di (3-tert-butyl-4-methoxy-5-methyl) phenyl sulfoxide ( 13.8 g (yield 76%) of white powder was obtained in the same manner as in Example 1 except that 20.9 mmol) was used. As a result of analysis by ¹ H-NMR, ¹³ C-NMR, IR, and HPLC, this product was found to be (4-phenylthiophenyl) -di (3-tert-butyl-4-methoxy-5) of the above formula. -Methyl) phenylsulfonium nonafluorobutanesulfonate with a purity of 99.1%. This is designated as sulfonium salt A-2. ¹ H-NMR (300 MHz, DMSOd-6) δppm: 1.26 (s, 18H), 2.32 (s, 6H), 3.81 (s, 6H), 7.36 (d, 2H), 7.40 (d, 2H), 7.50 -7.58 (m, 5H), 7.60 (d, 2H), 7.66 (d, 2H)

Example 3: Synthesis of (4-phenylthiophenyl) -di (4-tertbutyl) phenylsulfonium nonafluorobutanesulfonate

Except that instead of di [2-methoxy-5- (1,1,3,3-tetramethyl) butyl] phenyl sulfoxide, 6.57 g (20.9 mmol) of di (4-tertbutyl) phenyl sulfoxide was used. In the same manner as in Example 1, 11.4 g (yield 70%) of white powder was obtained. As a result of analysis by ¹ H-NMR, ¹³ C-NMR, IR, and HPLC, this product was found to be (4-phenylthiophenyl) -di (4-tertbutyl) phenylsulfonium nonafluorobutanesulfoform having the above formula. It was naruto and had a purity of 95.1%. This is designated as sulfonium salt A-3. ¹ H-NMR (300 MHz, DMSOd-6) δppm: 1.30 (s, 18H), 7.39 (d, 3H), 7.49-7.60 (m, 4H), 7.66-7.81 (m, 10H)

Example 4: Synthesis of phenyl- [4- (4-trifluorophenylthio) phenyl]-(4-trifluoromethyl) phenylsulfonium nonafluorobutanesulfonate

Instead of di [2-methoxy-5- (1,1,3,3-tetramethyl) butyl] phenyl sulfoxide of Example 1, 5.65 g (20.9 mmol) of (4-trifluoromethyl) phenyl sulfoxide, 13.0 g (yield 77%) of the desired product was obtained in the same manner as in Example 1 except that 5.49 g (21.6 mmol) of (4-trifluoromethyl) phenyl sulfoxide was used instead of diphenyl sulfide. As a result of analysis by ¹ H-NMR, ¹³ C-NMR, IR, and HPLC, this product was obtained as phenyl- [4- (4-trifluorophenylthio) phenyl]-(4-trifluoromethyl) phenyl, which is the target product. It was sulfonium nonafluorobutanesulfonate and had a purity of 98.1%. This is designated as sulfonium salt A-4. ¹ H-NMR (300 MHz, DMSOd-6) δppm: 7.33 (d, 1H), 7.43-7.53 (m, 3H), 7.58 (d, 1H), 7.65 (d, 1H), 7.68-7.91 (m, 8H), 7.94-8.03 (m, 2H), 8.13 (d, 1H)

Example 5: Solubility of sulfonium salt in water Using each of the sulfonium salts of Examples 1 to 5 and trisulfone sulfonium trifluoromethanesulfonate salt as the sulfonium salt of Comparative Example 1, the solubility in water at 20 ° C was as follows. It measured as follows.

[Method of measuring solubility in water]
0.5 g of each of the sulfonium salts of Examples 1 to 4 and Comparative Example 1 was put in a glass container with a 30 ml cap, and 20 g of ion-exchanged water and a stirrer were added. The temperature was adjusted to 20 ° C., stirred for 30 minutes, and then allowed to stand at 20 ° C. for 6 hours. Each supernatant is taken with a 3.0 ml syringe, filtered through a membrane filter (0.2 μm), and the concentration of the sulfonium salt dissolved in this filtrate is measured by HPLC, whereby the solubility (ppm) in water is determined. Asked.

  The results are shown in Table 1. From this result, it was found that the sulfonium salt of the present invention was very low in water solubility and substantially insoluble in water as compared with the sulfonium salt of Comparative Example 1. Therefore, it can be seen that in a resist for immersion exposure using water as an immersion liquid, if the sulfonium salt of the present invention is used as a photoacid generator, elution of the sulfonium salt from the resist film into water can be substantially prevented.

According to the present invention, a sulfonium salt that is substantially insoluble in water can be provided. Therefore, the sulfonium salt obtained by the present invention is suitable as a photoacid generator for a chemically amplified resist for semiconductor microfabrication, particularly a resist for immersion exposure using water as an immersion liquid.

Claims (5)

General formula (1)

(In the formula (1), R ₁ and R ₂ are the same or different from each other, alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms, or a perfluoroalkyl group having 1 to 8 carbon atoms M and n are the same as or different from each other and represent an integer of 1 to 5, m R _{1 s} may be the same or different from each other, and n R _{2 s} are the same as each other R ₃ may be an alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms, a perfluoroalkyl group having 1 to 8 carbon atoms, or the formula (2)

When R ₃ represents an alkyl group having 1 to 18 carbon atoms, an alkoxy group having 1 to 18 carbon atoms or a perfluoroalkyl group having 1 to 8 carbon atoms, k represents an integer of 0 to 5. And k R _{3 s} may be the same as or different from each other. When R ₃ represents a group represented by the formula (2), k represents 1 and X ⁻ represents an anion paired with sulfonium. In formula (2), R ₁ , R ₂ , m, n and X ⁻ are the same as defined above. ) A triarylsulfonium salt represented by X ⁻ is an anion moiety as a conjugate base of at least one compound selected from fluorinated alkanesulfonic acid, fluorinated arenesulfonic acid, fluorinated alkanesulfonylimide and fluorinated alkanesulfonylmethide. Sulfonium salt. The at least one of m R ₁ is an alkyl group having 4 to 18 carbon atoms, and at least one of the n R ₂ is an alkyl group having 4 to 18 carbon atoms. 2 sulfonium salts.   The sulfonium salt according to any one of claims 1 to 3, wherein the solubility in water (20 ° C) is 100 ppm or less.   A photoacid generator for a chemically amplified photoresist, comprising the sulfonium salt according to claim 1.