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[0012] The EUV lithography resist must meet high sensi› tivity, high resolution and low LWR at the same time. As the acid diffusion distance is reduced, LWR is reduced, but sensitivity becomes lower. For example, as the PEB tem› perature is lowered, the outcome is a reduced LWR, but a lower sensitivity. As the amount of quencher added is increased, the outcome is a reduced LWR, but a lower sensitivity. It is necessary to overcome the tradeoff relation between sensitivity and LWR. 

[0013] Patent Document 1: JP-A 2001-194776 

[0014] Patent Document 2: JP-A 2002-226470 

[0015] Patent Document 3: JP-A 2002-363148 

[0016] Patent Document 4: JP-A 2015-090382 

[0017] Non-Patent Document 1: SPIE Vol. 7639 p76390 W (2010) 

[0018] For the acid-catalyzed chemically amplified resist material, it is desired to develop a quencher capable of providing a high sensitivity and improving LWR or CDU. 

[0019] An object of the invention is to provide a resist composition which exhibits a high sensitivity, reduced LWR and improved CDU, independent of whether it is of positive tone or negative tone; and a pattern forming process using the same. 

[0020] The inventors have found that using a sulfonium salt of thiophenecarboxylic acid as the quencher, a resist material having a high contrast, high resolution, reduced LWR, improved CDU, and wide process margin is obtain› able. 

[0021] In one aspect, the invention provides a resist com› position comprising a base polymer and a sulfonium salt having the formula (A). 

Herein R 1 , R 2 and R 3 are each independently hydrogen, hydroxyl, cl -C6 alkyl, cl -C6 alkoxy, C2-C6 acyl, C2-C6 alkoxycarbonyl, C 6 -C$_{10 }$aryl, halogen, nitro, amino, or cyano group, in which at least one hydrogen may be substituted by C 1 -C 6 alkyl, hydroxyl, halogen, nitro or amino moiety, or at least one carbon may be substituted by an ether bond or thioether bond, R 1 and R 2 may bond together to form an alicyclic or aromatic ring with the carbon atom to which they are attached, at least one hydrogen on the ring may be substituted by C 1 -C 6 alkyl, hydroxyl, halogen, nitro or amino moiety; X is a single bond or a C 1 -C$_{10 }$ divalent aliphatic hydrocarbon group in which at least one hydrogen may be substituted by halogen, or at least one carbon may be substituted by an ether bond, ester bond or carbonyl moiety; R 4 , Rs and R 6 are each independently halogen or a C 1 -C$_{20 }$monovalent hydrocarbon group which may contain a 

## CITATION LIST 

## DISCLOSURE OF INVENTION 

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heteroatom, any two of R 4 , Rs and R 6 may bond together to form a ring with the sulfur atom to which they are attached. 

[0022] In a preferred embodiment, the resist composition may further comprise an acid generator capable of generat› ing sulfonic acid, imide acid or methide acid and/or an organic solvent. 

[0023] In a preferred embodiment, the base polymer com› prises recurring units having the formula (al) or recurring units having the formula (a2). 

Herein RA is each independently hydrogen or methyl, Y’ is a single bond, phenylene group, naphthylene group, or C 1 -C$_{12 }$ linking group containing an ester bond or lactone ring, Y 2 is a single bond or ester bond, R $^{11 }$and R 12 each are an acid labile group. 

[0024] The resist composition may further comprise a dissolution inhibitor. The composition is typically a chemi› cally amplified positive resist composition. 

[0025] In another preferred embodiment, the base polymer is free of an acid labile group. The resist composition may further comprise a crosslinker. The composition is typically a chemically amplified negative resist composition. 

[0026] Often the resist composition further comprises a surfactant. 

[0027] In a preferred embodiment, the base polymer fur› ther comprises recurring units of at least one type selected from the formulae (fl) to (f3). 

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