Apr. 18, 2019  2  US 2019/0113844 Al  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).  [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)  [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.  [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  [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).  [0026]  Often the resist composition further comprises a  surfactant.  [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.  [0024]  The resist composition may further comprise a  dissolution inhibitor. The composition is typically a chemi› cally amplified positive resist composition.  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.  CITATION LIST  DISCLOSURE OF INVENTION  (al)  RA  ~  yl  OAO  I  RI!  (a2)  RA  ~  y2  OR12  (A)  (fl)  ~  zl  I  R21_ 8 +_R22  M-