1  US 10,754,248 B2  2  SULFONIUM SALT, RESIST COMPOSITION,  AND PATTERNING PROCESS  TECHNICAL FIELD  BACKGROUND  SUMMARY OF THE INVENTION  Problem to be Solved by the Invention  Means for Solving the Problem  The present invention relates to a sulfonium salt having a  specific partial structure, a resist composition containing the  sulfonium salt, and a patterning process using the resist  composition.  Recent higher integration and speed of LSI integrated  circuits has required more advanced micropatterning in  patterning conditions and thus high-resolution resist pat› terns. In addition to lithography characteristics such as  pattern shapes, contrast, mask error enhancement factor  (MEEF), and roughness in particular, the improvement in  defects (surface defect) of developed resist patterns has been  more increasingly urgent than conventionally required. Such  defects are known as faults detected with surface defect  viewing instrument ("KLA", Product from KLA-Tencor  Corporation) that observes developed resist patterns from  the above, such as scum, bubbles, dust, and bridges between  resist patterns in developed resist patterns. The resulting  defects are partially attributed to low solubility to a cast  solvent in a resist composition such as a photo acid generator  and residual defects after the use of a developer.  One common means for reducing defects caused by the  use of a photo acid generator is to improve the solubility to  a developer and a resist solvent by modification from a photo  acid generator. The improvement in solubility to an organic  solvent may require introduction of a linear alkyl group, a  fluorine atom and others, while the alkaline developer solu- bility can be improved by introducing a polar group such as  a hydroxy group. A known effective method for improving  the solubility to a solvent is to introduce a hexafluoroalcohol  unit, e.g., those having excellent compatibility in the fol› lowing sulfonium cation illustrated in Patent Documents 1  and 2. As a resist composition, the use of the photo acid  generators disclosed in these documents can certainly reduce  defects, but it is very hard to obtain satisfactory lithography  performance due to swelling and increasing acid diffusion  length. Likewise, anion modification is expected to improve  the compatibility, while the resulting acid diffusion promo› tion can degrade lithography performance,  Amid growing recent demands for high-resolution resist  patterns, it is hard to satisfy both defect reduction and stable  lithography performance in a resist composition using a  conventional photo acid generator. The introduction of a  fluorine atom or an alkyl group is effective in defect reduc- lO tion, but this approach unfortunately fails to control acid  diffusion and thus obtain a satisfactory lithography perfor› mance such as mask error factor (MEF), exposure latitude  (EL), line width roughness (LWR), and critical dimension  uniformity (CDU).  The present invention was made in view of the situation  to solve the problems, and has an object to provide a  sulfonium salt capable of providing a resist composition  having few defects by its use as a photo acid generator in  photolithography where a high energy beam is used as a  light source, and excellent in lithography performance by  controlling acid diffusion, a resist composition containing  the sulfonium salt, and a patterning process using the resist  composition.  To solve these problems, the present invention provides a  sulfonium salt including an anion and a cation, the cation  having a partial structure represented by the following  general formula (1),  wherein, each of  R’a  and  w1’  independently represents a  fluoroalkyl group having 1 to 4 carbon atoms;  Rxa  represents  a hydrogen atom or an acid labile group; each of  Ra  and  Rb  independently represents a hydrogen atom, or a monovalent  hydrocarbon group having 1 to 10 carbon atoms optionally  containing a heteroatom,  Ra  and  Rb  may be bonded to form  a ring together with a carbon atom bonded thereto; Ala  represents an ether bond or a thioether bond; L  la  represents  a single bond, or a divalent linking group having 1 to 20  carbon atoms optionally containing a heteroatom;  "*" rep- resents a bond, and the sulfonium salt doesn’t correspond to  a sulfonium salt having a cation represented by the follow› ing general formula (1 ’),  PRIOR ART DOCUMENTS  Patent Documents  Patent Document  1:  JP-B-4621806  Patent Document 2: JP-A-2014-122204  wherein,  R’a  and  w1’  represent the same meanings as before.  (1’)  Ra  Rfa  •-L!a_A!a++o-RXa  Rb  RJb  (1) 