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## SULFONIUM SALT, RESIST COMPOSITION, AND PATTERNING PROCESS 

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, 

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Patent Document 1: JP-B-4621806 Patent Document 2: JP-A-2014-122204 

## TECHNICAL FIELD 

## BACKGROUND 

## PRIOR ART DOCUMENTS 

## Patent Documents 

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). 15 

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 ’), 

wherein, R’a and w1’ represent the same meanings as before. 

## SUMMARY OF THE INVENTION 

## Problem to be Solved by the Invention 

## Means for Solving the Problem 

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