US 9,250,518 B2  5  6  The resist composition may further comprise an organic  solvent, the composition being a chemically amplified nega› tive resist composition. Further a crosslinker may be added.  In any embodiment, a surfactant may be added.  In a preferred embodiment, the base polymer has further  copolymerized therein recurring units of at least one type  selected from sulfonium salt units (fl) to (f3), as represented  by the general formula (3).  Herein R  020 , R  024 , and R  028  each are hydrogen or methyl, R is  a single bond, phenylene, ---O-R 033 -, or ----C(=O)› Y1-R033-, Y 1  is oxygen or NH, R 033  is a straight, branched  or cyclic C 1  -C 6  alkylene group, alkenylene group or phe› nylene group, which may contain a carbonyl (--CO-), ester  (--COO-), ether (-0-), or hydroxyl moiety, R 022 , R 023 ,  Ro2s, Ro26, Ro27, Ro29, Ro30, andR031 are each independently  a straight, branched or cyclic C 1  -C$_{12 }$alkyl group which may  contain a carbonyl, ester or ether moiety, a C 6 -C 12  aryl group,  a C$_{7}$-C$_{20 }$aralkyl group, or a thiophenyl group, A  1  is a single  bond, -A 0 -C(=O)---0-, -A 0 -O-or-A 0 -O----C(=O)-,A 0  is a straight, branched or cyclic C 1  -C$_{12 }$alkylene group which  may contain a carbonyl, ester or ether moiety, A  2  is hydrogen,  CF 3  or carbonyl group, Z 1  is a single bond, methylene, eth› ylene, phenylene, fluorinated phenylene, -O-R 032 -, or  -C(=O)-Z 2 -R 032 -, Z 2  is oxygen or NH, R 032  is a  straight, branched or cyclic C 1  -C 6  alkylene, phenylene, flu› orinated phenylene, trifluoromethyl-substituted phenylene or  alkenylene group, which may contain a carbonyl, ester, ether  or hydroxyl moiety, M is a non-nucleophilic counter ion,  subscripts fl, f2 and f3 are numbers in the range: Osfls0.5,  Osf2s0.5, Osf3s0.5, and O<fl+f2+f3s0.5.  In another aspect, the invention provides a process for  forming a pattern comprising the steps of applying the resist  composition defined above onto a substrate, baking to form a  resist film, exposing the resist film to high-energy radiation,  and developing the exposed film with a developer. Typically,  the high-energy radiation is ArF excimer laser radiation of  193 nm wavelength, KrF excimer laser radiation of 248 nm  wavelength, EB, or extreme ultraviolet radiation of3 to 15 nm  wavelength.  By virtue of a high dissolution contrast, a photoresist film  containing a sulfonium or iodonium salt of a carboxylic acid  having an acid-reactive group of specific structure offers  improved resolution, wide focus margin and minimal LWR  when used as a positive resist film subject to alkaline devel› opment, a negative resist film, and a negative resist film sub› ject to organic solvent development.  As used herein, the singular forms "a," "an" and "the"  include plural referents unless the context clearly dictates  otherwise. "Optional" or "optionally" means that the subse- quently described event or circumstances may or may not  occur, and that description includes instances where the event  or circumstance occurs and instances where it does not. The  notation  (Cn-Cm)  means a group containing from n to m  carbon atoms per group. The term "(meth)acrylate" refers  collectively to acrylate and methacrylate.  The abbreviations and acronyms have the following mean- ing.  Mw: weight average molecular weight  Mn: number average molecular weight  GPC: gel permeation chromatography  Mw/Mn: molecular weight distribution or dispersity  PEB: post-exposure bake  PAG: photoacid generator  EB: electron beam  EUV: extreme ultraviolet  LWR: line width roughness  LER: line edge roughness  DOF: depth of focus  The invention pertains to a resist composition, specifically  chemically amplified positive or negative resist composition,  comprising a base polymer, a sulfonium or iodonium salt of a  carboxylic acid having an acid-reactive group of specific  structure, and preferably an acid generator capable of gener› ating a sulfonic acid, imidic acid (imide acid) or methidic acid  (methide acid). A pattern is formed by applying the resist  composition onto a substrate, pre-baking the composition to  evaporate off any Uilllecessary solvent and to form a resist  film, exposing the resist film to high-energy radiation, PEB,  and developing the exposed film with alkaline water to form  a positive pattern or with an organic solvent to form a negative  pattern.  The sulfonium salt according to the invention is an acid  generator in that it is capable of generating a carboxylic acid  of specific structure upon light exposure and at the same time,  a quencher in that it has an amino group. Since the carboxylic  acid has an acidity insufficient to induce deprotection reaction  of acid labile groups, it is recommended to separately add an  acid generator capable of generating a sulfonic acid, imidic  acid or methidic acid serving as strong acid enough to induce  deprotection reaction of acid labile groups. The acid genera› tor capable of generating a sulfonic acid, imidic acid or  methidic acid may be of addition type or bound type (i.e.,  bound in a base polymer).  When a mixture of an acid generator capable of generating  carboxylic acid and an acid generator capable of generating  perfluoroalkylsulfonic acid (which is a superstrong acid) is  exposed to light, these acid generators generate carboxylic  acid and perfluoroalkylsulfonic acid, as shown in formulae  (3)-1 and (3)-2 below, respectively. Since not all acid genera- tors are decomposed, undecomposed acid generators are left  in their proximity. As shown in formula (3)-3, where a sulfo› nium salt capable of generating carboxylic acid and perfluo› roalkylsulfonic acid are co-present, ion exchange occurs ther› e between whereby a sulfonium salt of perfluoroalkylsulfonic  acid is created and carboxylic acid is released. This is because  the salt of perfluoroalkylsulfonic acid having a higher acid  strength is stabler. On the other hand, as shown in formula  Advantageous Effects oflnvention  DESCRIPTION OF EMBODIMENTS  (3) 