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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). 

<img src='content_image/892296.jpg'>

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 

## Advantageous Effects oflnvention 

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 

Mw/Mn: molecular weight distribution or dispersity 

GPC: gel permeation chromatography 

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 

## DESCRIPTION OF EMBODIMENTS 

