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Herein R 51 , R 55 and R 59 each are hydrogen or methyl; R 52 is a sin§le bond, phenylene, ---O-R 63 -, or ----C(=O)› Y1-R6 -, Y 1 is ---0- or -NH-, R 63 is a straight, branched or cyclic C 1 -C 6 alkylene or alkenylene group which may contain a carbonyl, ester, ether or hydroxyl moiety or phenylene group• R 53 R 54 R 56 R 57 R 58 R 60 R 61 , a~d R 62 are each indep;nde~tly a’strai~ht, b~ancl~ed o; cyclic C 1 -C$_{12 }$ alkyl group which may contain a carbonyl, ester or ether moiety, or a C 6 -C 12 aryl group, C 7 -C 20 aralkyl group or mercaptophenyl group; A 1 is a single bond, -A 0 - C(=O)-O-, -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 hydro› gen or trifluoromethyl; Z 1 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, -O-R 64 -, or -C(=O)-Z 2 -R 64 -, Z 2 is -0- or-NH-, R 64 is a straight, branched or cyclic C 1 -C 6 alkylene or alkenylene group which may contain a carbonyl, ester, ether or hydroxyl moiety, or phenylene, fluorinated phenylene or trifluorom› ethyl-substituted phenylene group; M- is a non-nucleophilic counter ion, and fl, f2 and f3 are numbers in the range: Osfls0.5, Osf2s0.5, Osf3s0.5, and O<fl+f2+f3s0.5. 

The resist composition may further comprise a surfactant. 

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 radia› tion, and developing the exposed film in a developer. 

Typically, the high-energy radiation is ArF excimer laser radiation of wavelength 193 nm, KrF excimer laser radiation of wavelength 248 nm, EB, or EUV of wavelength 3 to 15 nm. 

Since a resist film containing a sulfonium or iodonium salt of formula (A) exhibits a high dissolution contrast, it offers improved resolution, a wide focus margin, a reduced LWR, and no dimensional changes on PPD as a positive or negative tone resist film subject to alkaline development and as a negative tone resist film subject to organic solvent development. 

As used herein, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates 

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## Advantageous Effects of Invention 

## DESCRIPTION OF EMBODIMENTS 

-continued 

otherwise. "Optional" or "optionally" means that the sub› sequently 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. Me stands for methyl, Ac for acetyl, and Ph for phenyl. 

The abbreviations and acronyms have the following meaning. 

EB: electron beam EUV: extreme ultraviolet 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 PPD: post PEB delay PAG: photoacid generator LWR: line width roughness 

## Resist Composition 

The resist composition of the invention is defined as comprising a base polymer and a sulfonium or iodonium salt of sulfonic acid containing a morpholino group. Although the sulfonium or iodonium salt is an acid generator capable of generating sulfonic acid containing a morpholino group upon light exposure, it functions as a quencher because of inclusion of nitrogen atom. Since the sulfonic acid does not possess a sufficient acidity to induce deprotection reaction of an acid labile group, it is recommended to separately add an acid generator capable of generating a strong acid such as a-fluorinated sulfonic acid, imide acid or methide acid, as will be described later, in order to induce deprotection reaction of an acid labile group. The acid generator capable of generating a-fluorinated sulfonic acid, imide acid or methide acid may be either of separate type which is added to the base polymer or of bound type which is bound in the base polymer. 

When a resist composition containing the sulfonium or iodonium salt of sulfonic acid containing a morpholino group in admixture with an acid generator capable of generating a perfluoroalkylsulfonic acid or superstrong acid is exposed to radiation, sulfonic acid containing a mor› pholino group and perfluoroalkylsulfonic acid generate. Since the acid generator is not entirely decomposed, the undecomposed acid generator is present nearby. When the sulfonium or iodonium salt capable of generating sulfonic acid containing a morpholino group co-exists with the perfluoroalkylsulfonic acid, ion exchange takes place whereby a sulfonium salt of perfluoroalkylsulfonic acid is created and sulfonic acid containing a morpholino group is released. This is because the salt of perfluoroalkylsulfonic acid having a high acid strength is more stable. In contrast, when a sulfonium salt of perfluoroalkylsulfonic acid co› exists with a sulfonic acid containing a morpholino group, no ion exchange takes place. 

The ion exchange reaction according to the acid strength series occurs not only with sulfonium salts, but also simi› larly with iodonium salts. Likewise, ion exchange takes place not only with the perfluoroalkylsulfonic acid, but also similarly with arylsulfonic acid, alkylsulfonic acid, imide acid and methide acid having a higher acid strength than the sulfonic acid containing a morpholino group. 

While the resist composition of the invention should essentially contain the sulfonium or iodonium salt of sulfo- nic acid containing a morpholino group, another sulfonium or iodonium salt may be separately added as the quencher. Examples of the sulfonium or iodonium salt to be added as 

