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Herein RA is each independently hydrogen or methyl. Z$^{1 }$is a single bond, phenylene group, -O-Z 11 , ----C(=O)› O-Z11- or ----C(=O)-NH-Z 11 -, wherein Z 11 is a C$_{1 }$-C$_{6 }$ aliphatic hydrocarbylene group or phenylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety. Z 2 is a single bond or ester bond. Z 3 is a single bond, -Z$^{31}$----C(=O)-O-, -Z $^{31}$---0- or -Z$^{31}$-O----C(=O)-, wherein Z 31 is a C$_{1}$-C$_{12 }$ saturated hydrocarbylene group which may contain a carbonyl moiety, ester bond, ether bond, iodine or bromine. Z 4 is methylene, 2,2,2-trifluoro-l,1-ethanediyl or carbonyl. Z 5 is a single bond, methylene, ethylene, phenylene, fluorinated phe› nylene, ---O-Z 51 -, -C(=O)-O-Z 51 -, or -C(=O)-NH-Z 51 -, wherein Z 51 is a C$_{1}$-C$_{6 }$ aliphatic hydrocarbylene group, phenylene group, fluorinated phe› nylene group, or trifluoromethyl-substituted phenylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety. R 21 to R$^{28 }$are each inde› pendently a C$_{1 }$-C$_{20 }$hydrocarbyl group which may contain a heteroatom, any two ofR 23 , R $^{24 }$and R$^{25 }$or any two ofR26 , R $^{27 }$and R $^{28 }$may bond together to form a ring with the sulfur atom to which they are attached. M- is a non-nucleophilic counter ion. 

[0030] The resist composition may further comprise an acid generator, organic solvent, quencher, and/or surfactant. 

[0031] In another aspect, the invention provides a process for forming a pattern comprising the steps of applying the positive resist composition defined above onto a substrate to form a resist film thereon, exposing the resist film to high-energy radiation, and developing the exposed resist film in a developer. 

[0032] Preferably, the high-energy radiation is i-line, KrF excimer laser, ArF excimer laser, EB, or EUV of wavelength 3 to 15 nm. 

[0033] The positive resist composition can enhance the decomposition efficiency of an acid generator, has a remark› able acid diffusion-suppressing effect, a high sensitivity, and a high resolution, and forms a pattern of good profile with improved edge roughness and size variation after exposure and development. By virtue of these properties, the resist composition is fully useful in commercial application and best suited as a micropatterning material for photomasks by EB lithography or for VLSis by EB or EUV lithography. The resist composition may be used not only in the lithography for forming semiconductor circuits, but also in the formation of mask circuit patterns, micromachines, and thin-film mag› netic head circuits. 

[0034] As used herein, the singular forms "a," "an" and "the" include plural referents unless the context clearly 

-continued 

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## Advantageous Effects of Invention 

## DESCRIPTION OF EMBODIMENTS 

dictates otherwise. "Optional" or "optionally" means that the subsequently 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. As used herein, the term "iodized" or "brominated" compound indicates a compound containing iodine or bromine or a compound substituted with iodine or bromine. In chemical formulae, Me stands for methyl, and Ac for acetyl. 

[0035] The abbreviations and acronyms have the follow› ing meaning. 

[0036] EB: electron beam 

[0037] EUV: extreme ultraviolet 

[0038] Mw: weight average molecular weight 

[0039] Mn: number average molecular weight 

[0040] Mw/Mn: molecular weight dispersity 

[0041] GPC: gel permeation chromatography 

[0042] PEB: post-exposure bake 

[0043] PAG: photoacid generator 

[0044] LER: line edge roughness 

[0045] LWR: line width roughness 

[0046] CDU: critical dimension uniformity 

## Positive Resist Composition 

[0047] One embodiment of the invention is a positive resist composition comprising a base polymer comprising recurring units (a) having the structure of an ammonium salt of a carboxylic acid having an iodized or brominated hydro› carbyl group, with the proviso that the hydrocarbyl group does not contain an iodized or brominated aromatic ring, and recurring units of at least one type selected from recurring units (bl) having a carboxyl group in which the hydrogen atom is substituted by an acid labile group and recurring units (b2) having a phenolic hydroxyl group in which the hydrogen atom is substituted by an acid labile group. It is noted that the carboxylic acid having an iodized or bromi› nated hydrocarbyl group is sometimes referred to as "iodized or brominated carboxylic acid." 

[0048] Preferably, the recurring units (a) have the formula (a). 

<img src='content_image/906561.jpg'>

[0049] In formula (a), RA is hydrogen or methyl. X 1 A is a single bond, ester bond or amide bond. X 1 B is a single bond or a C$_{1 }$-C$_{20 }$ di- or trivalent hydrocarbon group which may contain an ether bond, carbonyl moiety, ester bond, amide bond, sultone ring, lactam ring, carbonate moiety, halogen, hydroxyl moiety or carboxyl moiety. 

[0050] The C$_{1 }$-C$_{20 }$ di- or trivalent hydrocarbon group represented by X 1 B may be straight, branched or cyclic and may be either aliphatic or aromatic. Examples thereof include C$_{1 }$-C$_{20 }$ alkanediyl groups, C$_{3}$ -C$_{10 }$ cyclic saturated hydrocarbylene groups, C$_{1 }$-C$_{20 }$ alkanetriyl groups, C$_{3}$ -C$_{10 }$ 

