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[0027] Herein RA is each independently hydrogen or methyl; Z 1 is a single bond, phenylene, ---O-Z 11 -, -C(=O)---0-Z 11 - or----C(=O)-NH-Z 11 -, Z $^{11 }$is a C$_{1 }$-C$_{6 }$ alkanediyl group, C$_{2}$ -C$_{6 }$ alkenediyl group, or phe› nylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety: Z $^{2 }$is a single bond or a C$_{1 }$-C$_{12 }$ divalent group which may contain an ester bond, ether bond or lactone ring; Z 3 is a single bond, methylene, ethylene, phenylene, fluorinated phenylene, ---O-Z 31 -, -C(=O)---0-Z 31 - or ----C(O)-NH-Z 31 -, Z 31 is a C$_{1 }$-C$_{6 }$ alkanediyl group, C$_{2}$ -C$_{6 }$ alkenediyl group, or phe› nylene group, which may contain a carbonyl moiety, ester bond, ether bond or hydroxyl moiety; Rf 1 to Rf 1 are each independently hydrogen, fluorine or trifluoromethyl, at least one of Rf 1 to Rf 1 being fluorine; R 21 to R 28 are each independently a C$_{1 }$-C$_{20 }$ monovalent hydrocarbon group which may contain a heteroatom, any two of R 23 , R $^{24 }$and R$^{25 }$or any two of R 26 , R $^{27 }$and R$^{28 }$may bond together to form a ring with the sulfur atom to which they are attached, and M- is a non-nucleophilic counter ion. 

[0028] The positive resist composition may further com› prise an acid generator, organic solvent, quencher, and/or surfactant. 

[0029] In another aspect, the invention provides a pattern forming process comprising the steps of applying the posi› tive resist composition defined above to form a resist film on a substrate, exposing the resist film to high-energy radiation, and developing the exposed resist film in a developer. 

[0030] Typically, the high-energy radiation is i-line, KrF excimer laser. ArF excimer laser, EB, or EUV of wavelength 3 to 15 nm. 

[0031] The positive resist composition has a high decom› position efficiency of the acid generator, a remarkable 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 

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## Advantageous Effects of Invention 

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. 

[0032] 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 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. Me stands for methyl, and Ac for acetyl. 

[0033] The abbreviations and acronyms have the follow› ing meaning. 

[0034] EB: electron beam 

[0035] EUV: extreme ultraviolet 

[0036] Mw: weight average molecular weight 

[0037] Mn: number average molecular weight 

[0038] Mw/Mn: molecular weight distribution or dis- persity 

[0039] GPC: gel permeation chromatography 

[0040] PEB: post-exposure bake 

[0041] PAG: photoacid generator 

[0042] LWR: line width roughness 

[0043] LER: line edge roughness 

[0044] CDU: critical dimension uniformity 

[0045] Positive Resist Composition 

[0046] One embodiment of the invention is a positive resist composition comprising a base polymer comprising recurring units having a carboxyl group in which the hydro› gen is substituted by a nitrogen-containing tertiary hydro› carbon group. The nitrogen-containing tertiary hydrocarbon group is preferably a nitrogen-containing tertiary cyclic hydrocarbon group because a resist film having a satisfac› tory acid diffusion-suppressing effect and a high dissolution contrast is obtainable. 

[0047] Preferably, the recurring units have the formula (a). The recurring units having formula (a) are also referred to as recurring units (a). 

[0048] In formula (a), RA is hydrogen or methyl. X$^{1 }$is each independently a single bond, phenylene, naphthylene, or a C$_{1 }$-C$_{12 }$linking group containing an ester bond, ether bond or lactone ring. 

[0049] Examples of the monomer from which recurring units (a) are derived are shown below, but not limited thereto. Herein RA is as defined above, and R will be defined below. 

## DESCRIPTION OF EMBODIMENTS 

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