US 2020/0192221 Al  3  Jun.  18,2020  (d3)  -continued  (d2)  R23  I  +s-R24  I  R2s  [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  Advantageous Effects of Invention  [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.  (a)  DESCRIPTION OF EMBODIMENTS  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). 