Feb.4,2021  121  US 2021/0033971 Al  carbonyl, cyclic acetal, ether bond, ester bond, sulfonic acid  ester bond, cyano, amide bond, -0----C(=O)-S-, and  -0----C(=O)-NH-.  5.  The resist composition of claim  1  wherein the base  polymer further comprises recurring units of at least one  type selected from recurring units having the formulae ( dl)  to (d3):  nylene group, which may contain a carbonyl moiety,  ester bond, ether bond or hydroxyl moiety,  6.  The resist composition of claim  1,  further comprising  an acid generator.  7.  The resist composition of claim  1,  further comprising  an organic solvent.  8.  The resist composition of claim  1,  further comprising  a quencher.  9.  The resist composition of claim  1,  further comprising  a surfactant.  10.  A process for forming a pattern comprising the steps  of applying the positive resist composition of claim  1  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.  11.  The process of claim  10  wherein the high-energy  radiation is i-line,  KrF excimer laser, ArF excimer laser, EB,  or EUV of wavelength 3 to 15 nm.  * *  * * *  wherein  RA  is each independently hydrogen or methyl,  Z 1  is a single bond, phenylene group,  ---O-Z 11 -,  ----C(=0)---0-Z 11 - or  ----C(=0)-NH-Z 11 -,  Z $^{11 }$is a C$_{1 }$-C$_{6 }$ aliphatic hydrocarbylene group or phe- M- is a non-nucleophilic counter ion.  R $^{21 }$to  R $^{28 }$are each independently a  C$_{1}$-C$_{20 }$ hydrocarbyl  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  Z 5  is a single bond, methylene, ethylene, phenylene,  fluorinated phenylene,  -O-Z 51 -,  ----C(=0)-0- Z51-, or ----C(=0)-NH-Z 51 -, Z 51  is  a  C 1 -C$_{6 }$ aliphatic hydrocarbylene group, phenylene group, fluo› rinated phenylene group, or trifluoromethyl-substituted  phenylene group, which may contain a carbonyl moi› ety, ester bond, ether bond or hydroxyl moiety,  Z 4  is methylene, 2,2,2-trifluoro-l,1-ethanediyl or carbo› nyl,  Z 3  is a single bond, -Z 31 -C(=0)---0-, -Z 31 -0- or -Z 31 ---0-C(=0)-, 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 2  is a single bond or ester bond,  (dl)  (d2)  R23  I  +s-R24  I  R2s  (d3) 