Feb.4,2021  117  US 2021/0033970 Al  9.  The resist composition of claim  1  wherein the base  polymer comprises recurring units of at least one type  selected from recurring units having the formulae  (fl) to  (f3):  3.  The resist composition of claim  1,  further comprising  an acid generator capable of generating a sulfonic acid,  imide acid or methide acid.  4.  The resist composition of claim  1,  further comprising  an organic solvent.  5.  The resist composition of claim  1  wherein the base  polymer comprises recurring units having the formula (al)  or recurring units having the formula (a2):  10.  The resist composition of claim  1,  further comprising  a surfactant.  11.  A process for forming a pattern comprising the steps  of applying the resist composition of claim  1  onto a substrate  6.  The resist composition of claim  5  which is a chemically  amplified positive resist composition.  7.  The resist composition of claim  1  wherein the base  polymer is free of an acid labile group.  8.  The resist composition of claim  7  which is a chemically  amplified negative resist composition.  M- is a non-nucleophilic counter ion.  A 1  is hydrogen or trifluoromethyl, and  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 ofR 26 , R 27  and R 28  may  bond together to form a ring with the sulfur atom to  which they are attached,  Z 3  is a single bond, methylene, ethylene, phenylene,  fluorinated phenylene, ---O-Z 31 -, ----C(=0)- 0-Z31-, or ----C(=0)-NH-Z 31 -, Z 31  is a  C$_{1 }$-C$_{6 }$ aliphatic hydrocarbylene group, phenylene  group, fluorinated phenylene group, or trifluorom› ethyl-substituted phenylene group, which may con› tain a carbonyl moiety, ester bond, ether bond or  hydroxyl moiety,  Z 2  is a single bond, -Z 21 ----C(=0)---0-,  -Z 21 - 0- or -Z 21 ---0-C(=0)-,  z 21  is a  cl  -C12 satu› rated hydrocarbylene group which may contain a  carbonyl moiety, ester bond or ether bond,  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  phenylene group, which may contain a carbonyl,  ester bond, ether bond or hydroxyl moiety,  wherein  RA  is each independently hydrogen or methyl,  wherein  RA  is each independently hydrogen or methyl,  R$^{11 }$and R 12  each are an acid labile group, Y 1  is a single  bond, phenylene group, naphthylene group, or C$_{1 }$-C 12  linking group containing at least one moiety selected  from ester bond and lactone ring, and Y 2  is a single  bond or ester bond.  R 3  is a  C$_{1}$-C$_{20 }$ (q$^{2}$+1)-valent aliphatic hydrocarbon  group which may contain at least one moiety  selected from fluorine, chlorine, hydroxyl, carboxyl,  CcC 12  aryl, ether bond, ester bond, carbonyl, amide  bond, carbonate, urethane bond, and urea bond.  X 3  is a single bond or a C$_{1}$-C 20  (q$^{1}$+1)-valent hydro› carbon group which may contain a heteroatom exclu› sive of iodine and bromine,  X $^{2 }$is a single bond, ether bond, ester bond, amide bond,  carbonyl group or carbonate group,  XBI  is iodine or bromine,  R 2  is hydrogen, nitro, or a  C 1 -C 20  hydrocarbyl group  which may contain at least one moiety selected from  hydroxyl, carboxyl, thiol, ether bond, ester bond,  nitro, cyano, halogen and amino moiety, in case of  p$^{1}$=1 or 2, two R 2  may bond together to form a ring  with the nitrogen atom to which they are attached,  the ring optionally containing a double bond, oxy› gen, sulfur or nitrogen, or R 2  and X 1  may bond  together to form a ring with the nitrogen atom to  which they are attached, the ring optionally contain› ing a double bond, oxygen, sulfur or nitrogen,  (fl)  (f2)  R23  I  +s-R24  I  R2s  (fJ)  (al)  (a2)  ~  y2  OR12 