Sep. 10,2020  38  US 2020/0285152 Al  wherein  RA  is hydrogen or methyl, R$^{21 }$is a C$_{1 }$-C$_{8 }$ straight or  branched alkyl group, pis an integer of 1 to 3, and  XA  is an  acid labile group other than the group having the formula  (bl):  -continued  wherein R$^{21 }$and pare as defined above, and the broken line  designates a valence bond.  2.  The resist composition of claim  1  wherein  XA  is an acid  labile group having the formula (Ll), (L2) or (L3):  3.  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 ( c) to  (e):  4.  The resist composition of claim  1,  further comprising  0.8 to 20.0 parts by weight of a third onium salt having the  formula (3):  wherein  RA  is each independently hydrogen or methyl,  R 23  and R 24  are each independently hydrogen or  hydroxyl, R$^{25 }$is a substituent group containing a lac› tone structure, R  26  and R  27  are each independently  hydrogen or C$_{1}$-C$_{15 }$ alkyl, at least one of R$^{26 }$and R 27  being C$_{1}$-C$_{15}$, alkyl, R$^{26 }$andR$^{27 }$may bond together to  form a ring with the carbon atom to which they are  attached, a combination ofR$^{26 }$and R$^{27 }$being a C$_{2}$ -C$_{15 }$ alkanediyl group, R$^{28 }$is a C$_{1}$-C$_{20 }$divalent hydrocarbon  group in which any constituent  -CH$_{2}$ - may be  replaced by ---0- or  -C(O)-,  R 29  is  -CH$_{2}$ -,  ----CH$_{2}$CH$_{2}$ - or  -0-,  or two separate  -H,  and the  dotted line designates a single bond or divalent organic  group between the norbomane ring, bicyclo[2.2.2]oc› tane ring, 7 -oxanorbomane ring or cyclohexane ring  structure and the y-butyrolactone ring structure, or a  structure sharing one or two constituent carbon atoms  between these ring structures.  wherein the broken line designates a valence bond, RL 01  to  RL $^{03 }$are each independently hydrogen or a C$_{1 }$ -C$_{12 }$ alkyl group,  RL $^{04 }$is hydrogen or a C$_{1 }$-C$_{3 }$ straight or  branched alkyl group,  RLos  to RL $^{15 }$are each indepen› dently hydrogen or a C$_{1 }$-C$_{6 }$ monovalent hydrocarbon  group.  wherein A 2  and A 3  are each independently hydrogen or  trifluoromethyl, q is an integer of 1 to 3,  MB+  is a  sulfonium, iodonium or ammonium cation, and R$^{31 }$is  a C$_{1 }$-C$_{40 }$ monovalent hydrocarbon group which may  contain a heteroatom.  t-to  $^{0 }$\ $_{R2s }$ (d)  (e)  (bl)  RLOI  I  L02  ----C-R  I  RL03  (Ll)  --lg  (L2)  RLIO  ---- RL06  9  RL05  R  RL07  (L3)  (c)  (3) 