US 8,968,979 B2  40  39  (IA-4-1)  -continued  (L4-4-endo)  (IA-4-2)  (IA-4-4)  (L4-1-endo)  (L4-2-endo)  (L4-3-endo)  RsB  has a lactone structure as its partial structure and pref- erably stands for a group containing a 5 or 6-membered  lactone ring. Exemplary groups are shown below, but not  limited thereto.  An appropriate proportion "s" of repeat units having acid  labile group  RsA  is 0<ss0.8, and more preferably 0.1 <ss0.5.  A proportion "r" of the acid labile group-containing repeat  units of formula (1) or (2) and a proportion "s" ofrepeatunits  having acid labile group  RsA  are preferably so related to  provide the range: 0.1 <r+ss0.8, and more preferably 0.2<r+  ss0.65. Although values of rands outside the range need not  be positively excluded, sometimes a balance of properties  required of resist material may be broken.  Examples of the tertiary C$_{4}$ -C$_{20 }$alkyl groups, trialkylsilyl  groups in which each alkyl moiety has 1 to 6 carbon atoms,  and C$_{4}$ -C$_{20 }$oxoalkyl groups are as exemplified for  RL 04 .  Illustrative examples of the acid labile group of formula  (L4) are given below.  Each of formulas (L4-1) to (L4-4), (L4-3-l) and (L4-3-2),  and (L4-4-1) to (L4-4-4) collectively represents an enanti› omer thereof and a mixture of enantiomers.  It is noted that in the above formulas (L4-1) to (L4-4),  (L4-3-l) and (L4-3-2), and (L4-4-1) to (L4-4-4), the bond 35  direction is on the exo side relative to the bicyclo[2.2.1]  heptane ring, which ensures high reactivity for acid catalyzed  elimination reaction (see JP-A 2000-336121). In preparing  these monomers having a tertiary exo-alkyl group ofbicyclo  [2.2.1 ]heptane skeleton as a substituent group, there may be  contained monomers substituted with an endo-alkyl group as  represented by the following formulas (L4-1-endo) to (L4-4- endo ). For good reactivity, an exo proportion of at least 50  mo!  % is preferred, with an exo proportion of at least 80 mo!  45  % being more preferred.  -- --