US 9,097,971 B2  28  27  preferable that the hetero atom is not adjacent to the atom  within the base dissociable portion which has its bond cleaved  by the action of a base.  The alkylene group is the same as defined for the linear or  branched alkylene group for X in the formula  (I).  The hetero atom is most preferably an oxygen atom.  Among the above examples, R$^{53 }$is preferably a group in  which a base dissociable portion is boned to a group or atom  which is not base dissociable.  R 54  represents a divalent linking group, and examples  thereof include the same divalent linking groups as those  described above for X in the formula  (I).  Among these, an  alkylene group or a divalent aliphatic cyclic group is prefer› able, and an alkylene group is particularly desirable.  R$^{55 }$represents a group having an acid dissociable group.  The acid dissociable group is an organic group which can  be dissociated by the action of an acid. The acid dissociable  group is not particularly limited, and any group which has  been proposed as an acid dissociable, dissolution inhibiting  group of a base resin for a chemically amplified resist can be  used. Specific examples include the same acid dissociable,  dissolution inhibiting groups as those described later for the  structural unit ( a  1  ),  such as a cyclic or chain-like tertiary alky  1  ester-type acid dissociable group or an acetal-type acid dis› sociable group ( e.g., an alkoxyalkyl group). Among these, a  tertiary alkyl ester-type acid dissociable group is particularly  desirable.  The group having an acid dissociable group may be either  the acid dissociable group itself, or a group in which an acid  dissociable group is bonded to a group or atom which is not  acid dissociable (a group or atom which remains bonded to  the acid generator even after the dissociation of the acid  dissociable group). Examples of the group or atom which is  not acid dissociable include the same divalent linking groups  as those described above for X in the formula  (I).  In formula (I-1-27), W $^{2 }$represents a single bond or a diva› lent linking group, t represents 0 or  1,  and R$^{62 }$represents a  group which is not dissociable by acid (hereafter, referred to  as "acid non-dissociable group").  Examples of the divalent linking group for W $^{2 }$include the  same divalent linking groups as those described above for X  in the formula  (I).  Among these, as  W 2 ,  a single bond is  preferable.  t is preferably 0.  The acid non-dissociable group for R $^{62 }$is not particularly  limited as long as it is a group which is not dissociable by acid.  The acid non-dissociable group is preferably an acid non- dissociable hydrocarbon group which may have a substituent,  more preferably a cyclic hydrocarbon group which may have  a substituent, and still more preferably a group in which one  hydrogen atom has been removed from adamantane.  In formulae (I-1-28) and (I-1-29), each of R 9  and R 10  independently represents a phenyl group or naphthyl group  which may have a substituent, an alkyl group of  1  to 5 carbon  atoms, an alkoxy group or a hydroxy group; and u represents  an integer of  1  to 3, most preferably  1  or 2.  In formula (I-1-30), Y $^{10 }$represents a cyclic hydrocarbon  group of 5 or more carbon atoms which may have a substitu› ent, and is an acid dissociable group which may be dissoci› ated by the action of an acid; each of R$^{56 }$and R$^{57 }$indepen› dently represents a hydrogen atom, an alkyl group or an aryl  group, provided that R$^{56 }$and R$^{57 }$may be mutually bonded to  form a ring; each ofY$^{11 }$and Y$^{12 }$independently represents an  It is particularly desirable that each ofY$^{11 }$and Y$^{12 }$repre› sents a pheny  1  group or a naphthy  1  group. Further, Y $^{11 }$and Y  12  may be mutually bonded to form a ring, like in the case of the  aforementioned R 1 " to R 3 ".  Examples of the alkyl group or aryl group for Y$^{11 }$and Y 12  include the same alkyl groups and aryl groups as those  described above for R  1 " to R 3 ".  Each ofY 11  and Y 12  independently represents an alkyl  group or an aryl group.  Among the above-mentioned examples, it is particularly  desirable that both R$^{56 }$and R$^{57 }$represent a hydrogen atom.  Examples of the alkyl group or aryl group for R$^{56 }$and R 57  include the same alkyl groups and aryl groups as those  described above for R 1 " to R 3 ". Further, R $^{56 }$to R$^{57 }$may be  mutually bonded to form a ring, like in the case of the afore- mentioned R  1 " to R 3 ".  Each ofR$^{56 }$and R$^{57 }$independently represents a hydrogen  atom, an alkyl group or an aryl group.  As such aliphatic cyclic groups, groups in which two or  more hydrogen atoms have been removed from a monocy› cloalkane or a polycycloalkane such as a bicycloalkane, tri› cycloalkane or tetracycloalkane which may or may not be  substituted with an alkyl group of  1  to 5 carbon atoms, a  fluorine atom or a fluorinated alkyl group, may be used.  Specific examples include groups in which two or more  hydrogen atoms have been removed from a monocycloalkane  such as cyclopentane and cyclohexane; and groups in which  two or more hydrogen atoms have been removed from a  polycycloalkane such as adamantane, norbornane, isobor- nane, tricyclodecane or tetracyclododecane.  The "aliphatic cyclic group" may or may not have a sub› stituent. Examples of the substituent include an alkyl group of  1  to 5 carbon atoms, an alkoxy group of  1  to 5 carbon atoms,  a fluorine atom, a fluorinated alkyl group of  1  to 5 carbon  atoms, and an oxygen atom (=0).  Examples of the "aliphatic cyclic group" include monocy› clic groups or polycyclic groups which have no aromaticity,  and polycyclic groups are preferable.  The cyclic hydrocarbon group may have a substituent, and  the carbon atom(s) within the substituent is not included in the  number of carbon atoms of the "carbon atom of 5 or more  carbon atoms".  A "tertiary alkyl ester" refers to a structure in which a  tertiary carbon atom within a cyclic hydrocarbon group of 5  or more carbon atoms is bonded to the terminal oxygen atom  of-C(R$^{56}$)(R$^{57}$)----C(=O)-O-. In this tertiary alkyl ester,  the action of acid causes cleavage of the bond between the  oxygen atom and the tertiary carbon atom.  Examples ofY$^{10 }$include groups which form a cyclic ter› tiary alkyl ester with-C(R$^{56}$)(R$^{57}$)----C(=O)-O-.  Y$^{10 }$represents a cyclic hydrocarbon group of 5 or more  carbon atoms which may have a substituent, and is an acid  dissociable group which may be dissociated by the action of  an acid. By virtue of the Y$^{10 }$group being a cyclic hydrocarbon  group of 5 or more carbon atoms which may have a substitu› ent, and is an acid dissociable group which may be dissoci- ated by the action of an acid, various lithography properties  such as resolution, LWR, exposure latitude (EL margin) and  resist pattern are improved.  alkyl group or anaryl group, provided thatY$^{11 }$andY$^{12 }$maybe  mutually bonded to form a ring. 