Jul.  18,  2019  12  US 2019/0219920 Al  -continued  [0174]  Structural Unit (al):  [0175]  The component (Al) may include, in addition to  the structural unit (a0), a structural unit (al) containing an  acid decomposable group that exhibits increased polarity by  the action of acid.  [0176]  The term "acid decomposable group" refers to a  group in which at least a part of the bond within the structure  thereof is cleaved by the action of an acid.  [0177]  Examples of acid decomposable groups which  exhibit increased polarity by the action of an acid include  groups which are decomposed by the action of an acid to  form a polar group.  [0178]  Examples of the polar group include a carboxy  group, a hydroxy group, an amino group and a sulfa group  (-SO$_{3}$H). Among these, a polar group containing -OH in  the structure thereof (hereafter, referred to as "OH-contain› ing polar group") is preferable, a carboxy group or a  hydroxy group is more preferable, and a carboxy group is  particularly desirable.  [0179]  More specifically, as an example of an acid decom› posable group, a group in which the aforementioned polar  group has been protected with an acid dissociable group  (such as a group in which the hydrogen atom of the  OH-containing polar group has been protected with an acid  dissociable group) can be given.  [0180]  The "acid dissociable group" refers to both (i) a  group in which the bond between the acid dissociable group  and the adjacent atom is cleaved by the action of acid; and  (ii) a group in which one of the bonds is cleaved by the  action of acid, and then a decarboxylation reaction occurs,  thereby cleaving the bond between the acid dissociable  group and the adjacent atom.  [0181]  It is necessary that the acid dissociable group that  constitutes the acid decomposable group is a group which  exhibits a lower polarity than the polar group generated by  the dissociation of the acid dissociable group. Thus, when  the acid dissociable group is dissociated by the action of  acid, a polar group exhibiting a higher polarity than that of  the acid dissociable group is generated, thereby increasing  the polarity. As a result, the polarity of the entire component  (Al) is increased. By the increase in the polarity, the  solubility in an alkali developing solution changes, and the  solubility in an alkali developing solution is relatively  increased, whereas the solubility in an organic developing  solution is relatively decreased.  [0182]  Examples of the acid dissociable group for the  structural unit (al) include acid dissociable groups which  have been proposed for a base resin of a chemically ampli› fied resist composition, provided that the acid dissociable  group represented by the aforementioned general formula  (a0-rl-1) is excluded.  [0183]  Specific examples of acid dissociable groups for  the base resin of a conventional chemically amplified resist  include "acetal-type acid dissociable group", "tertiary alkyl  ester-type acid dissociable group" and "tertiary alkyloxy› carbonyl acid dissociable group" described below.  [0184]  -Acetal-Type Acid Dissociable Group  [0185]  Examples of the acid dissociable group for protect› ing the carboxy group or hydroxy group as a polar group  include the acid dissociable group represented by general  formula (al-r-1) shown below (hereafter, referred to as  "acetal-type acid dissociable group").  [0173]  Examples of other structural units include a struc› tural unit (al) containing an acid decomposable group that  exhibits increased polarity by the action of acid, (provided  that the structural unit (a0) is excluded); a structural unit (a2)  containing a lactone-containing cyclic group, an -SO$_{2}$ - containing cyclic group or a carbonate-containing cyclic  group; a structural unit (a3) containing a polar group› containing aliphatic hydrocarbon group (provided that the  structural units (a0), (al) and (a2) are excluded); a structural  unit (a4) containing an acid non-dissociable aliphatic cyclic  group; and a structural unit derived from styrene or a  derivative thereof.  [0172]  The component (Al) may be further include a  structural unit other than the structural unit (a0).  [0171]  <<Other Structural Units>>  [0170]  When the amount of the structural unit (a0) is at  least as large as the lower limit of the above-mentioned  preferable range, a resist pattern may be reliably obtained,  and sensitivity and various lithography properties such as  critical dimension uniformity (CDU) and circularity may be  improved. On the other hand, when the amount is no more  than the upper limit of the above preferable range, a good  balance may be achieved with the other structural units.  [0169]  In the component (Al), the amount of the structural  unit (a0) based on the combined total (100 mo! %) of all  structural units constituting the component (Al) is prefer› ably 10 to 70 mo! %, more preferably 20 to 65 mo! %, still  more preferably 25 to 60 mo! %, and most preferably 30 to  50 mo!%.  [0168]  As the structural unit (a0) contained in the com› ponent (Al), 1 kind of structural unit may be used, or 2 or  more kinds of structural units may be used.  [0167]  Among these examples, as the structural unit (a0),  at least one member selected from the group consisting of  structural units represented by formulae (a0-1-1) to (a0-1-3)  is most preferable.  [0166]  Among these, as the structural unit (a0), at least  one member selected from the group consisting of structural  units represented by general formulae (a0-1-1) to (a0-1-9)  shown below is preferable, and at least one member selected  from the group consisting of structural units represented by  general formulae (a0-1-1) to (a0-1-6) shown below is more  preferable.  (a0-1-12) 