Jan.  28,  2021  4  US 2021/0026244 Al  [0050]  Note that the monomer unit (A) is preferably a  structural unit that is derived from a fluoroalkyl ester of  a-chloroacrylic acid from a viewpoint of further improving  main chain scission properties of the copolymer upon irra› diation with extreme ultraviolet light. Furthermore, among  structural units derived from fluoroalkyl esters of a-chloro› acrylic acid, a structural unit that is derived from 2,2,3,3,3- pentafluoropropyl a-chloroacrylate is more preferable as the  monomer unit (A). When the monomer unit (A) is a struc› tural unit that is derived from 2,2,3,3,3-pentafluoropropyl  a-chloroacrylate, the sensitivity of a resist film to extreme  ultraviolet light can be especially increased.  [0051]  Monomer Unit (B)  [0052]  The monomer unit (B) is a structural unit that is  derived from a monomer (b) represented by general formula  (IV), shown below.  [0059]  On the other hand, from a viewpoint of further  increasing the sensitivity of a resist film to extreme ultra› violet light, the plurality of R 2  and/or R 3  groups that are  present in general formula (II) and general formula (IV)  preferably include a fluorine atom or a fluorine atom› substituted alkyl group, and more preferably include a  fluorine atom or a fluorine atom-substituted alkyl group  having a carbon number of not less than 1 and not more than  5.  [0060]  In particular, from a viewpoint of ensuring ease of  production of the copolymer while also further increasing  the sensitivity of a resist film to extreme ultraviolet light, it  is preferable that, in general formula (II) and general for› mula (IV), p is 5, q is 0, and the five R 2  groups include a  fluorine atom or a fluorine atom-substituted alkyl group,  more preferable that one of the five R  $^{2 }$groups is a fluorine  atom or a fluorine atom-substituted alkyl group and the  remaining four R 2  groups are hydrogen atoms, and even  more preferable that one of the five R  $^{2 }$groups is a fluorine  atom and the remaining four R $^{2 }$groups are hydrogen atoms.  [0061]  Examples of the monomer (b) represented by the  previously described general formula (IV) that can form the  monomer unit (B) represented by the previously described  general formula (II) include, but are not specifically limited  to, a-methylstyrene (AMS) and derivatives thereof such as  (b-1) to (b-11), shown below. Of these examples, a-meth› ylstyrene and 4-fluoro-a-methylstyrene, which is in accor› dance with general formula (b-2), are preferable as the  monomer (b ).  [0053]  (In general formula (IV), R 2 , R 3 , p, and q are the  same as in general formula (II).)  [0054]  The proportion constituted by the monomer unit  (B) among all monomer units of the copolymer is not  specifically limited but can, for example, be set as not less  than 30 mo!  % and not more than 70 mo! %.  [0055]  Examples of fluorine atom-substituted alkyl groups  that can constitute R 2  or R 3  in general formula (II) and  general formula (IV) include, but are not specifically limited  to, groups having a structure in which all or some of the  hydrogen atoms of an alkyl group are replaced by fluorine  atoms.  [0056]  Examples of unsubstituted alkyl groups that can  constitute R 2  or R 3  in general formula (II) and general  formula (IV) include, but are not specifically limited to,  unsubstituted alkyl groups having a carbon number of not  less than 1 and not more than 5. Of such alkyl groups, a  methyl group or an ethyl group is preferable as an unsub› stituted alkyl group that can constitute R 2  or R 3 .  [0057]  From a viewpoint of improving the ease of pro› duction of the copolymer, the plurality of R 2  and/or R 3  groups that are present in general formula (II) and general  formula (IV) are each preferably a hydrogen atom or an  unsubstituted alkyl group, more preferably a hydrogen atom  or an unsubstituted alkyl group having a carbon number of  not less than 1 and not more than 5, and even more  preferably a hydrogen atom.  [0058]  Note that in general formula (II) and general for› mula (IV), from a viewpoint of improving the ease of  production of the copolymer, it is preferable that pis 5, q is  0, and the five R 2  groups are each a hydrogen atom or an  unsubstituted alkyl group, more preferable that the five R 2  groups are each a hydrogen atom or an unsubstituted alkyl  group having a carbon number of not less than 1 and not  more than 5, and even more preferable that the five R 2  groups are each a hydrogen atom.  (b-1)  F  (b-2)  l  QF  (b-3)  l  F~F  (b-4)  (IV)  CH3 H  I  I  C=C  0 1  H  f--.:::-’f.  (R$^{2}$)p  (OR\ 