Jan.  28,  2021  3  US 2021/0026244 Al  atom-substituted alkyl group, R 3  is a hydrogen atom, an  unsubstituted alkyl group, or a fluorine atom-substituted  alkyl group, p and q are each an integer of not less than 0 and  not more than 5, and p+q=5.  [0035]  Note that although the copolymer may also include  any monomer units other than the monomer unit (A) and the  monomer unit (B), the proportion constituted by the mono› mer unit (A) and the monomer unit (B) among all monomer  units of the copolymer is, in total, preferably 90 mo!  % or  more, and is preferably 100 mo! % (i.e., the copolymer  preferably only includes the monomer unit (A) and the  monomer unit (B)). The total number of fluorine atoms  included in the monomer unit (A) and the monomer unit (B)  is preferably 5 or 6. This is because the sensitivity in a  method of forming a resist pattern can be further increased  due to further increased sensitivity of a resist film in a case  in which the total number of fluorine atoms included in the  monomer unit (A) and the monomer unit (B) is 5 or 6.  [0036]  Although the copolymer may be a random copo› lymer, a block copolymer, an alternating copolymer (ABAB  . . . ), or the like, for example, without any specific  limitations so long as it includes the monomer unit (A) and  the monomer unit (B), a copolymer including 90 mass  % or  more (upper limit of 100 mass %) of an alternating copo› lymer is preferable.  [0037]  The copolymer undergoes main chain scission to  lower molecular weight upon irradiation with extreme ultra› violet light as a result of including the specific monomer unit  (A) and monomer unit (B). Moreover, the copolymer dis› plays excellent sensitivity to extreme ultraviolet light when  used as a resist as a result of at least the monomer unit (A)  including 5 or more fluorine atoms. In particular, as a result  of at least the monomer unit (A) including 5 or more fluorine  atoms,  the  copolymer  enables  especially  significant  improvement of sensitivity in a case in which extreme  ultraviolet light is used compared to sensitivity in a case in  which the number of fluorine atoms in the monomer unit (A)  is 4 or fewer and sensitivity in a case in which an electron  beam or the like is used.  [0038]  Note that although it is not clear why the copoly› mer can display excellent sensitivity to extreme ultraviolet  light as a result of at least the monomer unit (A) including  5 or more fluorine atoms, it is presumed that absorption  efficiency of extreme ultraviolet light is improved through  the presence of fluorine atoms, thereby enabling rapid main  chain scission.  [0039]  Monomer Unit (A)  [0040]  The monomer unit (A) is a structural unit that is  derived from a monomer (a) represented by general formula  (III), shown below.  (In general formula (III), 10 1s the same as in general  formula (I).)  [0041]  The proportion constituted by the monomer unit  (A) 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! %.  [0049]  Examples of the monomer (a) represented by the  previously described general formula (III) that can form the  monomer unit (A) represented by the previously described  general formula (I) include, but are not specifically limited  to, fluoroalkyl esters of a-chloroacrylic acid such as 2,2,3,  3,3-pentafluoropropyl  a-chloroacrylate,  3,3,4,4,4-pen› tafluorobutyl a-chloroacrylate, lH-1-(trifluoromethyl)trif› luoroethyl  a-chloroacrylate,  1 H, 1 H,3H-hexafluorobutyl  a-chloroacrylate,  1,2,2,2-tetrafluoro-l-(trifluoromethyl)  ethyl a-chloroacrylate, and 2,2,3,3,4,4,4-heptafluorobutyl  a-chloroacrylate; fluoroalkoxyalkyl esters of a-chloro› acrylic acid such as pentafluoroethoxymethyl a-chloroacry› late and pentafluoroethoxyethyl a-chloroacrylate; and fluo› roalkoxyalkenyl esters of a-chloroacrylic acid such as  pentafluoroethoxyvinyl a-chloroacrylate.  [0048]  The fluoroalkoxyalkenyl group is, for example, a  pentafluoroethoxyviny I group.  [0047]  The fluoroalkoxyalkyl group is, for example, a  pentafluoroethoxymethyl group or a pentafluoroethoxyethyl  group.  [0046]  The fluoroalkyl group is, for example, a 2,2,3,3,3- pentafluoropropyl group (number of fluorine atoms: 5; car› bon number: 3), a 3,3,4,4,4-pentafluorobutyl group (number  of fluorine atoms: 5; carbon number: 4), a lH-1-(trifluorom› ethyl)trifluoroethyl group (number of fluorine atoms: 6;  carbon number: 3), a 1H,1H,3H-hexafluorobutyl group  (number of fluorine atoms: 6; carbon number: 4), a 2,2,3,3,  4,4,4-heptafluorobutyl group (number of fluorine atoms: 7;  carbon number: 4), or a 1,2,2,2-tetrafluoro-l-(trifluorom› ethyl)ethyl group (number of fluorine atoms: 7; carbon  number: 3), of which, a 2,2,3,3,3-pentafluoropropyl group  (number of fluorine atoms: 5; carbon number: 3) is prefer› able.  [0045]  More specifically, R  1  in general formula (I) and  general formula (III) is preferably a fluoroalkyl group, a  fluoroalkoxyalkyl group, or a fluoroalkoxyalkenyl group,  and is more preferably a fluoroalkyl group. When R$^{1 }$is any  of the groups set forth above, main chain scission properties  of the copolymer upon irradiation with extreme ultraviolet  light can be sufficiently improved.  [0044]  The carbon number ofR $^{1 }$is preferably not less than  2 and not more than 10, more preferably not less than 3 and  not more than 4, and even more preferably 3. Solubility in  a developer can be sufficiently improved when the carbon  number is not less than any of the lower limits set forth  above. Moreover, lowering of the glass-transition point can  be inhibited and sufficient clarity of an obtained resist  pattern can be ensured when the carbon number is not more  than any of the upper limits set forth above.  [0043]  The number of fluorine atoms included in 10 in  general formula (I) and general formula (III) is preferably 6  or more from a viewpoint of improving main chain scission  properties of the copolymer upon irradiation with extreme  ultraviolet light and further improving sensitivity in a case in  which extreme ultraviolet light is used. Moreover, the num› ber of fluorine atoms included in R  1  in general formula (I)  and general formula (III) is preferably 7 or fewer from a  viewpoint of improving the clarity of a resist pattern  obtained when the copolymer is used as a positive resist.  [0042]  R $^{1 }$in general formula (I) and general formula (III)  is required to be an organic group including 5 or more  fluorine atoms from a viewpoint of improving main chain  scission properties of the copolymer upon irradiation with  extreme ultraviolet light.  (III)  Cl  H  I  I  C=C  I  I  R 1 ooc  H 