Jan.  28,  2021  6  US 2021/0026244 Al  weight of copolymer that precipitates in the mixed solvent  can be increased by increasing the proportion of the good  solvent in the mixed solvent.  [0072]  The copolymer including the monomer unit (A)  and the monomer unit (B) set forth above can be produced,  for example, by carrying out polymerization of a monomer  composition that contains the monomer (a) and the mono› mer (b), and then optionally purifying the obtained polym› erized product.  [0073]  The composition, molecular weight distribution,  weight-average molecular weight, and number-average  molecular weight of the copolymer can be adjusted by  altering the polymerization conditions and the purification  conditions. In one specific example, the composition of the  copolymer can be adjusted by altering the proportion in  which each monomer is contained in the monomer compo› sition used in polymerization. In another example, the  weight-average molecular weight and the number-average  molecular weight can be reduced by raising the polymer› ization temperature. In yet another example, the weight› average molecular weight and the number-average molecu› lar weight can be reduced by shortening the polymerization  time.  [0074]  The monomer composition used in production of  the copolymer may be a mixture containing a monomer  component that includes the monomer (a) and the monomer  (b ), an optionally useable solvent, a polymerization initiator,  and optionally added additives. Polymerization of the mono› mer composition may be carried out by a known method. In  particular, in a case in which a solvent is used, it is preferable  that cyclopentanone or the like is used as the solvent.  Moreover, it is preferable that a radical polymerization  initiator such as azobisisobutyronitrile is used as the polym› erization initiator.  [0076]  The method of purification in a case in which the  obtained polymerized product is purified may be, but is not  specifically limited to, a known purification method such as  reprecipitation or colunm chromatography. Of these purifi› cation methods, purification by reprecipitation is preferable.  [0077]  Note that purification of the polymerized product  may be performed repeatedly.  [0078]  Purification of the polymerized product by repre› cipitation is, for example, preferably carried out by dissolv› ing the resultant polymerized product in a good solvent such  as tetrahydrofuran, and subsequently dripping the resultant  solution into a mixed solvent of a good solvent, such as  tetrahydrofuran, and a poor solvent, such as methanol, to  cause precipitation of a portion of the polymerized product.  When purification of the polymerized product is carried out  by dripping a solution of the polymerized product into a  mixed solvent of a good solvent and a poor solvent as  described above, the molecular weight distribution, weight› average molecular weight, and number-average molecular  weight of the resultant copolymer can easily be adjusted by  altering the types and/or mixing ratio of the good solvent and  the poor solvent. In one specific example, the molecular  [0086]  In the resist film formation step, the positive resist  composition for EUV lithography set forth above is applied  onto a workpiece, such as a substrate, that is to be processed  using a resist pattern, and the applied positive resist com› position for EUV lithography is dried to form a resist film.  The substrate is not specifically limited and may, for  example, be a mask blank including a light shielding layer  on a substrate or a substrate including an electrically insu- [0084]  As a result of a resist film containing the specific  fluorine atom-containing copolymer being developed and  subsequently rinsed using a rinsing liquid having a surface  tension of 20.0 mN/m or less in the presently disclosed  method of forming a resist pattern, resist pattern collapse can  be sufficiently inhibited while also favorably inhibiting the  presence of residues in the resist pattern. Moreover, as a  result of a resist film formed using the presently disclosed  positive resist composition for EUV lithography set forth  above being exposed using extreme ultraviolet light in the  presently disclosed method of forming a resist pattern, a  resist pattern can be efficiently formed.  [0083]  The presently disclosed method of forming a resist  pattern uses the presently disclosed positive resist compo› sition for EUV lithography set forth above and extreme  ultraviolet light. More specifically, the presently disclosed  method of forming a resist pattern includes: a step of  forming a resist film using the above-described positive  resist composition for EUV lithography containing the spe› cific fluorine atom-containing copolymer (resist film forma› tion step); a step of exposing the resist film using extreme  ultraviolet light ( exposure step); a step of developing the  resist film that has been exposed (development step); and a  step of rinsing the resist film that has been developed  (rinsing step). A feature of the presently disclosed method of  forming a resist pattern is that the rinsing step is performed  using a rinsing liquid having a surface tension of 20.0 mN/m  or less.  [0075]  A polymerized product obtained through polymer› ization of the monomer composition may be used as the  copolymer as obtained or may, without any specific limita› tions, be collected by adding a good solvent such as tetra› hydrofuran to a solution containing the polymerized product  and subsequently dripping the solution to which the good  solvent has been added into a poor solvent such as methanol  to coagulate the polymerized product, and then the polym› erized product may be purified as described below.  [0081]  The solvent may be any known solvent so long as  it is a solvent in which the copolymer set forth above is  soluble. Of such solvents, a pentyl ester of an organic acid,  a hexyl ester of an organic acid, or a mixture thereof is  preferable as the solvent from a viewpoint of obtaining a  positive resist composition having appropriate viscosity and  improving coatability of the positive resist composition for  EUV lithography, with pentyl acetate, hexyl acetate, or a  mixture thereof being more preferable, and isoamyl acetate  (isopentyl acetate) being even more preferable.  [0079]  Also note that in a situation in which the polym› erized product is purified by reprecipitation, polymerized  product that precipitates in the mixed solvent of the good  solvent and the poor solvent may be used as the presently  disclosed copolymer, or polymerized product that does not  precipitate in the mixed solvent (i.e., polymerized product  dissolved in the mixed solvent) may be used as the presently  disclosed copolymer. Polymerized product that does not  precipitate in the mixed solvent can be collected from the  mixed solvent by a known technique such as concentration  to dryness.  [0071]  Production Method of Copolymer  [0080]  -Solvent- [0082]  (Method of Forming Resist Pattern)  [0085]  <Resist Film Formation Step> 