[0071] Production Method of Copolymer 

[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. 

[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. 

[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 

weight of copolymer that precipitates in the mixed solvent can be increased by increasing the proportion of the good solvent in the mixed solvent. 

[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. 

[0080] -Solvent- 

[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. 

[0082] (Method of Forming Resist Pattern) 

[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. 

[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. 

[0085] <Resist Film Formation Step> 

[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- 

