Oct. 21, 2010  56  US 2010/0266955 Al  dose with fixed pitch and changing the mask size ( e.g., the line  width of a line and space pattern or the hole diameter of a  contact hole pattern).  [0556]  DOF is the range of depth of focus in which a resist  pattern having a predetermined size within the range corre› sponding to the target size can be formed when the exposure  focus is moved upwardly or downwardly with the same expo› sure dose, i.e., the range in which a resist pattern faithful to the  mask pattern can be obtained. Larger DOF is more preferable.  [0557]  <<Method of Forming a Resist Pattern>>  [0558]  The method of forming a resist pattern according to  a second aspect of the present invention includes: applying a  positive resist composition of the present invention to a sub› strate to form a resist film on the substrate; conducting expo› sure of the resist film; and alkali-developing the resist film to  form a resist pattern.  [0559]  More specifically, the method for forming a resist  pattern according to the present invention can be performed,  for example, as follows.  [0560]  More specifically, the method for forming a resist  pattern according to the present invention can be performed,  for example, as follows. Firstly, a positive resist composition  of the present invention is applied onto a substrate using a  spinneror the like, and a pre bake (post applied bake (PAB)) is  conducted under temperature conditions of 80 to 150°  C. for  40 to 120 seconds, preferably 60 to 90 seconds to form a resist  film. Then, for example, using anArF exposure apparatus or  the like, the resist film is selectively exposed with an ArF  exposure apparatus, an electron beam exposure apparatus, an  EUV exposure apparatus or the like through a mask pattern or  directly irradiated with electron beam without a mask pattern,  followed by post exposure bake (PEB) under temperature  conditions of 80 to 150°  C. for 40 to 120 seconds, preferably  60 to 90 seconds. Subsequently, developing is conducted  using an alkali developing solution such as a 0.1 to 10% by  weight aqueous solution of tetramethylammonium hydroxide  (TMAH), preferably followed by rinsing with pure water, and  drying. If desired, bake treatment (post bake) can be con› ducted following the developing. In this manner, a resist  pattern that is faithful to the mask pattern can be obtained.  [0561]  The substrate is not specifically limited and a con› ventionally known substrate can be used. For example, sub› strates for electronic components, and such substrates having  wiring patterns formed thereon can be used. Specific  examples of the material of the substrate include metals such  as silicon wafer, copper, chromium, iron and aluminum; and  glass. Suitable materials for the wiring pattern include cop› per, aluminum, nickel, and gold.  [0562]  Further, as the substrate, any one of the above-men› tioned substrates provided with an inorganic and/or organic  film on the surface thereof may be used. As the inorganic film,  an inorganic antireflection film (inorganic BARC) can be  used. As the organic film, an organic antireflection film ( or› ganic BARC) and an organic film such as a lower-layer  organic film used in a multilayer resist method can be used.  [0563]  Here, a "multilayer resist method" is method in  which at least one layer of an organic film (lower-layer  organic film) and at least one layer of a resist film (upper resist  film) are provided on a substrate, and a resist pattern formed  on the upper resist film is used as a mask to conduct patterning  of the lower-layer organic film. This method is considered as  being capable of forming a pattern with a high aspect ratio.  More specifically, in the multilayer resist method, a desired  thickness can be ensured by the lower-layer organic film, and  as a result, the thickness of the resist film can be reduced, and  an extremely fine pattern with a high aspect ratio can be  formed.  [0564]  The multilayer resist method is broadly classified  into a method in which a double-layer structure consisting of  an upper-layer resist film and a lower-layer organic film is  formed ( double-layer resist method), and a method in which  a multilayer structure having at least three layers consisting of  an upper-layer resist film, a lower-layer organic film and at  least one intermediate layer (thin metal film or the like) pro› vided between the upper-layer resist film and the lower-layer  organic film (triple-layer resist method),  [0565]  The wavelength to be used for exposure is not par› ticularly limited and the exposure can be conducted using  radiation such as ArF excimer laser, KrF excimer laser, F  2  excimer laser, extreme ultraviolet rays (EUV), vacuum ultra› violet rays (VlN), electron beam (EB), X-rays, and soft  X-rays. The positive resist composition of the present inven› tion is effective to KrF excimer laser, ArF excimer laser, EB  and EUV, and particularly effective to ArF excimer laser.  [0566]  The exposure of the resist film can be either a gen› eral exposure ( dry exposure) conducted in air or an inert gas  such as nitrogen, or immersion exposure (immersion lithog› raphy).  [0567]  In immersion lithography, the region between the  resist film and the lens at the lowermost point of the exposure  apparatus is pre-filled with a solvent (immersion medium)  that has a larger refractive index than the refractive index of  air, and the exposure (immersion exposure) is conducted in  this state.  [0568]  The immersion medium preferably exhibits a  refractive index larger than the refractive index of air but  smaller than the refractive index of the resist film to be  exposed. The refractive index of the immersion medium is not  particularly limited as long at it satisfies the above-mentioned  requirements.  [0569]  Examples of this immersion medium which exhibits  a refractive index that is larger than the refractive index of air  but smaller than the refractive index of the resist film include  water, fluorine-based inert liquids, silicon-based solvents and  hydrocarbon-based solvents.  [0570]  Specific examples of the fluorine-based inert liquids  include liquids containing a fluorine-based compound such as  C$_{3}$HC1$_{2}$F$_{5}$ ,  C$_{4}$ F$_{9}$OCH$_{3}$ ,  C$_{4}$F$_{9}$OC$_{2}$H$_{5 }$ or C$_{5}$H$_{3}$F$_{7 }$ as the main  component, which have a boiling point within a range from 70  to 180°  C. and preferably from 80 to 160°  C. A fluorine-based  inert liquid having a boiling point within the above-men› tioned range is advantageous in that the removal of the  immersion medium after the exposure can be conducted by a  simple method.  [0571]  As a fluorine-based inert liquid, a perfluoroalkyl  compound in which all of the hydrogen atoms of the alkyl  group are substituted with fluorine atoms is particularly desir› able. Examples of these perfluoroalkyl compounds include  perfluoroalkylether compounds and perfluoroalkylamine  compounds.  [0572]  Specifically, one example of a suitable perfluoro› alkylether compound is perfluoro(2-butyl-tetrahydrofuran)  (boiling point 102°  C.), and an example of a suitable perfluo› roalkylamine compound is perfluorotributylamine (boiling  point 174°  C.).  [0573]  As the immersion medium, water is preferable in  terms of cost, safety, environment and versatility. 