225  US 9,291,896 B2  226  (iii’) a step of developing the exposed film by using an  organic solvent-containing developer to form a negative pat› tern.  The exposure in the step (ii) may be immersion exposure.  The pattern forming method of the present invention pref› erably includes (iv) a heating step after the exposure step (ii).  The pattern forming method of the present invention may  further include ( v) a step of performing development by using  an alkali developer when the developer in the step (iii) is an  organic solvent-containing developer, and on the other hand, 10  may further include (v) a step of performing development by  using an organic solvent-containing developer when the  developer in the step (iii) is an alkali developer.  In the pattern forming method of the present invention, the  exposure step (ii) may be performed a plurality of times.  In the pattern forming method of the present invention, the  heating step (v) may be performed a plurality of times.  The resist film is formed of the above-described actinic  ray-sensitive or radiation-sensitive resin composition of the  present invention and, more specifically, is preferably formed  on a substrate. In the pattern forming method of the present  invention, the step of forming a film on a substrate by using  the actinic ray-sensitive or radiation-sensitive resin composi› tion, the step of exposing the film, and the development step  can be performed by generally known methods.  For example, the composition is coated on such a substrate  as used in the production of a precision integrated circuit  device, an imprint mold or the like ( e.g., silicon/silicon diox› ide-coated substrate, silicon nitride and chromium-deposited  quartz substrate) by using a spinner, a coater or the like. The  coating is thereafter dried, whereby an actinic ray-sensitive or  radiation-sensitive film can be formed.  Before forming the resist film, an antireflection film may be  previously provided by coating on the substrate.  The antireflection film used may be either an inorganic film  type such as titanium, titanium dioxide, titanium nitride,  chromium oxide, carbon and amorphous silicon, or an  organic film type composed of a light absorber and a polymer  material. A commercially available organic antireflection  film such as DUV30 Series and DUV-40 Series produced by  Brewer Science, Inc. andAR-2, AR-3 andAR-5 produced by  Shipley Co., Ltd. may be also used as the organic antireflec› tion film.  The pattern forming method also preferably includes, after  film formation, a pre-baking step (PB) before entering the  exposure step.  Furthermore, the pattern forming method also preferably  includes a post-exposure baking step (PEB) after the expo› sure step but before the development step.  As for the heating temperature, both PB and PEB are  preferably performed at 70 to 120°  C., more preferably at 80  to  110°c.  The heating time is preferably from 30 to 300 seconds,  more preferably from 30 to 180 seconds, still more preferably  from 30 to 90 seconds.  The heating can be performed using a device attached to an  ordinary exposure/developing machine or may be performed  using a hot plate or the like.  The reaction in the exposed area is accelerated by the  baking and in turn, the sensitivity or pattern profile is  improved.  It is also preferred to include a heating step (post baking)  after the rinsing step. By the baking, the developer and rinsing  solution remaining between patterns as well as in the inside of  the pattern are removed.  Examples of the actinic ray or radiation include infrared  light, visible light, ultraviolet light, farultraviolet light, X-ray,  and electron beam. An actinic ray or radiation having, for  example, a wavelength of250 nm or less, particularly 220 nm  or less, is preferred. Such an actinic ray or radiation includes,  for example, KrF excimer laser (248 nm), ArF excimer laser  (193 nm), F$_{2 }$ excimer laser (157 nm), X-ray, and electron  beam. The actinic ray or radiation is preferably, for example,  KrF excimer laser, ArF excimer laser, electron beam, X-ray or  EUV light, more preferably electron beam, X-ray or EUV  light.  In the present invention, an immersion exposure method  can be applied in the step of performing exposure.  The immersion exposure method is a technique to increase  the resolution, and this is a technique of performing exposure  by filling a space between the projection lens and the sample  with a high refractive-index liquid (hereinafter, sometimes  referred to as an "immersion liquid").  As for the "effect of immersion", assuming that  A$_{0 }$ is the  wavelength of exposure light in air, n is the refractive index of  the immersion liquid for air, 8 is the convergence half-angle  ofbeam and NA$_{0}$=sin 8, the resolution and the depth of focus  in immersion can be expressed by the following formulae.  Here, k$_{1 }$ and k$_{2 }$ are coefficients related to the process.  That is, the effect of immersion is equal to use of an  exposure wavelength of 1/n. In other words, in the case of a  projection optical system having the same NA, the depth of  focus can be made n times larger by immersion. This is  effective for all pattern profiles and furthermore, can be com- bined with the super-resolution technology under study at  present, such as phase-shift method and modified illumina› tion method.  In the case of performing immersion exposure, a step of  washing the film surface with an aqueous chemical may be  performed (1) before the exposure step after forming the film  on a substrate and/or (2) after the step of exposing the film  through an immersion liquid but before the step of baking the  film.  The immersion liquid is preferably a liquid being transpar› ent to light at the exposure wavelength and having as small a  temperature coefficient of refractive index as possible in order  to minimize the distortion of an optical image projected on the  film. In particular, when the exposure light source is ArF  excimer laser (wavelength: 193 nm), water is preferably used  in view of easy availability and easy handleability in addition  to the above-described aspects.  In the case of using water, an additive (liquid) capable of  decreasing the surface tension of water and increasing the  interface activity may be added in a small ratio. This additive  is preferably an additive that does not dissolve the resist layer  on the wafer and at the same time, gives only a negligible  effect on the optical coat at the undersurface of the lens  element.  Such an additive is preferably, for example, an aliphatic  alcohol having a refractive index substantially equal to that of  water, and specific examples thereof include methyl alcohol,  ethyl alcohol and isopropyl alcohol. By virtue of adding an  alcohol having a refractive index substantially equal to that of  water, even when the alcohol component in water is evapo› rated and its content concentration is changed, the change in  the refractive index of the liquid as a whole can be advanta› geously made very small.  On the other hand, if a substance opaque to light at 193 nm  or an impurity greatly differing in the refractive index from  water migrates into the water, this gives rise to distortion of an  (Resolution)~k  $_{1 }$•(t,rJn )!NA$_{0 }$ (Depth  offacus)~–k$_{2}$•(Ar/n)!NAa2 