Abstract:
A resist pattern forming method includes preparing a photomask for generating near-field light having an intensity distribution. The photomask has a light-transmissible base member, and a light-blocking film. The film has a micro-aperture adapted to expose an object to near-field light seeping out from the micro-aperture. The photomask has a periodic structure and a shift of a phase. The shift exists between recesses or projections adjacent to the micro-aperture. A difference in the intensity distribution of the near-field light in the area of the aperture is reduced. The photomask is arranged close to a photoresist film on a substrate. Light from a light source irradiates the photoresist film by way of the photomask to form a latent image based on the micro-aperture, and the photoresist film is developed to form a resist pattern on the substrate based on the latent image.

Description:
This application is a divisional application of U.S. patent application Ser. No. 10/842,451, filed May 11, 2004, now U.S. Pat. No. 7,374,844 issued May 20, 2008. 
     This application also claims priority from Japanese Patent application No. 2003-132517 filed on May 12, 2003, the entire contents of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a photomask for exposure to an optical near-field, to a method of controlling an optical near-field intensity distribution using such a photomask, to a pattern preparing method, and also to a pattern preparing apparatus. 
     2. Related Background Art 
     With the current advancement of technology brought forth to realize semiconductor memories having a higher memory capacity and CPUs that operate at a very high speed, and show an enhanced degree of integration, photolithography is inevitably required to accommodate micro-processing operations with smaller dimensions. Generally, the dimensional limit of micro-processing of a photolithography apparatus approximately corresponds to the wavelength of the light source to be used for photolithography. Therefore, a near-ultraviolet laser is often used as a light source for a photolithography apparatus in order to reduce the dimensional limit. Thus, currently, photolithography apparatuses are feasible for micro-processing on the order of about 0.1 μm. 
     While the dimensional limit of micro-processing is reduced for photolithography, there are still a number of problems to be solved for photolithography if it is to be used for micro-processing of 0.1 μm and less. For example, a light source having a shorter wavelength may be needed. Then, lenses that can be used for such a short wavelength may have to be developed. 
     Near-field exposure methods have been proposed as a possible solution for these problems. For example, U.S. Pat. No. 6,171,730 proposes an exposure method and an exposure apparatus that utilize an elastically deformable mask having a micro-aperture pattern of an opening with a width not greater than 100 nm on the front surface and made of an elastic material so as to be deformable along the normal line relative to the mask surface. The method and the apparatus as disclosed in the above-cited patent document are excellent, and have contributed greatly to the technological field to which the present invention relates. 
     However, when g-rays having a wavelength of 436 nm or i-rays having a wavelength of 365 nm are used for a micro-aperture pattern with a width not greater than 100 nm, as disclosed in U.S. Pat. No. 6,171,730, the width of the openings is less than one half of the wavelength. 
     In an operation of near-field exposure using a near-field formed by light by way of a micro-aperture in a metal film, the optical near-field intensity distribution can be different from the contour of the aperture. 
     Meanwhile, U.S. Pat. No. 6,236,033 proposes a photolithography mask formed having a metal film with an aperture and a surface profile that undulates periodically so as to interact with surface plasmon modes and intensifies the transmission of light through the aperture for the purpose of transferring an image. However, while the above-cited patent proposes to boost light obtained through the aperture, it does not propose to weaken light and make uniform the intensity distribution of light. 
     SUMMARY OF THE INVENTION 
     The present invention provides a photomask for exposure to an optical near-field, a method of controlling an optical near-field intensity distribution using such a photomask, a pattern preparing method and also a pattern preparing apparatus, as will be described hereinafter. 
     In an aspect of the invention, there is provided a photomask for exposure to an optical near-field having a micro-aperture and adapted to expose an object of exposure to light by using light seeping out from the micro-aperture, the mask having periodically arranged recesses or projections so as to make uniform the optical near-field intensity distribution in the micro-aperture. 
     For the purposes of the invention, the aperture width of the micro-aperture may be not greater than one-half of the wavelength of light from the light source. 
     For the purpose of the invention, the optical near-field intensity distribution may be controlled by way of the positions and/or sizes of the recesses or projections relative to the micro-aperture. 
     The recesses or projections may be arranged periodically relative to the micro-aperture and the optical near-field intensity distribution may be controlled by way of the extent of shift of the pitch and/or the phase of the period. 
     The pitch of the period may be made shorter than the intra-medium wavelength of the light used for the exposure within the mask base member of the photomask. 
     For the purpose of the invention, it may be so arranged that the micro-aperture includes a micro-aperture group of a plurality of micro-apertures and the photomask has the recesses or projections in the vicinity of an area of a weak optical near-field intensity that is produced when light for exposure is applied in the micro-aperture group. 
     Alternatively, it may be so arranged that the micro-aperture includes a micro-aperture group of a plurality of micro-apertures and the photomask has the recesses or projections in the vicinity of an area of a strong optical near-field intensity that is produced when light for exposure is applied in the micro-aperture group. 
     For the purposes of the invention, it may be so arranged that the micro-aperture includes a micro-aperture group of a plurality of micro-apertures and the photomask has at least a first recess or a projection in the vicinity of an area of a weak optical near-field intensity that is produced when light for exposure is applied in the micro-aperture group, the first recess or projection and the second recess or projection being different from each other in terms of relative position and/or size relative to the micro-aperture. 
     Still, alternatively, it may be so arranged that the micro-aperture includes a micro-aperture group of a plurality of micro-apertures and the photomask has first recesses or projections arranged periodically in the vicinity of an area of a weak optical near-field intensity that is produced when light for exposure is applied in the micro-aperture group and second recesses or projections arranged periodically in the vicinity of an area of a strong optical near-field intensity that is produced when light for exposure is applied in the micro-aperture group, the first recess or projection and the second recess or projection being different from each other in terms of the pitch and/or phase of period. 
     For the purposes of the invention, it may be so arranged that the micro-aperture is slit-shaped and the photomask has the area of a weak optical near-field intensity in the vicinity of each of the ends of the slit-shaped micro-aperture, the recess or projection being formed in the vicinity of each of the ends of the slit-shaped micro-aperture. 
     For the purposes of the invention, it may be so arranged that the micro-aperture shows an isolated pattern having a longitudinal dimension and a transversal dimension smaller than the wavelength, and the area of weak optical near-field intensity is located in the vicinity of the isolated pattern, whereas the recess or projection is formed in the vicinity of the isolated pattern. 
     For the purposes of the invention, it may be so arranged that the phase of the period forms a discontinued section in the periodically arranged recesses or projections and the discontinued section is arranged in the vicinity of the micro-aperture. 
     For the purposes of the invention, it may be so arranged that the micro-aperture includes a micro-aperture group of a plurality of micro-apertures and the photomask is provided with a first periodic structure having a discontinued section in the phase of the period in the vicinity of an area of a weak optical near-field intensity that is produced when light for exposure is applied in the micro-aperture group and a second periodic structure having a discontinued section in the phase of a period in the vicinity of an area of a strong optical near-field intensity that is produced when light for exposure is applied in the micro-aperture group, the amount of discontinuation of phase being different between the first periodic structure and the second periodic structure. 
     In another aspect of the present invention, there is provided a method of controlling an optical near-field intensity distribution using a photomask for exposure to an optical near-field according to the invention and adapted to control the intensity distribution of the optical near-field according to the invention and adapted to control the intensity distribution of the optical near-field by adjusting the coupled relation of light being propagated through the micro-aperture and plasmon polaritons on the surface of the photomask. 
     In still another aspect of the invention, there is provided a pattern preparing method comprising arranging a photomask for exposure to an optical near-field on a substrate to be processed, the substrate carrying a photoresist film thereon with a thickness not greater than the wavelength of light from a light source, irradiating light for exposure from the light source onto the photoresist film by way of the photomask, and forming/transferring a latent image on the photoresist film on the basis of the aperture pattern formed in the photomask by controlling the intensity distribution of the optical near-field. 
     In still another aspect of the invention, there is provided a pattern preparing apparatus comprising a stage adapted to carry thereon a photomask for exposure to an optical near-field according to the invention, a light source for exposure, a specimen table adapted to carry thereon a substrate to be processed, the substrate carrying a photoresist film thereon with a thickness not greater than the wavelength of light from the light source, and a distance control means for controlling the distance between the substrate to be processed and the photomask. 
     Thus, according to the invention, it is possible not only to control the intensity distribution of the optical near-field, at will, but it is also possible to provide a photomask for exposure to an optical near-field that can control the intensity distribution so as to make it uniform, a method of controlling the intensity distribution of an optical near-field by using such a photomask, a pattern preparing method and a pattern preparing apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the first embodiment of a photomask for exposure to an optical near-field according to the invention; 
         FIG. 2  is a schematic illustration of a photomask for exposure to an optical near-field obtained by modifying the first embodiment of the invention; 
         FIG. 3  is a schematic illustration of the second embodiment of a photomask for exposure to an optical near-field according to the invention; 
         FIG. 4  is a schematic illustration of the third embodiment of a photomask for exposure to an optical near-field according to the invention; 
         FIGS. 5A ,  5 B,  5 C,  5 D,  5 E,  5 F,  5 G and  5 H are numerical computation models of an optical near-field in a near-field micro-aperture produced by periodically arranged recesses and projections provided for the purpose of explaining the first embodiment of the invention; 
         FIG. 6  is a graph obtained by computationally determining the intensity distribution of an optical near-field provided for the purpose of explaining the first embodiment of the invention; 
         FIG. 7  is a contour graph illustrating the pitch lambda (nm) of the periodic structure and the intensity of light immediately below the aperture, relative to the phase, as provided for the purpose of explaining the first embodiment of the invention; 
         FIG. 8  is a schematic illustration of the pattern preparing apparatus used in Example 1; and 
         FIGS. 9A ,  9 B,  9 C and  9 D are schematic illustrations of the pattern preparing method used in Example 2. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described in greater detail by referring to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is an enlarged partial view of the first embodiment of a photomask for exposure to an optical near-field according to the invention. The photomask comprises a base member  101  that is transparent relative to the wavelength of light from a light source to be used for exposure and a metal thin film  102  that blocks light from the light source. The thin film has a thickness between 40 and 200 nm. 
     The base member  101  is also a thin film having a thickness between 0.1 and 100 μm that is supported along the outer peripheral section thereof by a support member (not shown). The metal thin film  102  has a slit-shaped aperture  103 . The width of the slit is smaller than half of the wavelength of light from the light source, whereas the length of the slit is greater than the wavelength of light from the light source. 
     Recesses/projections  104  are formed periodically in the vicinity of each of the ends of the slit of the metal film  102  in a direction perpendicular to the longitudinal direction of the slit (in the direction of AA′ in  FIG. 1 ). 
     The mask is made to tightly adhere to the thin film resist applied to a substrate. Then, light is irradiated onto it in a direction perpendicular to it so that the pattern is exposed to light. Now, the behavior of light in the vicinity of the mask will be discussed in detail below. 
     When light for exposure is irradiated onto the mask from the side of the base member, an optical near-field is produced in the vicinity of the aperture on the front surface of the mask (the surface at the lower side in the cross-sectional view of  FIG. 1 ). 
     The periodic recesses/projections  104  at each of the ends of the slit of this embodiment are designed to boost the intensity of light at a part of the slit located near the recesses/projections, so as to minimize the difference in the optical near-field intensity over the entire area of the mask aperture and to realize a quasi-flat light intensity distribution pattern. 
     The distributed optical near-field is weak in the vicinity of each of the ends of the slit when there are no periodic recesses/projections  104 , for the reason described below. 
     Assume that light is irradiated onto the mask of  FIG. 1  without controlling polarization of the light. The incident light can be divided into a parallel component whose electrical field vector is parallel to the slit and a perpendicular component whose electrical field vector is perpendicular to the slit. The slit does not allow the component of light whose electrical field vector is parallel to the slit (and, hence, parallel to the metal surfaces, or the lateral surfaces, of the slit) to pass through it. On the other hand, the component of light whose electrical field vector is perpendicular to the slit is propagated through the slit and produces an optical near-field on the front surface of the mask. However, the propagation loss of light is increased in the slit, because the electrical field vector runs parallel with the metal surface at each of the ends of the slit. Such a loss affects the range from the ends of the slit to the wavelength of light from the light source. 
     The concept of controlling the intensity of light by means of periodic recesses/projections  104  arranged near the slit is based on the principle as described below. The periodic recesses/projections  104  control coupling of interface plasmon polaritons and light that is being propagated through the slit on the surface of the metal film (the interface of the metal film and the base member or the interface of the photoresist and the metal film when the metal film tightly adheres to the photoresist). 
     More specifically, interface plasmon polaritons are scattered by the periodic recesses/projections to give rise to a standing wave. The intensity of an optical near-field is boosted if the position of the slit is found near a local maximum of the standing wave, whereas it is weakened if the position of the slit is found near a local minimum of the standing wave. 
     This effect will be described further by way of specific numeral computation models, as shown in  FIGS. 5A through 5H . Chromium (Cr, complex refractive index; 1.775-4.035i) is used as a metal, and a value of 436 nm is assumed for the wavelength in a vacuum of light from a light source. For the purpose of simplicity, two-dimensional models where the refractive index of the base member is equal to one, and the slit has an infinite length, are used. 
     First, two parameters of the pitch Λ and the phase Φ of recesses/projections that are used for describing the profile of periodic recesses/projections in a case wherein there is no slit will be explained by referring to  FIGS. 5A through 5D . A slit is to be arranged along the dotted chain line in each of  FIGS. 5A through 5D . However, the phase and the pitch are defined on the basis of the profile of the recesses/projections before arranging a slit. The pitch Λ is the distance for a period of a recess and a projection, as shown in  FIG. 5A . Phase Φ=0 is defined for the periodic structure of  FIG. 5A  where the phase does not show any discontinuity at the axis of symmetry and the latter is the center of a recess. On the other hand, phase Φ=π/4 is defined for the periodic structure of  FIG. 5B , phase Φ=π/2 defined for that of  FIG. 5C , and phase Φ=π is defined for that of  FIG. 5D . 
     While the phase Φ is equal to π also at the left side of the slit, and there is no discontinuity of phase in the periodic recesses/projections of  FIG. 5D , the periodic structure of  FIG. 5D  differs from that of  FIG. 5A  in terms of phase. 
       FIGS. 5E through 5H  show periodic structures where a slit is arranged at the axis of symmetry and correspond, respectively, to  FIGS. 5A through 5D . Note, however, that the recess where the slit is arranged is buried in each of  FIGS. 5E and 5F . Two recesses are arranged at each side of the slit. 
     Then, light is irradiated onto the structure form the side where the recesses are formed periodically, and the intensity distribution of an optical near-field is determined by numerical analysis, using the FDTD method. The slit has a width of 80 nm and the light shielding film has a thickness of 60 nm. 
       FIG. 6  shows the computationally obtained values of the intensity of light (a squared amplitude of the electrical field) at positions separated from the mask surface by 10 nm. In  FIG. 6 , the solid line indicates the intensity distribution of an optical near-field when no periodic recesses are arranged. It is provided as a reference for comparison. On the other hand, the intensity of light is increased by about 1.5 times relative to the reference value when recesses are arranged periodically with a pitch of Λ=410 nm and a phase of Φ=π/2 (dotted chain line), whereas it is reduced to about 0.6 times relative to the reference value when recesses are arranged periodically with a pitch of Λ=340 nm and a phase of Φ=0 (broken line). The pitch that is used to increase the intensity of light is close to the wavelength of surface plasmons along the vacuum/Cr interface, or
 λ spp=λ   0 [(∈ m +∈ d )/(∈ m ×∈ d )] 1/2 =428.4 nm, 
but does not completely agree with the latter. In the above formula, ∈ m  represents the dielectric constant of Cr and ∈ d  represents the dielectric constant of the dielectric substance involved (e.g., a vacuum in the computation models).
 
       FIG. 7  is a contour graph illustrating the pitch lambda (nm) of the periodic structure, as shown in any of  FIGS. 5E through 5H , and the intensity of light immediately below the aperture relative to the phase (deg), where the intensity of light is made equal to one when there is no periodic structure. As in the case of the numerical computation models of  FIGS. 5E through 5H , chromium (Cr, complex refractive index; 1.775-4.035i) is used as a metal, and a value of 436 nm is assumed for the wavelength in a vacuum of light from a light source. Again, for the purpose of simplicity, two-dimensional models where the refractive index of the base member is equal to one, and the slit has an infinite length, are used. 
     For the effect of intensifying an optical near-field by means of periodic recesses/projections, as used in the above computation models, to appear only in regions surrounding the respective ends of the slit where the intensity of the optical near-field falls, it is necessary to form periodic recesses/projections only in the vicinity of each of the ends of the slit of the mask. When the refractive index of the base material of the photoresist that is tightly adhering to the mask is not one, as used in the above-described numerical computation models but n, it is necessary to multiply the above-cited pitch Λ of the arrangement of periodic recesses/projections by 1/n to obtain Λ/n. 
     In this way, it is possible to obtain a distribution of an intensity that approximately reflects the shape of the slit, and hence, to form a resist pattern that approximately reflects the shape of the slit, by appropriately controlling the optical near-fields intensity distribution, using a photomask on which the pitch and/or the phase of the periodically arranged recesses are appropriately selected. While periodic recesses/projections are formed only on the surface of the metal film that is irradiated with light in the above description, periodic recesses/projections may alternatively be formed on the front surface of the mask, although the effect will be weakened to a certain extent. Still, alternatively, periodic recesses/projections may be formed on the opposite surfaces of the metal film. 
     While recesses are formed in the above-described instance, it may be needless to say that a similar effect is obtained by forming periodic projections. 
       FIG. 2  shows a modified arrangement of periodic recesses, where square recesses are formed around a single micro-aperture to obtain a mask pattern that can effectively increase the optical near-field intensity due to the arrangement of the periodic recesses. 
     Second Embodiment 
       FIG. 3  is an enlarged partial view of the second embodiment of a photomask for exposure to an optical near-field, according to the invention. With this embodiment, the intensity of an optical near-field can be reduced by means of periodic recesses for the purpose of controlling the intensity of the optical near-field. 
     The mask pattern of  FIG. 3  is formed by arranging periodic recesses  304  in the vicinity of a middle part of slit-shaped aperture  303 , where the intensity of the optical near-field is high. In this case, both the intensity of light propagated through the slit and that of the optical near-field on the surface of the mask are reduced by shifting the pitch and the phase of the periodic recesses  304 . 
     With this arrangement again, it is possible to control the optical near-field intensity distribution in the vicinity of the slit and to obtain an exposure pattern that approximately reflects the shape of the slit. 
     In a case of controlling the optical near-field intensity distribution not only for a single slit, but the respective optical near-field intensity distributions for a plurality of slits whose shapes may vary from each other, so as to obtain a resist pattern, as the whole mask pattern, having a desired profile, the use of such a pattern that can reduce the intensity of the optical near-field is effective. 
     Third Embodiment 
       FIG. 4  is an enlarged partial view of the third embodiment of a photomask for exposure to an optical near-field according to the invention. With this embodiment, the intensity of the optical near-field can be reduced in certain regions and, at the same time, increased in other regions on the mask by using a plurality of micro-apertures. 
     The mask pattern illustrated in  FIG. 4  is formed by arranging a group of slit-shaped apertures  403  that run in parallel with each other. Recesses  404  are arranged periodically in the vicinity of each of the ends of the group of slit-shaped apertures  403  for the purpose of intensifying an optical near-field, whereas recesses  405  are arranged periodically in the vicinity of a central part of the group of slits for the purpose of reducing the intensity of an optical near-field. 
     With the periodic arrangement of recesses, it is possible to control not only the optical near-field intensity distribution in a single slit, but also, the difference in the intensities of an optical near-field among the plurality of slits, so as to expose a resist pattern that corresponds to the shapes of the slit to light. 
     Now, the present invention will be described further by way of examples. 
     EXAMPLE 1 
       FIG. 8  is a schematic illustration of the pattern preparing (exposure) apparatus used in Example 1. 
     In  FIG. 8 , reference symbol  801  denotes a photomask according to the invention as described above by way of embodiments. The front surface (the lower surface shown in  FIG. 8 ) of the photomask  801  is directed to the outside of pressure adjustable container  805 , whereas the rear surface (the upper surface shown in  FIG. 8 ) of the photomask  801  is directed to the inside of the pressure adjustable container  805 . The internal pressure of the pressure adjustable container  805  can be adjusted by a pressure adjusting means  813 . 
     The object of exposure in this example was a substrate  806  carrying a resist film  807  that was formed on the surface thereof. The resist  807 /substrate  806  was placed on a stage  808  and the stage  808  was driven so as to align the substrate  806  relative to the photomask  701  in intra-planar two-dimensional directions of the mask. Subsequently, the stage  808  was driven in a direction along the normal line relative to the mask surface so as to make the photomask  801  tightly adhere to the resist  807  on the substrate  806 . 
     Then, the evanescent light exposure mask  801  was made to tightly adhere to the resist  807  on the substrate  806  by adjusting the internal pressure of the pressure adjustable container  805  by the pressure adjusting means  813  until the gap between the front surface of the mask  801  and the corresponding surface of the resist  807  became not greater than 100 nm over the entire surface area of the mask  801 . 
     Thereafter, light for exposure  810  emitted from light source  809  was collimated by a collimator lens  811 , and then made to pass through a glass window  812  so as to be introduced into the pressure adjustable container  805  and irradiated onto the evanescent light exposure mask  801  from the rear surface thereof (upper surface in  FIG. 8 ). The resist  807  was exposed to an optical near-field produced in the vicinity of the micro-apertures on the front surface of the photomask  801 . It was possible to transfer the different patterns  804  on the photomask on the substrate  806  as patterns of clear contrast by using the photomask  801 . 
     EXAMPLE 2 
       FIGS. 9A through 9D  are schematic illustrations of the pattern preparing method including a single buffer layer used in Example 2. 
       FIG. 9A  shows the photomask and the object of exposure of this example. The photomask  904  is a photomask according to the invention as described above by way of the embodiments. 
     In this example, a positive type photoresist was applied onto an Si substrate  901  by means of a spin coater. Subsequently, the photoresist was heated at 120° C. for thirty minutes to produce the first layer  902 , which had a film thickness of 700 nm. 
     Thereafter, a negative type photoresist that contained Si was applied onto the first layer  902  and pre-baked to produce the second layer  903 , which had a film thickness of 40 nm. Thus, the photoresist had a two-layered structure. 
     Then, the Si substrate  901  carrying the two-layered photoresist thereon, which was formed as a result of the application process, was brought close to the photomask  904  by means of the exposure apparatus as shown in  FIG. 8 , and pressure was applied thereto in order to make the resist layer  903  tightly adhere to the photomask  904 . 
     Light for exposure  905  (the photomask was prepared to meet the wavelength of light) was irradiated onto the photoresist layer  903  on the substrate  901  by way of the photomask. In other words, the photoresist layer  903  was exposed to light by way of the patterns on the photomask  904  ( FIG. 9B ). 
     Subsequently, the photomask was removed from the surface of the photoresist layer  903 , which was then subjected to a development process and a post-baking process. As a result, the patterns on the photomask were transferred as resist patterns ( FIG. 9C ). 
     Thereafter, the first photoresist layer  902  was subjected to an oxygen reactive ion etching process, using the patterns formed on the second photoresist layer  903  as an etching mask ( FIG. 9D ). 
     The Si contained in the second photoresist layer  903  was oxidized in the oxygen reactive ion etching process to raise the etching-resistance of the layer. 
     As a result of following the above-described procedure, it was possible to transfer the different patterns on the photomask on the substrate  901  as patterns of clear contrast. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the claims.