Patent Publication Number: US-2013248489-A1

Title: Microstructure manufacturing method

Description:
The entire disclosure of Japanese Patent Application No. 2007-039870, filed Feb. 20, 2007 is expressly incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a technology for manufacturing a microstructure, in particular, a structure (nanostructure) on the order of nanometers, using laser interference exposure. This technology is applicable to processes of manufacturing various nanostructures such as polarization beam splitters, phase retarders, and antireflective surfaces. 
     2. Related Art 
     If light enters a structure (subwavelength structure) on the order smaller than a wavelength (order of 380 to 780 nm) of visible light, a phenomenon such as polarization separation, birefringence, antireflection, or plasmon propagation occurs. As one of the methods for manufacturing a subwavelength structure, exposure technologies using laser interference are known (for example, see JP-A-2006-093644). In the exposure technology described in JP-A-2006-093644, a photosensitive film is exposed to interference light (light having interference fringes) obtained by crossing two laser beams. In this case, the period of a pattern formed by the exposure to the interference light depends on a wavelength λ of the laser beams and the cross angle between the laser beams. In theory, the shortest possible period that is realized by interference exposure is equal to λ/2. For example, if the wavelength is 266 nm, the shortest possible period of a formed pattern is 133 nm in theory. 
     In order to realize a pattern with a period shorter than 133 nm using the above-described interference exposure, the wavelength of the laser beams must be shorter than 133 nm. However, it is difficult to reduce the wavelength of the laser beams under the current circumstances. One of the reasons is that there is no source of highly coherent lasers with a wavelength sufficiently shorter than 266 nm. Among excimer lasers used in the fields of semiconductor lithography and the like are ones with a wavelength of 193 nm, 157 nm, or the like. However, such excimer lasers do not have sufficiently high coherence in the present time. Therefore, it is difficult to form a sharp latent image pattern using these excimer lasers. Another reason is that if the wavelength is shorter than 200 nm, it is difficult to perform exposure in an atmosphere. It is also difficult to obtain materials suitable for optical components (lens, mirror, etc.) included in a laser interference exposure system. This is because, for example, the absorption edge of quartz glass lies around 180 nm. Therefore, a technology is desired that forms a sharp latent image pattern and realizes a fine pattern using this pattern without excessively pursuing a laser beam of a shorter wavelength. 
     SUMMARY 
     An advantage of the invention is to provide a technology for realizing a microstructure with high resolution. 
     According to one aspect (hereafter referred to as a “first aspect” for convenience) of the invention, a microstructure manufacturing method includes (a) generating first light including an interference fringe by crossing two laser beams, (b) forming a denatured region and a non-denatured region on an object having thermal non-linearity by applying the first light onto the object, so that the denatured region and the non-denatured are disposed so as to correspond to a period of the interference fringe of the first light, and (c) etching the object so that the denatured region or the non-denatured region is selectively eliminated. 
     Here, the “object having thermal non-linearity” refers to a base material (substrate), a coating, a structure, or the like made of a material such as PtO, ZnS—SiO2, Ge—Sb—Te, Ge—Sb—Te—S, Te—TeO2-Ge—Sn, Te—Ge—Sn—Au, Ge—Te—Sn, Sn—Se—Te, Sb—Se—Te, Sb—Se, Ga—Se—Te—Ge, In—Se, In—Se—Tl—Co, Ge—Sb—Te, In—Se—Te, Ag—In—Sb—Te or TeO2-Pb. The “denatured region” refers to, for example, a crystalline region, and the “non-denatured region” refers to, for example, a non-crystalline region. 
     In the first aspect, by applying the first light having an interference fringe (that is, light intensity distribution) onto the object having thermal non-linearity, a denatured region and a non-denatured region corresponding to a region where the light intensity is relatively high and a region where the light intensity is relatively low, respectively, are obtained. The period of the denatured region and non-denatured region is reduced by shortening the wavelength of the crossed two laser beams or increasing the cross angle. By using the object having thermal non-linearity, a sharper boundary is obtained between the denatured region and non-denatured region than that obtained in a case where a photoresist film or the like is used as is done in related art manufacturing methods, even if the period of the denatured region and non-denatured region is reduced (for example, the order of 100 to 200 nm). Also, since a difference in etching rate is made between the denatured region and non-denatured region, a well-resolved microstructure is formed by etching the object. 
     According to the first aspect, the microstructure manufacturing method preferably further includes, after step (b) and before step (c), (d) generating second light including an interference fringe by making a phase difference between the two laser beams and then crossing the two laser beams, and (e) forming a denatured region and a non-denatured region by applying the second light onto the object, so that the denatured region and the non-denatured region are disposed so as to correspond to a period of the interference fringe of the second light. 
     By making a phase difference between the two beams, the interference fringe of the first light and that of the second light are displaced from each other by a fraction of the fringe pitch (for example, ½ pitch, ¼ pitch, etc.). By applying the first light and second light as described above one after another onto the object having thermal non-linearity, the period of the denatured region and non-denatured region is further reduced. Even if the period is further reduced in this way, a sharp boundary is obtained between the denatured region and non-denatured region, since the object having thermal non-linearity is used. As a result, a well-resolved microstructure is obtained. 
     In the microstructure manufacturing method according to the first aspect, in step (b), third light as well as the first light is preferably applied onto the object. Likewise, in step (d), third light as well as the second light is preferably applied onto the object. While the “third light” is preferably light of a single wavelength, such as a laser beam (laser light), it may be light including components of multiple wavelengths. 
     By additionally applying third light different from the first light and second light when applying the first light or second light, the light intensity distribution is biased. Thus, the denatured region and non-denatured region are securely formed even if the intensity of the first light or second light is not enough high. 
     According to another aspect (hereafter referred to as a “second aspect” for convenience) of the invention, a microstructure manufacturing method includes (a) forming a coating having thermal non-linearity on an object, (b) generating first light including an interference fringe by crossing two laser beams, (c) forming a denatured region and a non-denatured region on the coating by applying the first light onto the coating, so that the denatured region and the non-denatured region are disposed so as to correspond to a period of the interference fringe of the first light, (d) etching the coating so that the denatured region or non-denatured region is selectively eliminated, and (e) etching the object using the coating from which the denatured region or non-denatured region has been eliminated. 
     Here, the “coating having thermal non-linearity” refers to a coating (thin film or thick film) made of a material such as PtO, ZnS—SiO2, Ge—Sb—Te, Ge—Sb—Te—S, Te—TeO2-Ge—Sn, Te—Ge—Sn—Au, Ge—Te—Sn, Sn—Se—Te, Sb—Se—Te, Sb—Se, Ga—Se—Te—Ge, In—Se, In—Se—Tl—Co, Ge—Sb—Te, In—Se—Te, Ag—In—Sb—Te or TeO2-Pb. The “denatured region” refers to, for example, a crystalline region, and the “non-denatured region” refers to, for example, a non-crystalline region. 
     In the second aspect, by applying the first light having the interference fringe onto the coating having thermal non-linearity, a denatured region and a non-denatured region corresponding to a region where the light intensity is relatively high and a region where the light intensity is relatively low, respectively, are obtained. The period of the denatured region and non-denatured region is reduced by shortening the wavelength of the crossed two laser beams or increasing the cross angle. By using the coating having thermal non-linearity, a sharper boundary is obtained between the denatured region and non-denatured region than that obtained in a case where a photoresist film or the like is used as is done in related art manufacturing methods, even if the period of the denatured region and non-denatured region is reduced (for example, the order of 100 to 200 nm). Also, since a difference in etching rate is made between the denatured region and non-denatured region, the denatured regions or non-denatured region is eliminated by etching the coating. Thus, the eliminated region is made into an opening. By etching the object using the coating having the aperture obtained in this way as an etching mask, a well-resolved microstructure is formed 
     The second aspect preferably further includes, after step (c) and before step (d), (f) generating second light including an interference fringe by making a phase difference between the two laser beams and then crossing the two laser beams, and (g) forming a denatured region and a non-denatured region on the coating by applying the second light onto the coating, so that the denatured region and non-denatured region are disposed so as to correspond to a period of the interference fringe of the second light. 
     By making a phase difference between the two beams, the interference fringe of the first light and that of the second light are displaced from each other by a fraction of the fringe pitch (for example, ½ pitch, ¼ pitch, etc.). By applying the first light and second light as described above one after another onto the coating having thermal non-linearity, the period of the denatured region and non-denatured region is further reduced. Even if the period is further reduced in this way, a sharp boundary is obtained between the denatured region and non-denatured region, since the object having thermal non-linearity is used. As a result, a better-resolved microstructure is obtained. 
     In the second aspect, in step (c), third light as well as the first light is preferably applied onto the coating. Likewise, in step (g), third light as well as the second light is preferably applied onto the coating. While the “third light” is preferably light with a single wavelength, such as a laser beam (laser light), it may be light including components of multiple wavelengths. 
     By additionally applying third light different from the first light and second light when applying the first light or second light, the light application intensity is biased. Thus, the denatured region and non-denatured region are securely formed even if the intensity of the first light or second light is not enough high. 
     According to yet another aspect (hereafter referred to as a “third aspect” for convenience) of the invention, a method for applying light to a processing target includes (a) generating first light including an interference fringe by crossing two laser beams, and (b) forming a denatured region and a non-denatured region on the processing target having thermal non-linearity by applying the first light onto the processing target, so that the denatured region and the non-denatured region are disposed so as to correspond to a period of the interference fringe of the first light. 
     Here, specific examples of the “processing target having thermal non-linearity” are the same as those of the “object having thermal non-linearity” in the microstructure manufacturing method according to the first aspect of the invention. Also, respective specific examples of the “denatured region” and “non-denatured region” are the same as those in the first aspect of the invention. 
     In the third aspect, by applying the first light having interference fringes onto the processing target having thermal non-linearity, a denatured region and a non-denatured region corresponding to a region where the light intensity is a relatively high and region where the light intensity is relatively low, respectively, are obtained. The period of the denatured region and non-denatured region is reduced by shortening the wavelength of the crossed two laser beams or increasing the cross angle. By using the processing target having thermal non-linearity, a sharper boundary is obtained between the denatured region and non-denatured region than that obtained in a case where a photoresist film or the like is used as is done in related art manufacturing methods, even if the period of the denatured region and non-denatured region is reduced (for example, the order of 100 to 200 nm). Therefore, according to this light application method, a microstructure including a denatured region and a non-denatured region periodically disposed at a sub-micron pitch is formed. Also, since the difference in etching rate is made between the denatured region and non-denatured region, a microstructure with high resolution is formed by etching the processing target. 
     The light application method according to the third aspect preferably further includes, after step (b), (c) generating second light including an interference fringe by making a phase difference between the two laser beams and then crossing the two laser beams, and (d) forming a denatured region and a non-denatured region on the processing target by applying the second light onto the processing target, so that the denatured region and non-denatured region are disposed so as to correspond to a period of the interference fringe of the second light. 
     By making a phase difference between the two beams, the interference fringe of the first light and that of the second light are displaced from each other by a fraction of the fringe pitch (for example, ½ pitch, ¼ pitch, etc.). By applying the first light and second light as described above one after another onto the processing target having thermal non-linearity, the period of the denatured region and non-denatured region is further reduced. Even if the period is further reduced in this way, a sharp boundary is obtained between the denatured region and non-denatured region, since the processing target having thermal non-linearity is used. Also, a microstructure with a shorter period is obtained by etching this processing target. 
     In the third aspect, in step (b), third light as well as the first light is preferably applied onto the processing target. Likewise, in step (d), third light as well as the second light is preferably applied onto the processing target. While the “third light” is preferably light of a single wavelength, such as a laser beam (laser light), it may be light including components of multiple wavelengths. 
     By additionally applying third light different from the first light and second light when applying the first light or second light, the light application intensity is biased. Thus, the denatured region and non-denatured region are securely formed even if the intensity of the first light or second light is low. 
     According to still another aspect (hereafter referred to as a “fourth aspect” for convenience) of the invention, a microstructure manufacturing method includes (a) forming an amorphous, inorganic material layer on a metal film, (b) applying light generated by crossing a first laser beam and a second laser beam, onto the inorganic material layer, and heating a first portion of the inorganic material layer, the first portion corresponding to a period of an interference fringe of the light, up to a crystallization temperature or higher so that the first portion is denatured into a crystalline portion, (c) eliminating the first portion on the metal film and leaving a second portion of the inorganic material layer, the second portion having not been denatured into the crystalline portion, and (d) eliminating a region of the metal film, the region not overlapping the second portion, so that a structure with a pitch equal to the period of the interference fringe is formed. 
     In step (c), the second portion may be eliminated while leaving the first portion and, in step (d), a region of the metal film that does not overlap the first portion may be eliminated. This is because which of these portions is eliminated depends on, for example, the material of the inorganic material layer. 
     In the fourth aspect, the light having the interference fringe (that is, light intensity distribution) is generated by crossing the first and second laser beams. By applying this light including the interference fringe onto the amorphous, inorganic material layer, the first portion corresponding to a region where the light intensity is relatively high and a region where the light intensity is relatively low is denatured into a crystalline portion. The period of the first portion that has been denatured into a crystalline portion and the second portion that has not been denatured into a crystalline portion is reduced by shortening the wavelength of the crossed two laser beams or increasing the cross angle. By using the processing target having thermal non-linearity, a sharper boundary is obtained between the first portion (crystalline portion) and the second portion (non-crystalline portion) than that obtained in a case where a photoresist film or the like is used as is done in related art manufacturing methods, even if the period of the first and second portions is reduced (for example, the order of 100 to 200 nm). Also, since the difference in etching rate is made between the denatured region and non-denatured region, a microstructure with high resolution is formed by etching the object. 
     In step (b), the first portion is preferably completely heated up to a crystallization temperature or higher in a film thickness direction. 
     Thus, the first portion is completely crystallized in the film thickness direction. 
     The metal film preferably includes aluminum, and the structure is preferably a striped grid. 
     Thus, a microstructure that is usable as an optical element such as a polarization beam splitter is obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a block diagram showing a configuration of an exposure apparatus (microstructure manufacturing apparatus). 
         FIG. 2  is a side view of a diffractive optical element. 
         FIG. 3  is a side view of a phase difference plate. 
         FIG. 4  is a drawing schematically showing an aspect in which interference light generated by crossing two diffraction beams is applied onto an object. 
         FIG. 5  is a graph showing the relation represented by Eq. 1. 
         FIG. 6  is a diagram showing one example of a material having thermal nonlinearity. 
         FIG. 7  is a drawing schematically showing exposure of the material having thermal non-linearity. 
         FIG. 8  is a graph showing the depth of a phase change caused according to a change in temperature. 
         FIG. 9  is a diagram schematically showing an aspect of a surface-relief pattern formed according to the period of interference fringes. 
         FIGS. 10A to 10C  are schematic sectional views showing a microstructure manufacturing process according to a first embodiment of the invention. 
         FIG. 11A to 11D  are schematic sectional views showing a microstructure manufacturing process according to a second embodiment of the invention. 
         FIGS. 12A and 12B  are schematic sectional views showing a microstructure manufacturing process according to a third embodiment of the invention. 
         FIG. 13  is a diagram schematically showing an aspect of a surface-relief pattern formed according to the period of interference fringes. 
         FIG. 14  is a schematic sectional view showing a microstructure manufacturing process according to a fourth embodiment of the invention. 
         FIG. 15  is a graph showing the depth of a phase change caused according to a change in temperature. 
         FIG. 16  is a diagram schematically showing an aspect of a surface-relief pattern formed according to the period of interference fringes. 
         FIG. 17  are drawings schematically showing a structure of a polarization beam splitter that is one example of a microstructure. 
         FIG. 18  is a graph showing characteristics of the polarization beam splitter. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the invention will now be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing a configuration of an exposure apparatus (microstructure manufacturing apparatus) for use in the embodiments of the invention. An exposure apparatus  1  for use in the embodiments, shown in  FIG. 1 , includes a light source (pulse laser)  10 , a shutter  11 , mirrors  12  and  13 , a diffractive optical element  14 , a first lens  15 , a spatial filter  16 , a phase difference plate  17 , and a second lens  18 . 
     The light source  10  outputs laser beams of a short wavelength. For example, a Q switch pulse YAG laser (wavelength: 266 nm) is suitably used as the light source  10 . The laser average output of the light source  10  is, for example, approximately 1 W (at the pulse repetition rate of 1 kHz) and the pulse width is, for example, 1×10 −9  sec, or more and 1×10 −7  sec. or less. While pulse laser beams are used in the embodiments, the invention is not limited thereto and continuous wave (CW) laser beams may be used. A laser beam emitted from the light source  10  passes through the shutter  11  and then its path (optical path) is folded by each of the mirrors  12  and  13  by 90°. Subsequently, the laser beam enters the diffractive optical element  14 . The passage/interruption of the laser beam is controlled by the shutter  11 . 
     The diffractive optical element  14  splits the incident laser beam into multiple laser beams (diffraction beams).  FIG. 2  is a side view of the diffractive optical element  14 . As shown in  FIG. 2 , the diffractive optical element  14  has a binary structure that has two levels of a gap h (e.g., 266 nm) in a period P (e.g., 0.50 μm) and a surface thereof has a periodic structure. The diffractive optical element  14  is manufactured on a quartz substrate by direct laser-writing and ion-etching. Note that the structure of the diffractive optical element  14  is not limited to a binary structure and a surface thereof may have a sine (cosine)-curved, periodic structure. Or, the diffractive optical element  14  may have a flat appearance and a periodic structure in which the index of refraction is periodically distributed inside the diffractive optical element  14 . 
     The first lens  15  collects the split multiple diffraction beams so as to collimate the beams. In the embodiments, a focal length f 1  of the first lens  15  is, for example, 50 mm. The spatial filter  16  passes only two of the multiple diffraction beams therethrough. 
     The phase difference plate  17  gives a phase difference between the two beams that have passed through the spatial filter  16 .  FIG. 3  is a side view of the phase difference plate  17 . As shown in  FIG. 3 , the phase difference plate  17  has a gap with a depth g on a surface thereof. In other words, the phase difference plate  17  has different thicknesses in its right and left regions in the drawing. Thus, a predetermined phase difference (optical path difference) is made between the diffraction beam that has passed through the phase difference plate  17  and the diffraction beam that has not passed therethrough. In the embodiments, a phase difference φ between the diffraction beam that has passed through a region of the phase difference plate  17  that has a relatively large thickness and the diffraction beam that has not passed through the phase difference plate  17  is set to an integer multiple of 2π, namely, 0. Also, the phase difference φ between the diffraction beam that has passed through a region of the phase difference plate  17  that has a relatively small thickness and the diffraction beam that has not passed through the phase difference plate  17  is set to π. The phase difference plate  17  is manufactured on the quartz substrate by laser drawing and ion-etching. Note that the shape of the phase difference plate  17  is not limited to a stepped shape having different thicknesses. For example, the phase difference plate  17  may have a flat appearance and have different diffraction indexes in its right and left regions. If the phase difference is set to π, a ½ wavelength plate may be used as the phase difference plate  17 . 
     The second lens  18  collects the two diffraction beams that have passed through the phase difference plate  17 . A focal length  12  of the second lens  18  is, for example, 28 ram. The two diffraction beams collected by the second lens  18  are applied to an object  19 . The two diffraction beams interfere with each other at a predetermined cross angle θ so as to generate periodic interference fringes (interference light intensity distribution). Thus, a periodic fine pattern corresponding to the interference light intensity distribution is formed on a surface of the object. The time of exposure to the diffraction beams is, for example, on the order of several milliseconds and is controlled by the shutter  11 . 
     In the above-described exposure apparatus  1  for use in the embodiments, the two split diffraction beams are close to each other and the time required to perform interference exposure is as short as the order of several milliseconds. For these reasons, the diffraction beams are very stable against disturbances and resistant to vibrations and air flows. Generally, a laser interference exposure system is sensitive to air flows. In order to keep such a system stable, it must be placed on a vibration isolation bench. Further, a measure such as covering the system and the bench with a robust must be taken. Thus, a large facility cost is required. In the embodiments, a stable exposure system is realized by a simplified apparatus configuration. 
       FIG. 4  is a drawing schematically showing an aspect in which interference light generated by crossing the two diffraction beams is applied onto the object. The period P of the interference fringes F obtained when crossing the two diffraction beams at the cross angle θ is given by the following equation. 
         P =λ/(2 sin θ)  Equation 1 or Eq. 1
 
     As shown in  FIG. 4 , the two diffraction beams B 1  (first laser beam) and B 2  (second laser beam) enter the object  19  in a manner that these beams are symmetrical with respect to the normal to the application surface of the object  19 . Thus, the application depth of the interference field and the width thereof, the pitch P of the interference fringes F, and the like are made more uniform. 
       FIG. 5  is a graph showing the relation represented by Eq. 1 above. The horizontal axis corresponds to the cross angle θ, and the vertical axis corresponds to the period P of the interference fringes F. As shown in  FIG. 5 , for example, if the wavelength is 266 nm and the cross angle θ is 72°, the period P of the interference fringes F is 140 nm. By applying the interference light including the interference fringes with such a narrow pitch to the object  19 , a desired treatment can be performed on the object  19 . In the embodiments, a material having thermal nonlinearity is used as the object  19 . The material having thermal nonlinearity will be described below. 
       FIG. 6  is a diagram showing one example of the material having thermal nonlinearity. Specifically,  FIG. 6  shows the measurement result of a differential thermal analysis of the material (e.g., platinum oxide (PtO)) having thermal nonlinearity. As illustrated, for example, if platinum oxide is heated, a phase change occurs around a temperature of 550° C. and the platinum oxide is changed from a non-crystalline state (non-denatured state) into a crystalline state (denatured state). The temperature at which such a phase change occurs is referred to as a transition point Tc. Also, in the embodiments, a material in which a phase change abruptly (that is, non-linearly) occurs due to a rise in temperature as described above is referred to as a “material having thermal non-linearity. If the object  19  having both non-crystalline regions and crystalline regions based on the presence/absence of a phase change as described above is immersed into an alkali solution, for example, the crystalline regions are etched faster, since there is a difference in etching rate between these two types of regions. Thus, convex portions and concave portions, which correspond to the non-crystalline regions and crystalline regions, respectively, are generated. In the embodiments, the material having such thermal non-linearity is exposed to the above-described interference light. 
       FIG. 7  is a drawing schematically showing exposure of the material having thermal non-linearity. As shown in  FIG. 7 , the interference light generated by crossing the two diffraction beams is applied onto the object  19  formed on the substrate  20  (e.g., glass substrate). Thus, periodic phase changes corresponding to the pitch P of the interference fringes F of the interference light occur on a surface of the object  19  made of a material having thermal non-linearity. Specifically, regions of the object  19  in which the intensity of the interference light is relatively high are heated so that the transition point Tc is exceeded by the temperatures in these regions. This causes phase transitions in these regions, thereby putting these regions into a crystalline state. On the other hand, regions of the object  19  in which the intensity of the interference light is relatively low are not heated very much so that the transition point Tc is not exceeded by the temperatures in these regions. Thus, these regions cause no phase transitions and remain in a non-crystalline state. As shown in  FIG. 8 , the depth of a caused phase change is a depth dc from the surface of the object  19 . The temperature of the object  19  exceeds the transition point Tc up to the depth dc. Taking this into account, the thickness of the object  19  is previously set to the dc or less so that the object  19  is completely heated up to the transition point Tc or higher in the film thickness direction. After undergoing application of the interference light, the object  19  is immersed into an alkali solution. Thus, a surface-relief pattern (with the depth dc) having a period equal to the period P of the interference fringes is formed on the surface of the object  19 . This aspect is schematically shown in  FIG. 9 .  FIG. 9  shows a case where regions in which the light intensity is relatively low and that remain in a non-crystalline state are left. Note that there is also a case where regions in which the light intensity is relatively high are left. In that case, the convex portions in the surface-relief pattern are arranged so as to correspond to the high positions in the light intensity distribution. Since the phase change curve has high non-linearity as shown in  FIG. 6 , a sharp surface-relief pattern with a small width is realized by properly controlling the conditions of application of the laser beams. 
     In addition to the above-described platinum oxide, the following inorganic materials may be used as the material having thermal non-linearity. In the embodiments, a material that has once been crystallized need not be decrystallized again; therefore, materials that cause an irreversible phase change may also be used. Among such irreversible change materials are ZnS—SiO 2 , Ge—Sb—Te, Ge—Sb—Te—S, Te—TeO 2 —Ge—Sn, Te—Ge—Sn—Au, Ge—Te—Sn, Sn—Se—Te, Sb—Se—Te, Sb—Se, Ga—Se—Te—Ge, In—Se, In—Se—Tl—Co, Ge—Sb—Te, In—Se—Te, Ag—In—Sb—Te, and TeO 2 —Pb. Any of these materials may be used as the object  19 . 
     The configuration of the exposure apparatus  1  for use in the embodiments and its exposure principles (that is, light application method) have been described above. A microstructure manufacturing process (manufacturing method) according to a first one of the embodiments will now be described. 
       FIGS. 10A to 10C  are schematic sectional views showing the microstructure manufacturing process according to this embodiment. Interference light (first light) is generated by crossing two diffraction beams at a predetermined cross angle θ using the above-described exposure apparatus  1 . Then, the interference light is applied onto the object  19  on the substrate  20  ( FIG. 10A ). In this case, the phase difference φ between the diffraction beam B 2  that is one of the two diffraction beams and passes through the phase difference plate  17  and the diffraction beam B 1  that is the other diffraction beam and does not pass therethrough is set to 0. Note that in order to illustrate the phase difference plate  17  in  FIG. 10 , the phase difference plate  17  is drawn obliquely relative to the diffraction beam B 2  for convenience. However, as shown in  FIG. 1 , the phase difference plate  17  is actually disposed so that the diffraction beam B 2  is orthogonal to a surface of the phase difference plate  17 . The same goes for the drawings to be described below. 
     The two diffraction beams B 1  and B 2 , which are crossed at the cross angle θ, interfere with each other so that periodic interference fringes (interference light intensity distribution) are generated (see  FIG. 4 ). Application of such interference light having the interference light intensity distribution to the object  19  causes phase transitions in regions of the object  19  in which the light intensity is high. Thus, these regions (first portions)  21  are put into a crystalline state ( FIG. 10B ). The period of the regions  21  that have been put into a crystalline state (hereafter referred to as “crystalline regions  21 ”) is equal to the period of the interference light intensity distribution. The application time (exposure time) of the interference light is, for example, on the order of several milliseconds and is controlled by the shutter  11 . 
     Next, etching is performed by immersing the object  19  into an alkali solution. At that time, a difference in etching rate is made between the crystalline regions  21  and other regions (that is, non-crystalline regions). Thus, for example, the crystalline regions  21  are eliminated faster than the non-crystalline regions (second portions). As a result a pattern (for example, platinum oxide pattern) of the non-crystalline regions  19 ′ with a period (in this example, 140 nm) corresponding to the period of the interference fringes of the interference light ( FIG. 10C ). A microstructure of such a subwavelength order has various applications. It should be noted that the difference in etching rate made between the crystalline regions  21  and non-crystalline regions  19 ′ depends on the material of the object  19  or the relation between the etching solution and the material. For this reason, the non-crystalline regions  19 ′ are eliminated faster than the crystalline regions  21  in some cases. In those cases, a pattern of the crystalline regions  21  is obtained. 
     As described above, according to this embodiment, by applying the interference light (first light) having interference fringes to the object having thermal non-linearity, denatured regions (crystalline regions) and non-denatured regions (non-crystalline regions) corresponding to regions in which the light intensity is relatively high and regions in which the light intensity is relatively low, respectively, are obtained. The period of these denatured regions and non-denatured regions is reduced by shortening the wavelength of the two crossed laser beams or increasing the cross angle θ. By using the object having thermal non-linearity, sharper boundaries are obtained between the denatured regions and non-denatured regions than those obtained in a case where a photoresist film or the like made of an organic material is used as is done in related art manufacturing methods, even if the period of the denatured regions and non-denatured regions is as short as the order of 100 to 200 nm. Also, since a difference in etching rate is made between the denatured regions and non-denatured regions, a well-resolved microstructure is formed by etching the object. 
       FIGS. 11A to 11D  are schematic sectional views showing a microstructure manufacturing process according to a second one of the embodiments. While it is assumed in the first embodiment shown in  FIG. 10  that the pattern (for example, a platinum oxide pattern) itself of the non-crystalline regions  19 ′ obtained by etching is used as a microstructure, the fine pattern of the non-crystalline regions  19 ′ (or crystalline regions  21 ) may be transferred to another object by using the pattern as an etching mask. Such a case will now be described. The same steps as those in the above-described manufacturing process will be described in a simplified manner as appropriate. 
     In this embodiment, an object  22  is previously formed on the substrate  20 , as shown  FIG. 11A . Also, a coating  19  (for example, a platinum oxide film) is previously formed on a surface of the object  22 . While the material of the object  22  is not limited to a particular one, it is assumed in this embodiment that the object  22  is a metal film made of aluminum or silver. Also, the substrate  20  itself may be an object. As with the first embodiment, interference light (first light) is generated by crossing two diffraction beams at the cross angle θ. Then, the interference light is applied onto the coating  19 . Thus, as with the first embodiment, the crystalline regions  21  are formed on the coating  19  ( FIG. 11B ). 
     Next, by performing etching as in the first embodiment, the pattern (for example, a platinum oxide pattern) of the crystalline regions  21  with a period corresponding to the period of the interference fringes is eliminated, and the pattern of the non-crystalline regions  19 ′ of the coating  19  is left ( FIG. 11(C) ). Subsequently, the object  22  is etched using the pattern of the non-crystalline regions  19 ′ as an etching mask. A specific technique for the etching is preferably selected as appropriate depending on the object  22 . For example, dry etching is performed in this embodiment. Thus, the pattern of the non-crystalline regions  19 ′ is transferred to the aluminum film serving as the object  22  ( FIG. 11D ). That is, portions of the aluminum film that do not overlap the non-crystalline regions  19 ′ (second portions) are eliminated. Thus, a striped aluminum film  23  (aluminum grid) with a period of 140 nm is formed on the substrate  20 . 
     If the same steps as those shown in  FIGS. 11A to 11C  are performed without previously forming the object  22 , the pattern of the non-crystalline regions  19 ′ (or crystalline regions  21 ) is transferred to a surface of the substrate  22  (not shown). 
     As described above, by using the coating having thermal non-linearity, sharper boundaries are obtained between the denatured regions and non-denatured regions than those obtained in a case where a photoresist film or the like is used as is done in related art manufacturing methods, even if the period of the crystalline regions (denatured regions) and non-crystalline regions (non-denatured regions) is reduced (for example, the order of 100 to 200 nm). Also, since a difference in etching rate is made between the crystalline regions and non-crystalline regions, the crystalline regions (or non-crystalline regions) are eliminated by etching the coating so that apertures are made in the crystalline regions. By etching the object using the coating having the apertures obtained in this way as an etching mask, a well-resolved microstructure is formed 
       FIGS. 12A and 12B  are schematic sectional views showing a microstructure manufacturing process according to a third one of the embodiments. It is also preferable to perform the interference light application step twice or more in the first and second embodiments described with reference to  FIGS. 10A to 10C  and  FIGS. 11A to 11D  and further as shown in  FIG. 12  and to make a phase difference between the two diffraction beams B 1  and B 2  in each such step. 
     Specifically, in a step shown in  FIG. 12A , as with the first and second embodiments, the phase difference φ between the two diffraction beams B 1  and B 2  is set to φ (arbitrary value) and the two diffraction beams are crossed. Interference light (first light) generated in this way is applied onto the object  19 . Assume that the state of φ=0 is a reference state (state in which the phase difference is 0). Next, in a step shown in  FIG. 12B , the phase difference φ between the two diffraction beams B 1  and B 2  is set to φ+π and the two diffraction beams are crossed. Interference light (second light) generated in this way is applied onto the object  19 . The setup of the phase difference is realized by changing the disposition of the phase difference  17  as described above. By performing such multiple exposures, crystalline regions that repeat at the period P so as to correspond to a light intensity distribution (indicated by a solid line in the drawing) obtained by the first exposure are obtained as shown in  FIG. 13 . Also, crystalline regions that repeat at the period P so as to correspond to a light intensity distribution (indicated by a dotted line in the drawing) obtained by the second exposure are obtained in the positions displaced from the crystalline regions obtained by the first exposure by P/2. Thus, as a whole, a pattern of crystalline regions that repeat at the period P/2 is obtained. By etching the object  19  in such a state, a surface-relief pattern with the P/2 period is obtained. Since a reaction of the object  19  to light is non-linear as described above (see  FIG. 6 ), the pattern of the crystalline regions is prevented from losing its sharpness even after performing the second exposure. Thus, a pattern with a short pitch is obtained. 
     In comparison with this embodiment, think about an object whose reaction to light is linear (for example, an ordinary photoresist film, etc.). If the object is multiple times exposed to interference light with a pitch similar to that described above, no periodic pattern will be formed in the object. This is because the process in which a photoresist or the like reacts to light is one-photon absorption. Incidentally, in this embodiment, a case in which the amount of phase shift between the diffraction beams is set to n and a periodic pattern with a ½ wavelength is formed has been described. However, if the amount of phase shift is set to π/2, a periodic pattern with a ¼ wavelength is formed. Further, if the amount of phase shift is set to π/4, a periodic pattern with a ⅛ wavelength is formed. 
     As described above, by making a phase difference between the two beams, the interference fringes of the first light and those of the second light are displaced from each other by a fraction of the fringe pitch (for example, ½ pitch, ¼ pitch, etc.). By applying such first light and second light to the coating having thermal non-linearity one after another, the period of the crystalline regions (denatured regions) and non-crystalline regions (non-denatured regions) is further reduced. Even if the period is further reduced in this way, sharp boundaries are obtained between the crystalline regions and non-crystalline regions, since the coating having thermal non-linearity is used. As a result, a well-resolved microstructure is obtained. By using thermal non-linearity and the phase shift of interference light as described above, the resolution limit due to the wavelength of a laser is circumvented and a fine pattern is realized in theory. Also, in practice, a much finer pattern than patterns formed in related art manufacturing processes is realized. 
       FIG. 14  is schematic sectional views showing a microstructure manufacturing process according to a fourth one of the embodiments. It is also preferable to apply, to the object (coating)  19 , another light (third light) as well as the interference light generated using two diffraction beams in the step of applying interference light (laser interference exposure step) in the first and second embodiments described with reference to  FIGS. 10A to 10C  and  FIGS. 11A to 11D  and further as shown in  FIG. 14 . 
     For example, a laser beam B 3  different from the diffraction beams B 1  and B 2  used to perform interference exposure is applied onto the object  19 . By doing this, another laser beam B 3  serves to bias the interference intensity of the two diffraction beams B 1  and B 2 . Thus, as shown in  FIG. 15 , the depth dc of regions of the object  19  that exceed the transition point Tc is further increased. By forming crystalline regions of the object  19  in this way and etching these regions, a deeper surface-relief pattern than those formed in the first and second embodiments described with reference to  FIGS. 10A to 10C  and  FIGS. 11A to 11D  is formed on the object  19 , as shown in  FIG. 16 . This method is also effective if an attempt is made to form a surface-relief pattern over a wide area if the transition point Tc of the material selected as the object  19  is high. Also, even if the intensities of the diffraction beams B 1  and B 2  used to generate interference light are not enough high, a surface-relief pattern can be formed over a wide area of the surface of the object  19  by performing the above-described interference bias exposure. Note that while a case where the regions in which the light intensity is relatively low and that remain in a non-crystalline state are left is shown in  FIG. 16 , there is also a case where the regions in which the light intensity is relatively high and that have been put into a crystalline state are left. In the latter case, convex portions of a surface-relief pattern are arranged so as to correspond to bright portions in the light intensity distribution. 
     Also, by using the surface-relief pattern formed on the object  19  as an etching mask, the surface-relief pattern may be transferred to an object (see  FIG. 11 ) previously provided beneath the object  19 . 
     Various types of light other than the above-described laser beam may be used as the third light as long as the light serves to bias the temperature of the object  19 . While it is preferable in terms of effective biasing that a wavelength λ 2  of the third light be the same as or near a wavelength λ 1  of the diffraction beams B 1  and B 2  as the first light, the wavelength λ 2  is not limited thereto. Also, the third light need not be light with a single wavelength and may be light including components of multiple wavelengths. For example, an excimer laser (wavelength: 308 nm) may be used as the light source of the third light. By additionally performing such a bias exposure, an area that can be exposed to interference light is expanded. Thus, a pattern is formed over a wider area all at once. 
     A specific example in which the above-described embodiments are applied to a process of manufacturing a metal grid-type polarization splitter will now be described. 
     An aluminum film is formed on a quartz glass substrate and then a platinum oxide film is formed on the aluminum film. For example, the thickness of the aluminum film is 150 nm and that of the platinum oxide film is 100 nm. A Q switch nano pulse YAG laser (wavelength: 266 nm) is used as the light source  10 . The average output of the laser is 1 W (at the pulse repetition frequency of 1 kHz). A laser beam emitted from the light source  10  is split into two laser beams and then the two split laser beams (diffraction beams) are crossed. Thus, interference light is generated. The interference angle θ is set to 72°. According to Eq. 1 described above, the period of interference fringes on a surface of the platinum oxide film serving as an object is 140 nm. The first exposure is performed using this interference light. Note that a bias exposure may also be performed using the third light as appropriate. Subsequently, by performing etching using an alkali solution, a platinum oxide pattern with the same period as that of the interference fringes, that is, a period of 140 nm is formed. Then, by dry-etching the aluminum film using the platinum oxide pattern as an etching mask, the platinum oxide pattern is transferred to the aluminum film formed therebelow. Thus, a striped aluminum film (aluminum grid) with a period of 140 nm is formed on the quartz glass substrate. A microstructure manufactured in this way is used, for example, as a polarization beam splitter. 
     Also, if interference light is generated under the above-described conditions, the phase difference between the beams for the first exposure is set to φ (arbitrary value) and that for the second exposure is set to φ+π. The phase difference between the first and second exposures is set to n. After doing this, the second exposure is performed. A bias exposure may also be performed using the third light as appropriate. Subsequently, by performing etching using an alkali solution, a platinum oxide pattern with half the period of the interference fringes, that is, with a period of 70 nm is formed. Then, by dry-etching the aluminum film using this platinum oxide pattern as an etching mask, the platinum oxide pattern is transferred to the aluminum film formed therebelow. Thus, a striped aluminum film (aluminum grid) with a period of 70 nm is formed on the quartz glass substrate. A microstructure manufactured in this way is used, for example, as a polarization beam splitter. 
       FIGS. 17A and 17B  are drawings schematically showing a structure of a polarization beam splitter manufactured as described above. Specifically,  FIG. 17A  is a perspective view showing a function of the polarization beam splitter and  FIG. 17B  is a partial schematic sectional view of the polarization beam splitter. In the polarization beam splitter, an aluminum film with a thickness of 150 nm is formed into stripes (one-dimensional grip pattern) on a glass substrate (SiO 2 ). The period P of the grid of the aluminum film is 70 nm or 140 nm. The complex dielectric constant N of the glass substrate is 1.5−j0.0 and that of the aluminum film is 0.59−j5.34.  FIG. 18  shows polarization beam-splitting characteristics of this polarization beam splitter. In  FIG. 18 , the vertical axis represents the transmittance (left) and contrast (right), and the horizontal axis represents the ratio of the grid period P to the wavelength λ. The contrast is defined as the ratio of the transmittance of transverse-electric (TE) polarized light to that of transverse magnetic (TM) polarized light. As is understood from this result, the polarization beam splitter exhibits higher performance (that is, the transmittance becomes higher or the contrast becomes higher) as the period P of the grid pattern (subwavelength structure) of the polarization beam splitter is reduced. In  FIG. 18 , a characteristic corresponding to the period P of 70 nm and a characteristic corresponding to the period P of 140 nm are indicated by the respective arrows in an upper part of the graph. 
     Both the polarization beam splitters exhibit excellent polarization beam-splitting characteristics. In particular, it is understood that the polarization beam splitter with the period P of 70 nm is much more excellent in transmittance and contrast characteristics than that with the period P of 140 nm. 
     The invention is not limited to the above-described embodiments and various changes and modifications can be made thereto without departing from the spirit and scope of the invention. While the polarization beam splitter is used as a specific example in which the embodiments of the invention is applied, the invention is also useful when manufacturing various devices in which micro- and nano-patterns need be formed. Among such devices are optical thin film devices, semiconductor thin film devices, tracking marks, and fine mechanical devices. Applications to optical thin film devices include antireflective films for use in a liquid crystal application device, phase difference plates, optical waveguides, polarizing elements, alignment layers, photonic structures for use in a light-emitting element such as a light-emitting diode (LED) or electroluminescent (EL) element, and high-density optical disks. 
     While the crystalline regions are used as one example of the denatured regions and the crystalline regions are used as one example of the denatured regions in the above-described embodiments, the denatured regions and non-denatured regions are not limited thereto. Any denatured regions and non-denatured regions that make a difference in etching rate therebetween due to differences in physical properties (physical or chemical natures) therebetween may be used.