Abstract:
A micro-shape transcription method has preparing a mold having a transcription face on which a concavo-convex pattern is formed, pressing the transcription face against a base material softened by heating, then forcibly separating the mold from the base material to transcribe a reverse pattern of the concavo-convex pattern to the surface of the base material, wherein when assuming a temperature for pressing the mold against the base material as T 1  (° C.), a temperature for separating the mold from the base material as T 2  (° C.), thermal expansion coefficients of the mold and the base material as α a  and α b , and the maximum distance between the transcription center of the transcription face and the concavo-convex pattern as d (mm), the following relations (1) and (2):
 
T 1 ≧T 2    (1)
 
|α a −α b |·( T   1   −T   2 )· d ≦4×10 −2    (2)
 
are simultaneously satisfied.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for micro-fabricating a surface of a base material and a method for manufacturing an optical component whose surface is micro-fabricated such as an optical waveguide, diffraction grating, or polarizer. 
     2. Related Art of the Invention 
     Because optical-unit related markets such as those of optical communication, optical disk, display, and optical sensor have been progressed, coexistence of high performance and low cost is requested for an optical component. Particularly, price reduction is requested for a passive optical component that itself does not operate. 
     Particularly, optical waveguides and diffraction gratings require a very fine and accurate pattern for their surfaces. Dry etching frequently used for a semiconductor process is generally used to form the above pattern. As an example of micro-fabrication by dry etching, a process for manufacturing a single-mode optical waveguide for optical communication is described below by referring to the accompanying drawings. 
       FIG. 7(   a ) is a top view of a general quartz-based single-mode optical waveguide and  FIG. 7(   b ) is a sectional view of the waveguide in  FIG. 7(   a ), taken along the line A—A in  FIG. 7(   a ). Because a core  71  has a refractive index higher than that of a clad  72 , the light meeting a specific condition is transmitted by being confined in a core pattern. By patterning the core  71  as shown in  FIG. 7(   a ), it is possible to constitute an optical circuit. In a wavelength band of 1.3 to 1.55 μm, the core  71  generally has a square cross section one side of which is approx. 8 μm. A core shape and a core-surface roughness greatly influence a light propagation performance. 
       FIGS. 8(   a ) to  8 ( c ) are process charts showing a general manufacture method for a conventional quartz-based optical waveguide (refer to Kawauchi, OPTRONICS No. 8,851, 1988). In the illustrated steps, a core film  81  is formed on a quartz substrate  82  also serving as a lower clad layer in accordance with the flame deposition method ( FIG. 8(   a )). However, when using a substrate other than a quartz substrate, a lower clad layer is previously formed in accordance with the flame deposition method. Then, a core film is formed into a predetermined pattern through photolithography and dry etching ( FIG. 8(   b )). An upper clad layer  83  is then formed in accordance with the flame deposition method ( FIG. 8(   c )). An optical waveguide having a small loss has been manufactured so far in accordance with the above method. 
     In recent years, however, not only quartz but also resin have been studied as an optical-waveguide material. At present, resin is inferior to quartz in transparency and reliability. Resin is easily molded and superior in transparency at a wavelength of approx. 650 to 850 nm, however. Therefore, it is a very prospective optical-waveguide material. Polymethyl methacrylate (PMMA) superior in transparency is known as a specific resin. Recently, it is also studied to realize low absorption in a wavelength area of 1.3 to 1.55 μm by performing deuteration or fluorination on the basis of PMMA. 
     To manufacture an optical waveguide by using resin, a method is generally used in which a core layer and a clad layer are formed mainly by spin coating and the core layer is patterned by dry etching. Resin has a high productivity because it has a short film deposition time and a low annealing temperature of 200 to 300° C. compared to quartz. 
     As described above, dry etching is used to pattern a core of quartz or resin in the case of conventional optical-waveguide manufacturing. 
     However, dry etching is a complex process and requires a lot of equipment. Therefore, when considering cost, it cannot be said that dry etching is suitable for manufacturing a passive optical component, setting aside a semiconductor device. Therefore, various methods are proposed for manufacturing an optical component. Particularly, pressing-molding and injection-molding methods are prospective. 
     Proposed examples of press-molding are exclusively directed to glass materials and include a method disclosed in the specification of Japanese Patent Laid-Open No. 8-320420. The method is an optical-waveguide manufacturing method of simultaneously forming groove portions serving as core patterns by pressing a mold  91  on which a predetermined core pattern  92  is formed against a base material  93  also serving as a lower clad at a high temperature as shown in  FIG. 9 . The method makes it possible to efficiently form an optical waveguide by omitting the photolithography and dry-etching steps which have been used so far. 
     A method is also proposed in which a diffraction grating is manufactured by transcripting a pattern on a metal reflection film using pressing-molding (the specification of Japanese Patent Laid-Open No. 10-96808). 
     A micro-pattern transcription method using pressing-molding disclosed in the specification of Japanese Patent Laid-Open No. 8-320420 is briefly described below. First, a base material (workpiece) is heated and softened and brought into contact with a mold. While keeping the above state, the base material is cooled and separated from the mold when the shape of the base material is fixed. In this case, as shown in  FIG. 10 , a convex portion  103  formed on a mold  101  is provided correspondingly to a required waveguide pattern and pressed against a resin substrate  102 , and a reverse pattern is formed on the resin substrate  102 . 
     However, when using the above transcription method, a high pattern accuracy may not be obtained. Deterioration of the pattern accuracy seems to be caused by a difference between thermal expansion coefficients. That is, when bringing the base material softened by heating into contact with the mold and cooling the base material and the mold, a thermal stress is generated due to a difference between thermal expansion coefficients of the base material and mold. As a result, the accuracy of a pattern transcribed to the base material is lowered and the mold is broken if the mold is weak in strength. This problem becomes remarkable when using resin because resin has a thermal expansion coefficient 1 to 2 digits larger than that of quartz used as the material of a mold. 
     According to specific study by the present inventor, the width of a groove is increased at a micron-order level and the shape of the groove is deformed by transcribing a pattern to a resin substrate by using a mold. It is estimated that this phenomenon results from the fact that the resin substrate contracts toward the center of transcription because the substrate contracts more than the mold. This phenomenon remarkably occurs when a transcription face is a plane. Moreover, when the cross-section of a micro shape is rectangular, a pattern is further deformed. 
     Thus, resin can be molded at a low temperature and it is advantageous in the manufacturing cost. However, it has a problem that a micro pattern cannot be accurately transcribed when transcribing a pattern by pressing-molding. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the conventional problems and its object is to provide a method for efficiently transcribing a high-accuracy micro shape. 
     It is another object of the present invention to provide a method for fabricating an optical component such as an optical waveguide by using the above transcription method. 
     To achieve the above objects, a micro-shape transcription method of the present invention is a micro-shape transcription method comprising preparing a mold with a transcription face and including a concavo-convex pattern formed on the transcription face, pressing the transcription face against a base material softened by heating and then forcibly separating the mold from the base material, to transcribe a reverse pattern of the concavo-convex pattern to the surface of the base material, wherein when assuming a temperature for pressing the mold against the base material as T 1 (° C.), a temperature for separating the mold from the base material as T 2 (° C.), thermal expansion coefficients of the mold and the base material as α a  and α b , and the maximum distance between the transcription center of the transcription face and the concavo-convex pattern as d (mm), the following relations (1) and (2) are simultaneously satisfied.
 
T 1 ≧T 2   (1)
 
|α a −α b |·( T   1   −T   2 )· d≦ 4×10 −2   (2).
 
     The present invention is particularly preferable when the transcription face of a mold is a plane or a stepped plane. 
     Moreover, the present invention is particularly preferable when the sectional form of a concavo-convex pattern is rectangular. 
     Furthermore, the present invention is particularly preferable when |a a |a b | is equal to or higher than 50×10 −7 /° C. 
     Furthermore, the present invention is particularly preferable when the line width of a concavo-convex pattern is equal to or less than 100 μm. 
     Furthermore, the present invention is particularly preferable when the depth of a concavo-convex pattern is equal to or more than 1 μm. 
     Furthermore, in the above described method of the present invention, a base material is preferably an optically transparent thermoplastic resin or glass. It is preferable to use any one of polyolefin, polymethyl methacrylate, polycarbonate, norbornane, and acrylic resins as the thermoplastic resin. 
     A micro-shape transcription apparatus of the present invention comprises a first mold provided with a transcription face having a micro-shape, a second mold facing the first mold, a mechanism for driving either of the first and second molds, a heating source for controlling the temperatures of the first and second molds, and a vacuum chuck for attracting and fixing a base material to the second mold. 
     An optical-component manufacturing method of the present invention forms a pattern for controlling light in accordance with the above-described micro-shape transcription method. An optical component of the present invention includes an optical waveguide, diffraction grating, polarizer, or lens. When the optical component is an optical waveguide, a pattern corresponding to a core is formed in accordance with a micro-shape transcription method of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing a configuration of a pressing machine used for an embodiment of a micro-shape transcription method of the present invention; 
         FIGS. 2(   a ) and  2 ( b ) are illustrations showing a pattern formed on a transcription face of a mold used for an embodiment of a micro-shape transcription method of the present invention; 
         FIG. 3  is a sectional perspective view showing an example of an optical waveguide formed by the machine shown in  FIG. 1 ; 
         FIG. 4  is an SEM photograph of a cross section of a resin substrate obtaining a preferable transcription quality from an embodiment of the present invention; 
         FIG. 5  is an SEM photograph of a cross section of a resin substrate obtaining a bad transcription quality from an embodiment of the present invention; 
         FIG. 6  is a photograph showing a result of measuring the surface shape of a resin substrate obtaining a preferable transcription quality by an embodiment of the present invention; 
         FIG. 7(   a ) is a top view of a general quartz-based single-mode optical waveguide and  FIG. 7(   b ) is a sectional view of the waveguide in  FIG. 7(   a ); 
         FIGS. 8(   a ) to  8 ( c ) are process charts showing a method for manufacturing a conventional general optical waveguide; 
         FIG. 9  is a sectional view showing a conventional method for manufacturing an optical waveguide through pressing; and 
         FIG. 10  is a conventional pattern used for a method for manufacturing an optical waveguide through pressing. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are described below by referring to the accompanying drawings. 
       FIG. 1  shows a pressing machine used for a micro-shape transcription method of the present invention. The machine is provided with a pair of upper and lower heater blocks  11  and  12 . The upper heater block  11  is vertically movable and the lower heater block  12  is fixed. A cope  13  is fixed to the upper heater block  11  with the transcription face turning downward. The cope  13  is made of quarts having a size of 20×20 mm and has a micro shape formed through dry etching on the flat transcription face.  FIGS. 2(   a ) and  2 ( b ) show a pattern formed on the transcription face of a cope  21 .  FIG. 2(   a ) is a top view of the pattern and  FIG. 2(   b ) is a sectional view of the pattern in  FIG. 2(   a ), taken along the line A—A in  FIG. 2(   a ). In this case, the pattern is convex. The convex pattern  22  has a depth of approx. 5 μm and a width of approx. 8 μm. 
     In this case, a polyolefin-based thermoplastic resin is used as a base material  14  to be formed. This resin is optically transparent and has a glass-transition temperature of 140° C. and a load deflection temperature of 123° C. The base material has a size of 8×8 mm and is set on a planar drag  15  fixed on the lower heater block  12 . The base material  14  is fixed to the drag  15  by a vacuum chuck. 
     A micro-shape transcription method is described below by referring to  FIG. 1 . 
     The quartz mold  13  serving as a cope and the resin substrate  14  serving as a workpiece are overlapped at the molding machine so that the transcription face of the quartz mold  13  contacts with the resin substrate  14 . In this case, the center of the quartz mold  13  is almost adjusted to the center of the resin substrate  14 . Under the above state, the upper and lower heater blocks  11  and  12  are electrified, the resin substrate  14  is heated up to 180° C. to be softened, and the upper heater block  11  to which the quartz mold  13  is fixed is moved downward and pressed. While a convex pattern of the quartz mold  13  bites into the resin substrate  14 , heater temperature is lowered to 150° C. by controlling electrification of upper and lower heaters and the upper heater block  11  is raised to forcibly separate the cope  13  from the resin substrate  14 . In this case, the resin substrate  14  is fixed to the drag  15  by a vacuum chuck and smoothly separated from the cope  13  without being distorted or deformed. Thereafter, the resin substrate  14  is cooled up to approx. room temperature and taken out of the molding machine. 
     As a result of observing the surface and cross section of the taken-out resin substrate by an optical microscope and an electron microscope, the groove of a micro-pattern to which the convex pattern of the quartz mold is accurately transcribed is confirmed over the entire surface of a resin substrate having a diameter of approx. 20 mm. 
     Then, epoxy resin having a refractive index approx. 0.3% higher than that of the resin substrate is embedded in the groove of the resin substrate and a flat resin substrate made of the same material as the above resin substrate is bonded onto the above resin substrate. Thus, the optical waveguide shown in  FIG. 3  is fabricated. 
     The optical waveguide is provided with epoxy resin as a core  32  and polyolefin resin substrates as clads  31 ,  33 . The optical waveguide has a sufficient serviceability. 
     Moreover, molding is studied by using a molding temperature which is a temperature for pressing a mold against a resin substrate and a mold releasing temperature which is a temperature for releasing the mold from the resin substrate as parameters and the resin substrate is observed after molding. Table 1 shows an example of observation results. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Molding 
                 Mold releasing 
                 Transcription-quality 
               
               
                 temperature 
                 temperature 
                 evaluation 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 160 
                 80 
                 X 
               
               
                 160 
                 90 
                 X 
               
               
                 160 
                 100 
                 ◯ 
               
               
                 160 
                 120 
                 ◯ 
               
               
                 160 
                 140 
                 ◯ 
               
               
                 180 
                 80 
                 X 
               
               
                 180 
                 100 
                 X 
               
               
                 180 
                 120 
                 ◯ 
               
               
                 180 
                 150 
                 ◯ 
               
               
                   
               
             
          
         
       
     
     In the case of Table 1, the quality of a transcription shape is observed at a position 1 cm separate from the transcription center of a transcription face. In this case, an object transcribed at a submicron accuracy is judged as ◯ and an object not transcribed at a submicron accuracy is judged as x. 
       FIGS. 4 and 5  show electron micrographs of the cross sections of a groove transcribed onto a resin substrate. 
     In the case of  FIGS. 4 and 5 , cross sections are obtained by coating the formed resin substrate with a metallic film, embedding it in epoxy resin, and polishing it, and lower portions show the formed substrate. A while boundary line shows the metallic film.  FIG. 4  is obtained at a molding temperature of 180° C. and a mold releasing temperature of 150° C. at which the transcription quality is judged to be good, and  FIG. 5  is obtained at a molding temperature of 180° C. and a mold releasing temperature of 80° C. at which the transcription quality is judged to be bad. 
       FIG. 6  shows the result of measurement of the surface roughness of a groove pattern, which was obtained at a molding temperature of 180° C. and a mold releasing temperature of 150° C. at which the transcription quality is judged to be good, by a non-contact roughness meter. 
     The photograph is overlapped with a photograph when viewing a groove from the surface, the sectional form is shown by leader lines L 1  to L 5 , and the groove has a width of 7.8 μm and a depth of 4.68 μm. The irregularity of the surface of the molded resin substrate is originally present on a quartz mold. 
     Thereby, it is confirmed that the groove is vertical and transcribed at a roughness of a submicron level or lower. That is, it is found that transcription is smoothly performed even if measuring methods are changed. 
     The following Equation 1 is calculated on the above result. 
     [Equation 1]
 
 M=|α   a −α b |·( T   1   −T   2 )· d   Equation 1
 
     In the above Equation, α a  denotes a thermal expansion coefficient of quartz serving as a mold and α b  denotes a thermal expansion coefficient of polyolefin-based resin serving as a workpiece, in which α a  is equal to 5×10 −7 /° C. and α b  is equal to 600×10 −7 /° C. Moreover, T 1  denotes a molding temperature and T 2  denotes a mold releasing temperature. Symbol d (mm) denotes the distance between the position at which transcription quality is evaluated and the transcription center. 
     As a result of calculating the Equation 1 on each molding condition in Table 1, it is found that a preferable transcription quality is obtained in the case of M≦4×10  −2 . As for the above relation, it is found that the same result is obtained even if using one of polymethyl-methacrylate-, polycarbonate-, acrylic-, and norbornane-based thermoplastic resins or glass for a substrate to be molded, using one of various metals including nickel, various ceramics including zirconium, WC-based cemented carbide, diamond film, and DLC for a mold base material, combining various mold materials with a material to be molded, and changing molding conditions. That is, it is clarified that a micro shape can be transcribed as long as a molding condition meets the condition of Expression 1. 
     In this embodiment, manufacturing of an optical waveguide is described. However, a method of the present invention is not restricted to the above case but it is suitable for manufacturing all components requiring a micro pattern including an optical component, mechanical component, and micromechanics-related component. It is particularly suitable for manufacturing a high-accuracy optical component including an optical waveguide, diffraction grating, polarizer and lenz. 
     Moreover, for this embodiment, a mold provided with a convex pattern is used. However, the same advantage can be obtained by using a concave pattern or a concavo-convex pattern. A pattern sectional form is not restricted. For example, the mold is effective for a trapezoidal shape, triangular shape, or wave shape. The mold is particularly effective for a micro shape having a rectangular cross section on a flat transcription face that is easily influenced by thermal expansion and on which a pattern easily collapses. 
     Moreover, for this embodiment, the flat pattern shape shown in  FIG. 2  is described. However, a pattern shape is not restricted to the flat pattern shape. 
     Furthermore, as a result of studying various materials, it is found that a method of the present invention is particularly effective when a difference between thermal expansion coefficients of a mold and a base material is 50×10 −7 /° C. or more, a pattern has a width of 100 μm or less, and a concavo-convex pattern has a depth of 1 μm or more. That is, under the above conditions, separation cannot be made by a conventional method but separation can be realized by the present invention. 
     Moreover, though a material of a mold or a material to be molded is not restricted, the mold is particularly effective for a thermoplastic resin having a comparatively large thermal expansion coefficient. By applying protective coating to a mold and performing molding in an inert-gas atmosphere, it is possible to transcribe a micro pattern to glass. 
     As described above, the present invention makes it possible to efficiently transcribe a high-accuracy micro pattern. 
     Moreover, the present invention makes it possible to efficiently manufacture an optical component such as an optical waveguide. 
     Particularly, the present invention makes it possible to accurately transcribe a micro pattern to a wide area of resin having a large thermal expansion coefficient that is a problem as a material of an optical component. 
     Thus, according to the present invention, it is possible to efficiently manufacture various optical components including an optical waveguide and a diffraction grating.