Abstract:
The invention provides an imprinting jig for transferring a pattern onto a thermoplastic sheet. The imprinting jig is provided with: a first die having a surface with a pattern; a die holder for supporting the first die; an elastic member placed between the first die and the die holder; and a second die opposing to the first die. The thermoplastic sheet is placed on the surface with the pattern of the first die. The imprinting jig optionally includes an adhesion reducing means between the elastic member and the first die or between the elastic member and the die holder.

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The invention relates to an imprinting jig for putting a thermoplastic sheet therebetween and an imprinting apparatus. Particularly it relates to the imprinting jig and apparatus used for a thermal imprinting process for transferring or printing a fine pattern onto the thermoplastic sheet by heating and applying pressure. 
   (2) Related Art 
   Professor Stephen Y. Chou et al. of, Princeton University have made a number of reports on a nano-imprinting process which prints a fine pattern having a high aspect ratio at a moderate price. The nano-imprinting process includes mainly an optical nano-imprinting process in which ultraviolet curing resin is used for printing a pattern, and a thermal nano-imprinting process in which heat and pressure are applied to a thermoplastic substance to print a pattern. Especially the latter process has been attracting attention since the process can produce flat panel displays, electronic cards and the like at a low cost since a sheet material is used (hereinafter, this process will be referred to as “sheet thermal nano-imprinting process”). 
   In the field of recording media, attention has been paid to a technique in which an optical disk pattern is transferred onto a thin polycarbonate (PC) sheet such as an optical disk substrate through the sheet thermal nano-imprinting process, and the thin PC sheet is directly used as a recording medium, thereby recording capacity per unit volume is dramatically increased. For example, JP-A-2003-91970 proposes a mass storage record reproduction system configured to combine a thin optical disk having a thickness not greater than 100 μm with a stabilizer for stably rotating the thin optical disk with little run out of its surface. 
   A substrate of an optical disk is generally produced by injection molding. When the thin substrate as described above is produced by injection molding, however, resin cannot be filled in a gap between a mold and a first die (hereinafter, referred to as “stamper”), since the gap is extremely narrow. Thus, it has been technically difficult to apply the injection molding process to producing a thin substrate. In addition, the technique for patterning a thin substrate by a 2P method is expensive although it is possible to produce the thin substrate. On the contrary, the sheet thermal imprinting process, which directly print a pattern on a thin sheet substrate, is remarkably advantageous technically and in cost in relation to the conventional techniques described above. 
   SUMMARY OF THE INVENTION 
   In producing optical disks by means of the sheet thermal nano-imprinting process, a fine pattern is printed onto a thermoplastic sheet to produce an extremely thin optical disk, when the sheet is formed thin. The sheet thermal nano-imprinting process prints a fine pattern formed on a stamper onto a surface of the sheet softened by applying pressure to the sheet at a temperature higher than a glass transition temperature of the sheet. Subsequently, the sheet is cooled to a temperature lower than the glass transition temperature, and the sheet is separated from the die or stamper. A jig used therein is called “imprinting jig”. The imprinting jig comprising: a first die for transferring a fine pattern (hereinafter, referred to as “stamper”); a second die (hereinafter, referred to as “plate”) opposing the surface with the fine pattern of the first die (stamper); and a die holder (hereinafter, referred to as “stamper holder” or “holder plate”) for supporting the first die. JP-A-2006-26928 discloses that a thermoplastic sheet is fixed in the imprinting jig including the stamper on which a predetermined fine pattern is formed, and the imprinting jig is placed between parallel plates in the pressing apparatus, and pressed while heating, since it takes time to adjust a temperature of an entire pressing apparatus in using. Subsequently, the imprinting jig is taken out from the pressing apparatus for separating the sheet from the stamper, and only the imprinting jig is cooled. 
   A fine pattern should be surely transferred over a wide area, such as a whole circular area of φ120 mm, for producing optical disks, and it is necessary to accurately control the degree of parallelism between the sheet and the stamper. If these are pressed in a non-parallel state, the whole sheet may not be uniformly pressed, and the pattern may not be accurately transferred onto the sheet, or the sheet may be deformed. 
   To avoid such problems, as described below, a rubber sheet (hereinafter, referred to as “elastic member”) is positioned as a buffer between the stamper and the stamper holder for supporting the stamper. Consequently, the whole surface of the sheet can be uniformly pressed, which allows for a large area transfer. 
   As described below, however, the following problem has become clear through experiments by the inventors. That is, in cooling and separating step after transferring the pattern, a very large force is necessary to separate the already-transferred sheet from the stamper of the imprinting jig, when using the rubber sheet for buffering. Thus, the separated sheet may be deformed. Furthermore, if the already-transferred sheet is separated from the stamper at a tilt angle to each other, the transferred fine pattern may be damaged, and thus it is necessary to separate them while they are kept in parallel to each other. However, when the sheet is separated from the stamper while keeping them in parallel to each other, a larger force is required for separate them, compared with the case of separation by tilting them to each other. When a force necessary for separation turns large as described above, a problem arises that a large-scale separating apparatus becomes necessary. 
   According to a first aspect of the invention, an imprinting jig is provided comprising: a first die having a surface with a pattern for transferring the pattern onto a thermoplastic sheet; a die holder for supporting the first die; an elastic member placed between the first die and the die holder; and a second die opposing to the first die, wherein the sheet is placed on the surface with the pattern of the first die. Preferably, the second die has a mirror finished surface for contacting with the sheet. Alternatively, the second die may be formed with a pattern to be transferred on the sheet. When the second die has the mirror finished surface, the plastic sheet may be surely separated from the stamper without the plastic sheet being separated from the second die at the time of separation of the plastic sheet from the stamper, since the plastic sheet is firmly adhered to the second die. 
   According to a second aspect of the invention, an imprinting jig is provides, comprising: a first die having a surface with a pattern for transferring the pattern onto a thermoplastic sheet; a die holder for supporting the first die; an elastic member placed between the first die and the die holder; a second die opposing to the first die, wherein the sheet is placed on the surface with the pattern of the first die, wherein an adhesion reducing means is provided between the elastic member and the first die or between the elastic member and the die holder. Preferably, the second die has a mirror finished surface for contacting with the sheet. Alternatively, the second die may be formed with a pattern to be transferred on the sheet. When the second die has the mirror finished surface, the plastic sheet may be surely separated from the stamper without the plastic sheet being separated from the second die at the time of separation of the plastic sheet from the stamper, since the plastic sheet is firmly adhered to the second die. 
   Since the imprinting jig according to the invention includes the adhesion reducing means, adhesion between the elastic member and the stamper and/or between the elastic member and the stamper holder becomes low, and as the result, the stamper and the plastic sheet can be easily separated from each other. For example, after heating and pressurizing the plastic sheet between the stamper and the second die of the imprinting jig through a sheet thermal nano-imprinting process described below, the stamper and the plastic sheet are adhered to each other. Thus, a very large force becomes necessary for separating the plastic sheet from the stamper. According to the invention, the adhesion reducing means lowers the adhesion between the elastic member and the stamper and/or the elastic member and the stamper holder. Thus, at the start of separation of the plastic sheet from the stamper, the elastic member is slightly separated from the stamper holder at a portion where the elastic member is not fixed to the stamper holder (for example, the central portion of the elastic member). At this time, a portion of the elastic member which is fixed to the stamper holder (for example, a peripheral portion of the elastic member) cannot be separated from the stamper holder. Consequently, only the central portion of the elastic member is deformed to bend toward the side opposite to the stamper holder. The deformation generates a small gap between the stamper and the plastic sheet at a peripheral portion of the stamper. It is considered that the gap formed in such a manner facilitates separation of the plastic sheet from the stamper. 
   According to an embodiment of the invention, the elastic member may be made of silicon rubber, but the elastic member may be made of other rubber having thermal resistance. In each case, the elastic member may be made of rubber having thermal resistance (silicon rubber, fluorinated rubber and the like), and the imprinting jig can be used in a high-temperature environment necessary for the sheet thermal nano-imprinting process. 
   According to an embodiment of the invention, the adhesion reducing means may comprise irregularity formed on a surface of the stamper holder on the elastic member side. A height of the irregularity may be approximately 1 μm. In this case, the surface of the stamper holder is roughened so that the irregularity having a height of e.g. approximately 1 μm is formed, thereby adhesion between the stamper holder and the elastic member may be reduced, and further the plastic sheet may be easily separated from the stamper. 
   The term “irregularity” referred to in the description means protrusions and/or depressions, and the height of the irregularity means difference of elevation in a direction perpendicular to a surface on which the irregularity is formed (e.g. the surface of the stamper holder on the elastic member side). 
   According to an embodiment of the invention, the adhesion reducing means may comprise irregularity formed on a surface of the stamper holder on the elastic member side. A height of the irregularity may be approximately 1 μm. In this case, the surface of the stamper holder is roughened so that irregularity having a height of e.g. approximately 1 μm is formed, thereby adhesion between the stamper holder and the elastic member may be reduced, and further, the plastic sheet may be easily separated from the stamper. 
   According to an embodiment of the invention, the adhesion reducing means may comprise irregularity formed on a surface of the elastic member on the stamper side. A height of the irregularity may be approximately 1 μm. In this case, the surface of the elastic member is roughened so that irregularity having a height of e.g. approximately 1 μm is formed, thereby adhesion between the stamper holder and the elastic member may be reduced, and further, the plastic sheet may be easily separated from the stamper. 
   According to an embodiment of the invention, the adhesion reducing means may comprise a release agent. The adhesion reducing means may also comprise a thin film of fluorine resin between the elastic member and the stamper and/or between the elastic member and the stamper holder. The adhesion reducing means may also comprise a metallic thin film between the elastic member and the stamper holder and/or between the elastic member and the stamper, on which surface or surfaces irregularity is formed. In each case, adhesion between the stamper holder and the elastic member may be reduced, and further, the plastic sheet may be easily separated from the stamper. 
   According to an embodiment of the invention, a surface of the second die facing the stamper may be mirror finished. In this case, at the time of separation of the plastic sheet from the stamper, the plastic sheet may be surely separated from the stamper without the plastic sheet being separated from the second die, since the plastic sheet and the second die are firmly adhered to each other. 
   When a pattern is transferred onto a large area through the sheet thermal nano-imprinting process using the imprinting jig according to the invention, the sheet can be separated from the jig by exerting a comparatively small force without damaging the sheet. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a schematic diagram of an imprinting jig according to the invention; 
       FIG. 2  is a schematic diagram showing a state in which a PC sheet is put in the imprinting jig; 
       FIG. 3  is a schematic diagram of a pressing apparatus; 
       FIG. 4A  is a schematic diagram showing a state in which the imprinting jig with the PC sheet is mounted on the pressing apparatus; 
       FIG. 4B  is a schematic diagram showing a state in which the imprinting jig is pressed by the pressing apparatus; 
       FIG. 5  is a schematic diagram showing a state in which the imprinting jig is mounted on a separating apparatus; 
       FIG. 6  is a schematic diagram showing a state of the imprinting jig when the PC sheet is separated from the stamper; 
       FIG. 7  is a schematic diagram of the imprinting jig with a release agent; and 
       FIG. 8  is a schematic diagram of the imprinting jig with two rubber sheets. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of an imprinting jig according to the invention will be described with reference to  FIG. 1 . As shown in  FIG. 1 , an imprinting jig  100  mainly includes a stamper holder assembly  101 , and a mirror plate  102  placed above the stamper holder assembly  101 . 
   The mirror plate  102  is a substantially disk-shaped member made of stainless steel. A lower surface  102   a  of the mirror plate  102  facing the stamper holder assembly  101  is mirror finished. A flange  102   b  is provided around the periphery of the mirror plate  102 . The flange has a smaller thickness than that of the central portion of the mirror plate  102 . In handling the mirror plate  102 , flange  102   b  may be gripped to prevent a careless contact with the mirror-finished lower surface  102   a.    
   The stamper holder assembly  101  includes: a holder plate  103  that is a substantially disk-shaped member made of stainless steel; a rubber sheet  104  or an elastic member, made of heat-resisting rubber, placed on an upper surface of the holder plate  103  (on the side facing the mirror plate  102 ); a stamper  105  placed on an upper surface of the rubber sheet  104 ; and a clamp  106  (or hold portion, or peripheral hold portion) for fixing the rubber sheet  104  and the stamper  105  on an upper surface of the holder plate  103 . The rubber sheet  104  is formed of silicon rubber, and placed between the stamper  105  and the holder plate  103 . As described below, the rubber sheet  104  serves as a buffer during a heating and pressing process in production of optical disk substrates, so that the stamper  105  uniformly presses a whole plastic sheet having thermal plasticity to form an optical disk substrate. 
   The stamper  105  is a substantially disk-shaped thin member made of nickel, which has a surface formed with a predetermined fine pattern. In the heating and pressing process described below, the pattern is transferred onto a plastic sheet having thermal plasticity to form an optical disk substrate. The rubber sheet  104  and the stamper  105  are stacked and placed on the upper surface of the holder plate  103 , and a peripheral portion of the stamper  105  is clamped by the clamp  106 . Since the central portions of the rubber sheet  104  and the stamper  105  are not directly biased against the holder plate  103 , the central portions may bend and be deformed so that the central portions leave away from the holder plate  103 , as described below. 
   Next, a process will be described in which the imprinting jig  100  described above is employed to form an optical disk substrate (which is referred to as “the sheet thermal nano-imprinting process”). First, as shown in  FIG. 2 , a plastic sheet having thermal plasticity (PC sheet)  201  formed of polycarbonate (PC) having a thickness of about 100 μm is placed on the upper surface of the stamper holder assembly  101 , that is, on the upper surface of the stamper  105 , the peripheral portion of which is fixed by the clamp  106  (on the surface with the pattern). Furthermore, the mirror-finished lower surface  102   a  of the mirror plate  102  is brought into contact with an upper surface of the PC sheet  201  to fix the PC sheet  201  in the imprinting jig  100 . 
   Then, as shown in  FIG. 4A , the imprinting jig  100  sandwiching the PC sheet  201  is mounted on a pressing apparatus  210 . As shown in  FIG. 3 , the pressing apparatus  210  includes: an upper stage  211 A and a lower stage  211 B having a predetermined thickness, which are plate-like members made of a metal; heaters  212  provided on the upper stage  211 A and the lower stage  211 B, respectively; a ball screw  213  rotatably provided in the upper stage  211 A to move the upper stage  211 A up and down; and a motor (not shown) for the ball screw  213  and accommodated in the lower stage  211 B. The pressing apparatus  210  may press an object between the upper stage  211 A and the lower stage  211 B, with heating the object at a predetermined temperature by means of the heaters  212 . 
   The upper stage  211 A, as shown in  FIG. 4B , is moved in a direction toward the lower stage  211 B with use of the motor for the ball screw  213  to apply a predetermined load to the imprinting jig  100  between the upper stage  211 A and the lower stage  211 B. The imprinting jig  100  is pressed with the predetermined load or pressure, and consequently the PC sheet  201  put in the imprinting jig  100  is also subjected to the predetermined load. While the pressing apparatus  210  presses the imprinting jig  100  and the PC sheet  201 , the heaters  212  in the pressing apparatus  210  keep the imprinting jig  100  and the PC sheet  201  at a temperature higher than a glass transition temperature Tg of the PC sheet  201  (in the heating and pressing process). As described above, pressing of the PC sheet  201  at a temperature higher than the glass transition temperature Tg allows the predetermined pattern on the upper surface of the stamper  105  to be transferred onto a lower surface of the PC sheet  201  in contact with the upper surface of the stamper  105 . 
   After the pressing apparatus  210  applies the predetermined load to the imprinting jig  100  and the PC sheet  201  for a predetermined period, e.g. for about 10 seconds in the embodiment, the upper stage  211 A of the pressing apparatus  210  is separated from the lower stage  211 B, and the imprinting jig  100  (and the PC sheet  201 ) between the stages is taken out. The imprinting jig  100  and the PC sheet  201  are cooled to a temperature lower than the glass transition temperature Tg of the PC sheet  201  (a cooling process). When the PC sheet  201  is cooled to a temperature sufficiently lower than the glass transition temperature Tg, the pattern transferred onto the PC sheet  201  may be easily damaged in a separating process described below. Thus, a cooling temperature is preferably kept at a temperature a little lower than the glass transition temperature Tg of the PC sheet  201 . 
   After the imprinting jig  100  and the PC sheet  201  are cooled to a predetermined temperature, they are mounted on a separating apparatus  230  as shown in  FIG. 5 . The separating apparatus  230  includes: a stage  231  for placing an object thereon; a clamp  232  for fixing the object placed on the stage  231 ; a vacuum-contacting portion  233  for vacuum-contacting the object fixed on the stage  231 ; a vacuum pump  234 ; and a moving portion  235  for moving up the vacuum-contacting portion  233  in relation to the stage  231 . The vacuum-contacting portion  233  is placed to adhere to the object with forming a predetermined closed space between itself and the object. The vacuum pump  234  reduces a pressure in the space between the vacuum-contacting portion  233  and the object so that the vacuum-contacting portion  233  is vacuum-contacted to the object. 
   As shown in  FIG. 5 , the imprinting jig  100  is placed on the stage  231 . The stamper holder assembly  101  of the imprinting jig  100  is fixed to the stage  231  by the clamp  232 , and the vacuum-contacting portion  233  is vacuum-contacted to the upper surface of the mirror plate  102  of the imprinting jig  100 . Subsequently, the vacuum-contacting portion  233  is moved up with respect to the stage  231  by means of the moving portion  235 , and the mirror plate  102  is separated from the stamper holder assembly  101 . During the step, the mirror plate  102  and the stamper holder assembly  101  are separated from each other while keeping substantially parallel to each other. 
   The PC sheet  201  is firmly adhered to the mirror plate  102  since the surface of the mirror plate  102  in contact with the PC sheet  201  is mirror finished. Consequently, if a force is applied in a direction vertical to the surfaces of the mirror plate  102  and the PC sheet  201 , it is difficult to separate them from each other. Thus, at the time of pulling up the mirror plate  102  to which the vacuum-contacting portion  233  is vacuum-contacted, the mirror plate  102  does not separated from the PC sheet  201 , and the mirror plate  102  and the PC sheet  201  are pulled up in one in relation to the stamper holder assembly  101 . 
   On the other hand, when the mirror plate  102  to which the vacuum-contacting portion  233  is attached is pulled up as described above, the upper surface of the stamper holder assembly  101  (that is, the upper surface of the stamper  105 ) is separated from the lower surface of the PC sheet  201  since the stamper holder assembly  101  is fixed on the stage  231  (a separating process). Subsequently, the PC sheet  201  is separated from the mirror plate  102 , and there is provided an optical disk substrate, on one of which surfaces the fine pattern is formed. 
   Between the mirror plate  102  and the stamper holder assembly  101  from which the PC sheet  201  has been separated is placed another PC sheet  201 , and they are mounted again on the pressing apparatus  210 . The imprinting jig  100  and the pressing apparatus  210  are independent apparatuses, and the imprinting jig  100  may be easily taken out from the pressing apparatus to cool the PC sheet  201  while keeping the pressing apparatus  210  at a predetermined temperature. Since the imprinting jig  100  has a small heat capacity, it can be cooled in a short time. Consequently, it is not necessary to wait for the pressing apparatus  210  having a large heat capacity to be cooled in the cooling process of the PC sheet  201 , as contrasted to the case where the imprinting jig  100  is incorporated in the pressing apparatus  210 . Furthermore, when a plurality of imprinting jigs  100  are used, the cooling process of the PC sheet  201  mounted on some imprinting jig  100  and the heating and pressing process of the PC sheet  201  mounted on another imprinting jig  100  using the pressing apparatus  210  kept at the predetermined temperature can be concurrently performed, and work time may be shortened. 
   Since the pressing apparatus  210  and the imprinting jig  100  are not integrated, however, the PC sheet  201  mounted on the imprinting jig  100  is not always pressed uniformly over the whole surface of PC sheet  201  even if the upper stage  211 A and the lower stage  211 B of the pressing apparatus  210  are formed with a high machining accuracy. It is because uniformity of the pressure applied to the PC sheet  201  is subjected to errors depends on a machining accuracy of the imprinting jig  100  (tolerance), an accuracy of attachment of the PC sheet  201  to the imprinting jig  100 , and/or an accuracy of attachment of the imprinting jig  100  to the pressing apparatus  210 . When the errors described above are large, and the PC sheet  201  is not uniformly pressed over the entire surface thereof, the upper surface of the stamper  105  and the lower surface of the PC sheet  201  may not uniformly contact to each other, and the fine pattern on the stamper  105  may not be transferred onto the lower surface of the PC sheet  201 . In order to prevent this, the rubber sheet  104  for buffering may be placed between the holder plate  103  and the stamper  105  in the embodiment to absorb the errors described above, thereby the PC sheet  201  may be uniformly pressed entirely over the surface thereof. 
   While the PC sheet  201  is mounted on the imprinting jig  100 , a predetermined fine pattern formed on the upper surface of the stamper  105  may be transferred onto the lower surface of the PC sheet  201  by pressing them with the pressing apparatus  210 . Subsequently, when the imprinting jig  100  is taken out from the pressing apparatus  210 , the PC sheet  201  and the imprinting jig  100  are adhered to each other, and they have to be separated from each other as described above. When the PC sheet  201  is separated from the stamper  105  of the imprinting jig  100 , if a force tilted to a direction of the surfaces of the stamper  105  and the PC sheet  201  (hereinafter, referred to as merely “surface direction”) may be applied to the stamper  105  or the PC sheet  201 , as if a sticker is stripped, the pattern transferred onto the lower surface of the PC sheet  201  may be possibly damaged while they may be separated from each other. Furthermore, when a very large force may be applied to separate the stamper  105  and the PC sheet  201  from each other (peeling force), the peeled PC sheet  201  may be deformed. 
   Thus, it is desirably to separate the stamper  105  and the PC sheet  201  from each other while keeping them parallel to each other using a force as weak as possible, when the PC sheet  201  is separated from the stamper  105 . The separating apparatus  230  is used for separation in the embodiment, and separation may be performed while keeping the stamper  105  and the PC sheet  201  parallel to each other. However, in order keep the stamper  105  and the PC sheet  201  parallel to each other during separation, it is necessary to apply to them with a force vertical to the surface direction for separation, and a larger peel force is required, compared with the case where a force tilted to the surface direction is applied to separate them from each other. If such a large peeling force is applied to the PC sheet, the separated PC sheet  201 , as described above, may be deformed. 
   With respect to the problem, the inventors have found, based on the experimental results shown below, that the force required for separation (peeling force) largely vary depending on roughness (Ra) of the upper surface of the holder plate  103 . That is, it has been found that the force required for separating the PC sheet  201  from the stamper  105  does not depend on surface roughness of a portion of the imprinting jig  100  in contact with the PC sheet  201 , but on the surface roughness of a portion in contact with the rubber sheet  104 . That is, it has been found that the peel force depends on adhesion between the holder plate  103  and the rubber sheet  104 . 
   Experiments was performed in which the holder plate  103  having the upper surface having different surface roughness (Ra) and separation conditions were studied in separating the PC sheet  201  from the stamper  105  using the separating apparatus  230  described above. Table 1 shows the summary of the experimental results. From the results, it is understood that when Ra is small (when Ra is 20 nm or 400 nm), a very large peel force becomes necessary, and the PC sheet  201  cannot be separated from the stamper  105  using an existing separating apparatus. On the contrary, when Ra is large (for example, Ra is 600 nm or 1000 nm), a small peeling force may separate the PC sheet  201  from the stamper  105 . That is, it is understood that an increase in the roughness (Ra) of the upper surface of the holder plate  103  allowed the stamper  105  and the PC sheet  201  to be separated from each other with a small peeling force. 
   
     
       
             
             
           
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               surface roughness: Ra (nm) 
               Result of separation 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               20 
               impossible to separate by 
             
             
                 
               existing device 
             
             
               400 
               impossible to separate by 
             
             
                 
               existing device 
             
             
               500 
               possible to separate, but 
             
             
                 
               large peeling force is 
             
             
                 
               required 
             
             
               600 
               Possible to separate with 
             
             
                 
               small peeling force 
             
             
               1000 
               Possible to separate with 
             
             
                 
               small peeling force 
             
             
                 
             
           
        
       
     
   
   Next, an effect of the roughness of the upper surface of the holder plate  103  on the separation of the PC sheet  201  from the stamper  105  will be discussed. When the PC sheet  201  is separated from the stamper  105 , the mirror plate  102  is pulled up in relation to the stamper holder assembly  101 . At this time, the PC sheet  201  integrated with the mirror plate  102  will pull up the stamper  105 . Although the peripheral portion of the stamper  105  is biased toward the holder plate  103  by the clamp  106 , the central portions thereof are not directly biased toward the holder plate  103 . Furthermore, the rubber sheet  104  adheres to the stamper  105 , and they are firmly attached to each other. However, when the upper surface of the holder plate  103  is roughened with fine irregularity, adhesion between the holder plate  103  and the rubber sheet  104  becomes low. Then, it is conceivable that when a force is applied in a direction shown by the arrows shown in  FIG. 6  by the separating apparatus  230  to separate the PC sheet  201  from the stamper  105 , the stamper  105  and the rubber sheet  104  becomes integrated so that the central portions thereof are slightly deformed in an upwardly convex shape. 
   When the stamper  105  and the rubber sheet  104  are pulled up as shown in  FIG. 6 , the central portions thereof are slightly deformed in an upwardly convex shape, while the periphery portion of the stamper  105  is fixed on the holder plate  103  and is not deformed. Then, a small gap is generated between the periphery portion of the stamper  105  and the periphery portion of the PC sheet  201 . It is thought that the gap may give a source to facilitate separation of the PC sheet  201  from the stamper  105 . That is, it is thought that the formation of the gap may cause an equivalent effect as pulling the stamper  105  and the PC sheet  201  diagonally at a very little angle. Alternately, it is also thought that the formation of the gap may allow an air to be supplied into a contact surface (an interface) between the stamper  105  and the PC sheet  201  to facilitate separation thereof. 
   As described above, it is thought to be necessary that the central portion of the stamper  105  moves relative to the periphery portion of the stamper  105  in a direction in which the central portion of the stamper  105  is pulled away from the holder plate  103  in order to reduce the peeling force required for separation of the PC sheet  201  from the stamper  105 . That is, it is necessary for the stamper  105  to be slightly deformed in an upwardly convex shape. A means for improving the separation of the PC sheet  201  from the stamper  105  is not limited to the formation of small irregularity (with Ra of about 600 nm to about 1 μm) on the surface of the holder plate  103  in contact with the rubber sheet  104 , such as the means in the embodiment described above, but various means described below may be employed. 
   The inventors have found, based on the experimental results described above, that the force required for separation (peeling force) largely varies depending on the roughness (Ra) of the upper surface of the holder plate  103 . That is, it has been found that the force required for separation of the PC sheet  201  from the stamper  105  depends on adhesion force between the rubber sheet  104  and the holder plate  103 . As described above, focusing on the adhesion between the rubber sheet  104  and the holder plate  103 , small irregularity (with Ra of about 600 nm to about 1 μm) may be formed on the surface of the rubber sheet  104  in contact with the holder plate  103  in addition to, or instead of, the formation of small irregularity (Ra is about 600 nm to about 1 μm) on the surface of the holder plate  103  in contact with the rubber sheet  104 . The formation of the irregularity may reduce the adhesion force between the rubber sheet  104  and the holder plate  103 . In this case, when the PC sheet  201  is separated from the stamper  105 , the central portions of the rubber sheet  104  and the stamper  105  may be deformed in an upwardly convex shape, and separation of the PC sheet  201  from the stamper  105  may be facilitated because of the reason described above. 
   As shown in  FIG. 7 , a release agent  180  may be applied between the rubber sheet  104  and the holder plate  103 . In this case, separation of the rubber sheet  104  from the holder plate  103  may be also facilitated, and when the PC sheet  201  is separated from the stamper  105 , the central portions of the rubber sheet  104  and the stamper  105  may be deformed in an upwardly convex shape. Then, the separation of the PC sheet  201  from the stamper  105  may be easily carried out. Also, in addition to, or instead of, the application of the release agent  180 , a thin film, such as fluorine resin, a metal thin film having a surface or surfaces on which fine irregularity is formed, or the like may be placed between the rubber sheet  104  and the holder plate  103 , and thereby the separation of the PC sheet  201  from the stamper  105  may be also facilitated. 
   As shown in  FIG. 8 , another rubber sheet  104 A may be further placed between the rubber sheet  104  and the holder plate  103 . In this case, fine irregularity may be formed on a surface of the rubber sheet  104  and/or the rubber sheet  104 A, and a release agent  180  may be applied between these rubber sheets. Alternatively, the thin films described above may be inserted between these rubber sheets. In each case, the separation of the PC sheet  201  from the stamper  105  may be also facilitated. 
   Hitherto, the cases have been described where the separation of the PC sheet  201  from the stamper  105  is facilitated by the means for reducing the adhesion between the contacting surfaces of the rubber sheet  104  and the holder plate  103 . However, the means described above may be applied between the stamper  105  and the rubber sheet  104 , and thereby the adhesion therebetween may be reduced. In this case, the rubber sheet  104  may be adhered to the holder plate  103 , but the central portion of the stamper  105  may be pulled away from the rubber sheet  104  to be deformed in an upwardly convex shape, and thus, the separation of the PC sheet  201  from the stamper  105  may be facilitated. 
   The irregularity formed on the rubber sheet  104 , the holder plate  103  or the stamper  105  stated above may be formed over the entire surfaces thereof, or on a part of the central portions thereof, for example. Furthermore, the irregularity may be formed on two or more surfaces, for example, on both surfaces of the rubber sheet  104 . Furthermore, material of the rubber sheet  104  is not limited to silicon rubber, but may be rubber having thermal resistance such as fluorinated rubber or Viton. In addition, the rubber sheet  104 A may be of the same material as the rubber sheet  104 , or formed of another material having thermal resistance. 
   Furthermore, the holder plate, the mirror plate and the like are not limited to the shape and the material in the embodiments described above. For example, a fine pattern in a nano-level similar to that of the stamper may be formed on the surface of the mirror plate in contact with the PC sheet. The embodiments have been described above referring to the PC sheet as an example for raw material for forming an optical disk substrate, but the raw material may be a plastic sheet having thermal plasticity, for example, a PET sheet. 
   The specific embodiments of the invention have been described above, but the invention is not limited to the embodiments. Those skilled in the art may make various modifications and changes to the configuration and the function of the invention without departing from the spirit of the invention and the scope of the claims.