Patent Publication Number: US-2006013117-A1

Title: Multilayered optical information-recording media and process for manufacture thereof

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a mutilayered optical information-recording medium (hereafter, optical disk) that has two or more information-recording surfaces in the thickness direction of a light transmitting substrate and a process for the manufacture thereof.  
      2. Description of the Prior Art  
      Optical disks which record optically readable information and allow reading of the recorded information using a laser beam spot have heretofore been available. Optical disks including compact disks (CDs) and CD-ROMs have become widespread in use, particularly over the recent years.  
      CD-ROMs recently have come to be used not only in computers but also in multimedia game CD-ROMs and are increasingly replacing magnetic disks (floppy disks) and ROM cartridges both in computer and game applications. Furthermore, a high density CD version called the DVD (digital videodisk) is about to enter the field of movies and multimedia.  
      Recently proposals have been made of multilayered optical disks that enable the recording of massive quantities of information. In contrast to the conventional CD that has a single-layer information-recording surface in which information signals are recorded only on one surface thereof on a substrate, the mutilayered optical disks are structured to have a multiple number of information-recording surfaces in the thickness direction of the substrate.  
      Referring to  FIG. 1 , the aforementioned multilayered optical disk is now described below.  
       FIG. 1  is a drawing illustrating the structure of a conventional multilayered optical disk.  FIG. 1  shows a partial cross-sectional view of the optical disk in its tracking direction.  
      As illustrated in  FIG. 1 , the conventional multilayered optical disk  101  has a first reflecting layer  105 , a transparent layer  106 , a second reflecting layer  107 , and a protective layer  108  laminated together in this sequence onto a light transmitting substrate  104 .  
      Crenulated pits  104 A corresponding to information are generated on the substrate  104 , and pits  104 B corresponding to information are generated on the above transparent layer  106 . In other words, the optical disk  101  has two information-recording surfaces in the thickness direction of the substrate  104 , where the surface on which pits  104 A are formed on the light transmitting substrate  104  is the first information-recording surface  102 , and the surface on which pits  104 B are formed on the light transmitting substrate  106  is the second information-recording surface  103 .  
      The first reflecting layer  105  which is formed between the first information-recording surface  102  and the second information-recording surface  103  is made of a material with a certain degree of light transmittance to enable light to be incident on the second information-recording surface  103  or to be reflected.  
      If three or more information-recording surfaces are to be formed, a second transparent layer is formed on the second reflecting layer  107  on the above transparent layer  106 , followed by forming the pits on the second transparent layer, generating a third recording surface, and a similar procedure is used to form a fourth information-recording surface and beyond.  
      Incident playback laser beam for reading information recorded on each of the above information-recording surfaces ( 102 ,  103 ) is directed through the underside of the above substrate  104 . The playback light beam pickup is equipped to accurately recognize the spacing between adjacent information-recording surfaces, which may be as narrow as several tens of microns (μm), and to be focused onto the desired information-recording surface, so as to read out the information present on each information-recording surface.  
      As described above, the multilayered optical disk which is structured to have two or more information-recording surfaces in the thickness direction of substrate  104  can record a greater amount of information than can a conventional single layer structured optical disk.  
      Referring to  FIG. 2 , a conventional process for the manufacture of optical disk  101  having the above construction is described as follows.  
      First, as illustrated in  FIG. 2 (A), a stamper (not illustrated) with a pattern negative to pits  104 A on the first information-recording surface is used to mold, for example, by injection molding, an optical disk substrate  104  having pits  104 A on its surface.  
      Then, as illustrated in  FIG. 2 (B), a first reflecting layer  105  is formed on the surface of substrate  104  having pits  104 A formed thereon by a film-forming method such as sputtering, vacuum vapor depositing, spin coating or the like. It should be recalled that as mentioned above, the first reflecting layer  105  is a reflecting film that has a certain degree of light transmittance.  
      Then, a second information-recording surface is formed on top of the above first reflecting layer  105 . The second information-recording surface is formed by the process well-known in the art as “the 2P process.” 
      That is, as illustrated in  FIG. 2 ( c ), a stamper  141  with crenulated  141 A pattern negative to pits  104 B of the second information-recording surface is used, a UV-curable resin  142  is deposited as a droplet onto the first reflecting layer  105  of the above substrate  104 , and then the stamper  141  is pressed at its signal-bearing surface against the substrate  104  having the UV-curable resin  142  deposited thereon so as to spread the UV-curable resin  142  to a uniform thickness.  
      Incidentally a nozzle (not illustrated) is used to suction off any resin that has overflown from the outer peripheral of substrate  104  as the stamper  141  is pressed.  
      Then, when the above UV-curable resin  142  is cured by irradiating UV from the substrate  104  side, followed by peeling off the stamper  141 , an optical disk substrate is obtained on which is generated pits  104 B that provide a second information-recording surface  103 , as illustrated in  FIG. 2 (D).  
      Lastly, a second reflecting layer  107  is formed as a film of aluminum, gold, or the like, by sputtering or by vacuum film formation such as vacuum vapor deposition or the like, followed by forming a protective film  108  onto the second reflecting layer  107  and print a label (not illustrated) on the protective layer  108 , thereby completing the formation of a multilayered optical disk as illustrated in  FIG. 2  (E).  
      It should be noted that the playback principle imposed on a multilayered optical disk as described above requires that the spacing between adjacent information-recording surfaces be under strict control. For example, a 40 μm thick film spacing between adjacent information-recording surfaces allows a variation in thickness of only about ±2 μm.  
      However, “the 2P process” mentioned above even with improved precision could achieve at best a level of ±5 μm. In addition, “the 2P process” is inferior in productivity relative to that of injection molding, requiring as long as 2 minutes per surface for molding a second and subsequent information-recording surfaces.  
      It may be possible to increase in practice the molding rate of “the 2P process”, but that would lead to problems such as reduced film thickness precision, trapping of air bubbles, unsatisfactory suction-removal of the resin overflow, and the like.  
      That is, manufacture of the above multilayered optical disk by a conventional manufacturing process would be plagued with problems in terms of both precision and productivity.  
     BRIEF SUMMARY OF THE INVENTION  
      1. Object of the Invention  
      This invention relates to a multilayered optical disk with two or more information-recording surfaces in the thickness direction of a light transmitting substrate, and aims to provide a multilayered optical disk effective in terms of both precision and productivity and a process for the manufacture thereof.  
      2. Brief Summary  
      The first aspect of this invention provides an optical information-recording medium that has at least two or more information-recording surfaces in the thickness direction of a light transmitting substrate and that allows reading of the information recorded on each information-recording surface of the two or more information-recording surfaces by varying the focal position of a playback laser beam incident on and passing through said light transmitting substrate comprising: 
          a transparent layer that is laminated onto said light transmitting substrate and is constituted of a thermoplastic resin sheet of uniform thickness; wherein an information-recording surface other than the surface of said light transmitting substrate is formed on said transparent layer.        

      Also, the second aspect of this invention provides a process for the manufacture of an optical information-recording medium that has at least two or more information-recording surfaces in the thickness direction of a light transmitting substrate and that allows reading of the information recorded on each information-recording surface of the two or more information-recording surfaces by varying the focal position of a playback laser beam incident on and passing through said light transmitting substrate comprising: 
          a first step in which a thermoplastic resin sheet of uniform thickness is positioned between said light transmitting substrate and a stamper having a crenulated pattern formed thereon corresponding to pits or guide grooves; and     a second step in which said light transmitting substrate and said stamper are heated, followed by pressing the light transmitting substrate and stamper against said thermoplastic sheet, thereby transferring the crenulated pattern formed on said stamper to the surface of said thermoplastic resin sheet and forming an information-recording surface on a surface other than that of said light transmitting substrate.        

      The above and other related objects and features of the invention will be apparent from a reading of the following description of the disclosure found in the accompanying drawings and the novelty thereof pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a drawing illustrating a conventional multilayered optical disk;  
       FIG. 2  is a drawing illustrating an embodiment of a process for the manufacture of a conventional multilayered optical disk;  
       FIG. 3  is a drawing illustrating the structure of an optical disk of a first example of this invention;  
       FIG. 4  is a drawing illustrating an embodiment of a process for the manufacture of the optical disk of this invention which is shown in  FIG. 1 ;  
       FIG. 5  is a schematic view for the essential parts of a molding device for the optical disk of this invention which is shown in  FIG. 1 ;  
       FIG. 6  is a drawing illustrating the structure of an optical disk of a second example of this invention; and  
       FIG. 7  is a drawing illustrating another embodiment of a process for the manufacture of the optical disk of this invention which is shown in  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Examples of this invention are described below referring to the attached drawings.  
     EXAMPLE 1 FOR OPTICAL DISK  
      First, the structure of an optical disk of this invention is described using  FIG. 3 .  FIG. 3  illustrates part of the cross-sectional view of an optical disk in its tracking direction. For simplification, the multilayered optical disk described below will be one with two information-recording surfaces.  
      As shown in  FIG. 3 , the optical disk  1  of this Example has a first reflecting layer  5 , a transparent layer  6 , a second reflecting layer  7 , and a protective layer  8  laminated together in this sequence on a light transmitting substrate  4 .  
      On substrate  4  are formed crenulated pits  4 A corresponding to information; on the above transparent layer are formed crenulated pits  4 B corresponding to information. That is, the optical disk  1  has two information-recording surfaces in the thickness direction of the substrate  4 , wherein the surface on which the pits  4 A of the above light transmitting substrate  4  are formed is the first information-recording surface  2 , and the surface on which the pits  4 B of the above transparent layer  6  are formed is the second information-recording surface  3 .  
      In addition, a first reflecting layer formed between the first information surface  2  and the second information-recording surface  3  is made of a material that has a certain degree of light transmittance, so as to permit light to be incident on the second information-recording surface  3  or to be reflected.  
      If three or more information-recording surfaces are to be formed, a second transparent layer is formed on the second reflecting layer  7  on the above transparent layer  6 , and pits are formed on the second transparent layer to make a third information-recording surface, and fourth and subsequent information-recording surfaces can be similarly formed.  
      A notable aspect of the above optical disk  1  is that a thermoplastic resin is chosen as the material for constructing the transparent layer  6 . This is because a thermoplastic resin sheet is used to manufacture the optical disk  1  to be described later.  
      The thermoplastic resin sheet that constitutes the transparent layer  6  is chosen, for a playback principle, from those that have a double pass birefringence of not more than ±50 nm, preferably not more than ±30 nm. This choice is made because as the double pass birefringence increases beyond ±50 nm, the resultant optical distortion will increasingly degrade playback signals, thereby increasing error rates. Therefore, the material to be used as the thermoplastic resin sheet is preferably an acrylic resin (PMMA), polycarbonate resin(PC), or an amorphous polyolefin resin.  
      A thermoplastic resin sheet equal to or less than 20 μm in film thickness would reduce ease of handling and also tend to wrinkle during molding. Since the film thickness of the above transparent layer  6  is nearly equal to that of the thermoplastic resin sheet used, it would be difficult for a light pickup of a playback device to track if the thermoplastic resin sheet were 100 μm or thicker. This, in turn, could require a more complicated mechanism for the light pickup, making it undesirable to use any thermoplastic resin sheet film thicker than 100 μm. Therefore, the film thickness of the thermoplastic sheet used should be about 20 μm to about 100 μm, preferably about 25 μm to about 75 μm so as to result in transparent layer  6  with a film thickness of about 20 μm to about 100 μm.  
      Incident playback laser beams for reading the information recorded on each of the information-recording surfaces thus constructed on the optical disk  1  are directed from the underside of substrate  4 . The light pickup of a playback device can accurately recognize the spacing between the adjacent information-recording surfaces, in which the spacing may be as narrow as several tens of microns (μm), and are focused onto the desired information-recording surface, so as to allow reading out the information on each information-recording surface.  
      Since the information-recording surface (second information-recording surface  3 ) on a surface other than that of substrate  4  is formed on transparent layer  6  which is constituted of a thermoplastic resin sheet with a double birefringence within ±50 nm and a uniform film thickness in the range of about 20 μm to about 100 μm, it is possible to produce an optical disk in which the degradation of playback signals due to an optical distortion is slight and the variation in spacing between the adjacent information-recording surfaces is very low.  
      That is, the variation in the film thickness of transparent layer  6  is about equal to that of the thermoplastic resin, so that the use of a thermoplastic resin sheet of uniform film thickness will allow the variation in the spacing between the adjacent information-recording surfaces to fall within the allowable range. Therefore, this construction will lead to a reduction in errors due to a variation in spacing between the adjacent information-recording surfaces and also to a reduction in requirements for the construction of a playback device.  
     EXAMPLE 1 OF A PROCESS FOR THE MANUFACTURE OF AN OPTICAL DISK  
      Next, the process for the manufacture of the above optical disk  1  is described.  
       FIG. 4  is a drawing illustrating an embodiment of the process for the manufacture of the optical disk of this invention shown in  FIG. 3 . For simplicity, the description is made of a two-layered disk.  
      As illustrated in  FIG. 4 (A), an optical disk substrate  4  with pits  4 A on the surface thereof is molded, for example, by injection molding using a stamper (not illustrated) with a pattern negative to pits A of the first information-recording surface.  
      Next, as illustrated in  FIG. 4 (B), a first reflecting layer  5  is formed on the surface on which are formed pits  4 A of substrate  4  by a film-forming method such as sputtering, vacuum vapor depositing, spin coating, or the like. As described above, the first reflecting film is a reflecting layer having a certain degree of light transmittance.  
      Then, a second information-recording surface is formed on the first reflecting layer using a molding device  31  as illustrated in  FIG. 5 .  FIG. 5  is used to explain the molding device  31 .  
       FIG. 5  is a schematic drawing showing essential parts of the optical disk molding device shown in  FIG. 3 .  
      As shown in  FIG. 5 , the inside of a vacuum chamber  32  of the molding device for the optical disk  1  is constructed in a way that allows adjustment to a specific vacuum level by means of a vacuum pump  35 .  
      Molds  33  and  34  are also positioned in the vacuum chamber  32 . The mold  33  is fixed in the vacuum chamber  32  and is used to mount substrate  4  having the first information-recording surface  2  thereon. Onto the mold  34  is mounted a stamper  36  to form the second information-recording surface  3 . The mold  34  is mounted movably in the vertical direction in the vacuum chamber  32  in such a way that it can move the mounted stamper  36  while holding it horizontally until the stamper  36  comes into contact with the substrate  4  mounted on the above mold  33 .  
      In addition, thermoplastic resin sheet  37  which is used to generate a transparent layer  6  is unfolded and positioned between the molds  33  and  34 . As described above, the thermoplastic resin sheet  37 , which is constituted of a thermoplastic resin material having a double pass birefringence of not more than ±50 nm, has a uniform film thickness in the range of about 20 μm to about 100 μm, is in continuous sheet form wider than the diameter of the above substrate  4 , and is stored as a wound roll, as illustrated by  FIG. 5 , within the vacuum chamber  32  (or outside of the vacuum chamber  32 ). The wound thermoplastic resin sheet  37  is unwound to a suitable length and unfolded to be held, via guide rollers  39 A and  39 B, between the mounted substrate  4  and the stamper  36 .  
      Furthermore, the substrate  4  mounted on mold  33  and the stamper  36  mounted on mold  34  are equipped with heaters, which are not illustrated, and the thermoplastic resin sheet  37  is equipped with a means for heating, not illustrated, such as an indirect infrared heater so that these can be heated to any designated temperatures. This setup is made so as to melt the surface of the thermoplastic resin sheet  37  when the stamper  36  is pressed against the sheet as will be described later.  
      Thus, substrate  4 , stamper  36 , and thermoplastic resin sheet  37  are each heated so that the thermoplastic resin sheet  37  should not come in contact with, but be apart from, the surfaces of the molds  33  and  34 , the mounted substrate  4 , and the stamper  36 . It is preferred to keep the thermoplastic resin sheet under some tension in the lengthwise direction so as to prevent the thermoplastic resin sheet  37  between molds  33  and  34  from slacking between guide rollers  39 A and  39 B.  
      Stopping the heating by the above heating means will allow the thermoplastic resin sheet  37  to cool and solidify, where some cooling means may be provided to facilitate cooling the thermoplastic resin sheet  37 .  
      As illustrated in  FIG. 4 (C), the substrate  4  on which the first reflecting layer is formed is mounted on mold  33 , and the stamper  36  having crenulated pits  36 A formed with a pattern negative to pits  4 B of the second information-recording surface  3  is mounted on mold  34 . As also shown in  FIG. 4 (C), the substrate  4  is mounted with its surface having the first reflecting layer  5  formed thereon (the first information-recording surface  2 ) upward, and the stamper  36  is mounted with its surface having a crenulated pattern  36  A formed thereon downward. As described above, thermoplastic resin sheet  37  is positioned between the mounted substrate  4  and the stamper  36 .  
      Then, the inside of the vacuum  32  is brought to a specific level of vacuum, and mold  34  is moved downward so as to interpose the thermoplastic resin sheet  37  between the substrate  4  and the stamper  36  and to apply a specified pressure. Since the substrate  4 , the stamper  36 , and the thermoplastic resin sheet  37  are each heated, the two surfaces of the thermoplastic resin sheet  37  will melt. The two molten surfaces allow pits  4 B to be transferred to that surface of thermoplastic resin sheet  37  in contact with the stamper and the other surface thereof in contact with the substrate  4  to be adhered to substrate  4 . After applied heat and pressure is held for a given period of time, followed by suitably cooling to solidify the thermoplastic resin sheet  37 , the stamper  36  is peeled off, and the thermoplastic resin sheet  37  is cut along the outer edge of substrate  4 , as shown in  FIG. 4  (D), resulting in the formation of a substrate having pits  4 B of the second information-recording surface  3  formed on the surface thereof.  
      The formation of the optical disk  1  is completed ( FIG. 4 (E)) by forming a film of aluminum, gold, or the like on the transparent layer  6  on which pits  4  have been formed by means of vacuum film formation such as sputtering, vacuum vapor depositing, or the like, thereby forming a second reflecting layer  7 , by spin coating on the second reflecting layer  7  with a film of a UV curable resin or the like to form a protective layer  8 , and then by printing a label (not illustrated) on the protective layer  8 .  
      The optical disk as manufactured as above will have the spacing between adjacent information-recording surfaces to be nearly identical to the thickness of the original thermoplastic resin sheet  37 . Since the thermoplastic resin sheet  37  has a variation in film thickness of about ±1 μm, the spacing between the adjacent information-recording surfaces can also be achieved at a similar magnitude of variation. This construction can eliminate a need for a rigorous control of the spacing between adjacent information-recording surfaces, which will lead to shortening of the time needed in the molding steps to form a second information-recording surface  3  and will also allow manufacturing a multilayered optical disk with a very low variation in spacing between adjacent information-recording surfaces (the variation in film thickness of transparent layer  6 ).  
      Although the above description of the process calls for molding the thermoplastic resin sheet  37  in a vacuum, it would obviously be possible to mold without a vacuum. Nevertheless, molding the thermoplastic resin sheet  37  in a vacuum offers some advantages. Such an operation will allow the sheet  37  to be uniformly laminated without any chance of trapping air bubbles at the moment when the sheet  37  and the stamper  36 , and the substrate  4  and sheet  37 , are joined together at the interfaces. Uneven lamination, if any, would result in film thickness irregularities for the transparent layer  6 , causing an out-of-focus problem, but sheet molding in a vacuum can avoid such problems and provide a uniform lap even for a large diameter disk.  
     EXAMPLE 2 FOR AN OPTICAL DISK  
      As shown in  FIG. 6 , both the first and second information-recording surfaces may be formed by molding thermoplastic resin sheet  37 .  
      The first information-recording surface  2  of an optical disk  11  is formed in the same manner as that of the second information-recording surface of the above optical disk  1 . That is, a mirror-surface substrate  12  free of any crenulated features made by pits or the like is mounted on mold  33  of the above molding device; a stamper used to form pits  12 A of a first information-recording surface is mounted on mold  34  and is pressed with heat in the manner described above to transfer the pits  12 A, followed by cooling and cutting out the thermoplastic resin sheet  37  into a first transparent layer  13 . A first reflecting layer  5  is formed on the first transparent layer  13 , followed by forming sequentially, as in the optical disk  1 , a second transparent layer  14 , a second reflecting layer  7 , and a protective film  8 , thereby generating optical disk  11 .  
     EXAMPLE 2 OF A PROCESS FOR THE MANUFACTURE OF AN OPTICAL DISK  
      A reflecting film may be precoated to one surface of the above thermoplastic resin sheet in the above process for the manufacture of optical disks  1  and  11 , which are shown in  FIG. 3  and  FIG. 6 , respectively.  
      As illustrated in  FIG. 7  (A), a reflecting film  38  is precoated to one surface of the thermoplastic resin sheet  37 , and the sheet is placed to position the side having the reflecting film  38  formed thereon to face the stamper  36 . The reflecting film  38  coated to the thermoplastic resin sheet  37  becomes upon molding a second reflecting layer  7 . When the thermoplastic resin sheet  37  having the reflecting layer coated thereto is processed with heat and pressure as described above and rapidly cooled, the pattern of crenulated pits  36 A of the stamper  36  is transferred to the surface of the thermoplastic resin sheet  37  having the reflecting film  38  formed thereon. Cutting out the molded thermoplastic resin  37  along the outer periphery of substrate  4  results in the formation of a transparent layer  6  having the reflecting layer coated thereto, thereby forming a second information-recording surface  3 . Formation of a protective film  8  on the second reflecting layer  7  on the transparent layer  6  gives optical disks  1  and  11 .  
      If it is necessary to have different reflecting layers depending on the information-recording surfaces to be formed, thermoplastic resin sheets with different reflecting films coated thereto may be prepared in several types so as to permit selection of the type appropriate to the information-recording surface to be formed.  
      Since the film of the reflecting layer is formed in a vacuum, it is possible to integrate the film-forming device of the reflecting layers into the vacuum chamber of the molding device  31 . This will permit forming the reflecting layer film and moving to the molding step without removing it out into the atmosphere in a one-pass equipment operation to consolidate the film formation and molding and will improve the productivity by an order of magnitude.  
      Optical disks in these examples of this invention are now described in more detail using specific experimental examples as below:  
     Experiment Example 1  
      Two pieces of nickel stamper with digitized image signals inscribed thereinto in a 0.84 μm track pitch and a pit length 0.45 μm or longer were used. One of them was used to produce a first information-recording surface by injection molding polycarbonate, thereby forming a substrate  4  with a 120 mm diameter and a 1.2 mm thickness having pits  4 A of the first information-recording surface formed thereon. A high transmitting first reflecting layer  5  was formed on the surface of the substrate  4  having the formed pits.  
      Then, the substrate was mounted on mold  33  of the above molding device  31  not equipped with either the above vacuum chamber  32  or vacuum pump  35 . The mold  33  was heated to 70° C. Acrylic resin (PMMA) sheet (thermoplastic sheet)  37  with a double pass birefringence in the plane of not more than ±15 nm was used, which was available as a 15 cm wide and 500 m length cut roll with a film thickness of 50 μm (film thickness variation range ±1 μm). This sheet was unfolded over the substrate  4 .  
      Mold  34  with stamper  36  to be used to form a second information-recording surface  3  was positioned to face substrate  4  in the mold  33 . Incidentally, the pit-formed surface of substrate  4 , sheet  37 , the underside of stamper  36  were held parallel to, and apart from, each other.  
      The surface of the stamper  36  was heated to 85° C. and the sheet  37  in the atmosphere was indirectly infrared-heated (not illustrated) to 75° C. Then, mold  34  was moved to press against mold  33  and was held for 5 seconds to compress the sheet  37  interposed between the molds. The mold  33  was then rapidly cooled to 50° C. and the mold  34  down to 60° C.; and the sheet  37  was cut out along a 120 mm outer diameter by a circular blade (not illustrated) placed at an outer periphery of mold  33 , followed by separating the molded substrate from the original sheet  37 . Then, the two molds were opened to remove the substrate adhered to mold  33 . The completed substrate had pits  4 B of the second information-recording surface formed thereon in a 50 μm spacing apart from, and parallel to, the first information-recording surface  2 . The total molding time to form the transparent layer  6  to produce the second information-recording surface was 30 seconds.  
      A second reflecting layer (aluminum)  7  was sputtered to a film thickness of 70 nm onto the above transparent layer  6 , followed by spin coating the layer with SD-11 (manufactured by Dainippon Ink &amp; Chemicals, Inc.) to an 8 μm thick protective film  8 , and the layer was then UV-cured. Lastly a label was printed over it to complete the manufacture of the optical disk  1 .  
      The optical disk  1  was evaluated for playback using a disk tester equipped with a laser wavelength of 635 nm and an objective lens with a numerical aperture, NA0.52. Signals from the two surfaces were individually observed by changing the focal position of the pickup, and it was found that eye patterns were cleanly open in the both layers to permit stable playback. Playback jitter was 15% for the first layer and 12% for the second layer, indicating highly usable levels if operated in conjunction with an equalizer.  
     Experiment Example 2  
      This example used a rolled acrylic resin sheet (thermoplastic resin sheet) with one surface having a 70 nm film of a second reflecting layer (aluminum)  7  prcoated thereto (See  FIG. 7 (A)). This was used to replace the acrylic sheet used in Example 1 and molded with the aluminized surface to come in contact with stamper  36 . In this case, an operation similar to that of Example 1 was followed to fabricate a disk substrate, except that the inside of the vacuum chamber  32  of molding device  31  was held at 1 torr. Since the molded transparent layer  6  in this experiment example had already been aluminized, the formation of the second reflecting layer  7  was omitted, and a protective film  8  was spin coated over it to complete the manufacture of an optical disk  1 .  
      A similar playback testing of this example also confirmed a stable playback performance.  
      The optical disk in the above Experiment Example 1 may be sheet-molded in a vacuum; similarly, the optical disk in the Experiment example 2 may also be sheet-molded without the use of a vacuum.  
      Although the above examples and Experiment examples were described exclusively on read-only optical disks as illustrative examples, this invention is not limited to such applications, and can be applied to once-writable and rewritable type disks as well as to optical cards and other optical information-recording media. When the application to optical information-recording media involves a once-writable or rewritable type, the stamper  36  forms preformatting information such as guide grooves, and the like.  
      In addition, further applications may be made, such as the formation of an antistatic layer on the underside of substrate  4  or the upper side of protective film  8  or printing of a label on the uppermost protective film  8 .  
      [Advantageous effect of the Invention] 
     
         
          (1) According to the optical disk of this invention, a multilayered optical disk can be obtained with very little degradation of the playback signal caused by optical distortions and with a very low variation in spacing between adjacent information recording surfaces. That is, since the variation in film thickness of the transparent layer is about equal to that of the thermoplastic resin sheet, use of a uniform thickness thermoplastic resin sheet permits bringing the variation in spacing between adjacent information-recording surfaces to an allowable range of error.  
       
    
      Therefore, this construction can reduce error rates caused by the variations in spacing between adjacent information-recording surfaces and also to reduce the burden of requirements on the playback device to be used. 
      (2) The optical disk manufacturing process of this invention eliminates a rigorous control of the spacing between adjacent information-recording surfaces in its manufacture and will allow shortening of the time required to form an information-recording surface in the molding steps. Since the variation in spacing between adjacent information-recording surfaces is about equal to the variation in film thickness of the original thermoplastic resin sheet, the use of a uniform thickness thermoplastic resin sheet will permit manufacture of a multilayered optical disk with a very low variation in spacing between adjacent information-recording surfaces (the variation in thickness of the transparent layer). That is, this invention can improve both precision and productivity.