Patent Publication Number: US-6982016-B2

Title: Optical information recording medium and its manufacturing method and apparatus

Description:
This application is a divisional application of Ser. No. 10/446,052, filed May 28, 2003, which is a divisional application of Ser. No. 09/547,879, now U.S. Pat. No. 6,613,170 filed Apr. 12, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. (Field of the Invention) 
     The present invention relates to a method of and an apparatus for manufacturing an optical information recording medium. 
     2. (Description of the Prior Art) 
     Technology in which playback and recording of high-density information are performed by using a laser beam is known and is put to practical use mainly as an optical disk. Optical disks can be roughly classified into read-only type, write-once read multiple type and rewritable type. The read-only optical disk is commercially available as a compact disk for recording musical information and a laser disk for recording information on images, while the write-once read multiple type optical disk is commercially available for storing document files and still picture files. Furthermore, the rewritable type optical disk is commercially available for storing data files for a personal computer. 
     The optical disk usually has an arrangement in which an information layer is provided on a principal face of a transparent resinous substrate of 1.2 mm in thickness and a protective film, such as an overcoat, is provided on the information layer or alternatively, an arrangement in which a protective sheet having the same shape as a substrate is bonded to the substrate. 
     Meanwhile, in recent years, use of a shorter laser wavelength and an objective lens having a larger numerical aperture (NA) has been studied in order to achieve higher density of the optical disk. However, the shorter wavelength and the larger numerical aperture reduce an allowable value of an angle of inclination, (tilt) of the optical disk relative to a direction of incidence of the laser beam. Reduction of thickness of the substrate is effective for increasing the allowable value of the tilt. For example, in a digital video disk (DVD) having a laser wavelength of 650 nm and a numerical aperture of 0.60, the substrate has a thickness of 0.6 mm. Since mechanical strength of the single substrate of 0.6 mm in thickness is small, the two substrates are bonded to each other such that information recording faces of the substrates confront each other. 
     In order to bond the two substrates to each other, a method is mainly employed in which radiation cure resin is coated on one substrate, the other substrate is brought into close contact with the one substrate and then, radiation is irradiated over the substrates so as to cure the radiation cure resin. This method is referred to as a “radiation cure method”, hereinafter. Meanwhile, ultraviolet (UV) rays are generally used as radiation. Generally in the radiation cure method, radiation cure resin is coated on one substrate annularly by rotating the one substrate at low speed and the other substrate is placed on the one substrate such that the two substrates are formed integrally. Subsequently, after the radiation cure resin has been fully diffused between the two substrates by rotating the two substrates at high speed, radiation is irradiated to the substrates so as to cure the radiation cure resin. 
     However, in this method, since a position of diffusion of the radiation cure resin to an inner periphery of the substrates changes based on a position of coating of the radiation cure resin, a timing of placing the substrates on each other, high-speed rotational conditions of the substrates, etc., it is difficult to stop the resin at a predetermined radial position of the substrates. In a case where the resin is excessively diffused to the inner periphery of the substrates, the resin protrudes into central bores of the substrates. If the resin is cured in this state, the substrates become eccentric relative to a turntable when mounted on the turntable. Therefore, in this case, the resin should be cured after having been wiped from the central bores of the substrates. Meanwhile, in a case where the resin is insufficiently diffused to the inner periphery of the substrates, a disk clamp area of the substrates, which is used for clamping the substrates to the turntable, is not filled with the resin and thus, the mechanical strength of the optical disk becomes small. 
     In order to solve this problem, Japanese Patent Laid-Open Publication No. 8-321074 (1996) proposes that a stopper for preventing the radiation cure resin from protruding into the central bores is provided at an innermost periphery of the substrate. For example, an annular recess is formed at an inner peripheral side of the disk clamp area on at least one of opposed faces of the substrates. Thus, when the resin has been diffused to the inner periphery of the substrates, the resin is received in the recess acting as a resin reservoir and therefore, is prevented from being diffused further to the inner periphery of the substrates. Namely, even if the resin is filled in the disk clamp area, the resin does not protrude into the central bores of the substrates. Accordingly, it is possible to stably manufacture an optical disk having a large mechanical strength. 
     In order to manufacture an optical disk, a method is generally known in which two substrates are bonded to each other with ultraviolet (UV) cure resin. This method has such features that (1) since air bubbles or the like are not contained in the resin, an external appearance of the optical disk is good and (2) since the resin is instantaneously cured upon irradiation of UV rays thereto, working efficiency is excellent and tact time can be shortened. 
     Further, thin substrates represented by the DVD have been used in recent years. Since the mechanical strength of the thin substrate is small, it is desirable that the thin substrates are bonded to each other by filling the resin also in the disk clamp area of the substrates so as to be formed integrally. To this end, the UV cure resin should be coated on a neighborhood of the central bores of the substrates. 
     However, it has been difficult to stop the resin at a predetermined radial position of the substrate at all times. This is because diffusion speed of the UV cure resin to an inner periphery of the substrates changes based on a position of coating of the UV cure resin, timing of bonding of the substrates, rotational conditions of the substrates, change of viscosity of the UV cure resin due to temperature changes, etc. In a case where the resin is excessively diffused to an inner periphery of the substrates, the resin protrudes into the central bores of the substrates. If the resin is cured in this state, the substrates become eccentric relative to a turntable when mounted on the turntable. Therefore, in this case, the resin should be cured after having been wiped from the central bores of the substrates. Meanwhile, in a case where the resin is insufficiently diffused to the inner periphery of the substrates, the resin is not filled in the disk clamp area of the substrates and thus, mechanical strength of the optical disk becomes small as described above. 
     Meanwhile, Japanese Patent Laid-Open Publication No. 8-321074 (1996) discloses a method in which the radiation cure resin is filled in the disk clamp area stably without projecting into the central bores of the substrates. However, in this prior art document, it is essential that the stopper is provided on the substrate. In order to form the stopper on the substrate, a method in which an annular protuberance is provided on the substrate by printing or a method in which an annular recess is provided on the substrate by cutting the substrate is disclosed. At any rate, it is necessary to provide a step of working the substrate. However, the provision of the step of working the substrate results in adherence of dust thereto and rise of production cost. 
     Meanwhile, it is also possible to form the stopper on the substrate preliminarily. However, in a case where the substrate is manufactured by, for example, transfer from a stamper, optical characteristics of the substrate and properties of transfer from the stamper to the substrate may be adversely affected by the stopper. Namely, if an annular protuberance is provided on the stamper or a stamper holder, an annular recess acting as the stopper is formed on the substrate. However, in a case of an optical information recording medium, especially an optical disk, the substrate is manufactured by injection molding. Thus, if the protuberance is preliminarily provided on the stamper, flow of the resin at the time of molding of the substrate is different from that in a case where the protuberance is not provided on the stamper, so that double refraction and formation of signal recording pits and signal recording grooves of the manufactured substrate may be insufficient. Furthermore, in a case where a position of the stopper is required to be changed, the position of the protuberance on the stamper or the stamper holder should also be changed. As a result, it has been practically difficult to control resinous filling to an arbitrary position. 
     Meanwhile, in the case where the two substrates are bonded to each other such that the information recording faces of the substrates confront each other as described above, an optical disk in which a reflective layer made of aluminum or the like, as in prior art, is formed on the information recording face of one of the substrates and a thin translucent reflective layer made of gold or the like, is formed on the information recording layer of the other of the substrates such that playback on the two information recording faces is performed from the substrate having the translucent reflective layer is proposed and put to practical use. Another optical disk formed by the two substrates is also proposed in which not the metallic reflective layer, but a thin rewritable recording layer is formed on the information recording layer. 
     Furthermore, in order to achieve higher density, the use of a bluish purple laser beam source having a wavelength of about 400 nm is also proposed. In this case, an arrangement of the two substrates can be obtained in the same manner as described above. 
     In the DVD, the substrate has a thickness of 0.6 mm. However, a method is also proposed in which the substrate is set at a thickness of about 0.1 mm and a quite minute laser beam spot is formed by using a lens having an NA of about 0.85, so as to perform recording and playback of signals. Since it is difficult to provide a signal recording layer on the substrate of 0.1 mm in thickness, a thin translucent sheet is generally bonded, by using UV cure resin, to a substrate of about 1.1 mm in thickness, which is provided with the signal recording layer, such that recording or playback of signals is performed from the sheet. At this time, a sum of a thickness of the sheet and that of the UV cure resin is designed to reach 0.1 mm. 
     In order to bond the substrates to each other, there is a method in which UV cure resin is coated on one substrate, the other substrate is brought into close contact with the one substrate and then, UV rays are irradiated on the substrate so as to cure the UV cure resin. Generally in this method, the UV cure resin is annularly coated on the one substrate by rotating the one substrate at low speed and the other substrate is placed on the one substrate such that the two substrates are formed integrally. Subsequently, after the UV cure resin has been fully diffused and spread between the two substrates by rotating the substrates at high speed, UV rays are irradiated to the substrates so as to cure the UV cure resin. 
     However, in this conventional method, thickness of the UV cure resin varies based on a position of coating of the UV cure resin, timing of placing the substrates on each other, high-speed rotational conditions of the substrates, etc. These variations take place not only between optical disks, but in one optical disk. Generally, since the UV cure resin is spread by centrifugal force produced by high-speed rotation of the substrates, the thickness of the UV cure resin is small at an inner periphery of the substrates but is large at an outer periphery of the substrates. Such distribution of thickness of the UV cure resin poses a problem in a case where recording or playback is performed through a layer of the UV cure resin. Namely, in the case where recording or playback is performed from one of the two substrates bonded to each other or the thin translucent sheet is bonded, by using the UV cure resin, to the substrate having the signal recording layer such that recording and playback of signals are performed from the sheet. Namely, since variations of thickness of the UV cure resin result in variations of an optical path length of a laser beam, a shape of a laser beam spot on the signal recording layer varies, thereby resulting in variations of recording and playback characteristics. This adverse effect becomes greater in a case where the laser beam spot is reduced in diameter. Namely, in a case where a bluish purple laser beam is employed as the laser beam or an objective lens having an NA as large as 0.85 is used. 
     SUMMARY OF THE INVENTION 
     Accordingly, a first object of the present invention is to provide, with a view towards eliminating the above-mentioned drawbacks of prior art, a method of and an apparatus for manufacturing an optical information recording medium, in which filling of resin at an innermost periphery of substrates can be controlled to an arbitrary position, regardless of whether or not a stopper is provided at the innermost periphery of the substrates, or regardless of a position of the stopper, whereby an optical disk having a high mechanical strength and good external appearance, in which a filling position of the resin at the innermost periphery of the substrates is fixed, can be manufactured at high yield and at low cost. 
     A second object of the present invention is to provide a method and an apparatus of the above described type, in which a layer of UV cure resin has uniform thickness, such that recording and playback characteristics do not vary even when the recording or playback of signals is performed through the layer of UV cure resin. 
     In order to accomplish the first object of the present invention, a method of manufacturing an optical information recording medium, in which a first substrate having a first central bore and a second substrate having a second central bore are bonded to each other through radiation cure resin, according to the present invention comprises the steps of coating the radiation cure resin on the first substrate, bringing the first and second substrates into close contact with each other through the radiation cure resin so as to form the first and second substrates integrally, irradiating radiation to a neighborhood of the first and second central bores, and irradiating radiation to a whole of at least one of opposite outer faces of the integral first and second substrates so as to cure the radiation cure resin wholly. 
     Meanwhile, in order to accomplish the second object of the present invention, a method of manufacturing an optical information recording medium, including a disklike first substrate having a signal recording layer formed on one principal face thereof and a disklike second substrate, according to the present invention comprises the steps of bringing the first and second substrates into close contact with each other through radiation cure resin such that the signal recording layer is disposed between the first substrate and the radiation cure resin, rotating the first and second substrates while the first and second substrates are being held in close contact with each other so as to form the first and second substrates integrally, irradiating radiation to an inner peripheral region of the integral first and second substrates so as to cure a portion of the radiation cure resin, continuing the rotation of the first and second substrates so as to spread the radiation cure resin between the first and second substrates, and irradiating radiation so as to cure the radiation cure resin wholly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings in which: 
         FIGS. 1A ,  1 B and  1 C are perspective views showing steps of a method of manufacturing an optical information recording medium, according to a first embodiment of the present invention; 
         FIG. 2  is a sectional view of  FIG. 1B ; 
         FIG. 3A  is a perspective view showing a step of a modification of the method of  FIG. 1 ; 
         FIG. 3B  is a sectional view of  FIG. 3A ; 
         FIG. 4  is a sectional view showing a step of another modification of the method of  FIG. 1 ; 
         FIG. 5A  is a perspective view showing a step of a further modification of the method of  FIG. 1 ; 
         FIG. 5B  is a sectional view of  FIG. 5A ; 
         FIG. 6  is a sectional view showing a step of a still a further modification of the method of  FIG. 1 ; 
         FIGS. 7A ,  7 B,  7 C and  7 D are views showing steps of a method of manufacturing an optical information recording medium, according to a second embodiment of the present invention; 
         FIG. 8  is a schematic sectional view showing one example of a photodetector employed in the method of  FIG. 7 ; 
         FIGS. 9A and 9B  are schematic sectional views showing another example of the photodetector employed in the method of  FIG. 7 ; 
         FIG. 10  is sectional view showing a step of a modification of the method of  FIG. 7 ; 
         FIG. 11  is a sectional view showing a step of another modification of the method of  FIG. 7 ; 
         FIGS. 12A ,  12 B and  12 C are perspective views showing steps of a method of manufacturing an optical information recording medium, according to a third embodiment of the present invention; 
         FIG. 13  is a sectional view of  FIG. 12B ; 
         FIG. 14  is a perspective view showing a step of a method of manufacturing an optical information recording medium, according to fourth embodiment of the present invention; 
         FIG. 15  is a view similar to  FIG. 14 , particularly showing a modification thereof; 
         FIG. 16  is a perspective view showing a step of a method of manufacturing an optical information recording medium, according to a fifth embodiment of the present invention; and 
         FIG. 17  is a sectional view showing a step of a modification of the method of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout several views of the accompanying drawings. 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. 
     (First Embodiment) 
     Here, a method in which when UV cure resin is diffused between two substrates, a filling position of the UV cure resin at an innermost periphery of the substrates is controlled by irradiating UV rays only to an innermost peripheral region of the substrates, according to a first embodiment of the present invention is described with reference to  FIGS. 1A–2 . In  FIG. 1A , substrates  1  and  2  are identical with each other and each of the substrates  1  and  2  is a polycarbonate substrate produced by injection molding and having a thickness of 0.6 mm and a diameter of 120 mm. A central bore  20  of each of the substrates  1  and  2  has a diameter of 15 mm. Information signals are preliminarily recorded as pits on one face of each of the substrates  1  and  2 , which act as signal recording faces. A reflective layer  3 , mainly consisting of aluminum and having a thickness of about 100 nm, is provided on the signal recording face of the substrate  1 . Thus, by irradiating a laser beam from the other face of the substrate  1 , playback of the information signals can be performed. The substrate  2  is a dummy substrate which is bonded to the substrate  1 , so as to increase its mechanical strength. A reflective layer is not provided on the dummy substrate  2 . 
     Initially, UV cure resin  5  is dripped from a nozzle  4  so as to be coated on the reflective layer  3  of the substrate  1  annularly by rotating the substrate  1  at low speed, for example, at 20 to 120 rpm with a motor  150 . Alternatively, the nozzle  4  may also be rotated and the substrate  1  fixed. Then, the dummy substrate  2  is brought into close contact with the substrate  1  such that not only the central bores  20  of the substrates  1  and  2  are made concentric with each other, but the signal recording faces of the substrates  1  and  2  confront each other. 
     Thereafter, owing to a weight of the dummy substrate  2  and capillarity, UV cure resin  11  is diffused between the substrates  1  and  2  as shown in  FIG. 2 . At this time, if the substrates  1  and  2  are rotated at high speed, for example, at 300 to 6000 rpm by the motor  150 , diffusion of the UV cure resin  11  to an outer periphery of the substrates  1  and  2  is accelerated and thus, tact time can be reduced greatly. Meanwhile, it has also been proved that a thickness of the UV cure resin  11  at the time the substrates  1  and  2  are rotated at high speed can be made more uniform than that at the time the substrates  1  and  2  are not rotated. An excessive portion of the UV cure resin  11  is expelled as droplets  9  from the outermost periphery of the substrates  1  and  2  by centrifugal force of the high-speed rotation of the substrates  1  and  2 , as shown in  FIGS. 1B and 2 . 
     Further, the UV cure resin  11  is also diffused to an inner periphery of the substrates  1  and  2 . The present invention is characterized by a method of accurately controlling a position for stopping this diffusion of the UV cure resin  11  to the inner periphery of the substrates  1  and  2 . This method is described with reference to  FIGS. 1B and 2 . When the substrates  1  and  2  are rotated at a high speed by the motor  150 , UV rays  8  are irradiated by a UV lamp  6  only to a UV irradiation region  10 , i.e., a region of the substrate  2 , which is disposed radially inwardly of an innermost peripheral position for stopping diffusion of the UV cure resin  11 . By changing a height and size of a UV shielding cover  7  for shielding the UV rays  8  of the UV lamp  6 , the UV irradiation region  10  can be arbitrarily controlled regardless of a shape of the substrates  1  and  2 . 
     When the UV cure resin  11  has reached the UV irradiation region  10  through its diffusion to the inner periphery of the substrates  1  and  2  upon close contact of the substrate  2  with the substrate  1 , the UV cure resin  11  is cured by the UV rays  8  and thus, further diffusion of the UV cure resin  11  to the inner periphery of the substrates  1  and  2  is stopped. As a result, an annular cured region  12  in which the UV cure resin  11  is cured is formed at innermost periphery of the substrates  1  and  2 . 
     Speed of diffusion of the UV cure resin  11  to the inner periphery of the substrates  1  and  2  is not constant at all times, but varies according to minute changes of viscosity of the UV cure resin  11  and the shape of the substrates  1  and  2 . However, if the UV irradiation region  10  is made concentric with the central bores  20  of the substrates  1  and  2 , the diffusion of the UV cure resin  11  is stopped at a boundary of the UV radiation region  10  at all times. Therefore, the UV cure resin  11  can be filled at inner periphery of the substrates  1  and  2  stably, so that not only stable mechanical strength can be secured but good external appearance can be obtained. 
     After the UV cure resin  11  has been fully diffused between the substrates  1  and  2 , UV rays  14  are irradiated to a whole of the substrate  2  by a UV lamp  13 , as shown in  FIG. 1C , so as to cure the UV cure resin  11  wholly, so that bonding of the substrates  1  and  2  is completed and thus, an optical disk  15  is obtained. 
     Meanwhile, in the first embodiment, the UV cure resin  5  is dripped from the nozzle  4  so as to be coated on the substrate  1  annularly and then, the substrate  2  is brought into contact with the substrate  1 , such that not only the central bores  20  of the substrates  1  and  2  are made concentric with each other, but the signal recording faces of the substrates  1  and  2  confront each other. A modification of the method of the first embodiment is described with reference to  FIGS. 3A and 3B . In this modification, a substrate  21  having a reflective layer  23  formed thereon and a dummy substrate  22  are caused to confront each other with a minute gap formed therebetween, beforehand. A needlelike dispenser  24  is inserted into the gap and UV cure resin  25  is filled annularly between the substrates  21  and  22  by the dispenser  24 , while the substrates  21  and  22  are being rotated at low speed. Then, the substrates  21  and  22  are brought into close contact with each other so as to be formed integrally. In the method of the first embodiment, if the substrate  2  is brought into close contact with the substrate  1  rapidly, air bubbles may penetrate in between the substrate  2  and the UV cure resin  11 . However, in this modification, since the UV cure resin  25  is filled between the substrates  21  and  22  by the dispenser  24 , the penetration of air bubbles in between the substrates  21  and  22  does not take place and thus, such an advantage is obtained that tact time for bonding the substrates  21  and  22  to each other can be shortened. 
     Another modification of the method of the first embodiment is described with reference to  FIG. 4 . In this modification, when UV cure resin  38  is diffused between substrates  31  and  32 , by rotating the substrates  31  and  32  at high speed after the substrates  31  and  32  have been brought into close contact with each other, the UV cure resin  38  is sucked from the central bores  20  of the substrates  31  and  32 . More specifically, after the substrate  31  having a reflective layer  33  formed thereon and the dummy substrate  32  have been brought into close contact with each other, the substrates  31  and  32  are mounted on a boss  35  having a suction port  34 . In this case, the suction port  34  is set so as to be located at a level of a gap between the substrates  31  and  32 . The UV cure resin  38  is diffused between the substrates  31  and  32  by rotating the integral substrates  31  and  32  at high speed. At this time, a suction pump  36  is actuated and sucks the UV cure resin  38  from the suction port  34  so as to diffuse the UV cure resin  38  to inner periphery of the substrates  31  and  32 . Also in this modification, since UV rays  37  are irradiated to an innermost peripheral region of the substrates  31  and  32 , an annular cured region  39  in which the UV cure resin  38  is formed. In this modified method, since the UV cure resin  38  is diffused also to inner periphery of the substrates  31  and  32  in a short period, tact time for bonding the substrates  31  and  32  to each other can be shortened. 
     Meanwhile, in the first embodiment, at the time of diffusion of the UV cure resin  11 , the UV rays  8  are irradiated to the innermost peripheral region of the substrates  1  and  2  concentrically with the central bores  20  of the substrates  1  and  2  by the UV lamp  6  provided immediately above a center of the substrate  2 . A further modification of the method of the first embodiment is described with reference to  FIGS. 5A and 5B . In this modification, a UV spot light  46  is irradiated to an inner peripheral spot of substrates  41  and  42  while the substrates  41  and  42  are being rotated. More specifically, after the substrate  41  having a reflective layer  43  formed thereon and the dummy substrate  42  have been brought into close contact with each other, the substrates  41  and  42  are rotated at high speed so as to diffuse UV cure resin  47 . At this time, UV rays produced by a UV light source  44  are guided by an optical fiber  45  so as to be irradiated as the UV spot light  46  to the inner peripheral spot of the substrates  41  and  42 . Therefore, a UV irradiation region of the UV spot light  46  is a spot. However, since the substrates  41  and  42  are rotated, the UV cure resin  47  proceeding to inner periphery of the substrates  41  and  42  is cured at a radial position of the substrate  42 , where the UV spot light  46  is irradiated and thus, an annular cured region  48  in which the UV cure resin  47  is cured, is formed. In this modified method, by merely changing the radial position of the UV spot light  46  on the substrate  42 , position for stopping diffusion of the UV cure resin  47  to the inner periphery of the substrates  41  and  42  can be easily controlled. Furthermore, the optical fiber  45  can be introduced into a narrow space. Hence, in a bonding device, a coating section for coating the UV cure resin  47  or a diffusion section for diffusing the UV cure resin  47  can be restrained to a size substantially equal to that of prior art. 
     A still further modification of the method of the first embodiment is described with reference to  FIG. 6 . In this modification, when UV rays  59  are irradiated to a whole of a substrate  52 , so as to wholly cure UV cure resin  54  after the UV cure resin  54  has been fully diffused between substrates  51  and  52 , the integral substrates  51  and  52  are gripped between two flat plates, e.g., a substrate holder  56  and a transparent disk-like glass plate  57  so as to be subjected to a load. More specifically, the UV cure resin  54  is already filled between the substrate  51  having a reflective layer  53  formed thereon and the dummy substrate  52 . In the method of the present invention, since the UV cure resin  54  is diffused between the substrates  51  and  52  while UV rays are being irradiated to an innermost peripheral region of the substrate  52 , an annular cured region  55  in which the UV cure resin  54  is cured is formed at a boundary of the innermost peripheral region of the substrate  52 . In this state, the substrates  51  and  52  are placed on the substrate holder  56  and then, the glass plate  57  is placed on the substrates  51  and  52  so as to apply the load to the substrates  51  and  52  such that tilt of the substrates  51  and  52  is reduced for correction. Since UV rays are transmitted through the glass plate  57 , the UV rays  59  are irradiated to the whole of the substrate  52  from above the glass plate  57  by a UV lamp  58 , so as to wholly cure the UV cure resin  54 . A tilt of a thus obtained optical disk is made smaller than that in the case where a load is not applied to the substrates  51  and  52 . This effect of reducing tilt of the optical disk becomes greater as the substrates  51  and  52  became thinner. 
     (Second Embodiment) 
     A method in which when UV cure resin is diffused between two substrates, a filling position of the UV cure resin at an innermost periphery of the substrates is controlled by providing a detection means or detector for detecting a presence and absence of diffusion of the UV cure resin according to a second embodiment of the present invention, is described with reference to  FIGS. 7A–9B . In  FIG. 7A , substrates  61  and  62  are identical with each other and each of the substrates  61  and  62  is a polycarbonate substrate produced by injection molding and having a thickness of 0.6 mm and a diameter of 120 mm. A central bore  20  of each of the substrates  61  and  62  has a diameter of 15 mm. Information signals are preliminarily recorded as pits on one face of each of the substrates  61  and  62 , which act as signal recording faces. A reflective layer  63  mainly consisting of aluminum and having a thickness of about 100 nm is provided on the signal recording face of the substrate  61 . Thus, by irradiating a laser beam from the other face of the substrate  61 , playback of the information signals can be performed. The substrate  62  is a dummy substrate which is bonded to the substrate  61 , so as to increase its mechanical strength. A reflective layer is not provided on the dummy substrate  62 . 
     Initially, UV cure resin  65  is dripped from a nozzle  64  so as to be coated on the reflective layer  63  of the substrate  61  annularly by rotating the substrate  61  at low speed, for example, at 20 to 120 rpm with the motor  150 . Alternatively, the nozzle  64  may also be rotated and the substrate  61  fixed. Then, the dummy substrate  62  is brought into close contact with the substrate  61  such that not only the central bores  20  of the substrates  61  and  62  are made concentric with each other, but the signal recording faces of the substrates  61  and  62  confront each other. 
     Thereafter, owing to a weight of the dummy substrate  62  and capillarity, UV cure resin  66  is diffused between the substrates  61  and  62 . At this time, if the substrates  61  and  62  are rotated at high speed, for example, at 300 to 6000 rpm by the motor  150 , diffusion of the UV cure resin  66  to an outer periphery of the substrates  61  and  62  is accelerated and thus, tact time can be reduced greatly. Meanwhile, it has also been proved that thickness of the UV cure resin  66  at the time the substrates  61  and  62  are rotated at high speed can be made more uniform than that at the time the substrates  61  and  62  are not rotated. An excessive portion of the UV cure resin  66  is expelled as droplets  68  from the outermost periphery of the substrates  61  and  62  by centrifugal force of the high-speed rotation of the substrates  61  and  62  as shown in  FIG. 7B . 
     Further, the UV cure resin  66  is also diffused to an inner periphery of the substrates  61  and  62 . In the second embodiment, a detection means or detector for detecting this diffusion of the UV cure resin  66  to the inner periphery of the substrates  61  and  62  is provided. Thus, when the detection means has detected that the UV cure resin  66  is diffused to an innermost peripheral position for stopping diffusion of the UV cure resin  66 , rotation of the substrates  61  and  62  is stopped. Then, UV rays  70  are irradiated to a whole of the substrates  61  and  62  by a UV lamp  69  as shown in  FIG. 7D  so as to cure the UV cure resin  66  wholly, so that bonding of the substrates  61  and  62  is completed and thus, an optical disk  71  is obtained. 
     In order to detect diffusion of the UV cure resin  66  to the inner periphery of the substrates  61  and  62 , a laser beam  67  having a wavelength of, for example, 650 nm is irradiated to an inner peripheral position of the substrates  61  and  62  and a sensor for detecting at least one of reflected light or transmitted light from the substrates  61  and  62  is provided, as will be described later. The sensor detects a change of quantity of the reflected light or quantity of the transmitted light, which change is produced when the UV cure resin  66  has been diffused to the inner periphery of the substrates  61  and  62 . 
     Generally, it is known that in a case where light whose quantity is represented by “K” is incident upon a medium having a refractive index of n2 from a medium having a refractive index of n1, {Kx(n2−n1) 2 /(n2+n1) 2 } is reflected and [Kx(1−(n2−n1) 2 /(n2+n1) 2 }] is transmitted. If the substrates  61  and  62  have a refractive index of 1.6 and light whose quantity is represented by “L 0 ” is incident on the substrate  62  when the UV cure resin  66  is not filled in an inner peripheral region of the substrates  61  and  62 , as shown in  FIG. 7B , a quantity of reflected light R 1  and a quantity of transmitted light T 1  approximately assume (0.18×L 0 ) and (0.82×L 0 ), respectively. On the other hand, if the UV cure resin  66  has a refractive index of 1.6 and the UV cure resin  66  has been filled in the inner peripheral region of the substrates  61  and  62 , as shown in  FIG. 7C , a quantity of reflected light R 2  and a quantity of transmitted light T 2  approximately assume (0.10×L 0 ) and (0.90×L 0 ), respectively, for the following reason. Namely, since the refractive indexes of the substrates  61  and  62  are equal to that of the UV cure resin  66 , reflection at an interface between the substrates  61  and  62  and the UV cure resin  66  is substantially eliminated. Thus, upon filling of the UV cure resin  66  in the inner peripheral region of the substrates  61  and  62 , a quantity of reflected light changes from 18% to 10%, while a quantity of transmitted light changes from 82% to 90%. Such a change of quantity of light may be detected by a photodetector. 
     One example of the photodetector for detecting a change of quantity of reflected light or quantity of transmitted light is shown in  FIG. 8 , in which a dummy substrate  82  is bonded by UV cure resin  84  to a substrate  81  having a reflective layer  83  formed thereon. A laser beam  86  emitted from a laser diode  85  is turned into collimated rays by a collimator lens  87  and then, is transmitted through a polarization beam splitter (PBS)  88  so as to be incident upon the substrate  82  via a quarter-wave plate  89 . The laser beam  86  emitted from the laser diode  85  is linearly polarized light, but is turned into circularly polarized light upon its pass through the quarter-wave plate  89 . Reflected light  92  from the substrates  81  and  82  is again transmitted through the quarter-wave plate  89  so as to be turned back into linearly polarized light. However, since direction of polarization of the linearly polarized light  92  is orthogonal to the laser beam  86  during this process, the linearly polarized light  92  is not transmitted through the PBS  88  but is reflected by the PBS  88  so as to be guided to a photodetector  93 . The photodetector  93  detects a change of intensity of the reflected light  92 , which change is produced when the UV cure resin  84  has been diffused to an inner peripheral region of the substrates  81  and  82 . 
     Meanwhile, if a photodetector  91  is provided on an optical path of transmitted light  90  of the substrates  81  and  82 , a change of quantity of the transmitted light  90  can be detected by the photodetector  93 . However, in the case where only the transmitted light  90  is detected without detecting the reflected light  92 , it is not necessary to provide the PBS  88  and the quarter-wave plate  89 . 
     Another example of the photodetector for detecting a change of quantity of reflected light is shown in  FIGS. 9A and 9B , in which a dummy substrate  102  is bonded by UV cure resin  104  to a substrate  101  having a reflective layer  103 . In the example of  FIG. 8 , the laser beam  86  for detecting filling of the UV cure resin  84  between the substrates  81  and  82  is perpendicularly incident upon the substrate  82 . On the other hand, in  FIGS. 9A and 9B , a laser beam  105  is obliquely incident upon the substrate  102 . Namely, the laser beam  105  emitted from a laser diode  100  is turned into collimated rays by a collimator lens  106  and then, is obliquely incident upon an innermost peripheral portion of the substrate  102 . When the UV cure resin  104  is not diffused to an inner peripheral region of the substrates  101  and  102 , as shown in  FIG. 9A , the laser beam  105  is sequentially reflected by an upper face of the substrate  102 , an interface between a lower face of the substrate  102  and a gap  111  between the substrates  101  and  102 , an interface between the gap  111  and an upper face of the substrate  101  and a lower face of the substrate  101  so as to produce reflected light  107 , reflected light  108 , reflected light  109  and reflected light  110 , respectively. At this time, a photodetector  112  is provided on optical paths of the reflected light  108  and the reflected light  109  so as to obtain intensities of the reflected light  108  and the reflected light  109 . Supposing that the substrates  101  and  102  have a refractive index of 1.6, a sum of the intensities of the reflected light  108  and the reflected light  109  is about 9%. 
     It is assumed here that the UV cure resin  104  also has a refractive index of 1.6, identical with that of the substrates  101  and  102 . When the UV cure resin  104  has been diffused to a filling completion position in the inner peripheral region of the substrates  101  and  102 , as shown in  FIG. 9B , the laser beam  105  is sequentially reflected by the upper face of the substrate  102 , an interface between the lower face of the substrate  102  and the UV cure resin  104 , an interface between the UV cure resin  104  and the upper face of the substrate  101  and the lower face of the substrate  101 , so as to produce reflected light  113 , reflected light  114 , reflected light  115  and reflected light  116 , respectively. However, since the refractive index of the substrates  101  and  102  is identical with that of the UV cure resin  104  as described above, reflection of light at the interfaces between the UV cure resin  104  and the substrates  101  and  102  does not occur, so that intensities of the reflected light  114  and the reflected light  115  are substantially 0%. Therefore, it is possible to detect by the photodetector  112  that the UV cure resin  104  has been filled in the inner peripheral region of the substrates  101  and  102 . In this method, quantities of reflected light proceeding to the photodetector  112  at the time the UV cure resin  104  is not filled in the inner peripheral region of the substrates  101  and  102  and has been filled in the inner peripheral region of the substrates  101  and  102  are 9% and about 0%, respectively. Accordingly, such an advantage is gained that a large ratio of quantities of reflected light can be obtained. 
     Meanwhile, in the second embodiment, the UV cure resin  65  is dripped from the nozzle  64  so as to be coated on the substrate  61  annularly and then, the substrate  62  is brought into close contact with the substrate  61 , such that not only the central bores  20  of the substrates  61  and  62  are made concentric with each other, but the signal recording faces of the substrates  61  and  62  confront each other. However, in the second embodiment, the modification of  FIGS. 3A and 3B  in the first embodiment may be employed, in which the dispenser  24  is inserted into the minute gap between the dummy substrate  22  and the substrate  21  having the reflective layer  23 , so as to annularly fill the UV cure resin  26  between the substrates  21  and  22 , while the substrates  21  and  22  are being rotated at low speed and then, the substrates  21  and  22  are brought into close contact with each other so as to be formed integrally. In the method of  FIG. 7 , if the substrate  62  is brought into close contact with the substrate  61  rapidly, air bubbles may penetrate in between the substrate  62  and the UV cure resin  66 . However, in this modification, since the UV cure resin  25  is filled between the substrates  21  and  22  by the dispenser  24 , the penetration of air bubbles in between the substrates  21  and  22  does not take place and thus, such an advantage is achieved that tact time for bonding the substrates  21  and  22  to each other can be shortened. 
     Meanwhile, in the second embodiment, the refractive index of the substrate is substantially identical with that of the UV cure resin. However, in the second embodiment, the refractive index of the UV cure resin may assume any value other than 1, which is a refractive index of the gap between the substrates. 
     A modification of the method of the second embodiment is described with reference to  FIG. 10 . In this modification, when UV cure resin  128  is diffused between substrates  121  and  122 , by rotating the substrates  121  and  122  at high speed after the substrates  121  and  122  have been brought into close contact with each other, the UV cure resin  128  is sucked from the central bores  20  of the substrates  121  and  122 . More specifically, after the substrate  121  having a reflective layer  123  and the dummy substrate  122  have been brought into close contact with each other, the substrates  121  and  122  are mounted on a boss  125  having a suction port  124 . In this case, the suction port  124  is set so as to be located at a level of a gap between the substrates  121  and  122 . The UV cure resin  128  is diffused between the substrates  121  and  122  by rotating the integral substrates  121  and  122  at high speed. At this time, a suction pump  126  is actuated and sucks the UV cure resin  128  from the suction port  124  so as to diffuse the UV cure resin  128  to inner periphery of the substrates  121  and  122 . In this case, a laser beam  127  is irradiated to an innermost peripheral portion of the substrate  122 . Therefore, by monitoring at least one of reflected light or transmitted light of the laser beam  127  with a photodetector, it is possible to detect that the UV cure resin  128  has been diffused to an innermost peripheral region of the substrates  121  and  122 . When the photodetector detects that the UV cure resin  128  has been diffused to the innermost peripheral region of the substrates  121  and  122 , suction from the central bores  20  of the substrates  121  and  122  by the suction pump  126  and rotation of the substrates  121  and  122  are stopped and then, the step of irradiating UV rays to a whole of the substrate  122  is performed in the same manner as in  FIG. 7D . 
     Another modification of the method of the second embodiment is described with reference to  FIG. 11 . In this modification, when UV rays  138  are irradiated to a whole of a substrate  132 , so as to wholly cure UV cure resin  134  after the UV cure resin  134  has been fully diffused between substrates  131  and  132 , the integral substrates  131  and  132  are gripped between two flat plates, e.g., a substrate holder  135  and a transparent disk-like glass plate  136 , so as to be subjected to a load. More specifically, the UV cure resin  134  is already filled between the substrate  131  having a reflective layer  133  and the dummy substrate  132 . In the method of the present invention, since diffusion of the UV cure resin  134  is detected, diffusion of the UV cure resin  134  is stopped at a predetermined position in an innermost peripheral region of the substrates  131  and  132 . In this state, the substrates  131  and  132  are placed on the substrate holder  135  and then, the glass plate  136  is placed on the substrates  131  and  132  so as to apply the load to the substrates  131  and  132  such that tilt of the substrates  131  and  132  is reduced for correction. Since UV rays are transmitted through the glass plate  136 , the UV rays  138  are irradiated to the whole of the substrate  132  from above the glass plate  136  by a UV lamp  137  so as to wholly cure the UV cure resin  134 . A tilt of a thus obtained optical disk is made smaller than that in the case where a load is not applied to the substrates  131  and  132 . This effect of reducing tilt of the optical disk becomes greater as the substrates  131  and  132  become thinner. 
     Meanwhile, in the above mentioned first and second embodiments, a read-only optical disk in which only the reflective layer is provided on the substrate has been described. However, it is needless to say that the present invention can be also applied to a write-once read multiple optical disk and a rewritable optical disk. 
     Furthermore, in the first and second embodiments, a case in which the reflective layer is provided on only one of the substrates and the other substrate is the dummy substrate has been adopted. However, if the substrate having the reflective layer transmits UV rays therethrough, the reflective layer may be provided on each of the substrates. The inventors of the present invention have confirmed that even if transmittance of a substrate having a reflective layer is about 1%, the substrate is capable of curing the UV cure resin sufficiently and therefore, can be used in the present invention. Meanwhile, in a case where the reflective layer is provided on each of the substrates and both of the substrates are capable of transmitting UV rays therethrough, UV rays may be irradiated from both of the substrates, concurrently. A thin film may generate heat upon absorption of UV rays and one side of the UV cure resin, upon which UV rays are incident, is readily cured. Therefore, if UV rays are irradiated from both of the substrates as described above, a more symmetric optical disk having less tilt can be obtained. Meanwhile, in a case where UV rays are irradiated while a load is being applied to the substrates, as shown in  FIGS. 6 and 11 , the UV rays can be irradiated from both of the substrates, concurrently, if the substrate holders  56  and  135  are also made of translucent material such as glass. 
     Meanwhile, in the first and second embodiments, since the substrate and the dummy substrate are identical with each other, signals are also recorded on the dummy substrate. However, it is needless to say that the signals are not required to be recorded on the dummy substrate. Furthermore, a thickness of the dummy substrate may be different from that of the substrate. Especially, in a case where the dummy substrate has a thickness of not more than 0.2 mm, for example, the dummy substrate has a thickness of about 0.1 mm, a rigidity of the dummy substrate is small. Therefore, it is difficult to form a stopper on the dummy substrate for preventing the radiation cure resin from advancing into the inner peripheral region of the substrates and it is also difficult to perform work for forming grooves, etc. on the dummy substrate itself. In such a case, the method of the present invention in which the radiation cure resin is cured in the inner peripheral region of the substrates is especially effective for preventing the radiation cure resin from advancing into the inner peripheral region of the substrates. 
     Moreover, even if the reflective layer is provided on each of the substrates and neither of the substrates transmits UV rays therethrough, the present invention can be utilized as follows in a case where a transparent region free from the reflective layer exists in an innermost peripheral region or an outermost peripheral region of the substrates. Namely, by using an adhesive resin having both a thermosetting property and UV curing property, etc., UV rays are irradiated from one or both of the substrates, concurrently, so as to cure the transparent region and then, the transparent region is thermoset. 
     As is clear from the foregoing description of the first and second embodiments of the present invention in bonding of the substrates of the optical disk by the radiation cure resin, the filling of the resin at the inner most periphery of the substrates can be controlled to an arbitrary position regardless of whether or not a stopper for preventing the resin from protruding into the central bores of the substrates is provided at the innermost periphery of the substrates or regardless of position of the stopper. Consequently, an optical disk having a high mechanical strength and a good external appearance, in which the filling position of the resin at the innermost periphery of the substrates is fixed, can be manufactured at high yield and at low cost. 
     (Third Embodiment) 
     Hereinafter, third to fifth embodiments of the present invention, in which thickness of the UV cure resin is made uniform such that recording and playback characteristics do not vary even when recording or playback of signals is performed through the UV cure resin, are described. 
     A method of manufacturing an optical disk, according to the third embodiment of the present invention is described with reference to  FIGS. 12A–13 . In  FIG. 12A , a first substrate  201  is a polycarbonate substrate produced by injection molding and having a thickness of 1.1 mm and a diameter of 120 mm. The central bore  20  of the first substrate  201  has a diameter of 15 mm. A signal recording layer  202  is provided on one principal face of the first substrate  201 . In the signal recording layer  202 , information signals are preliminarily recorded as pits and a reflective layer made of aluminum and having a thickness of about 100 nm is further provided on the pits. Meanwhile, a second substrate  203  is a sheetlike polycarbonate substrate having a thickness of 90 μm and a diameter of 120 mm. The central bore  20  of the second substrate  203  has a diameter of 15 mm. Since a signal recording layer is not provided on the second substrate  203 , the second substrate  203  is flat. 
     Initially, UV cure resin  205  is dripped from a nozzle  204  so as to be coated on the signal recording layer  202  of the first substrate  201  annularly by rotating the first substrate  201  at low speed, for example, at 20 to 120 rpm with the motor  150 . Alternatively, the nozzle  204  may also be rotated and the first substrate  201  fixed. Then, the second substrate  203  is brought into close contact with the first substrate  201  such that the central bores  20  of the first and second substrates  201  and  203  confront each other concentrically. 
     Thereafter, owing to a weight of the second substrate  203  and capillarity, UV cure resin  211  is diffused between the first and second substrates  201  and  203 . At this time, if the substrates  201  and  203  are rotated at high speed, for example, at 300 to 6000 rpm by the motor  150 , diffusion of the UV cure resin  211  to an outer periphery of the substrates  201  and  203  is accelerated and thus, tact time can be reduced greatly. An excessive portion of the UV cure resin  211  is expelled as droplets  209  from the outermost periphery of the substrates  201  and  203  by centrifugal force of the high-speed rotation of the substrates  201  and  203 , as shown in  FIGS. 12B and 13 . 
     In a conventional method, the rotation of the substrates is stopped at the time a thickness of the UV cure resin has become substantially uniform upon diffusion of the UV cure resin from the inner periphery to the outer periphery of the substrates. Then, UV rays are irradiated to a whole of the substrates, so as to cure the UV cure resin and thus, bonding of the substrates is completed. However, in this known method, since the UV cure resin is diffused by centrifugal force of high-speed rotation of the substrates, the UV cure resin becomes thin at inner periphery of the substrates and thick at outer periphery of the substrates. The effect of such thickness distribution of the UV cure resin becomes greater in a case where a laser beam spot is reduced in diameter. Namely, in a case where a bluish purple laser beam is employed as a laser beam or an objective lens having an NA as large as 0.85 is employed. For example, supposing that playback of signals is performed from the second substrate  203  via the UV cure resin  211  by using a bluish purple laser beam having a wavelength of 400 nm and an objective lens having an NA of 0.85 in the third embodiment, a sum of a thickness of the second substrate  203  and a thickness of the UV cure resin  211  is required to fall within a range of about ±3 μm from its nominal value of, for example, 100 μm, i.e. 0.1 mm. However, in the known method, since the thickness of the UV cure resin at the outer periphery of the substrates is larger than that at the inner periphery of the substrates, it is difficult to set the sum of the thickness of the second substrate  203  and the thickness of the UV cure resin  211  within the range of ±3 μm from the nominal value. 
     Therefore, in the third embodiment, when the UV cure resin  211  has been spread to the inner periphery of the substrates  201  and  203 , UV rays  208  are irradiated by a UV lamp  206  to only an inner peripheral region of the substrates  201  and  203 , i.e., a UV irradiation region  210 , as shown in  FIG. 12B , so as to preliminarily cure the UV cure resin  211  at the inner peripheral region of the substrates  201  and  203  only. The UV lamp  206  is covered by a UV shielding cover  207 . By irradiating the UV rays  208  to the inner peripheral region of the substrates  201  and  203 , a cure region  212  in which the UV cure resin  211  is cured is formed, as shown in  FIG. 13 . Rotation of the substrates  201  and  203  is further continued for a predetermined period. The UV cure resin  211  at the inner peripheral region of the substrates  201  and  203  is already cured and therefore, does not further thin. However, the UV cure resin  211  from an intermediate peripheral region to an outer peripheral region of the substrates  201  and  203  is not yet cured and becomes thinner upon rotation of the substrates  201  and  203 . By rotating the substrates  201  and  203  for the predetermined period, the thickness of the UV cure resin  211  at the outer peripheral region of the substrates  201  and  203  can be made identical with that at the inner peripheral region of the substrates  201  and  203 . At this stage, the rotation of the substrates  201  and  203  is stopped and UV rays  214  are irradiated to a whole of the substrate  203  by a UV lamp  213 , as shown in  FIG. 12C . As a result, the bonding of the substrates  201  and  203  is completed such that thickness of the UV cure resin  211  is uniform from inner periphery to outer periphery of the substrates  201  and  203  and thus, an optical disk  215  is obtained. 
     In the third embodiment, after the substrates  201  and  203  have been brought into close contact with each other, the substrates  201  and  203  are rotated at 4000 rpm for two seconds. Then, the UV rays  208  are irradiated to the inner peripheral region of the substrates  201  and  203 . Furthermore, after the rotation of the substrates  201  and  203  has been continued for two seconds, the rotation of the substrates  201  and  203  is stopped and the UV rays  214  are irradiated to the whole of the substrate  203 . At this time, the sum of the thickness of the second substrate  203  and the thickness of the UV cure resin  211  assumes 100 μm, 101 μm and 102 μm at the inner peripheral region, the intermediate peripheral region and the outer peripheral region of the substrates  201  and  203 , respectively. Thereby, a excellent distribution of the sum of the thickness of the second substrate  203  and thickness of the UV cure resin  211  results. 
     Meanwhile, as comparative examples, a prior art method has been performed in which the UV cure resin  211  at the inner periphery of the substrates  201  and  203  is not preliminarily cured. As a first comparative example, after the substrates  201  and  203  have been rotated at 4000 rpm for two seconds, the rotation of the substrates  201  and  203  is stopped and then, the UV rays  214  are irradiated to the whole of the substrate  203  so as to cure the UV cure resin  211 . In a thus obtained optical disk, a sum of the thickness of the second substrate  203  and the thickness of the UV cure resin  211  assumes 100 μm, 105 μm and 115 μm at the inner peripheral region, the intermediate peripheral region and the outer peripheral region of the substrates  201  and  203 , respectively. Meanwhile, as a second comparative example, in a case where the rotation of the substrates  201  and  203  is stopped after the substrates  201  and  203  have been rotated at 4000 rpm for four seconds, a sum of the thickness of the second substrate  203  and the thickness of the UV cure resin  211  assumes 90 μm, 100 μm and 103 μm at the inner peripheral region, the intermediate peripheral region and the outer peripheral region of the substrates  201  and  203 , respectively. In both of the comparative examples, the sum of the thickness of the second substrate  203  and the thickness of the UV cure resin  211  varies greatly in an optical disk. 
     Meanwhile, in the third embodiment, the signal recording layer  202  is provided only on the first substrate  201 . However, the present invention can also be applied to manufacture an optical disk of a type in which a translucent signal recording layer is provided on the second substrate  203  and playback of signals of both of the signal recording layers is performed from the second substrate  203 . This is because playback of signals of the signal recording layer  202  of the first substrate  201  from the second substrate  203  should be performed through the UV cure resin  211 . 
     Furthermore, in the third embodiment, a read-only optical disk in which, in the signal recording layer, information signals are recorded as pits and the reflective layer of aluminum is provided on the pits has been described. However, it is needless to say that the present invention can also be applied to a recordable optical disk in which recording and playback of signals can be performed after completion of manufacture of the optical disk. 
     Meanwhile, in the third embodiment, the UV rays  214  are irradiated to the whole of the substrate  203  after a stop of rotation of the substrates  201  and  203 . However, the UV rays  214  may also be irradiated to the whole of the substrate  203  while the substrates  201  and  203  are being rotated. 
     (Fourth Embodiment) 
     A method of manufacturing an optical disk, according to the fourth embodiment of the present invention is described with reference to  FIG. 14 . In bonding of substrates of the optical disk, the thickness of UV cure resin is almost completely determined by the number and period of rotations of the substrates, which rotations of the substrates are performed after the substrates have been brought into close contact with each other. However, the thickness of the UV cure resin may scatter with a change of viscosity of the resin, due to temperature changes and an amount of warpage between prior to and after bonding of the substrates. A method of the present invention is described in which even in such a case, a scatter of thickness of the UV cure resin is restrained, so as to obtain an accurate thickness of the UV cure resin. 
     A step of initially dripping UV cure resin on the first substrate  201  from the nozzle and a step of bringing the second substrate  203  into close contact with the first substrate  201  are performed in the same manner as those of the third embodiment. After the substrates  201  and  203  have been brought into close contact with each other, the substrates  201  and  203  are rotated at high speed by the motor  150  so as to spread the UV cure resin  211 . At this time, a thickness of the UV cure resin  211  at an inner peripheral region of the substrates  201  and  203  is measured by a film thickness meter  220 , as shown in  FIG. 14 . The film thickness meter  220  is of reflection type and is capable of measuring a distance from the signal recording layer  202  to an upper face of the second substrate  203 , i.e., a sum of thickness of the UV cure resin  211  and thickness of the second substrate  203 . When the substrates  201  and  203  are rotated at high speed, the thickness of the UV cure resin  211  at the inner peripheral region of the substrates  201  and  203  decreases gradually. However, in the fourth embodiment, irradiation of the UV rays  208  to the inner peripheral region of the substrates  201  and  203  is adapted to be started at the time the film thickness meter  220  indicates that the sum of the thickness of the UV cure resin  211  and the thickness of the second substrate  203  has reached a preset value. As a result, even if there are variations of viscosity of the UV cure resin  211 , etc., thickness of the UV cure resin  211  at the inner peripheral region of the substrates  201  and  203  can be kept constant at all times. 
     Subsequently, in the same manner as the third embodiment, after rotation of the substrates  201  and  203  has been continued for a predetermined period such that thickness of the UV cure resin  211  at the inner peripheral region of the substrates  201  and  203  is made identical with that at the outer peripheral region of the substrates  201  and  203 , rotation of the substrates  201  and  203  is stopped and then, the UV rays  214  are irradiated to the whole of the substrate  203  by the UV lamp  213 . Consequently, bonding of the substrates  201  and  203  is completed such that the thickness of the UV cure resin  211  is uniform from inner periphery to outer periphery of the substrates  201  and  203  and thus, an optical disk is obtained. 
     Meanwhile, in the fourth embodiment, the signal recording layer  202  is provided only on the first substrate  201 . However, the present invention can also be applied to manufacture an optical disk of a type in which a translucent signal recording layer is provided on the second substrate  203  and playback of signals of both of the signal recording layers is performed from the second substrate  203 . In this case, when the film thickness meter  220  for detecting thickness of the UV cure resin  211  indicates that thickness of the UV cure resin  211  has reached a predetermined value, irradiation of the UV rays  208  to the inner peripheral region of the substrates  201  and  203  is started. 
     Meanwhile, in the fourth embodiment, the film thickness meter  220  is provided only at the inner peripheral region of the substrates  201  and  203 . A modification of the method of the fourth embodiment is described with reference to  FIG. 15 . In this modification, a film thickness meter  221  is provided also at the outer peripheral region of the substrates  201  and  203  in addition to the film thickness meter  220 . In a case where the substrates  201  and  203  are rotated at high speed by the motor  150 , so as to spread the UV cure resin  211  after the substrates  201  and  203  have been brought into close contact with each other, irradiation of the UV rays  208  to the inner peripheral region of the substrates  201  and  203  is started upon detection by the film thickness meter  220  that the sum of the thickness of the UV cure resin  211  and the thickness of the second substrate  203  at the inner peripheral region of the substrates  201  and  203  has reached the preset value. Thereafter, the rotation of the substrates  201  and  203  is continued while a sum of the thickness of the UV cure resin  211  and the thickness of the second substrate  203  at the outer peripheral region of the substrates  201  and  203  is being measured by the film thickness meter  221 . When the film thickness meter  221  indicates that the sum of the thickness of the UV cure resin  211  and the thickness of the second substrate  203  at the outer peripheral region of the substrates  201  and  203  has reached the preset value, rotation of the substrates  201  and  203  is stopped. Then, in the same manner as the fourth embodiment, the UV rays  214  are irradiated to the whole of the substrate  203  by the UV lamp  213 . As a result, bonding of the substrates  201  and  203  is completed such that thickness of the UV cure resin  211  is completely uniform from inner periphery to outer periphery of the substrates  201  and  203 . 
     Meanwhile, in the modification of  FIG. 15 , the film thickness meters  220  and  221  are, respectively, provided at the inner peripheral region and the outer peripheral region of the substrates  201  and  203 , but may also be provided at other radial positions. Furthermore, not less than three film thickness meters may be provided. 
     In addition, a plurality of film thickness meters may be provided at an inner peripheral radial position and other radial positions such that UV rays are irradiated to only the neighborhood of the respective radial positions when readings of the film thickness meters have reached a preset value. 
     (Fifth Embodiment) 
     A method of manufacturing an optical disk, according to the fifth embodiment of the present invention is described with reference to  FIG. 16 . In the third and fourth embodiments, the UV lamp  206  is used as a UV irradiation device and is covered by the UV shielding cover  207 , so as to irradiate the UV rays  208  to the inner peripheral region of the substrates  201  and  203  concentrically with the central bores  20  of the substrates  201  and  203 . In the fifth embodiment, the UV irradiation device for irradiating the UV rays to the inner peripheral region of the substrates  201  and  203  is constituted by a UV light source  230  and an optical fiber  231 . 
     In the same manner as the third embodiment, the UV cure resin  205  is initially dripped on the first substrate  201  from the nozzle  204  and then, the second substrate  203  is brought into close contact with the first substrate  201 . Subsequently, the substrates  201  and  203  held in close contact with each other are rotated at a high speed by the motor  150 , so as to spread the UV cure resin  211 . After a predetermined period, a UV spot light  232  is irradiated from the optical fiber  231  to the inner peripheral region of the substrates  201  and  203 . The UV spot light  232  is produced by the UV light source  230  provided at a location other than immediately above the second substrate  203 . Since the substrates  201  and  203  are rotated at the high speed, an annular cured region in which the UV cure resin  211  is cured is formed at an inner periphery of the substrates  201  and  203  concentrically with the central bores  20 . 
     Then, in the same manner as the third embodiment, after the thickness of the UV cure resin  211  at the inner peripheral region of the substrates  201  and  203  has been made identical with that at the outer peripheral region of the substrates  201  and  203  by continuing rotation of the substrates  201  and  203  for a predetermined period, rotation of the substrates  201  and  203  is stopped and the UV rays  214  are irradiated to the whole of the substrate  203  by the UV lamp  213 . As a result, bonding of the substrates  201  and  203  is completed such that thickness of the UV cure resin  211  is uniform from inner periphery to outer periphery of the substrates  201  and  203 , so that an optical disk is obtained. 
     By curing the UV cure resin  211  at the inner peripheral region of the substrates  201  and  203  by the UV spot light  232 , such advantages are gained that the UV light source  230  can be spaced away from a location for coating the UV cure resin  205 , a location for bringing the substrates  201  and  203  into close contact with each other, and a location for rotating the substrates  201  and  203 . There is no risk of cure of the UV cure resin due to leakage of the UV rays, thus resulting in facilitation of a layout of parts of an apparatus for manufacturing the optical disk. 
     It is needless to say that the film thickness meter of the fourth embodiment may be added in the fifth embodiment. 
     A modification of the method of the fifth embodiment is described with reference to  FIG. 17 . In this modification, after the UV spot light  232  has been irradiated to the inner peripheral region of the substrates  201  and  203  from the optical fiber  231  in the same manner as the fifth embodiment, the UV spot light  232  is gradually displaced to outer periphery of the substrates  201  and  203  in the direction shown by the arrow A and thus, the UV cure resin  211  can be cured wholly from the inner periphery to the outer periphery of the substrates  201  and  203 . At an initial stage of processing, thickness of the UV cure resin  211  is small at inner periphery of the substrates  201  and  203  but large at outer periphery of the substrates  201  and  203 . Therefore, if the UV spot light  232  is displaced to outer periphery of the substrates  201  and  203  with lapse of time, thickness of the UV cure resin  211  can be made uniform from the inner periphery to the outer periphery of the substrates  201  and  203 . In this modification, the UV lamp  213  for irradiating the UV rays  214  to the whole of the substrate  203  in the third embodiment can be eliminated and thus, the apparatus for manufacturing the optical disk can be advantageously structurally simplified. 
     In the third to fifth embodiments, the first substrate  201  has a thickness of 1.1 mm and the second substrate  203  has a thickness of 0.09 mm. However, thickness of the first substrate  201  and that of the second substrate  203  in the third to fifth embodiments are not restricted to the above mentioned values. However, for the following reason, the third to fifth embodiments are quite effective especially when thickness of the second substrate  203  is not more than 0.2 mm. Namely, when thickness of the second substrate  203  is larger than 0.2 mm, the rigidity of the second substrate  203  is large, so that thickness of the UV cure resin  211  depends considerably on the parallelism of the substrates  201  and  203 . However, when thickness of the second substrate  203  is not more than 0.2 mm, the rigidity of the second substrate  203  is small, so that thickness of the UV cure resin  211  is readily determined by the flow of the UV cure resin  211  at the time of high-speed rotation of the substrates  201  and  203 . Thus, the trend is that the UV cure resin  211  is thin at the inner periphery of the substrates  201  and  203 , but thick at the outer periphery of the substrates  201  and  203  which becomes dominant. Accordingly, the third to fifth embodiments are quite advantageous in that thickness of the UV cure resin  211  can be made uniform from the inner periphery to the outer periphery of the substrates  201  and  203 , even when the thickness of the second substrate  203  is not more than 0.2 mm. 
     Meanwhile, in the third to fourth embodiments, after the UV cure resin  205  has been annularly dripped from the nozzle  204  on the first substrate  201 , the second substrate  203  is placed on the first substrate  201  so as to be brought into close contact with the first substrate  201 . However, in the third to fifth embodiments, the arrangement of  FIG. 3  may also be employed in which the first and second substrates  201  and  203  are held in parallel with each other by preliminarily forming a gap of several mm in width therebetween. The needlelike dispenser  24  is then inserted into the gap between the first and second substrates  201  and  203  so as to discharge and fill the UV cure resin between the first and second substrates  201  and  203 , while the first and second substrates  201  and  203  are being rotated at a low speed. 
     Moreover, in the third to fifth embodiments, the arrangement of  FIG. 4  may also be employed in which when the UV cure resin is spread by rotating the substrates  201  and  203  at a high speed after the substrates  201  and  203  have been brought into close contact with each other by the UV cure resin. The UV cure resin between the substrates  201  and  203  is sucked from the central bores  20  of the substrates  201  and  203  by the suction pump  36  such that diffusion of the UV cure resin to inner periphery of the substrates  201  and  203  is accelerated. 
     In the first, third, fourth and fifth embodiments, when radiation is irradiated to the neighborhood of the central bores so as to preliminarily cure the radiation cure resin in the neighborhood of the central bores after the first and second substrates have been formed integrally, the radiation is preferably irradiated radially inwardly of a signal recording region of the optical disk for the following reason. Namely, when radiation is subsequently irradiated to the whole of the outer face of one of the first and second substrates so as to wholly cure the radiation cure resin, the signal recording region is cured uniformly and concurrently, so that variations of signal recording and playback characteristics due to nonuniform curing of the signal recording region can be minimized. 
     As was be seen from the foregoing description of the third to fifth embodiments of the present invention, the thickness of the UV cure resin for bonding the substrates to each other can be made uniform from the inner periphery to the outer periphery of the substrates. Therefore, in an optical disk of a type in which recording or playback is performed through the UV cure resin, recording and playback characteristics can be made uniform from the inner periphery to the outer periphery of the substrates. Consequently, the optical disk can be manufactured at high yield and at low cost.