Abstract:
A method for producing a multilayer optical disk including a plurality of information storage layers is disclosed. The method includes a mastering step of preparing a plurality of metal dies, a replicating step of producing a base plate onto which a desired pattern is transferred using the plurality of metal dies and forming recordable/reproducible information storage layers. The desired pattern includes a wobbling tract pattern that is formed by combining a plurality of wobble patterns each having the same fundamental frequency. The mastering step produces the plurality of metal dies for the plurality of information storage layers having a shape that specifies a track groove whose shape factor differs from one information storage layer to another.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/935,130, filed on Nov. 5, 2007, which is a divisional of U.S. patent application Ser. No. 10/398,092, filed Apr. 2, 2003, now U.S. Pat. No. 7,315,508, which is a Section 371 of International Application No. PCT/JP01/08477, filed Sep. 27, 2001, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an optical storage medium such as an optical disk and a method for producing the optical storage medium, and more particularly, to a multilayer optical disk comprising a plurality of information storage layers with guide grooves (track grooves). 
         [0003]    With the widespread use of compact disks (CD), the optical disk has won a position of an important storage medium. Readable/writable disks, such as CD-R and CD-RW disks, which can not only reproduce but also record information is also widely used. Research and development of optical disks of higher density is at full blast in recent years. 
         [0004]    In increasing the recording density of an optical disk, not only increasing the recording density of one information storage layer but also increasing the number of information storage layers is effective. In DVD family, there is a read-only optical disk that allows information recorded on two information storage layers to be read from one side. In addition to the read-only optical disk having two information storage layers, a readable/writable optical disk having two information storage layers is also under development. 
         [0005]    With reference to  FIG. 1 , a configuration of a readable/writable optical disk having two information storage layers will be explained. 
         [0006]    The readable/writable optical disk shown in  FIG. 1  has two information storage layers made of a phase change material whose optical characteristic changes between amorphous and crystalline phases. On each information storage layer, an amorphous pattern called “mark” is recorded by irradiation with a laser beam. 
         [0007]    The optical disk in  FIG. 1  comprises a first substantially transparent substrate  201  having a track groove (groove) and a second substantially transparent substrate  205  having a track groove which are bonded to each other. A semi-transparent first information storage layer  202  is formed on the first substantially transparent substrate  201 , and a second information storage layer  204  is formed on the second substantially transparent substrate  205 . Both substrates  201  and  205  are placed in such a way that the two information storage layers  202  and  203  face each other, and are bonded to each other by means of a substantially transparent bonding layer  203 . The bonding layer  203  functions as an intermediate layer that separates the first information storage layer  202  from the second information storage layer  203 . 
         [0008]    The track grooves of the respective information storage layers  202  and  204  wobbles at a predetermined frequency. When information is recorded/reproduced, a readout signal having the frequency is detected and a clock signal is generated. The clock signal is used to adjust the rotation speed of the disk with the read/write speed of the disk apparatus. 
         [0009]    In such a multilayer optical disk, compared to the amount of light incident from the optical head of the disk apparatus upon the optical disk, the amount of light that returns from each information storage layer to the photo-detection area of the optical head is quite small. This causes the readout signal obtained from the wobbling of the track grooves on the respective storage layers to become small. 
         [0010]    Furthermore, since the structure of the recording film (light transmittance and reflectance, etc.) differs from one information storage layer to another, the ratio of the amplitude of a readout signal to the noise level of the readout signal (CN ratio: Carrier to Noise Ratio) may vary considerably among a plurality of information storage layers. In this case, it is difficult to reproduce the clock signal accurately based on the wobbling shapes of the track grooves of the respective information storage layers. 
         [0011]    It is an object of the present invention to provide a multilayer optical disk capable of reliably reproducing a clock signal based on wobbling shapes of track grooves of respective information storage layers. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The multilayer optical disk according to the present invention is a multilayer optical disk including a plurality of information storage layers to/from which information is recorded and/or reproduced by an optical head, wherein the plurality of information storage layers are stacked through intermediate layers, each information storage layer includes a wobbling track groove, and shape factors of the track groove of at least one information storage layer of the plurality of information storage layers are different from the shape factors of track grooves of other information storage layers. 
         [0013]    In a preferable embodiment, the shape factors of the track groove include a wobbling amplitude of the track groove along the radial direction of the disk, a depth of the track groove and/or a slope angle of the side wall of the track groove. 
         [0014]    In another preferable embodiment, a different value is given to any one of the shape factors of the track groove for each information storage layer and the amplitude of a readout signal caused by the wobbling of the track groove is thereby adjusted. 
         [0015]    In a further preferable embodiment, the shape factors of the track groove are adjusted in such a way that the CN ratios related to the amplitudes of the readout signals caused by the wobbling of the track grooves of the plurality of information storage layers have substantially the same values and variations in the CN ratios are adjusted to within 30% among the information storage layers. 
         [0016]    In a further preferable embodiment, the wobbling of the track groove contains a basic frequency component, which oscillates in almost single cycle used for reproduction of a clock signal. 
         [0017]    In a further preferable embodiment, the wobbling of the track groove exhibits a shape that varies according to sub-information and contains a higher frequency component than the basic frequency component. 
         [0018]    In a further preferable embodiment, the sub-information contains positional information indicating addresses on the disk. 
         [0019]    In a further preferable embodiment, the wobbling shape of the track groove includes a combination of a sine wave and/or generally rectangular waveform. 
         [0020]    In a further preferable embodiment, the wobbling amplitude of the rectangular waveform is set to be greater than the wobbling amplitude of the sine wave. 
         [0021]    The method for producing a multilayer optical disk according to the present invention is a method for producing a multilayer optical disk including a plurality of information storage layers comprising a mastering step of preparing a plurality of metal dies and a replicating step of producing a substrate onto which a desired pattern is transferred using the plurality of metal dies and forming recordable/reproducible information storage layers, wherein in the mastering step, a plurality of metal dies for the plurality of information storage layers having a shape that specifies a track groove whose at least one shape factor differs from one information storage layer to another are produced. 
         [0022]    In a preferable embodiment, the mastering step includes a step of preparing a plurality of substrates to which a photosensitive material is applied, a recording step of forming a latent image of a pattern including the wobbling track groove by irradiating a selected area of the photosensitive material with recording light, a developing step of producing a plurality of master disks having the above-described pattern by developing the photosensitive material, and a step of producing a plurality of metal dies based on the plurality of master disks, wherein in the recording step, the amount of deflection of the recording light along the radial direction of the disk is changed for each substrate and the amplitude of wobbling of the track groove is thereby changed for each information storage layer. 
         [0023]    In a preferable embodiment, the mastering step includes a step of preparing a plurality of substrates to which a photosensitive material is applied, a recording step of forming a latent image of a pattern including the wobbling track groove by irradiating a selected area of the photosensitive material with recording light, a developing step of producing a plurality of master disks having the above-described pattern by developing the photosensitive material, and a step of producing a plurality of metal dies based on the plurality of master disks, wherein in the recording step, the thickness of photosensitive material is changed for each of the plurality of metal dies. 
         [0024]    In a preferable embodiment, the mastering step includes a step of preparing a plurality of substrates to which a photosensitive material is applied, a recording step of forming a latent image of a pattern including the wobbling track groove by irradiating a selected area of the photosensitive material with recording light, a developing step of producing a plurality of master disks having the above-described pattern by developing the photosensitive material, and a step of producing the plurality of metal dies based on the plurality of master disks, wherein the slope angle of the side wall of the track groove is changed for each of the plurality of metal dies. 
         [0025]    In a preferable embodiment, the slope angle of the side wall of the track groove is changed by applying heating processing to the master disk after the developing step. 
         [0026]    In a preferable embodiment, the slope angle of the side wall of the track groove is changed by applying plasma processing to the metal die after the mastering step and before the replicating step. 
         [0027]    In a preferable embodiment, argon and/or oxygen is used for the plasma processing. 
         [0028]    In a preferable embodiment, the recording light is deflected according to a pattern combining a sine waveform and rectangular waveform in the recording step. 
         [0029]    In a preferable embodiment, the amount of deflection of the recording light is changed between the sine waveform section and the rectangular waveform section. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0030]    In the drawings: 
           [0031]      FIG. 1  is a schematic view illustrating a structure of a conventional optical disk; 
           [0032]      FIG. 2  is a schematic view illustrating a track groove of a multilayer optical disk according to the present invention; 
           [0033]      FIG. 3  illustrates details of the above-described track groove; 
           [0034]      FIG. 4  is a cross-sectional view showing a structure of a first embodiment of a multilayer optical disk according to the present invention; 
           [0035]      FIG. 5  is a plan view illustrating wobbling patterns of track grooves of the first embodiment; 
           [0036]      FIG. 6  is a perspective view illustrating wobbling patterns of the track groove of the first embodiment; 
           [0037]      FIG. 7  is a process sectional view illustrating a method for producing the multilayer optical disk according to the first embodiment; 
           [0038]      FIG. 8  is a process sectional view illustrating a first replicating step according to the first embodiment; 
           [0039]      FIG. 9  is a process sectional view illustrating a second replicating step according to the first embodiment; 
           [0040]      FIG. 10  is a cross-sectional view showing a structure of a second embodiment of a multilayer optical disk according to the present invention; 
           [0041]      FIG. 11  is a graph illustrating a relationship between a depth of a track groove and a signal amplitude; 
           [0042]      FIG. 12  is a cross-sectional view showing a structure of a third embodiment of a multilayer optical disk according to the present invention; 
           [0043]      FIG. 13  is a plan view showing another example of track grooves; 
           [0044]      FIG. 14(   a ) is a plan view showing wobble pattern elements; 
           [0045]      FIG. 14(   b ) is plan view illustrating 4 types of wobble patterns formed by combining the above-described elements; 
           [0046]      FIG. 15  illustrates a basic configuration of an apparatus capable of identifying the type of a wobble pattern based on a wobble signal whose amplitude changes according to the wobbling of a track groove; 
           [0047]      FIG. 16  is a waveform diagram showing a wobble pattern, wobble signal and pulse signal of a track groove; and 
           [0048]      FIG. 17  illustrates a circuit configuration that separates a pulse signal and clock signal from a wobble signal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    The multilayer optical disk according to the present invention can accurately reproduce a signal based on the wobbling of a track groove by changing shape parameters (shape factors) of the track groove for each information storage layer. 
         [0050]    With reference now to the attached drawings, a configuration of a track groove of the optical disk will be explained in detail below. 
         [0051]    On a recording plane  1  of the optical disk medium according to the present invention, a track groove  2  is formed in a spiral shape as shown in  FIG. 2 .  FIG. 3  shows an enlarged view of part of the track groove  2 . In  FIG. 3 , a disk center (not shown) exists below the track groove  2  and a disk radial direction is indicated by the arrow a. The arrow b points a direction in which a read/write light beam spot, being formed on the disk, moves as the disk is rotated. In the following description, a direction parallel to the arrow a will be herein referred to as a “disk radial direction” (or “radial direction” simply), while a direction parallel to the arrow b will be herein referred to as a “tracking direction”. 
         [0052]    In a coordinate system in which the light beam spot is supposed to be formed at a fixed position on the disk, a part of the disk irradiated with the light beam (which will be herein referred to as a “disk irradiated part”) moves in the direction opposite to the arrow b. 
         [0053]    Hereinafter, the X-Y coordinate system illustrated in  FIG. 3  will be considered. In an optical disk according to the present invention, the Y coordinate of a position on a side face  2   a  or  2   b  of the track groove changes periodically as the X coordinate thereof increases. Such a periodic positional displacement on the groove side face  2   a  or  2   b  will be herein referred to as the “wobble” or “wobbling” of the track groove  2 . A displacement in the direction pointed by the arrow a will be herein referred to as a “radially-outwardly displacement”, while a displacement in the direction opposite to the arrow a will be herein referred to as a “radially-inwardly displacement”. Also, in the Figure, one wobble period is identified by “T”. The wobble frequency is inversely proportional to one wobble period T and is proportional to the linear velocity of the light beam spot on the disk. 
         [0054]    In the illustrated example, the width of the track groove  2  is constant in the tracking direction (as indicated by the arrow b). Accordingly, the amount to which a position on the side face  2   a  or  2   b  of the track groove  2  is displaced in the disk radial direction (as indicated by the arrow a) is equal to the amount to which a corresponding position on the centerline of the track groove  2  (as indicated by the dashed line) is displaced in the disk radial direction. For this reason, the displacement of a position on the side face of the track groove in the disk radial direction will be herein simply referred to as the “displacement of the track groove” or the “wobble of the track groove”. It should be noted, however, that the present invention is not limited to this particular situation where the centerline and the side faces  2   a  and  2   b  of the track groove  2  wobble to the same amount in the disk radial direction. Alternatively, the width of the track groove  2  may change in the tracking direction. Or the centerline of the track groove  2  may not wobble but only the side faces of the track groove may wobble. 
         [0055]    The optical disk of the present invention comprises a plurality of information storage layers and the above-described wobbling track groove is formed on each information storage layer. A main feature of the multilayer disk of the present invention is that shape factors of a track groove are not uniform but different from one information storage layer to another. 
         [0056]    Hereunder, embodiments in which three types of shape parameters of a track groove are adjusted will be explained more specifically. 
       Embodiment 1 
       [0057]    First, with reference to  FIGS. 4 and 5 , a first embodiment of the optical disk of the present invention will be explained. 
         [0058]    As shown in  FIG. 4 , the multilayer optical disk according to this embodiment is a multilayer optical disk including a plurality of information storage layers to/from which information is recorded and/or reproduced by an optical head. The plurality of information storage layers are stacked through intermediate layers and each information storage layer has a surface with a wobbling track groove and a storage layer.  FIG. 5  shows how track grooves wobble. The width of the track groove is approximately 0.10 to 0.25 μm and the depth is approximately 10 to 25 nm. 
         [0059]      FIG. 4  will be referenced again. More specifically, the optical disk of this embodiment comprises a polycarbonate sheet (80 μm thick)  301 , a first UV cure resin layer (10 μm thick)  302 , a first semitransparent information storage layer (10 μm thick)  303 , a second UV cure resin layer (20 to 40 μm thick)  306 , a second information storage layer  304  and a polycarbonate substrate  305 , in the order from the side on which the optical head is placed. 
         [0060]    The first UV cure resin layer  302  has track grooves with approximately 0.32 μm track pitches and pits formed in the inner area. Pit locations represent non-rewritable information. 
         [0061]    Both the first information storage layer  303  and second information storage layer  304  contain a phase change recording materials mainly composed of GeTeSb. 
         [0062]    The second UV cure resin layer  306  bonds the first information storage layer  303  and second information storage layer  304 , and also functions as an intermediate layer that separates the two information storage layers. 
         [0063]    On a first plane of the polycarbonate substrate  305 , track grooves with approximately 0.32 μm pitches are formed spirally or concentrically and non-rewritable pits are provided in the inner area of the disk. 
         [0064]    Recording/reproduction of the second information storage layer  304  by irradiation with a laser beam is conducted through the first information storage layer  303 . Thus, the first information storage layer  303  has a transmittance of approximately 50% with respect to the laser beam used for recording/reproduction. 
         [0065]    The various feature sizes such as thickness of each layer, width and depth of each track groove do not reflect their actual sizes. For example, the depth of the track groove is no more than a fraction of a wavelength of the laser beam used for recording/reproduction, while the thickness of the second UV cure resin layer  306  (that is, distance between the upper and lower information storage layers) ranges from several tens of times to even 100 times the above-described wavelength. 
         [0066]    A feature of the optical disk of this embodiment is that the amplitude of wobbling of a track groove of the first information storage layer  303  is different from the amplitude of wobbling of a track groove of the second information storage layer  304 . This point will be explained in more detail below. 
         [0067]    The track grooves of the respective information storage layers  303  and  304  wobbles in an almost sine waveform at a single frequency as shown in  FIG. 5 . A frequency of a clock signal is defined based on this frequency. That is, the wobbling frequency of a track groove represents clock information. Here, the “amplitude of wobbling” of the track groove refers to the amplitude of wobbling measured along the disk radial direction. 
         [0068]    The above-described clock information is reproduced by the optical head with a numerical aperture of 0.85, which emits a laser beam with a wavelength of 405 nm. More specifically, the laser beam reflected from the optical disk is detected by a photo-detection area divided into two portions to the right and left with respect to the track direction to produce a difference between the two signals (push-pull signal). This push-pull signal is used to control an optical pickup in such a way that a laser beam spot keeps track of the track groove. The push-pull signal includes a frequency component that follows the wobbling of the track groove, but the frequency band of the wobbling is higher than the frequency band of the signal component, which is important in tracking control. Thus, applying appropriate filtering to the push-pull signal makes it possible to separate/detect the clock information. It is possible to provide a portion where the track groove breaks at some region of the track groove to record information other than the clock information in that portion. 
         [0069]    In this embodiment, as shown in  FIG. 6 , the two information storage layers have identical pitches of track grooves (track pitches), but the amplitude of wobbling W 1  of the track groove of the first information storage layer  303  and the amplitude of wobbling W 2  of the track groove of the second information storage layer  304  are different. 
         [0070]    As a comparative example, assuming that the amounts of wobbling of the respective information storage layers W 1  and W 2  are equivalent to approximately 4% of the pitch of the track groove, the ratio of the intensity of the reflected light which is reflected by the first information storage layer  303  and detected by the photo-detection area of the optical head to the intensity of the laser light incident from the optical head was approximately 7%, while the ratio of the intensity of the reflected light which is reflected by the second information storage layer  304  and detected by the photo-detection area of the optical head was approximately 5%. 
         [0071]    Furthermore, when data in the second information storage layer  304  is recorded/reproduced, since the first information storage layer  303  exists in the path of the laser beam, an optical signal reflected by the second information storage layer  304  includes noise caused by the presence of the first information storage layer  303 . 
         [0072]    Thus, when the intensity of the reflected light from the second information storage layer  304  decreases and noise increases, the CN ratio of the reproduced signal decreases. Furthermore, reading the clock information based on the wobbling of the track groove satisfactorily requires a CN ratio of 30 dB or more. A relationship between the amplitude of wobbling of the track groove W and the CN ratio is shown in Table 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Amount of 
                 Amount of 
               
               
                   
                 wobbling 
                 wobbling 
               
               
                   
                 10 nm 
                 15 nm 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1st information storage layer 
                 33 dB 
                 — 
               
               
                   
                 (W1) 
               
               
                   
                 2nd information storage layer 
                 28 dB 
                 33 dB 
               
               
                   
                 (W2) 
               
               
                   
                   
               
             
          
         
       
     
         [0073]    As is appreciated from Table 1, when both the amplitude of wobbling of the track groove W 2  of the second information storage layer  304  and the amplitude of wobbling of wobbling of the track groove W 1  of the first information storage layer  303  were equally set to 10 nm, the CN ratio of the signal reproduced from the second information storage layer  304  fell below 30 dB. However, when the amplitude of wobbling W 1  was set to 10 nm and the amplitude of wobbling W 2  was set to 15 nm, the CN ratio of 33 dB was obtained for the signals reproduced from both information storage layers. 
         [0074]    If there is a considerable difference in the amount of light reflected by the respective information storage layers and detected by the photo-detection area of the optical head depending on the information storage layers, a problem occurs when the focal point of the optical head is moved between the two information storage layers when reading/writing data to/from different information storage layers. To avoid this problem, it is desirable to adjust the amount of the detected light in such a way that the amount of light with respect to the information storage layer where the amount of the detected light reaches a maximum is not more than twice the amount of the light with respect to the information storage layer where the amount of the detected light reaches a minimum. 
         [0075]    It is preferable to adjust a variation in the CN ratio with respect to the amplitude of the reproduced signal caused by the wobbling of the track groove to within 30% between the information storage layers. 
         [0076]    Then, the method of producing the multilayer optical disk of this embodiment will be explained with reference to  FIG. 7 . 
         [0077]    First, a first glass substrate (thickness: for example, approximately 6 mm)  501  and a second glass substrate (thickness: for example, approximately 6 mm)  502  are cleaned. Then, a first master disk  503  with a photoresist (thickness: approximately 10 to 40 mm) applied onto the first glass substrate  501  and a second master disk  504  with a photoresist (thickness: approximately 10 to 40 mm) applied onto the second glass substrate  502  are prepared. The photoresists formed on the glass substrates  501  and  502  have substantially the same thickness. 
         [0078]    Then, recording/developing steps of transferring predetermined patterns to the two master disks  503  and  504  are conducted. More specifically, a laser beam with a wavelength of 248 nm is focused on the photoresist for exposure. While rotating the master disks  503  and  504 , the position of the beam spot of the laser light on the photoresist is displaced in a radial direction of the disk. This periodical displacement is produced by deflecting the laser beam. In this way, a pattern of a wobbling track groove is transferred to the photoresist. By the way, modulating the intensity of the laser beam makes it possible not only to stop the formation of the track groove but also to control a physical shape such as the width of the track groove. Thus, a desired pattern including the track groove is transferred to the photoresist as a latent image. Then, the predetermined pattern is given to the photoresist after development and a first master disk  505  and second master disk  506  are produced. 
         [0079]    A Ni thin film is deposited on the master disks  505  and  506  using a sputtering method. Then, Ni electroforming is performed using the Ni thin film as an electrode and a Ni layer of approximately 300 μm in thickness is formed. After removing the Ni layer from the master disks  505  and  506 , the photoresist adhered to the Ni layer is removed and the back of the Ni layer is polished. Unnecessary parts that define the inner diameter and outer diameter of the disk are punched out from this Ni layer and a first stamper  507  and a second stamper  508  that function as metal dies of the optical disk are produced (mastering step). 
         [0080]    Then, a first substrate  509  on which a first information storage layer is formed is produced using the first stamper  507 . This step (first replicating step) will be explained with reference to  FIG. 8 . 
         [0081]    First, a polycarbonate substrate master  601  will be produced by injection molding using the first stamper  507 . A concavo-convex pattern of the first stamper  507  is transferred to the surface of the substrate master  601 . An aluminum film is deposited on the pattern transferred surface of the substrate master  601  using the sputtering method. 
         [0082]    On the other hand, a circular sheet  602  made of a polycarbonate sheet of approximately 80 μm in thickness is prepared and UV cure resin is discharged onto this circular sheet  602  in a doughnut shape. 
         [0083]    Then, the circular sheet  602  is laid on the substrate master  601  with the surface on which the aluminum film is formed facing the circular sheet  602 . By rotating the substrate master  601 , the extra UV cure resin is removed by a centrifugal force. Thus, the thickness of the UV cure resin between the substrate master  601  and the sheet  602  is adjusted to approximately 10 μm. 
         [0084]    After hardening the UV cure resin by irradiation with ultraviolet rays, the hardened UV cure resin and the sheet  602  are removed from the substrate master  601 . The UV cure resin and the sheet film  602  are bonded to constitute a sheet substrate  603 . On the surface of this sheet substrate  603 , the pattern of the first stamper  507  is transferred. 
         [0085]    On the pattern transferred surface of the sheet substrate  603 , a first dielectric film (thickness: approximately 50 to 1000 nm)  604 , a recording film (thickness: approximately 3 to 50 nm)  605 , a second dielectric film (thickness: approximately 50 to 1000 nm)  606  and a metal reflecting film (thickness: approximately 0 to 40 nm)  607  are stacked in that order. The metal reflecting film  607  can be omitted. Both the first dielectric film  604  and second dielectric film  606  are made of a material predominantly composed of ZnS and the recording film  605  is formed of a phase change recording material predominantly composed of GeTeSb. The metal reflecting film  607  is made of an Ag alloy film and has a thickness, which makes it semi-transparent to the laser beam used for recording/reproduction. All the layers that constitute this multilayer (information recording film) are preferably deposited using a sputtering method. 
         [0086]    The recording film  605  formed using the sputtering method is in an amorphous state immediately after film formation. To initialize the recording film  605 , a laser beam is focused and incident upon the recording film  605  to crystallize the recording film  605 . The first substrate  509  is produced in this way. 
         [0087]    Then, a second substrate  510  will be produced using the above-described second stamper  508 . This step (second replicating step) will be explained with reference to  FIG. 9 . 
         [0088]    First, a polycarbonate substrate master  701  of approximately 1.1 mm in thickness will be produced by injection molding using the second stamper  508 . A concavo-convex pattern of the second stamper  508  is transferred to the surface of the substrate master  701 . 
         [0089]    On the pattern transferred surface of the substrate master  701 , a metal reflecting film  705 , a second dielectric film  704 , a recording film  703  and a first dielectric film  702  are stacked in that order. These multilayers (information recording films) are preferably formed using a sputtering method. 
         [0090]    The metal reflecting film  705  is made of a metal film predominantly composed of aluminum and the first dielectric film  702  and the second dielectric film  704  are formed of a film predominantly composed of ZnS. The recording film  703  is formed of a phase change recording material mainly composed of GeTeSb. All these layers constituting the multilayer (information recording film) are preferably deposited using the sputtering method. 
         [0091]    As in the case with the first substrate, a laser beam is condensed and irradiated onto the recording film  703  to crystallize and initialize the recording film  703 . The second substrate  510  is produced in this way. 
         [0092]      FIG. 7  will be referenced again. 
         [0093]    UV cure resin is discharged concentrically onto the surface on which the information storage layer of the first substrate  509  is formed. Then, the substrate with the first information storage layer is laid on the substrate  510  with the second information storage layer in such a way that the surface on which the information storage layer of the substrate  510  is formed faces the substrate with the first information storage layer. By rotating these substrates and shaking off the extra UV cure resin by a centrifugal force, the thickness of the UV cure resin is adjusted to approximately 20 to 40 μm. 
         [0094]    By hardening the UV cure resin by irradiation with ultraviolet rays, both substrates are bonded together and an intermediate layer for separating both information storage layers is formed. The multilayer optical disk  511  having two information storage layers is formed in this way. 
         [0095]    By the way, the figure shows that the position of the track groove of the upper information storage layer perfectly aligns with the position of the track groove of the lower information storage layer, but this is not required in practice. Since tracking control is performed for each information storage layer, the positions of track grooves need not have a specific relationship with each other among different information storage layers. 
         [0096]    This embodiment requires that the amplitude of wobbling of a track groove should differ from one information storage layer to another. Thus, the amount of deflection of the laser beam is differentiated between the step of recording a track groove pattern on the photoresist on the first master disk  505  and the step of recording a track groove pattern on the photoresist on the second master disk  506 . More specifically, the amount of deflection of the laser beam is adjusted for the first master disk  505  so that the amplitude of wobbling shown in  FIG. 6  becomes W 1 . In contrast, the amount of deflection of the laser beam is adjusted for the second master disk  506  so that the amplitude of wobbling shown in  FIG. 1  becomes W 2  (≠W 1 ). This gives the first substrate  509  with the amplitude of wobbling of the track groove adjusted to W 1  and the second substrate  510  with the amplitude of wobbling of the track groove adjusted to W 2 . 
       Embodiment 2 
       [0097]    With reference to  FIG. 10 , a second embodiment of the multilayer optical disk according to the present invention will be explained. 
         [0098]    The multilayer structure of the multilayer optical disk according to this embodiment is substantially identical to that of the multilayer optical disk according to the first embodiment. However, in the case of the optical disk of this embodiment, the depth D 1  of a track groove of a first information storage layer  801  is different from the depth D 2  of a track groove of a second information storage layer  802 . 
         [0099]      FIG. 11  is a graph showing a relationship between the amplitude of a readout signal and the depth of the track groove originated from the wobbling of the track groove assuming that the wavelength of a laser beam emitted from the optical head is λ. The depth of the track groove is converted to an optical path length. As is apparent from the graph in  FIG. 11 , the amplitude of the readout signal reaches a maximum when the depth of the track groove is λ/8 and the amplitude of the readout signal decreases as the depth approximates to λ/4. The depth D 1  of the track groove of the first information storage layer and the depth D 2  of the track groove of the second information storage layer are preferably set to λ/8 or less. 
         [0100]    In the case of a multilayer optical disk, there is a tendency that noise included in the readout signal from the second information storage layer  802  becomes greater than noise included in the readout signal from the first information storage layer  801 . Assuming that the depth of the track groove D 1 =the depth D 2  of the track groove, the ratio of intensity of the laser beam reflected by the first information storage layer  801  and detected by the photo-detection area to the intensity of the incident laser beam output from the optical head is approximately 7%, whereas the proportion of intensity of the laser beam reflected by the second information storage layer  802  and detected by the photo-detection area is approximately 5%. 
         [0101]    A CN ratio when the optical head with a numerical aperture of 0.85 that emits a laser beam with a wavelength of 405 nm is used to perform recording/reproduction is shown in Table 2. The required CN ratio is 30 dB or greater. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Depth 16 nm 
                 Depth 18 nm 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1st information storage layer 
                 33 dB 
                 — 
               
               
                   
                 (D1) 
               
               
                   
                 2nd information storage layer 
                 27 dB 
                 31 dB 
               
               
                   
                 (D2) 
               
               
                   
                   
               
             
          
         
       
     
         [0102]    As is apparent from Table 2, by making the depth D 2  of the track groove of the second information storage layer  802  by approximately 10 to 20% greater than the depth D 1  of the track groove of the first information storage layer  801 , almost the same CN ratio of 30 dB or more was obtained for both information storage layers. When the amount of light reflected by the information storage layer and detected by the photo-detection area of the optical head varies considerably depending on the information storage layer, a problem occurs when the focus of the optical head is moved between the two information storage layers when recording/reproduction is performed on different information storage layers. To avoid this problem, it is preferable to adjust the amount of light corresponding to the information storage layer whose amount of light detected becomes a maximum to not more than two times the amount of light corresponding to the information storage layer whose amount of light detected becomes a minimum. 
         [0103]    Then, a method of producing a multilayer optical disk according to this embodiment will be explained. This method of producing a multilayer optical disk is substantially the same as the method of producing a multilayer optical disk according to the first embodiment. This embodiment is different in that the thicknesses of photoresists formed on the first master disk and second master disk are set to D 1  and D 2  (≠D 1 ), respectively. 
         [0104]    By the way, it is also possible to change the depth of the track groove for each information storage layer by making the thickness of the photoresist applied to the respective master disks greater than the depth of a desired track groove and changing the intensity of the laser beam in the exposure step between the two master disks. 
       Embodiment 3 
       [0105]    With reference to  FIG. 12 , a third embodiment of the multilayer optical disk according to the present invention will be explained. 
         [0106]    The multilayer structure of the multilayer optical disk according to this embodiment is substantially the same as that of the multilayer optical disk according to the first embodiment. However, in the case of the optical disk of this embodiment, an angle A 1  of the side wall of the track groove of a first information storage layer  1001  is different from an angle A 2  of the side wall of the track groove of a second information storage layer  1002 . 
         [0107]    The signal of the track groove detected at the optical head is obtained when the reflected light of the track groove interferes with the light refracted by the track groove on the photo-detection area of the optical head. When the angle of the side wall of the track groove increases, this interference state changes, consequently having the effect equivalent to the effect produced from the shallower track groove. 
         [0108]    Assuming that the angle A 1  of the side wall of the track groove=the angle A 1  of the side wall of the track groove, the proportion of intensity of the light reflected by the first information storage layer  1001  and detected by the photo-detection area to the intensity of the light incident from the optical head was approximately 7%, whereas the proportion of intensity of the light reflected by the second information storage layer  1002  and detected by the photo-detection area was approximately 5%. 
         [0109]    Assuming that the depth of the track groove in the respective information storage layers is 17 nm, a CN ratio when the optical head with a numerical aperture of 0.85 that emits a laser beam with a wavelength of 405 nm is used to perform recording/reproduction is shown in Table 3. The required CN ratio is 30 dB or greater. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Angle 45° 
                 Angle 60° 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1st information storage layer 
                 38 dB 
                 33 dB 
               
               
                   
                 (A1) 
               
               
                   
                 2nd information storage layer 
                 33 dB 
                 — 
               
               
                   
                 (A2) 
               
               
                   
                   
               
             
          
         
       
     
         [0110]    As is apparent from Table 3, by making the slope angle A 1  of the side wall of the track groove of the first information storage layer  1001  by approximately 15 degrees greater than the slope angle A 2  of the side wall of the track groove of the second information storage layer  1002 , it was possible to obtain almost the same CN ratio (33 dB) as that of the readout signal from the second information storage layer  1002 . Thus, almost the same CN ratio of 30 dB or more was obtained for the readout signals from both information storage layers. 
         [0111]    When the amount of light reflected by the information storage layer and detected by the photo-detection area of the optical head varies considerably depending on the information storage layer, a problem occurs when the focus of the optical head is moved between the two information storage layers when recording/reproduction is performed on different information storage layers. To avoid this problem, it is preferable to adjust the amount of light corresponding to the information storage layer whose amount of light detected becomes a maximum to not more than two times the amount of light corresponding to the information storage layer whose amount of light detected becomes a minimum. 
         [0112]    Then, a method of producing a multilayer optical disk according to this embodiment will be explained. This method of producing a multilayer optical disk is substantially the same as the method of producing a multilayer optical disk according to the first embodiment. However, this embodiment heats the master disk at a temperature close to the melting point of the photosensitive material (e.g., approximately 120° C.) for several minutes between the step of exposure and development and the step of plating on the photoresist. In this heating step, the surface of the photoresist is soften and the surface region of the photoresist is rounded by surface tension. Adjusting this temperature of heating processing and/or time of heating processing allows the angle of the side wall of the track groove of the master disk to be controlled. 
         [0113]    Even after producing the stamper, exposing the stamper to the plasma of an Ar gas allows the angle of the side wall of the track groove to be changed. This is because the plasma processing allows an electric field to concentrate on the corners of the track groove and allows the corners to be sputtered ahead of other parts and rounded. The variation in the shape of the corners through this processing depends on the time of plasma processing and state of plasma (ion density and ion irradiation energy). Thus, changing the time of plasma processing and power applied used for plasma generation from one stamper to another also makes it possible to produce a multilayer optical disk according to this embodiment. Furthermore, it is also possible to generate plasma using another gas (e.g., oxygen gas) instead of the above-described Ar gas or in addition to the Ar gas. 
         [0114]    In the aforementioned embodiments, shape parameters such as the amplitude of wobbling of the track groove, the depth and the side wall angle of the track groove are adjusted so as to reduce noise of the second information storage layer. However, noise of the first information storage layer may also increase depending on the structure or method of producing a multilayer optical disk. In such a case, it is also possible to adjust the shape parameters of the track groove so as to reduce noise of the first information storage layer. Moreover, it is possible not only to wobble both sides of the track groove but also to wobble each side independently or wobble only one side. 
       Embodiment 4 
       [0115]    The wobbling of a track groove need not be constructed of sine waveforms alone. Part of the wobbling can also be changed to a rectangular waveform as shown in  FIG. 13 . Giving features distinguished from a sine waveform to the track groove allows information other than clock information (sub-information such as address information) to be recorded in the track groove. Making the amount of amplitude of the rectangular waveform greater than the amount of amplitude of the sine waveform allows the sub-information to be detected with high quality. 
         [0116]    In order to give the shapes as shown in  FIG. 13  to the track groove, it is possible to expose the above-described photoresist to light using a deflector (e.g., deflector using electro-optical effects) that allows a laser beam to be deflected in a frequency band not smaller than 10 times a sine waveform frequency. 
         [0117]    Hereunder, the optical disk specified by a combination of displacement patterns of a plurality of types of track groove wobbling structure will be explained in detail with reference to the drawings. 
         [0118]    The surface shape of the track groove according to this embodiment not only comprises simple sine waveforms as shown in  FIG. 3  or  FIG. 5  alone, but also has a portion in the shape different from a sine waveform at least partially. The basic configuration of such a wobbled groove is disclosed in the Specifications of the patent applications filed by the present applicant (Japanese Patent Application No. 2000-6593, Japanese Patent Application No. 2000-187259 and Japanese Patent Application No. 2000-319009). 
         [0119]    Here,  FIG. 14(   a ) and  FIG. 14(   b ) will be referenced.  FIG. 14(   a ) illustrates the four types of basic elements that make up a wobble pattern of the track groove  2 . In  FIG. 14(   a ), smooth sine waveform portions  100  and  101 , a rectangular portion  102  with a steep radially-outward displacement and a rectangular portion  103  with a steep radially-inward displacement are shown. By combining these elements or portions with each other, the four types of wobble patterns  104  through  107  shown in  FIG. 14(   b ) are formed. 
         [0120]    The wobble pattern  104  is a sine wave with no rectangular portions. This pattern will be herein referred to as a “fundamental waveform”. It should be noted that the “sine wave” is not herein limited to a perfect sine curve, but may broadly refer to any smooth wobble. 
         [0121]    The wobble pattern  105  includes portions that are displaced toward the disk outer periphery more steeply than the sine waveform displacement. Such portions will be herein referred to as “radially-outward displaced rectangular portions”. 
         [0122]    In an actual optical disk, it is difficult to realize the displacement of a track groove in the disk radial direction vertically to the tracking direction. Accordingly, an edge actually formed is not perfectly rectangular. Thus, in an actual optical disk, an edge of a rectangular portion may be displaced relatively steeply compared to a sine waveform portion and does not have to be perfectly rectangular. As can also be seen from  FIG. 14(   b ), at a sine waveform portion, a displacement from the innermost periphery toward the outermost periphery is completed in a half wobble period. As for a rectangular portion, a similar displacement may be finished in a quarter or less of one wobble period, for example. Then, the difference between these shapes is easily distinguishable. 
         [0123]    It should be noted that the wobble pattern  106  is characterized by radially-inward displaced rectangles while the wobble pattern  107  is characterized by both “radially-inward displaced rectangles” and “radially-outward displaced rectangles”. 
         [0124]    The wobble pattern  104  consists of the fundamental waveform alone. Accordingly, the frequency components thereof are defined by a “fundamental frequency” that is proportional to the inverse number of the wobble period T. In contrast, the frequency components of the other wobble patterns  105  through  107  include not only the fundamental frequency components but also high-frequency components. Those high-frequency components are generated by the steep displacements at the rectangular portions of the wobble patterns. 
         [0125]    If the coordinate system shown in  FIG. 3  is adopted for each of these wobble patterns  105  through  107  to represent the Y coordinate of a position on the track centerline by a function of the X coordinate thereof, then the function may be expanded into Fourier series. The expanded Fourier series will include a term of a sin function having an oscillation period shorter than that of sin(2πx/T), i.e., a harmonic component. However, each of these wobble patterns includes a fundamental wave component. The frequency of the fundamental waveform will be herein referred to as a “wobble frequency”. The four types of wobble patterns described above have a common wobble frequency. 
         [0126]    In the present invention, instead of writing address information on the track groove  2  by modulating the wobble frequency, the multiple types of wobble patterns are combined with each other, thereby recording various types of information, including the address information, on the track groove. More specifically, by allocating one of the four types of wobble patterns  104  through  107  to each predetermined section of the track groove, four types of codes (e.g., “B”, “S”, “0” and “1”, where “B” denotes block information, “S” denotes synchronization information and a combination of zeros and ones represents an address number or an error detection code thereof) may be recorded. 
         [0127]    Next, the fundamentals of an inventive method for reading information, which has been recorded by the wobble of the track groove, from the optical disk will be described with reference to  FIGS. 15 and 16 . 
         [0128]    First,  FIGS. 15 and 16  will be referred to. 
         [0129]      FIG. 15  illustrates a main portion of a reproducing apparatus, while  FIG. 16  illustrates a relationship between the track groove and a read signal. 
         [0130]    The track groove  1200  schematically illustrated in  FIG. 16  is scanned by a read laser beam  1201  so that the spot thereof moves in the arrowed direction. The laser beam  1201  is reflected from the optical disk to form reflected light  1202 , which is received at detectors  1203  and  1204  of the reproducing apparatus shown in  FIG. 15 . The detectors  1203  and  1204  are spaced apart from each other in a direction corresponding to the disk radial direction and each output a voltage corresponding to the intensity of the light received. If the position at which the detectors  1203  and  1204  are irradiated with the reflected light  1202  (i.e., the position at which the light is received) shifts toward one of the detectors  1203  and  1204  with respect to the centerline that separates the detectors  1203  and  1204  from each other, then a difference is created between the outputs of the detectors  1203  and  1204  (which is “differential push-pull detection”). The outputs of the detectors  1203  and  1204  are input to a differential circuit  1205 , where a subtraction is carried out on them. As a result, a signal corresponding to the wobble shape of the groove  1200  (i.e., a wobble signal  1206 ) is obtained. The wobble signal  1206  is input to, and differentiated by, a high-pass filter (HPF)  1207 . As a result, the smooth fundamental components that have been included in the wobble signal  1206  are attenuated and instead a pulse signal  1208 , including pulse components corresponding to rectangular portions with steeps gradients, is obtained. As can be seen from  FIG. 16 , the polarity of each pulse in the pulse signal  1208  depends on the direction of its associated steep displacement of the groove  1200 . Accordingly, the wobble pattern of the groove  1200  is identifiable by the pulse signal  1208 . 
         [0131]    Next, referring to  FIG. 17 , illustrated is an exemplary circuit configuration for generating the pulse signal  1208  and a clock signal  1209  from the wobble signal  1206  shown in  FIG. 16 . 
         [0132]    In the exemplary configuration illustrated in  FIG. 17 , the wobble signal  1206  is input to first and second band-pass filters BPF 1  and BPF 2 , which generate the pulse and clock signals  1208  and  1209 , respectively. 
         [0133]    Supposing the wobble frequency of the track is fw (Hz), the first band-pass filter BPF 1  may be a filter having such a characteristic that the gain (i.e., transmittance) thereof reaches its peak at a frequency of 4 fw to 6 fw (e.g., 5 fw). In a filter like this, the gain thereof preferably increases at a rate of 20 dB/dec, for example, in a range from low frequencies to the peak frequency, and then decreases steeply (e.g., at a rate of 60 dB/dec) in a frequency band exceeding the peak frequency. In this manner, the first band-pass filter BPF 1  can appropriately generate the pulse signal  1208 , representing the rectangularly changing portions of the track wobble, from the wobble signal  1206 . 
         [0134]    On the other hand, the second band-pass filter BPF 2  has such a filtering characteristic that the gain thereof is high in a predetermined frequency band (e.g., in a band ranging from 0.5 fw to 1.5 fw and including the wobble frequency fw at the center) but is small at the other frequencies. The second band-pass filter BPF 2  like this can generate a sine wave signal, having a frequency corresponding to the wobble frequency of the track, as the clock signal  1209 . 
         [0135]    The track groove in this embodiment has the aforementioned complicated wobble shape, and therefore if the CN ratio of the signal reproduced from one of the two information storage layers is reduced compared to the CN ratio of the signal reproduced from the other, neither a clock signal nor address information, etc., may be reproduced accurately. 
         [0136]    Thus, as explained with respect to Embodiments 1 to 3, by adjusting the shape factors of the track groove layer by layer, it is possible to optimize the CN ratio of the readout signal. 
         [0137]    The present invention is not limited to a multilayer optical disk having two information storage layers, but is also applicable to a multilayer optical disk having three or more information storage layers. Furthermore, the shape factors of the track groove are not limited to the aforementioned elements and the method of changing each shape factor for each information storage layer is not limited to the aforementioned method, either. Moreover, the combination of a plurality of shape factors explained in Embodiments 1 to 3 can also be changed for each information storage layer. 
         [0138]    The present invention provides a type of multilayer optical disk which records information in the wobbling of track grooves, changes elements of changing the amplitude of a signal with respect to the degree of modulation of the signal according to the wobbling of the track groove, that is, the amplitude of the signal with respect to the amount of reflected light (shape factors of the track groove) for each information storage layer, and can thereby read the information recorded according to the wobbling of the track groove at a satisfactory CN ratio.