Patent Publication Number: US-2005122887-A1

Title: Optical information recording medium and method of recording bar code-like marks

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to an optical disc having a plurality of information recording layers and more particularly, to a disc for reproduction only, in which specific information is subjected to write-once recording.  
      2. Description of the Prior Art  
      In a conventional DVD-ROM for reproduction only, bar code-like marks called a burst cutting area (BCA) are worked by burning off a reflective film of an information recording layer with a laser such that write-once recording of data such as a serial number of the medium is performed. For example, Japanese Patent Laid-Open Publication No. 2000-76705 teaches one example of such working method.  
      However, in a known two-layer DVD-ROM disc, in case one layer (layer  1 ) is subjected to BCA working of a laser, the BCA working also causes damage to a reflective film of the other layer (layer  0 ) and thus, working traces are left on the reflective film of the layer  0  through its rupture. In the known DVD-ROM disc, necessary reproduction signal quality can be secured by its comparatively large reproduction spot. However, in a disc having a higher density, for example, a so-called “Blu-ray Disc” having a narrow distance between two information recording layers, such problems arise that shape of edges of BCA marks formed on one information recording layer exerts a strong influence on reproduction signals and the reproduction signals are adversely affected by the working traces left on the other information recording layer.  
       FIG. 4  shows one example of a configuration of a prior art DVD-ROM. This prior art DVD-ROM includes a first substrate  101  formed with pits on its one face, a first information recording layer  102  composed of the pits of the first substrate  101  and a reflective film formed on the pits, a second substrate  105  formed with pits on its one face, a second information recording layer  104  composed of the pits of the second substrate  105  and a reflective film formed on the pits, a space layer  103  disposed between the first and second information recording layers  102  and  104  and BCA marks  106  formed on the reflective film of the first information recording layer  102 . In the prior art DVD-ROM, information is reproduced from a side of the second substrate  105 . Meanwhile, the first information recording layer  102  is made of an Al alloy, while the second information recording layer  104  is made of Au.  
       FIG. 5  mimetically shows the BCA marks  106  formed on the first information recording layer  102  of the prior art DVD-ROM of  FIG. 4  by BCA working. In  FIG. 5 , the striped BCA marks  106  are formed on the first information recording layer  102  made of Al but working traces T are left on the second information recording layer  104  made of Au. It is considered that this phenomenon may happen because thermal diffusion in the second information recording layer  104  is insufficient due to the fact that a thermal conductivity of Au of the second information recording layer  104  is as small as about 1.3 times that of Al of the first information recording layer  102  as shown in Table 1 below.  
                           TABLE 1                                      Thermal Conductivity (W · m −1  · K −1 )                                     Material   0° C.   100° C.   300° C.   700° C.                                         Al   236   240   233   92       Au   319   313   299   272                    
       FIG. 6  shows a reproduced waveform of the BCA marks  106  of FIG.  5 . In  FIG. 6 , a reproduction beam spot assumes a diameter of about 30 μm. Thus, if the reproduction beam spot partially falls on the working traces T, a transmission factor of the second information recording layer  104  rises substantially due to disappearance of the reflective film. As a result, when the BCA marks  106  formed on the first information recording layer  102  are reproduced, the reproduced waveform is distorted at a start end portion A and a final end portion B of each of the BCA marks  106 .  
     SUMMARY OF THE INVENTION  
      Accordingly, an essential object of the present invention is to provide, with a view to eliminating the above mentioned drawbacks of prior art, an information recording medium which is provided with a plurality of information recording layers including one information recording layer subjected to BCA working such that a thermal conductivity of a material of the remaining information recording layers is made as large as not less than 1.5 times that of a material of the one information recording layer.  
      Meanwhile, in the information recording layers, the one information recording layer subjected to BCA working is disposed closest to one of opposite faces of the information recording medium such that BCA working is performed from the one of the opposite faces of the information recording medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      This object and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings in which:  
       FIG. 1  is a mimetic diagram showing a configuration of an optical disc according to an embodiment of the present invention;  
       FIG. 2  is a mimetic diagram of BCA marks formed on the optical disc of  FIG. 1 ;  
       FIG. 3  is a mimetic diagram showing a reproduced waveform of the BCA marks of  FIG. 2 ;  
       FIG. 4  is a mimetic diagram showing a configuration of a prior art optical disc;  
       FIG. 5  is a mimetic diagram of BCA marks formed on the prior art optical disc of  FIG. 4 ; and  
       FIG. 6  is a mimetic diagram showing a reproduced waveform of the BCA marks of  FIG. 5 . 
    
    
      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.  
     DETAILED DESCRIPTION OF THE INVENTION  
      Hereinafter, an embodiment of the present invention is described with reference to the drawings.  FIG. 1  mimetically shows a configuration of an optical disc according to the embodiment of the present invention. This optical disc has a configuration in which a polycarbonate substrate  11  formed with pits on its one face, a first information recording layer  12  composed of the pits of the substrate  11  and a reflective film formed on the pits, a space layer  13  formed with pits on its one face and made of ultraviolet curable resin and having a thickness of about 25 μm, a second information recording layer  14  composed of the pits of the space layer  13  and a reflective film formed on the pits and a cover layer  15  having a thickness of about 75 μm are stacked on one another. In the optical disc, information is reproduced from a side of the cover layer  15 . Meanwhile, bar code-like BCA marks  16  are formed on the reflective film of the first information recording layer  12 . The first information recording layer  12  is subjected to write-once recording of the BCA marks  16  and information is reproduced by reading the first information recording layer  12  with an optical head.  
      In this embodiment of the present invention, an Al alloy is used as a material of the reflective film of the first information recording layer  12 , while an Ag alloy is used as a material of the reflective film of the second information recording layer  14 . When a beam emitted from a YAG laser so as to be shaped into a rectangular form and having a wavelength of about 1064 nm is irradiated in a pulse state to the first information recording layer  12  from a side of the substrate  11  while the optical disc is being rotated synchronously by a rotary mechanism, the BCA marks  16  having intervals modulated by signals to be recorded are formed on the first information recording layer  12 .  
       FIG. 2  mimetically shows the BCA marks  16  formed on the first information recording layer  12  and the BCA marks  16  are formed in a pattern of stripes each having a width of about 10 μm. At this time, no working trace is left on the second information recording layer  14 .  
       FIG. 3  shows a waveform obtained by reproducing the BCA marks  16  of the first information recording layer  12  by using a blue laser beam having a wavelength of 405 nm and an objective lens having a numerical aperture of 0.85. In  FIG. 3 , pits having random lengths are recorded on a ground GND subjected to working of the BCA marks  16  and reproduction signals are usually modulated. Since the reflective film is burnt off by the YAG laser, the BCA marks are reproduced as portions having low reflectance.  
      No working trace is left on the second information recording layer  14  on the ground that a thermal conductivity of the first information recording layer  12  is quite different from that of the second information recording layer  14 . Table 2 below shows thermal conductivities at several temperatures in typical metals usable for the first and second information recording layers  12  and  14 .  
                           TABLE 2                                      Thermal Conductivity (W · m −1  · K −1 )                                     Material   0° C.    100° C.    300° C.   700° C.                                         Al   236   240   233   92       Ag   248   422   407   377       Ni   94   83   67   71       Cu   403   395   381   354                  
 
      When the first information recording layer  12  is subjected to BCA working, the reflective film of the first information recording layer  12  is heated by energy of the YAG laser. Therefore, in case the thermal conductivities of the first and second information recording layers  12  and  14  are compared with each other, it is necessary to make the comparison in several temperature ranges.  
      When Al used for the first information recording layer  12  is compared with Ag used for the second information recording layer  14 , Ag has a thermal conductivity which is 1.75 times that of Al over a wide temperature range. Thus, Al of the first information recording layer  12  is worked by a recording beam through local accumulation of heat, while heat is effectively diffused in the second information recording layer  14 , thereby presumably resulting in no working trace left on the second information recording layer  14 .  
      It follows from this that a thermal conductivity of material of the second information recording layer  14  set to about 1.3 times that of material of the first information recording layer  12  as described earlier in Table 1 of prior art is insufficient and the fine effect is gained if the thermal conductivity of material of the second information recording layer  14  is set to 1.75 times that of material of the first information recording layer  12 . Therefore, it is desirable that the thermal conductivity of material of the second information recording layer  14  is not less than 1.5 times that of material of the first information recording layer  12 .  
      Furthermore, in an optical disc as a comparative example, an Al alloy is used as the first information recording layer  12  and an Ag alloy is used as the second information recording layer  14  and BCA working of the first information recording layer  12  is performed by irradiating from a side of the cover layer  15  with the YAG laser. As a result, upon increase of strength of the YAG laser required for forming the satisfactory BCA marks  16  on the first information recording layer  12 , working traces are left on the second information recording layer  14 . It is considered that such a phenomenon happens because thermal diffusion in the second information recording layer  14  is not performed sufficiently by direct irradiation of a more powerful laser beam over the second information recording layer  14  from the YAG laser.  
      Accordingly, in case one information recording layer subjected to BCA working, for example, the first information recording layer  12  is disposed closeset to one of opposite faces of the optical disc, it is desirable that the laser beam of the YAG laser is irradiated from the one of the opposite faces of the optical disc so as to reach the first information recording layer  12  earlier than the other information recording layer not subjected to BCA working, i.e., the second information recording layer  14 .  
      Meanwhile, in this embodiment of the present invention, the Al alloy is used as the first information recording layer  12  and the Ag alloy is used as the second information recording layer  14  but the same effects can be achieved by other materials if a ratio of thermal conductivities of the materials is similar to that of the Al alloy and the Ag alloy. For example, from the thermal conductivities shown in Table 2, Ni or an alloy mainly consisting of Ni can be used as the first information recording layer  12 , while Cu or an alloy mainly consisting of Cu can be used as the second information recording layer  14 .  
      Moreover, this embodiment of the present invention employs the optical disc reproduced by using the objective lens having a numerical aperture of 0.85 and the blue laser beam having a wavelength of 405 nm. However, the present invention may be applied to also a conventional DVD-ROM in which it is possible to suppress formation of working traces on the other information recording layer (e.g., the second information recording layer  14 ) than the information recording layer (e.g., the first information recording layer  12 ) subjected to BCA working.  
      The present invention is applicable to usages in which permanent information is recorded on a reproduction type disc by burning off the reflective film with a laser or the like. Especially, the present invention can be used for applications in which write-once recording of information is performed on only a desired recording layer in discs for reproduction only each having a plurality of recording layers, which include a DVD-ROM and a reproduction type disc (ROM) such as a Blu-ray Disc.  
      In the present invention, the following effects are obtained. Firstly, in an arrangement in which the thermal conductivity of material of the one information recording layer subjected to BCA working is made smaller than that of the remaining information recording layers, diffusion rate of irradiation heat is retarded in the one information recording layer so as to make the irradiation heat stagnant in the one information recording layer such that the one information recording layer is worked readily, while thermal diffusion is promoted in the remaining information recording layers such that the remaining information recording layers are set to a state unsuitable for working and thus, damage to the remaining information recording layers can be lessened. As a result, the BCA marks can be satisfactorily formed on only the one information recording layer subjected to BCA working and it is possible to prevent working traces from being left on the remaining information recording layers.  
      Secondly, if the one information recording layer subjected to BCA working is disposed closest to the one of the opposite faces of the information recording medium in the information recording layers and BCA working is performed from the one of the opposite faces of the information recording medium, influence of BCA working on the remaining information recording layers can be lessened further.