Abstract:
A high-density optical disk includes reflective layers respectively formed at both information-recorded surfaces of a first substrate, and semitransparent layers respectively formed at one information-recorded surface of each of second and third substrates. The second and third substrates are respectively bonded at both sides of the first substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical recording medium from which recorded information is reproduced by using a laser and method of producing the same and, more particularly, to a high-density recording medium which can densely record the information and from which the information is reproduced. 
     2. Description of the Related Art 
     Recently, a study of a high-density optical disk which is spotlighted as audio and video recording media has actively been made. A digital video disk (DVD) system which is a representative example of an information transmission medium of high picture quality and high sound quality has brought about the concept of a multimedia age combining digital video information with audio information. The birth of the DVD system includes prospects for a vast commercial market as a substitute demand for video cassette recorders and laser disks which are analog video systems, in addition to achieving technical innovation regarding the digitization of the video and audio information. Therefore, a fabrication method of a high-density substrate which can increase recording capacity has been demanded to provide for a next generation multimedia market. 
     A technique for fabricating the high-density substrate is disclosed in Korean Patent Application No. 95-1802, assigned to the same assignee as the present invention. In the above Korean Patent Application No. 95-1802, as shown in FIG. 1 of the present application, a semitransparent layer 2 and a protective layer 4B are sequentially formed at the lower portion of a substrate 1 having grooves and pits formed at both of its surfaces. At the upper portion of the substrate 1, a reflective layer 3 and a protective layer 4A are sequentially formed. However, if there is dust or a contaminant in the incident direction of a light beam, the above optical disk substrate may generate an error during reproduction since a spot of the light beam is scattered. The optical disk shown in FIG. 1 has the protective layers 4A and 4B, each having a thickness of 30-40 μm. Thus, if there is dust having a thickness of 5 μm in the incident direction of a light beam, the spot of the laser beam can not be focused due to the dust. Moreover, since an information surface is exposed to the exterior, the information surface may be damaged when using the disk and it is necessary to supplement the durability of the optical disk. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a high-density optical disk which can increase recording capacity and ensure durability. 
     Another object of the present invention is to provide a high-density optical disk which can simplify work and raise yield. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     The foregoing objects of the present invention are achieved by providing a high-density optical disk which includes reflective layers respectively formed at both information-recorded surfaces of a first substrate, and semitransparent layers respectively formed at one information-recorded surface of each of second and third substrates. The second and third substrates are respectively bonded at both sides of the first substrate. 
    
    
     The present invention will be more specifically described with reference to the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a cross-sectional view of a conventional optical disk; and 
     FIG. 2 is a cross-sectional view of an optical disk according to an embodiment of the present invention. 
     FIG. 3 is a cross-sectional view of a conventional molding machine. 
     FIG. 4 is a cross-sectional view of a molding machine according to the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, well known functions and constructions which may obscure the present invention are not described in detail. 
     To fabricate a typical optical disk, there is needed a series of processes, that is, a mastering process, a stamper making process, a disk molding process, reflective layer and protective layer forming processes, a post process, etc. The mastering process is to make a glass master by forming minute pits on a photoresist covered on a glass platter by a photolithography process. The stamper making process is to transfer the minute pits formed on the glass master onto a stamper by a nickel plating process. The disk molding process is to reproduce optical disk substrates in large quantities by using the stamper. Various disk molding processes are compression molding, injection molding, photopolymerization (2P), etc. An example of a conventional molding machine is shown in FIG. 3 A substrate 20 is supported by a fixed mold 22. A moving mold 24 moves a stamper 26 to make minute pits in the substrate 20. The fixed mold 22 and the moving mold 24 are supported by a platen (not shown). The reflective layer and protective layer forming processes are to form a reflective layer and a protective layer. The reflective layer is typically formed by vacuum deposition or sputtering, etc. The protective layer is generally formed by spinning. 
     In a preferred embodiment of the present invention, unlike a conventional disk molding process (for example, injection molding), the disk is molded by fixing 2 stampers in which information is recorded to both surfaces of a molding machine. Thereafter, information-recorded pits are formed at both (two opposite) surfaces of one substrate. FIG. 4 shows an example of a molding machine having two stampers 26A and 26B. The substrate 20A (10 in FIG. 2) is supported on the stamper 26B. The moving mold 24 moves the stamper 26A so that both the stamper 26A and the stamper 26B make minute pits in respective surfaces of the substrate 20A. When forming the reflective layer, a target is put at both surfaces of a chamber of a sputter device so as to simultaneously form the reflective layers at both surfaces of one substrate. 
     In the stamper making process for making the stampers, a track pitch is set to 0.6 μm for example. Moreover, the substrate structure indicated in FIG. 1 is modified. That is, as shown in FIG. 2, substrates each having the thickness of 0.6 mm are respectively bonded at the upper and lower portions of a substrate of 0.6 mm or 1.2 mm having both information-recorded surfaces produced by a UV bonding method. Hence, there are 4 information surfaces. 
     FIG. 2 is a cross-sectional view of an optical disk according to the embodiment of the present invention. Referring to FIG. 2, on both surfaces of a substrate 10 in which pits are formed, Al reflective layers 12A and 12B, UV bonding layers 14A and 14B, semitransparent layers 16A and 16B, and substrates 18A and 18B having pits formed only at one of their surfaces are sequentially formed. Unlike a general molding method, the substrate 10 having the information-recorded pits formed at both of its surfaces is fabricated by fixing 2 stampers to both surfaces of the metal mold and molding the stampers. The substrates 18A and 18B having the information-recorded pits formed only at one of their surfaces are fabricated by fixing one stamper to one surface of the metal mold and molding the stamper. The Al or Al--Ti reflective layers 12A and 12B are respectively formed at both surfaces of the substrate 10. The SiN, SiC or Au semitransparent layers 16A and 16B are respectively formed at the pits-formed surface of each of the substrates 18A and 18B fabricated by the general molding method. After the substrate 10, and the substrates 18A and 18B are fabricated, the two substrates 18A and 18B are bonded at both surfaces of the substrate 10 by the UV bonding method. It is preferable that the thickness of each of the UV bonding layers 14A and 14B formed by the UV bonding method is 30-40 μm. 
     To reproduce the information from the optical disk of FIG. 2, a red laser beam of 635 nm is irradiated through the substrate 18A, thus to read the information of the upper substrate 18A. The information of an upper layer of the optical disk is reproduced by using the red laser beam passing through the semitransparent layer 16A and the UV bonding layer 14A. The information of the upper layer of the optical disk is sensed by a difference in a reflectance. The reflectance of the SiN semitransparent layer 16A (16B) is 30%, and the reflectance of the Al reflective layer 12A (12B) is 90%. The reflectance sensed by an optical pickup of the upper layer of the optical disk is 30% in case of an information layer of the semitransparent layer 16A (16B) and 44% in case of an information layer of the reflective layer 12A (12B). 
     In other words, the substrate 18A uses the SiC or SiN semitransparent layer 16A and the upper portion of the substrate 10 uses the Al or Al--Ti reflective layer 12A. Thus, when the red laser beam of 635 nm is detected, the reflectance sensed by the optical pickup of the upper layer of the optical disk is 30% in the case of an information layer of the semitransparent layer 16A and 44% in the case of an information layer of the reflective layer 12A. When the red laser is focused on the optical disk, the information layer of the semitransparent layer 16A is distinguishable from the information layer of the reflective layer 12A due to the difference of the reflectance therebetween. 
     The information of a lower layer of the optical disk is reproduced by an additional pickup installed thereat based on similar reflectances. 
     The following Table 1 shows the capacity of the inventive optical disk of FIG. 2 as compared with the conventional optical disk of FIG. 1. 
     
                       TABLE 1______________________________________minimum     track   channel          recordingpit length  pitch   bit length                         resolution                                capacity______________________________________conven-  0.4      0.74    0.1333  0.01   9.4tional disk  (μm)  (μm) (μm) (μm)                                  (GB/disk)inventive  0.36     0.60    0.1197  0.01   30disk   (μm)  (μm) (μm) (μm)                                  (GB/disk)______________________________________ 
    
     As shown in the above Table 1, while the conventional optical disk has a recording capacity of 9.4 Giga bytes (GB) per disk at a track pitch of 0.74 μm, the inventive optical disk has a recording capacity of 30 GB per disk at a track pitch of 0.6 μm. 
     The minimum pit length (pit length recorded on the optical disk) corresponds to 3 T and the channel bit length (bit length in processing data) corresponds to 1/3 of the minimum pit length. The factors having an effect on recording density are the minimum pit length, the track pitch and the number of information layers available for recording the information. The following calculation shows the recording capacity of the inventive optical disc of FIG. 2 as compared with the conventional optical disk of FIG. 1. 
     Recording density increment effects in view of the Minimum Pit Length 
     
         0.4 μm÷0.36 μm=1.333                             (a) 
    
     Effects in view of the Track Pitch 
     
         0.74÷0.60=1.233                                        (b) 
    
     The number of information Layers available for recording 
     
         4÷2=2                                                  (c) 
    
     Total Recording Capacity increment effects=(a)×(b)×(c)≈3.3 
     The minimum pit length and the track pitch are controlled by the capacity of a LBR (laser beam recorder). When the recording capacity of the conventional DVD is compared with the recording capacity of the present invention without respect to the above LBR of one factor having an effect on the recording density, the recording density of the present invention can be twice as much as that of the conventional DVD due to structural differences between the conventional DVD and the present invention through the disk molding process. 
     As stated previously, since there are 4 information layers, twice the recording capacity of the conventional DVD can be obtained. The DVD system has complicated processes since there is a need to fabricate a dual layer by the photopolymerization process. However, the inventive optical disk simplifies the work without the photopolymerization process and the yield can be raised. Furthermore, by bonding the substrates of 0.6 mm to the upper and lower portions of the substrate having both information-recorded surfaces, the durability can be improved. 
     In addition, the inventive optical disk has the substrates 18A and 18B each having a thickness of 0.6 mm, and not the protective layers of the conventional optical disk. Therefore, even though dust forms on the optical disk, having a thickness of 5 μm, for example, in the incident direction of a light beam, it is possible for the spot of the laser beam to be focused on the optical disk because a sufficient focusing distance exists between the substrate 18A or 18B and the substrate 10. 
     It should be understood that the present invention is not limited to the particular embodiment disclosed herein as the best mode contemplated for carrying out the present invention, but rather that the present invention is not limited to the specific embodiments described in this specification except as defined in the appended claims.