Abstract:
Information-bearing discs are fabricated from a first disc and a second disc. The first disc has a first substantially planar surface and a second surface opposite to the first surface. The second surface has first information recorded on it by means of local changes in the distance of the second surface from the first surface. The thickness of the second disc is not equal to the thickness of the first disc and has opposite, substantially planar, third and fourth surfaces. The fourth surface may have second information recorded on it by means of local changes in the distance of the fourth surface from the third surface. The second disc is secured to the first disc so that the fourth surface faces the second surface.

Description:
This is a continuation-in-part of application Ser. No. 09/146,861, filed Sep. 3, 1998, now U.S. Pat. No. 6,124,011 issued Sep. 26, 2000 which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to information recording media in the form of light-readable information-bearing discs, and more particularly to such media which are made up of a front-side disc and a back-side disc secured to one another back to back to form a two-sided stack. 
     Light-readable information recording media in the form of information-bearing discs are well known as shown, for example, by Kramer U.S. Pat. No. 5,068,846. Commercially available compact discs (“CDs”) and compact disc read-only memories (“CD-ROMs”) are examples of recording media of this general type. 
     More recently, digital versatile discs (“DVDs”) have emerged wherein light-readable information technology is employed to record movies and other similar real-time audio/visual programming (hereinafter “program information”) on discs. To increase program information storage capacity, information-bearing discs are formed from two discs (similar in appearance to conventional audio CDs) bonded back to back (see, for example, Marquardt, U.S. Pat. No. 5,729,533, entitled “Two-sided, Light-Readable Information Recording Disc Stacks And Methods Of Making Same,” the entire contents of which are hereby incorporated by reference). 
     Relatively long audio/visual programs will not fit on one disc of conventional audio CD size. As such, the “back-side” (i.e., the second disc bonded to the back of the “front-side” or first disc) of an information-bearing disc may often contain “spillover” audio/visual program information (i.e., information which would not fit on the front-side of the information-bearing disc). 
     During the formation of a one-program-information-side information-bearing disc (for example, an information-bearing disc wherein only the front-side disc in the two disc stack contains program information), a substantially transparent plastic disc (i.e., transparent enough to allow a light beam to remain detectable following round trip travel through the plastic disc) with a substantially planar first surface (i.e., planar enough to allow a light source to differentiate topographical features disposed on a surface of the plastic disc as the disc rotates) is provided. Opposite the first surface is a second surface having program information recorded on it by means of local changes in the distance of the second surface from the first surface. A light-reflecting coating (e.g., aluminum) is then deposited (e.g., sputtered) over the second surface, the light-reflecting coating conforming to the local changes of the second surface so that light is reflected back through the disc in accordance with the program information recorded by means of the local changes. 
     Once the front-side program-information-bearing disc has been fabricated, one-program-information-side information-bearing disc formation is completed by bonding a back-side non-program-information-bearing disc (e.g., a disc without program information such as a blank disc or a disc with artwork, labels, or other ornamentation) to the front-side disc. This type of non-program-information-bearing disc does not normally contain a light-reflecting coating so that bonding may be performed using a light-curable bonding material which is cured by transmitting light through the back-side plastic disc. Light can not be transmitted through the front-side disc because it contains a light-reflecting coating. 
     When a two-program-information-sides information-bearing disc (for example, an information-bearing disc wherein both discs contain program information) is formed, a light-reflecting coating must be deposited on the back-side disc so that program information recorded thereon may be read out. A light-reflecting coating may also be required if “pit art” (see previously incorporated Marquardt U.S. Pat. No. 5,729,533, entitled “Two-sided, Light-Readable Information Recording Disc Stacks And Methods Of Making Same”), “enhanced pit art” (see, for example, van Hoof et al. U.S. Pat. No. 5,958,651, entitled “Methods for Providing Artwork on Plastic Information Discs,” which is hereby incorporated by reference herein), or similar visible image information is contained on the back-side disc. (As used herein, the term “information,” unmodified, refers to both program information and visual image information such as pit art, enhanced pit art, holographic artwork, and other labels/ornamentation.) 
     Regardless of whether the information-bearing disc has information on one or both sides, it is important to the light-readability (i.e., playability) of the information-bearing disc that the front-side disc and back-side disc bond together to create a suitably flat information-bearing disc. Typically, when both front-side and back-side discs are formed by the same injection molding machine, both discs fit together to create a suitably flat information-bearing disc. However, due to slight variations between molding machines, front-side and back-side discs formed on different molding machines may not bond together to create a suitably flat information-bearing disc, causing rippling and buckling (this warping is also referred to as the “potato chip” effect) which reduces the light-readability of the information-bearing disc. 
     A need therefore exists for a method of fabricating information-bearing discs such that the effects of variations in molding machines are minimized. Such a method will allow bonding together of front-side and back-side discs to create a substantially flat information-bearing disc with improved light-readability. 
     In view of the foregoing, it is an object of this invention to provide substantially flat information-bearing discs and methods of fabrication. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are accomplished in accordance with the principles of the invention by providing a first disc and a second disc. The first disc has a first substantially planar surface and a second surface opposite to the first surface. The second surface has first information recorded on it by means of local changes in the distance of the second surface from the first surface. A second disc thicker than the first disc is provided. The second disc has opposite, substantially planar, third and fourth surfaces. The fourth surface may have second information recorded on it by means of local changes in the distance of the fourth surface from the third surface. The second disc is secured to the first disc so that the fourth surface faces the second surface. 
     During information-bearing disc formation, a first disc in the information-bearing disc is constructed generally as shown in Kramer U.S. Pat. No. 5,068,846 entitled “Reflective Optical Record Carrier.” This first disc therefore has light-readable program information recorded on it in the known fashion (e.g., local changes in distance between a first and a second surface of the disc covered with a light-reflecting coating so that light is reflected back through the disc in accordance with the program information recorded by means of the local changes). The second disc in the information-bearing disc may similarly contain program information or some other (e.g., pit art, enhanced pit art, or holographic artwork). 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of a conventional one-program-information-side information-bearing disc. 
     FIG. 2 shows a cross-sectional view of a conventional two-program-information-sides information-bearing disc. 
     FIG. 3 shows a cross-sectional view of a one-program-information-side information-bearing disc made in accordance with this invention. 
     FIG. 4 shows a cross-sectional view of a two-program-information-sides information-bearing disc made in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to understand how the present invention improves information-bearing disc formation, it is first necessary to understand how conventional information-bearing discs are formed. With this understanding, the problems with conventional information-bearing disc fabrication techniques will be apparent. 
     FIG. 1 shows a conventional one-program-information-side information-bearing disc  10  comprising a first disc  20  (the front-side disc) adhesively bonded to a second disc  30  (the back-side disc). FIG. 1 illustrates the case in which only one of these two discs (first disc  20 ) is needed for program information recording. Second disc  30  may be used for artwork, labeling, or left blank. 
     First disc  20  comprises a substantially smooth first surface  22   a  and a second surface  22   b  having program information recorded on it by means of local changes  24  in the distance of the second surface  22   b  from the first surface  22   a.  A light-reflecting coating  26  is deposited over second surface  22   b.    
     To form conventional one-program-information-side information-bearing disc  10 , second disc  30  is bonded to first disc  20 . Second disc  30  comprises substantially smooth third surface  32   a  and fourth surface  32   b  and is bonded to first disc  20  with a light-curable bonding material  40  disposed between second surface  22   b  and fourth surface  32   b.    
     As shown in FIG. 1, first disc  20  and second disc  30  have thicknesses a and b, respectively, which are approximately the same. For example, for a DVD embodiment of information-bearing disc  10 , typical values for a and b are approximately 0.6 mm. Thus, the ratio of thickness a/b is approximately 1. 
     FIG. 2 shows a conventional two-program-information-sides information-bearing disc  10 ′ (which may be a dual-sided optical disc) formed by bonding a first disc  20 ′ (the front-side disc) and a second disc  30 ′ (the back-side disc) to one another back to back. As with first disc  20  of FIG. 1, first disc  20 ′ of FIG. 2 comprises a substantially smooth first surface  22   a ′ and a second surface  22   b ′ having program information recorded on it by means of local changes  24 ′ in the distance of the second surface  22   b ′ from the first surface  22   a ′. A light-reflecting coating  26 ′ is deposited over second surface  22   b′.    
     To form conventional two-program-information-sides information-bearing disc  10 ′, second disc  30 ′ is bonded to first disc  20 ′, second disc  30 ′ comprising substantially smooth third surface  32   a ′ and fourth surface  32   b ′. Unlike fourth surface  32   b  of FIG. 1, fourth surface  32   b ′ of FIG. 2 has program information recorded on it by means of local changes  34 ′ in the distance of fourth surface  32   b ′ from third surface  32   a ′ (as second disc  30 ′ is a program-information-bearing disc). Additionally, a light-reflecting coating  36 ′ is deposited over second surface  32   b′.    
     As shown in FIG. 2, first disc  20 ′ and second disc  30 ′ have thicknesses c and d, respectively, which are approximately the same. For example, for a DVD embodiment of information-bearing disc  10 , typical values for c and d are approximately 0.6 mm. Thus, the ratio of thickness c/d is approximately 1. 
     As shown in FIG. 3, an illustrative one-information-side information-bearing disc  10  made in accordance with this invention has a first disc  20  (the front-side disc) and a second disc  30  (the back-side disc) bonded to one another back to back. First disc  20  and second disc  30  have thicknesses a′ and b′, respectively, where b′ is slightly larger than a′. For example, for a DVD embodiment of information-bearing disc  10 , preferred values for a′ and b′ are approximately 0.6 mm and 0.7 mm, respectively. Thus, the ratio of thickness a′/b′ is less than 1. First and second discs  20  and  30  of FIG. 3 are identical to first and second discs  20  and  30  of FIG. 1 with the exception that the ratio of thickness a′/b′ for the embodiment of FIG. 3 is less than the ratio of thickness a/b for the embodiment of FIG.  1 . 
     As shown in FIG. 4, an illustrative two-information-sides information-bearing disc  10 ′ made in accordance with this invention has a first disc  20 ′ (the front-side disc) and a second disc  30 ′ (the back-side disc) bonded to one another back to back. As shown in FIG. 4, first disc  20 ′ and second disc  30 ′ have thicknesses c′ and d′, respectively, where d′ is slightly larger than c′. For example, for a DVD embodiment of information-bearing disc  10 , preferred values for c′ and d′ are approximately 0.6 mm and 0.7 mm, respectively. Thus, the ratio of thickness c′/d′ is less than 1. First and second discs  20 ′ and  30 ′ are identical to first and second discs  20 ′ and  30 ′ of FIG. 2 with the exception that the ratio of thickness c′/d′ for the embodiment of FIG. 4 is less than the ratio of thickness c/d for the embodiment of FIG.  2 . 
     While especially preferred values for a′ and b′ in the case of a one-information-side information bearing disc stack  10  and for c′ and d′ in the case of a two-information-side information bearing disc stack  10 ′ are approximately 0.6 mm and 0.7 mm respectively, those skilled in the art will recognize that as long as certain criteria are met the invention may be practiced using other disc thickness values. More specifically, it is known that a DVD information bearing disc stack should have a thickness of approximately 1.2 mm, +0.3 mm, −0.06 mm. Thus, a disc stack  10  or  10 ′ may actually have a total thickness anywhere between approximately 1.14 mm and approximately 1.5 mm and still comply with the publicly established DVD industry standard. A total disc stack thickness of approximately 1.3 mm (0.6 mm+0.7 mm) is not necessarily required. 
     Also, if a′, b′, c′ or d′ for any disc is less than approximately 0.5 mm, it may not be possible to read the information on that disc. Thus 0.5 mm is the permissible minimum disc thickness, not necessarily 0.6 mm. According to these criteria, it is possible in accordance with the present invention to have a disc stack with a′ or c′ equal to approximately 0.5 mm while b′ or d′ is as thick as approximately 1.0 mm. Either of the discs can have a thickness between approximately 0.5 mm≦t≦1.0 mm as long as the total thickness of the two-disc stack is in the range of approximately 1.14 mm to approximately 1.5 mm. 
     Also, while there should be, in accordance with this invention, a significant difference in the thicknesses of the two discs a′ and b′ or c′ and d′, the invention can be practiced with discs that have thickness values that are closer together than the 0.5 mm/1.0 mm ratio described above. The difference in thickness between the two discs will preferably be at least approximately 0.1 mm. Thus, a′ or c′ could be equal to approximately 0.52 mm while the thickness of the mating disc is approximately 0.62 mm. 
     From the foregoing it can be seen that in accordance with the invention the thicker disc in a DVD can be anywhere from approximately 14.28% thicker than the thinner disc (when the thicknesses are 0.7 mm and 0.8 mm) to approximately 100% thicker than the thinner disc (when the thicknesses are 0.5 mm and 1.0 mm) . The constraints are: (1) maximum DVD thickness 1.5 mm, (2) minimum DVD thickness 1.14 mm, (3) minimum thickness of any constituent disc 0.5 mm, and (4) in accordance with the invention, minimum difference in thickness between the two discs 0.1 mm. The following table gives some other examples of suitable disc thicknesses (in mm) in a DVD in accordance with this invention. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Thinner Disc 
                 Thicker Disc 
                 Thickness 
                 % Thickness 
               
               
                   
                 Thickness 
                 Thickness 
                 Difference 
                 Difference 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0.52 
                 0.62 
                 0.1 
                 19.23% 
               
               
                   
                 0.53 
                 0.63 
                 0.1 
                 18.87% 
               
               
                   
                 0.50 
                 0.64 
                 0.14 
                 28%   
               
               
                   
                 0.6 
                 0.7 
                 0.1 
                 16.67% 
               
               
                   
                 0.6 
                 0.8 
                 0.2 
                 33.33% 
               
               
                   
                 0.6 
                 0.9 
                 0.3 
                 50%   
               
               
                   
                   
               
             
          
         
       
     
     In all of the foregoing discussion the dimensions respectively associated with discs a′ and b′ can be reversed. Similarly, the dimensions respectively associated with discs c′ and d′ can be reversed. In other words, it does not matter which of the two discs is the thicker disc and which of the two discs is the thinner disc. 
     The added thickness of the second disc (the back-side disc) of the embodiments shown in FIGS. 3 and 4 substantially eliminates the rippling and buckling problems often associated with bonding two discs formed on different molding machines. 
     It will be understood that the foregoing is only illustrative of the principles of this invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. For example, the various dimensions and materials mentioned herein are preferred, but other dimensions and materials can be used if desired.