Abstract:
An improved optical disc for use in the manufacture of a multilayer optical disc. The improved optical disc has a first side with a data layer and at least one projection extending outward from the first side. The projection separates the first side of the improved optical disc from another optical disc used in the manufacture of the multilayer optical disc, thereby providing a uniform adhesive layer between the discs. A mutilayer optical disc and a method of making a multilayer optical disc utilizing the improved optical disc is also described.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to optical disc manufacturing and more particularly, to the manufacture of multilayer optical discs.  
         BACKGROUND OF THE INVENTION  
         [0002]    Multilayer discs are now being produced that are an assembly of two discs, each with its own data layer of grooves or pits representing recorded and stored data or information. Referring to FIG. 5, a composite disc  18  is formed of a first, bottom disc  20  normally having a first, lower, sputtered semi-reflective data layer or surface  22 . A read laser either focuses on the first surface  22  or, alternatively, penetrates through the first surface  22  and focuses on a second, data layer or surface  24  on a second, top disc  26 . The distance between the two surfaces or focal planes  22 ,  24  is made up of a transparent bonding layer  28  joining the two discs  20 ,  26  together. Industry DVD specifications require that the transparent bonding layer be in the range of from 40 microns to 70 microns. Other specifications may be required to comply with other specific formats.  
           [0003]    The composite disc  18  is formed from two discs that are normally manufactured using an injection molding process. An ultraviolet, curable, liquid resin, for example, lacquer, that functions as an adhesive is first deposited on a lower rotating disc  20 . Upon striking the rotating disc surface  22 , the resin spreads across the disc surface  22  toward the inner centerhole  30  and the outer disc circumference  32 . However, the resin that first contacts the disc surface  22  will have experienced substantial spreading as the last of the resin is deposited on the surface  22  of the disc  20 . Thus, before the final resin deposited spreads, the first resin deposited may reach and spill over the inner hole  30  and/or the outer circumference  32  of the disc  20 . Any resin that does spill off of the disc  20  must somehow be handled and contained which adds cost to the process in terms of machine design, maintenance and the cost of the resin.  
           [0004]    The construction of the composite disc  18  continues by placing the other disc  26  on top of the resin coating on the upper surface  22  of the first disc  20 , thereby forming the multilayer disc  18 . The multilayer disc assembly  18  with the uncured resin layer therebetween is subjected to a high speed spinning process to achieve a desired thickness of the intermediate laquer layer. After the spinning process, the resin is cured; and the multilayer disc construction process is complete.  
           [0005]    The above spinning process presents several challenges with respect to consistently manufacturing a high quality multilayer disc. First, the spinning process has no process feedback and thus, is not subject to precise control from one disc to another. Further, the forces created by the spinning create a nonuniform resin thickness profile across the resin layer. The resin thickness profile changes from a thinner layer close to the centerhole  30  of the multilayer disc  18  to a thicker layer near its outer circumference  32 . The nonuniform resin layer between the discs  20 ,  26  does not permit a true noise-free replay of the disc.  
           [0006]    To hold the thickness of the resin layer within specifications, the resin layer of a finished multilayer disc is measured at many different locations. Those measurements are statistically processed, and the spinning process is adjusted over a period of time, as required, to maintain the resin layer within desired thickness specifications. Many manufacturing environments strive for a thickness specification that is more strict than the about 40-70 microns thickness required by the industry specification. And, the known manufacturing processes can lead to the production of out-of-specification discs while the spinning process is being tuned to hold the resin layer thickness within specification.  
           [0007]    Thus, the current multilayer disc bonding process presents several opportunities for improvement. First, there is no real-time control of the thickness of the resin bonding layer between the discs. Second, since the resin is continuously deposited over a single rotation, the first resin deposited reaches the inner hole and the outer edge sooner than the later deposited resin. Third, control over the thickness of the resin layer is further complicated because both of the discs are flat; and therefore, the resin flows freely in all directions and may flow into the inner hole and/or over the outer edge of the disc.  
           [0008]    Consequently, there is a need for an improved process for joining two optical discs to form a multilayer disc with an intermediate bonding fluid layer that has a more predictable thickness profile.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides an improved optical disc for use in a multilayer disc that provides a consistently high quality noise-free playback of data on the disc. Further, in the manufacture of a multilayer optical disc, the improved optical disc of the present invention can be adhered to another optical disc with a bonding layer thickness that is reliably and repeatably maintained within specification. Therefore, multilayer optical discs are manufactured with a minimum of scrap and thus, manufacturing costs are reduced. Further, the repeatably consistent bonding layer thickness provided by the present invention has the advantage of being able to consistently read data through the bonding layer.  
           [0010]    According to the principles of the present invention and in accordance with the described embodiment, the invention provides an optical disc for use in the manufacture of a multilayer optical disc. The optical disc has a first side with a data layer and at least one projection extending outward from the first side. The projection separates the first side from another optical disc used in the manufacture of the multilayer optical disc. The projection is effective to maintain a known and constant bonding layer separation between this optical disc and another optical disc bonded thereto in the manufacture of the multilayer optical disc. Maintaining a repeatably consistent bonding layer improves the consistency of the reading of data from the disc.  
           [0011]    In one aspect of the invention, the projection is comprised of a first projection located between a center hole and the data layer and a second projection located between the data layer and an outer circumferential edge of the optical disc. In another aspect of the invention, the first and second projections extend outward from the first side a distance of about 40-70 microns. The use of two projections radially separated on the first side of the optical disc provides a better control over the consistency of the thickness of the bonding layer. In addition, the inner projection substantially stops the migration of air from the centerhole into the bonding layer during a spinning process. Thus, the present invention provides a more consistent and pure bonding layer thickness across the whole diameter of the resulting multilayer optical disc. Such consistency further improves the noise-free readability of data from the optical disc.  
           [0012]    In another embodiment, the present invention provides a multilayer optical disc having a first optical disc with a first side and a second optical disc with a first side facing the first side of the first optical disc. An adhesive extends between the first sides of the first and second optical discs, thereby bonding the first and second optical discs together to form the multilayer optical disc. A spacing bridge is located between, and contacts, the first sides of the first and second optical discs. The spacing apparatus separates the first and second optical discs by a distance that is substantially equal to a desired thickness of the adhesive. Again, the multilayer optical disc has a more consistent bonding layer thickness across its whole diameter which further improves the noise-free readability of data from the multilayer optical disc.  
           [0013]    In another embodiment, the present invention provides a method of making a multilayer optical disc by first, applying a bonding agent to a first side of a first optical disc during substantially a single rotation of the first optical disc. Next, a first side of a second optical disc is placed on top of the first side of the first optical disc; and the first side of the second optical disc is supported on a spacing bridge extending between the first and second optical discs. The first and second optical discs are spun to spread the adhesive to a uniform, desired thickness therebetween.  
           [0014]    These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0016]    [0016]FIG. 1 is a diagrammatic diametric cross-sectional view of a multilayer, composite disc with spacing bridges in accordance with the principles of the present invention.  
         [0017]    [0017]FIG. 2 is an enlarged view of the inner bridge illustrated in FIG. 1.  
         [0018]    [0018]FIG. 3 is an enlarged view of the outer bridge illustrated in FIG. 1.  
         [0019]    [0019]FIG. 4 is a top plan view of a disc of the composite disc of FIG. 1 employing spacing bridges in accordance with the principles of the present invention.  
         [0020]    [0020]FIG. 5 is a diametric cross-sectional view of a known multilayer, composite disc. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring to FIG. 1, a composite disc  40  in accordance with the present invention is formed of a first, bottom disc  42  and a second, top disc  48 . The bottom disc  42  has a first, lower, sputtered semi-reflective data layer on a focal plane or surface  44 . The top disc  48  has a second, upper, fully reflective data layer on a focal plane or surface  46 . As discussed earlier, the distance between the two focal planes or surfaces  44 ,  46  is specified by DVD industry specifications to be in the range of about 40-70 microns.  
         [0022]    As shown in FIG. 1, the distance between the discs  42 ,  48  is fixed by spacing bridges  50 ,  54 . The annular, inner spacing bridge  50  is formed on the lower disc  42  and surrounds a centerhole  52 ; and the outer spacing bridge  54  is formed on the lower disc  42  adjacent an outer circumference  56 . The spacing bridges  50 ,  54  extend outward from surface  44  a distance or height of about 40-70 microns; and typically, the spacing bridges  50 ,  54  are about 55 microns in height.  
         [0023]    Referring to FIG. 2, the inner spacing bridge  50  is located immediately adjacent the centerhole  52  of the lower disc  42 . The centerhole  52  has a radius of about 7.5 mm, and the data layer on surface  44  normally begins at a radius of about 23 mm. The bridge  50  is a continuous annular boss or projection  60  extending above the surface  44  that is located at a radius between about 10 mm and about 23 mm. In cross-sectional profile, the projection has a first, angled or sloping side wall  62  that begins at a radius of about 10 mm from the centerhole  52 . The first side wall  60  slopes radially outward at an angle of about 45°. The projection  60  has a second, opposed, angled or sloping side wall  66  that slopes radially inward at an angle of about 45°. Thus, the side walls  62 ,  66  slope toward each other and intersect a top, supporting surface  68 . The cross-sectional profile of the projection  60  is principally determined by the manufacturing process used to form the projections  60  on the disc  42 . For example, the angle of the side walls  62 ,  66  is chosen to facilitate a release of the side walls  62 ,  66  from a mold. The radial width of the inner bridge  50  is normally chosen to be as small as possible, that is, the smallest width that can be reliably and repeatably molded. For example, the radial width of the inner bridge  50  is about 2 mm but may be thicker or thinner. The radial width of the inner bridge is normally a width that can be repeatably molded with sufficient accuracy. Thus, the projections  60  of the inner bridge  50  can have any shape as long as the inner bridge  50  separates the discs  42 ,  48  by a distance or spacing determined by industry specifications, for example, about 40-70 microns.  
         [0024]    Referring to FIGS. 3 and 4, the outer spacing bridge  54  is located immediately inside the outer edge  56  of the disc  42 . The outer edge  56  is normally located at a radius of about 60 mm, and the data layer on surface  44  normally extends to a radius of about 58 mm. The bridge  54  is comprised of a series of bosses or projections  70  extending above the surface  44 . In cross-sectional profile, each projection  70  has first and second angled side walls  72 ,  74  that intersect an upper supporting surface  76 . Like the inner bridge  50 , the cross-sectional profile of the projections  70  is principally determined by the manufacturing process used to form the projections  70  from the disc  42 . The angle of the side walls  72 ,  74  and the radial thickness of the outer bridge  54  are chosen in a manner similar to that previously described with respect to the inner bridge  50 . Thus, the projections  70  of the outer bridge  54  can have any shape that can be repeatably molded with reasonable accuracy and that separates the discs  42 ,  48  by a distance or spacing determined by industry specifications, for example, about 40-70 microns.  
         [0025]    The discs  42 ,  48  are normally made with a molding process, for example, an injection molding process. The data layers on surfaces  44 ,  46  are created by a stamping plate mounted on one of the molds in a known manner. The stamping plate surface must be hard and durable so that it may be used over a large number of molding cycles; and therefore, the molding surface on the stamping plate is often made of a nickel material. The outwardly extending spacing bridges  50 ,  54  on the disc  42  translate into matching indentations or dimples in the nickel molding surface of the stamping plate. Therefore, a die having projections or protuberances matching the desired bridges  50 ,  54  can be used in a press to form mating depressions in the nickel molding surface of the stamping plate. Thus, in addition to forming the data layers  44 ,  46 , the stamping plate is also used to form bridges  50 ,  54  in the injection molded polycarbonate used to make the disc  42 .  
         [0026]    In use, in the process of making the composite disc  40 , the lower disc  42  is supported and rotated as a liquid adhesive is deposited on the disc in a known manner. As the lower disc  42  rotates, the adhesive spreads toward the centerhole  52  and the inner spacing bridge  50  as well as the outer edge  56  and the outer spacing bridge  54 . The resin is normally dispensed during a full, or slightly less than full, revolution of the lower disc  42 . Therefore, the adhesive spreads across the disc  42  at different times. Adhesive that is dispensed first will be the first to contact the inner wall or spacing bridge  50 . The continuous annular nature of the spacing bridge  50  prevents adhesive from flowing into the centerhole  52 . The inner bridge  50  is also effective to avoid the migration of air from the centerhole  52  which is encouraged by the spinning process.  
         [0027]    As the disc  42  rotates, the adhesive is also forced radially outward toward the outer spacing bridge  54 . The presence of the outer spacing bridge  54  helps retain the adhesive initially dispensed onto the disc  42  from flowing over the outer edge  56  while the remainder of the adhesive is being dispensed. However, as shown in FIG. 4, the projections  70  comprising the spacing bridge  54  has spaces  73  intermediate the projections  70 . The spaces  73  permit excess adhesive to spill over the outer edge  56  during the adhesive application process.  
         [0028]    After the adhesive is applied, the upper disc  48  is carefully placed over the lower disc  42 . The upper surface  66  of the inner spacing bridge  50  contacts the surface  46  of the upper disc  48 . Similarly, the upper surface  76  of the outer bridge  54  also contacts the surface  46  adjacent the outer edge  56 . The discs  42 ,  48  are then spun to more evenly spread the adhesive therebetween. The bridges  50 ,  54  function to fix the spacing between the surfaces  44 ,  46  on the respective discs  42 ,  48 ; and during the spinning process, the adhesive spreads more evenly between the surfaces  44 ,  46 . Since the spacing bridges  50 ,  54  extend about 55 microns above the surface  44 , the distance between the surfaces  44 ,  46  and thus, the thickness of the intermediate adhesive layer  80 , is maintained at about 55 microns. Therefore, the thickness of the adhesive layer is substantially in the middle of the specified range of about 40-70 microns.  
         [0029]    The above-described optical disc for use in a multilayer disc provides a consistently high quality noise-free playback of data on the disc. Further, in the manufacture of a multilayer optical disc, the above-described optical disc is adhered to another optical disc with a bonding layer thickness that is reliably and repeatable maintained within specification. Therefore, multilayer optical discs are manufactured with a minimum of scrap and thus, manufacturing costs are reduced. Further, the repeatably consistent bonding layer thickness provided by the present invention has the advantage of being able to consistently read data through the bonding layer.  
         [0030]    With the above-described embodiment, the use of two projections radially separated on the first side of the optical disc provides a better control over the consistency of the thickness of the bonding layer. In addition, the inner projection substantially stops the migration of air from the centerhole into the bonding layer during a spinning process. Thus, the use of projections provides a more consistent and pure bonding layer thickness across the whole diameter of the resulting multilayer optical disc. Such consistency further improves the noise-free readability of data from the optical disc.  
         [0031]    While the invention has been illustrated by the description of one embodiment, and while the embodiment has been described in considerable detail, there is no intention to restrict nor in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those who are skilled in the art. For example, with the embodiment illustrated in FIG. 1, two spacing bridges  50 ,  54  are used; however, as will be appreciated, either one of the spacing bridges  50 ,  54  can be used alone. Further, a single spacing bridge can be placed at any radial location that is consistent with industry specifications for the disc  40 .  
         [0032]    Further, in the described embodiment, the inner bridge  50  is illustrated as a continuous annular projection extending around a circumference adjacent the centerhole  52 . However, as will be appreciated, the inner bridge  50  may be a segmented or discontinuous annular ring or any number of annularly disposed bumps, ridges or other protuberances that function to separate the discs  42 ,  48  by the desired spacing, that is, about 40-70 microns.  
         [0033]    Further, the outer bridge  54  is illustrated as a series of spaced apart projections  70  extending circumferentially adjacent the outer edge  56 . However, as will be appreciated, the outer bridge  54  can be formed by any number of circumferentially disposed bumps, ridges or other protuberances that function to separate the discs  42 ,  48  by the desired spacing, that is, about 40-70 microns.  
         [0034]    In the description of FIGS.  1 - 4 , the bridges  50 ,  54  are both disposed on the lower disc  42  so that, during the adhesive dispensing process, adhesive is restrained from flowing over the edge  64  of the inner hole  52  and/or the outer edge  56 . However, in alternative embodiments of the invention, it is contemplated that the spacing bridges  50 ,  54  can be located on the upper plate  48 . As a further alternative, one spacing bridge, for example, spacing bridge  50 , can be located on one disc, for example, upper disc  48 , and the other spacing bridge, for example, spacing bridge  54 , can be located on the lower disc  42  and vice versa.  
         [0035]    In a still further embodiment, spacing bridges  50 ,  54  may be located on both the lower disc  42  and the upper disc  48 . In one example, the inner bridge  50  may be formed by first arcuate segments on the lower disc  42  and second arcuate segments on the upper disc  48 . The segments being spaced such that when the disc  48  is placed over the lower disc  42 , the segments on the upper disc  48  are disposed between the arcuate segments on the lower disc  42 . A similar construction may be used to form the outer bridge  54 . In another embodiment, the inner bridge  50  may be formed by a single or multiple arcuate segment on the lower disc and one or more opposing arcuate segments on the upper disc  48 . Therefore, when the upper disc is placed on the lower disc  42 , the opposing segments contact each other to form the inner bridge  50 . A similar construction may also be utilized to form the outer bridge  54 .  
         [0036]    In a further embodiment, the inner bridge may be formed by causing the surface  44  to have randomly arranged protuberances such that the bridge  50  merely appears as a texture portion of the surface  44 . Again, such textured areas may be on one or the other of the discs  42 ,  48  or on areas of both of the discs  42 ,  48 .  
         [0037]    Therefore, the invention in its broadest aspects is not limited to the specific details shown and described. Consequently, departures may be made from the details described herein without departing from the spirit and scope of the claims which follow.