Abstract:
A Method for manufacturing an optical disc substrate comprises a first substrate with at least one structured surface, on which an anti-adhesive layer, preferably carbon, is deposited and first layer on top of said anti-adhesive layer. On a second substrate with a structured surface also a layer is deposited. Both substrates are bonded together with the layers facing each other. The separation now easily can take place afterwards alongside the adhesive layer. This way the first layer from the first substrate is being transferred to the second substrate.

Description:
[0001]     This invention relates to multilayer disks, in particular DVD14/DVD18-disks, Dual-Layer-DVD/Recordable disks and Dual-Layer-Blu-Ray Disk.  
       BACKGROUND OF THE INVENTION  
       [0002]     DVD-9 are produced by molding two separate disk halves with pits containing the information and then coating these disks with a semi-transparent and a reflective coating and bonding the two disk halves together. If more layers for higher capacity are desired, additional layers have to be added.  
       RELATED ART  
       [0003]     In a first process step the double layer is produced as usual in a DVD-9 manufacturing process, as shown in  FIG. 2 : This requires a dummy substrate  60  made of PMMA or a similar material with a surface  60   a.  A metal layer  62 , usually a fully reflective one, deposited on surface  60   a  adheres very weakly to the surface  60   a.  A second substrate  64  is made e. g. from polycarbonate PC by injection moulding. On this substrate  64  a layer  66 , e. g. a semi-reflective one is being coated on. In a bonding step both substrates  60  and  64  are being glued together with the coated sides facing each other by means of an adhesive  68 . After this bonding step the dummy substrate  60  can be separated easily from the metal layer  62  so that the sequence of layer  62 , adhesive  68 , layer  66  remains attached to substrate  64 . If two of such dual layer disk halves again are glued together in a further bonding step, this results in an optical disc, e. g. a DVD  18  with four information layers as shown in Prior Art  FIG. 1 , where the substrates  1 ,  9  each carry the aforementioned semi-reflective layers  2 ,  8  and fully reflective layers  3 ,  7 . Finally the substrates are bonded with adhesive  5 .  
         [0004]     Of course the combination of a double layer half disk and a single layer half disk is possible also.  
         [0005]     This production process is frequently used in the manufacturing of DVD-14 and DVD-18. In the following this production process is referred to as the “surface transfer process”.  
         [0006]     Such a process is described also in U.S. Pat. No. 6,117,284. This document also mentions the use of a release material to ease the separation of the dummy substrate  60  from the metal layer  62 . The &#39;284 patent suggests silicone as release material, which could be applied on the substrate before coating surface  60   a  of substrate  60 . However, surface tension effects may lead an inhomogenous distribution of the release material and thus to degrade the accuracy of the substrate&#39;s surface structure.  
         [0007]     Another production process for the production of DVD-14 and DVD-18 disks consists in coating two disk halves first with a semireflective coating each, then applying a thermoplastic layer of approximately 50 micrometer thickness, embossing the second information layer by e. g. hot pressing, finally coating this embossed information layer with a reflective layer and bonding the two disk halves with two information layers each. A variation of this technology is the so called 2P-process, which consists in the steps of depositing a semireflective layer on a first substrate, applying a photopolymer resin, embossing the second data surface with a stamper and hardening the photopolymer with UV light, finally remove the stamper and metallize the embossed surface as before. This 2P-process is commonly also used in the production of Dual-Layer-DVD/Recordable and Dual-Layer-Blu-Ray disks, with photopolymer thicknesses of approximately 50 micrometer and approximately 20 micrometer, respectively. The nature of the deposited layers (semireflective, metallic, dye etc.) depends on the type of disk to be manufactured. Similar to the surface transfer process, the stamper is made of a plastic material with very low adhesion like PMMA or cyclic polyolefins. This stamper has to be removed from the (coated) first substrate, similar to the surface transfer process, before further processing.  
         [0008]     The 2P-process can also be used in the production of disks with more than 2 layers by repeating the process steps described above.  
         [0009]     One critical process step in the surface transfer process as well as the 2P-process is the separation of the dummy substrate or stamper from the first substrate without damaging the first substrate or deposited layers. U.S. Pat. No. 6,117,284 proposes PMMA for the dummy substrate in the production of DVD-14 or DVD-18. PMMA, however, requires quite different molding parameters (temperatures, pressure, cooling time, pit shape etc) from the standard Polycarbonate and would preferably be avoided. In the 2P-process the stamper commonly is made by injection molding from cyclic polyolefins. Similar problems as in the production of PMMA occur, as different molding parameters as well as high cost of this material. In the production process it would be very convenient to have just one plastic material as base for the first and the dummy substrates or stamper, respectively. Since polycarbonate is the material used most commonly in the production of optical disks, this material should be used for the dummy substrate or stamper, respectively, as well. However, polycarbonate shows high adhesion and can not be separated in both production processes.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore proposed for manufacturing an optical disc substrate to deposit onto a first substrate with at least one structured surface an anti-adhesive layer, preferably carbon, and a first layer on top of said anti-adhesive layer. On a second substrate with a structured surface also a layer is deposited. Both substrates are bonded together with the layers facing each other. The separation now easily can take place afterwards alongside the adhesive layer. This way the first layer from the first substrate is being transferred to the second substrate.  
         [0011]     In a broader sense, this is a method for mapping the structure of the first substrate&#39;s surface onto a second substrate&#39;s surface, by introducing the first substrate in an evacuable chamber, reducing the pressure in the chamber to at least 10 3  mbar, introducing a working gas and depositing a carbon layer by means of magnetron sputtering onto the surface area to be mapped. After depositing a thin material layer onto the carbon layer this material layer can be transferred onto a second substrate, whereas the carbon layer remains at the first substrate.  
       SOLUTION ACCORDING TO THE INVENTION  
       [0012]     The disk separation method according to both production processes described above is improved by using Polycarbonate for the dummy substrate or stamper, respectively, coating it with a very thin film of carbon or a carbon containing compound conforming to the pit structure and then continue with the process steps necessary for the production of whatever disk has to be produced. In the case of the production of DVD-14 and DVD-18 the next process step is to coat with a reflective film, made e.g. of aluminum or silver. The carbon-based anti-adhesive film allows easy separation of the dummy substrate or stamper, respectively, after bonding with the first substrate.  
         [0013]     Different methods have been used for the deposition of the carbon or carbon containing anti-adhesive film:  
         [0014]     1) Deposition of a liquid film with high viscosity by a vapor deposition method  
         [0015]     A thermal evaporation source (Knudsen cell) was used to deposit a thin film of Silicon oil DC704 as used for diffusion pumps. The experiments showed, however, that the electrical signals from the DVD-substrate degraded to an unacceptable level for oil-thicknesses sufficient for simple separation from the first substrate. It is assumed that the liquid has a tendency to fill the pits of the substrate or forming islands.  
         [0016]     2) Deposition of a carbon film with a PECVD-method The idea was to form a teflon-like film on the substrate by polymerizing C 4 F 8  on the substrate. However initial results showed no anti-adhesive effect.  
         [0017]     In a second experiment, a plasma gun (as e. g. described in U.S. Pat. No. 5,017,835) was used to deposit a diamond like carbon layer on the substrate using C 2 H 2  as gas during deposition. With 20 to 30 nm of deposited CH x  a sufficient anti-adhesive effect was obtained with fair replication of pit structure. However the achievable deposition rates (up to 4 nm/sec) are not sufficient for a commercial use of this process at present, since standard DVD lines run at a cycle time of below or about 4 sec, whereas the carbon deposition requires about 7 sec to give reliable results. Furthermore, the electrical signals jitter increased somewhat for the thicknesses required for sufficient anti-adhesive effect.  
         [0018]     Another disadvantage of this PECVD-method is the comparatively high cost of the plasma gun itself.  
         [0019]     3) Sputter deposition of carbon  
         [0020]     Sputtering of carbon gave the best results for anti-adhesive effect and small effect on electrical signals as for example jitter. The experimental results are given in the table below. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     A preferred embodiment for the production method of an optical storage media, e. g. a DVD- 18 , is explained in more detail with reference to  FIG. 3 .  
         [0022]     A first substrate  10 , preferably made from polycarbonate (PC), is being produced, e. g. by injection molding. An anti-adhesive film  11  is applied to the surface of substrate  10 , and, in a further production step a further layer  12  is applied on top of layer  11 . Layer  12  may be reflective or semi reflective, preferably a thin metal layer.  
         [0023]     The second substrate  13  is being produced like described above for the DVD  9  process, showing a layer  14 , e. g. a semi-reflective layer. Adhesive  5  is being used to glue together first substrate  10  and second substrate  13 . The separation after this bonding step results in a transfer of layer  12  from substrate  10  to substrate  13 . Anti-adhesive film  11  should remain after separation preferably on subtrate  10 .  
         [0024]     The equipment used to pratice the inventive method may comprise: Vacuum deposition systems for depositing an anti-adhesive layer on a disk with structured pits, in particular a sputtering system, preferentially a DC magnetron sputter system.  
         [0025]     The combination of an anti-adhesive layer deposition chamber and a metal layer deposition chamber attached to a common transport chamber for the deposition of a removable metal layer on a structured substrate  
         [0026]     The usage of two sputter stations for depositing an anti-adhesive layer within the required cycle time (2.5 to 4 sec is commonly used in the production of DVD-9)  
         [0027]     The usage of a focus target for increased sputter rate and reduced sputter time (see Heinz et. al. U.S. Pat. No. 6,682,637).  
       FURTHER ADVANTAGES OF THE INVENTION  
       [0028]     DVD-14 and DVD-18 production normally is only a fraction of total DVD production; therefore it is desirable to produce DVD-18 on the same production lines as DVD-5 or DVD-9, with as little modification as possible. The present invention can be integrated easily into an existing DVD-9 line. The basic DVD-9 line can be used with two standard polycarbonate molding machines and a standard bonding module. The only change required is an additional anti-adhesive deposition module. This can be integrated into the sputter module, e. g. by replacing the single station sputter coating station with a dual station coating module. These new modules are compact enough to fit as a replacement in the space of the previously used single sputter module. Switching from DVD-18 to DVD-9 production requires no hardware modification (no change of PMMA to PC in the molding machine in particular). The anti-adhesive coating step can be skipped in the DVD-9 production. The additionally required substrate separation and final bonding step require the same offline equipment as the prior art DVD-18 process.  
         [0029]     An additional advantage is that the dummy substrate can be removed from the reflective layer without damage and reused several times after fresh coating.  
         [0030]     Recycling is simplified as the PC recycling is already in place and no different material has to be recycled.  
         [0031]     In the production of Dual-layer-DVD/Recordable and Dual-layer-Blu-Ray disks the usage of cyclic polyolefins in the 2P-process is discussed frequently. However, this material is difficult to mold, especially for the Dual-layer-DVD/Recordable process and the usage of polycarbonate for the stamper improves the quality of the disk. Furthermore, the cost of cyclic polyolefins is quite high compared to polycarbonate so that a significant reduction of production costs is achieved.  
         [0032]     Application in the Optical Mastering Process  
         [0033]     The anti-adhesive film may also find an application in the production of optical masters, used as stampers in the molding equipment for producing optical disks. Here we have the problem that the structured photoresist which is metallized first by sputtering and then by galvanization cannot be separated from the metal without leaving adhering particles on the metal master. An anti-adhesive layer applied by the method above of similar thickness, conforming to the-structured photoresist may also solve this problem, using corresponding equipment as described above. Under favorable circumstances the photoresist master may be reused as well.  
         [0000]     Experimental Results  
         [0034]     A standard UNAXIS sputtering system equipped with a sputter source of type ARQ131 was used to deposit carbon films with DC magnetron sputtering in an Ar atmosphere for the production of DVD-18 with the sputtered carbon layer.  
         [0035]     Process Parameters:  
         [0036]     Power 5 kw U: 670V I: 7.5 A  
         [0037]     Flow: 5 sccm Ar, p=10-3 mbar  
         [0038]     Sputter time: 2-10 sec  
         [0039]     Sputter rate: approx. 2-2.5 nm/sec  
         [0040]     Sputter target: Graphite  
         [0041]     L 1  layer: Ag alloy  
         [0042]     The first substrate and the dummy substrate were bonded with a vacuum bonding machine manufactured by Unaxis, using a standard bonding lacquer (DIC SD6200). After separation of the dummy substrate the first substrate was rebonded to simulate the production of a DVD-14 or DVD-18.  
         [0043]     Optimum results were obtained at 4-6 sec sputter time (approximately 8-12 nm film thickness of Carbon).  
         [0044]     The following table shows some experimental results of electrical signals, obtained with a CATS disc tester from Audio development. (PI Sum 8 ave refers to a DVD error rate (parity error)). For 3-6 sec sputter time the studied electrical signals were well within specification after separation and rebonding to a dummy substrate.  
                                                                                                                           Before   After rebonding to           Sample   separation   dummy substrate                Sputter   PI Sum   DC jitter   DC jitter   PI Sum 8   DC jitter   DC jitter           #   time (s)   8 ave   ave (%)   max (%)   sep   sep (%)   max (%)   % transfer                    144   2   3.8   6.34   8.16   n.m.   n.m.   n.m.   90%       145   3   4.5   6.46   8.24   14   6.166   8.58   95%       146   4   5.4   6.63   8.87   6.1   6.251   8.58   100%       147   5   14.3   6.596   8.75   14.5   6.4   8.89   100%       148   4   10.3   6.784   8.89   9.7   6.15   8.56   100%       149   6.0   5.3   6.61   8.97   7.5   6.148   8.62   100%       150   8.0   4.9   6.834   9.11   18.3   6.452   9.41   100%       151   10.0   4.6   6.859   9.15   27.9   6.705   10.35   100%                  
 
         [0045]     With lower film thickness (2-3 sec sputter time corresponding to approximately 4-6 nm thickness) separation is critical but can be improved by process optimization. At larger film thickness the electrical signals (in particular jitter) deteriorate, but could be improved by adjusting pit geometry to the film thickness. From the standpoint of compatibility with the DVD-9 process it is however preferable to use lower film thickness, so the same stamper geometry can be used for DVD-9 as for DVD-14/18.  
       REFERENCE NUMERALS  
       [0000]    
       
           1 ,  9  substrate  
           2 ,  8  semi-reflective data layer  
           3 ,  7  fully reflective data layer  
           5  glue, adhesive  
           10  first substrate  
           11  anti-adhesive layer, anti-adhesive film (dotted line in  FIG. 3 )  
           12  layer, semi-reflective/reflective (continous line in  FIG. 3 )  
           13  second substrate  
           14  layer, semi-reflective/reflective (continous line in  FIG. 3 )  
           60  substrate, with a surface  60   a  with embossed information  
           62  reflective layer  
           64  substrate, preferably comprising PC (polycarbonate)  
           66  semireflective layer  
           68  glue, adhesive