OPTICAL DISC AND MANUFACTURING METHOD

CROSS-REFERENCES TQ RELATED APPLICATIONS

This application claims priority to U.S. Provisional application S/N 61/126,322, "Optical Disc Manufacturing Method" filed on May 2, 2008, which is herein incorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates to a method of forming an optical disc and the disc having a composite substrate structure formed by the method.

BACKGROUND

A dual-layer optical disc, e.g., a Blu-ray disc (BD), has two data layers that can be accessed from one side of the disc. FIG. 1 depicts a cross sectional view of a dual-layer optical disc 100, having a substrate 102, a first reflective layer 104, a spacer or bonding layer 106, a second reflective layer 108, and a cover layer 1 10. For a BD, the substrate 102 is typically a 1.1mm polycarbonate disc, and the first reflective layer 104, which is highly or substantially totally reflective at the read-out wavelength, can be made of a silver alloy or aluminum with a thickness of less than about 50nm. The second reflective layer 108, which is partially reflective, can be made of a silver alloy with a thickness of less than about 20nm. The Blu-ray spacer layer 106 is made of a transparent resin material and has a thickness of about 25μm, and the cover layer is made of a transparent material with a thickness of about 75 μm, and may also include a protective hard coating.

Presently, there are two widely used methods for manufacturing dual-layer Blu-ray optical discs. One is a plastic stamper method, and the other is a "wet embossing" method. The plastic stamper method is also referred to as a "2P" process, which stands for photo polymerization (referring to one of the steps in the method). The method involves molding a first data layer in a first substrate, and a second data layer to a second substrate. After a reflective layer is formed over the first data layer, the two substrates are bonded to each other using an adhesive. The second substrate is then separated and discarded, leaving an

impression of the second data layer molded into the adhesive layer remaining on the first substrate. Additional processing steps are performed to complete the fabrication of the disc.

In the case of Blu-ray discs, the first substrate is a 1.1mm disc, and the second substrate may be a 0.6mm disc. Thus, for each Blu-ray disc manufactured, the second substrate is discarded. Not only is this process wasteful, but it is also not environmentally friendly due to the need of disposing the scrap substrate material.

The 2P method is further illustrated with reference to FIGS. 2a-d, which depict cross- sectional views showing various stages during the fabrication of an optical disc, e.g., a Blu- ray disc. FIG. 2a shows a first substrate 202, e.g., a 1.lmm thick polycarbonate disc, with a surface 202D having structures such as pits and lands representing data in a first data layer. A first reflective layer 204 is formed over the molded surface 202D of the substrate 202. A different substrate 250, e.g., a polycarbonate disc, typically with a thickness of 0.6mm, has a surface 250D with pits and lands representing data in a second data layer. Surfaces 202D and 250D are molded using respective stampers (not shown). The substrate 250 is bonded to the reflective layer 204 with an adhesive layer 206

(which may be made of two separate layers of different adhesive materials), as shown in FIG. 2b. After curing the adhesive by exposure to ultra-violet (UV) light (through photo polymerization of the adhesive), the substrate 250 is mechanically stripped from the reflective layer 204, leaving the data layer impression on a surface 206D of the adhesive layer 206, as shown in FIG. 2c. Since the substrate 250 is discarded after this fabrication, and effectively serves as a stamper for transferring the data layer onto the adhesive 206, it is also referred to as a sacrificial plastic stamper.

The substrate structure in FIG. 2c has two data layers - one molded on the substrate 202 and the other on adhesive layer 206. A reflective layer 208 is then formed over the data surface 206D of the adhesive layer 206, e.g., by sputtering, and a cover layer 210 is applied over the reflective layer 208, resulting in an optical disc 200, as shown in FIG. 2d.

Another conventional method for making discs is the "wet embossing" method, which starts by molding the first data layer in the 1.1 mm Blu-ray disc polycarbonate substrate 202 as shown in FIG. 2a. After applying the first reflective layer 204, one or two layers of lacquer (not shown), including a wet embossing lacquer, are applied over the reflective layer 204. The second data layer is embossed in the wet lacquer using a re-usable nickel stamper

and the lacquer is UV-cured on the stamper. The disc structure is subsequently removed from the stamper, and fabrication is completed by performing additional steps such as those described above in connection with FIG. 2c-d. This method eliminates the use of a sacrificial plastic stamper disc to form the second data layer. Thus, under the current state-of-the-art, the plastic stamper method has a material cost disadvantage compared to the wet-embossing method.

SUMMARY OF THE INVENTION

Embodiments of the present principles provide a method for forming an optical disc, which includes a component that serves both as a stamper during fabrication and a part of the final substrate.

One embodiment provides a method of forming an optical disc, which include: (a) providing a first substrate having a first surface with structures formed thereon and a second surface, (b) providing a second substrate having a first surface with structures formed thereon, (c) bonding the first substrate to the second substrate using at least one adhesive layer, with the first surface of the first substrate facing the first surface of the second substrate, (d) removing the second substrate, and (e) bonding a third substrate to the second surface of the first substrate.

Another embodiment provides an optical disc, which includes: a composite substrate having a first substrate bonded to a second substrate, at least one adhesive layer between the first substrate and a cover layer, in which the first substrate has a first data layer formed thereon, and the at least one adhesive layer has a second data layer formed thereon.

Yet another embodiment provides a computer readable medium having program instructions that, when executed by a processor, perform a method of forming an optical disc. The method includes: (a) providing a first substrate having a first surface with structures formed thereon and a second surface, (b) providing a second substrate having a first surface with structures formed thereon, (c) bonding the first substrate to the second substrate using at least one adhesive layer, with the first surface of the first substrate facing the first surface of the second substrate, (d) removing the second substrate, and (e) bonding a third substrate to the second surface of the first substrate.

BRIEF DESCRIPTION OF THE DRAWING

The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

Figure 1 depicts a cross sectional view of a conventional optical disc; Figures 2a-2d depict a sequence of steps during fabrication of the optical disc of

FIG. 1 in accordance with a conventional method;

Figures 3a-3d illustrate cross-sectional views during various stages of manufacture of an optical disc in accordance with the present principles; and

Figure 4 is a diagram illustrating a method according to the present principles. To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.

DETAILED DESCRIPTION

One embodiment of the present principles provides a method of forming a dual-layer optical disc, e.g., a Blu-ray disc (BD), in which a first substrate is used for forming a first data layer. A second substrate is used as a stamper for forming a second data layer on an adhesive layer, and then bonded to a blank side of the first substrate. The bonded substrates result in a final substrate thickness that is specified for the optical disc, e.g., according to a known or established standard, which, in the case of BD, is about 1.1mm. The dual-use of the second substrate ~ i.e., as a stamper during disc fabrication and a part of the final disc, provides a method that is more environmentally friendly than the existing 2P method, and also reduces the need of polycarbonate material by a factor of about 35%. Other embodiments also provide a computer readable medium with program instructions for performing the method, and a disc having a substrate structure formed by the method. FIGS. 3a-d depict a sequence of method steps for forming an optical disc, e.g., a BD, in accordance with the present principles.

Injection molding is performed on a first substrate 302 with a first stamper, and on a second substrate 350, with a second stamper. Molded structures on the first substrate 302 represent data in a first data layer, also referred to as Layer-0 (or LO), and the structures molded on the second substrate 350 represent data in a second data layer, also referred to as Layer- 1 (or Ll). In one embodiment, the first substrate 302 is a 0.6 mm polycarbonate disc,

and the second substrate 350 is a 0.5 mm polycarbonate disc. Other materials with suitable properties may also be used for forming one or more of these substrates, including, for example, cyclo-olefin polymers such as those manufactured by ZEON Corporation of Japan (e.g., ZEONOR ^® ). In one embodiment, injection molding of the two substrates are performed simultaneously or substantially at the same time, e.g., as a time-saving measure in a production sequence. However, it is also possible that the injection molding be done sequentially or at different times, if desired.

A reflective layer 304 is then formed, e.g., by sputtering a reflective material, on the data-bearing or structured surface 302D of the substrate 302. This reflective layer 304 is referred to as being associated with the first data layer (for data molded in substrate 302) because it is used to reflect an optical beam for reading the data. Various reflective materials may be used, e.g., silver alloy, aluminum, among others, with a thickness of less than about 50nm. The choice of the material and thickness combination depends on the specific optical disc formats and specifications.

As shown in FIG. 3a, the two discs or substrates 302, 350 are bonded together with their structured surfaces 302D and 35OD facing each other. At least one adhesive or bonding layer 306 is used with a suitable bonding technique, including for example, those used in conventional bonding of digital video discs (DVD). The substrate 350 is then removed from the adhesive layer 306 using conventional disc stripping techniques developed for DVD and BD manufacturing processes. The resulting structure, shown in FIG. 3 b, includes data pits or structures on the adhesive layer 306 (complementary to those molded on substrate 350), which correspond to the data layer Ll . In general, the adhesive layer 306 has properties that promote good adhesion to the metal layer 304 of substrate 302, and facilitate removal from the substrate 350 such that essentially no adhesive remains on the substrate 350.

In one embodiment, the bonding layer 306 is made of two layers of different adhesive materials (separated by a dashed line, as shown in FIG. 3a). In this case, a first bonding layer 306a can be spin-coated onto the reflective layer 304, and a second bonding layer 306b can be spin-coated onto the structured surface 350D of the second substrate 350. The two substrates 302 and 350 are bonded to each other via adhesive layers 306a and 306b. These

bonding layer materials are typically UV-curable acrylate resins designed with different adhesive properties, e.g., the first bonding layer 306a has good adhesion to the reflective layer 304; and the second bonding layer 306b has good adhesion to the first bonding layer 306a, but relatively poor adhesion to substrate 350. Thus, the substrate 350 can be separated from the bonding layer 306b without leaving any residual adhesive on the surface 35OD.

FIG. 3c shows that a second reflective layer 308 is formed, e.g., by sputtering, onto the adhesive layer 306. The reflective layer 308, which in this case, is associated with the data formed on the adhesive layer 306, is typically a partially reflective layer, e.g., a silver alloy or other suitable metals or materials, which allows sufficient transmission of a read-out beam for accessing the data surface 302D.

As shown in FIG. 3d, the substrate 350, which has previously been removed (see FIG. 3a-b), is bonded to the first substrate 302 using an adhesive or bonding layer 352, resulting in a disc 300 with a final or composite substrate (having components 302 and 350) of predetermined thickness, e.g., in accordance with disc specification or established standard for the specific disc format. The bonding layer 352 may have a thickness range of about 1 Oμm to 20μm.

Other embodiments may use different thickness combinations for the first and second substrates, with the sum of their thickness and those of other layers (e.g., bonding layer, cover layer, and so on) being equal to the final desired disc thickness. For optical discs with a required substrate thickness (e.g., specified by a standard), the first and second substrates 302, 350 are chosen such that a sum of their thickness and those of one or more bonding layers 352 between them is equal to the specified substrate thickness. Certain practical considerations (e.g., commercial availability of disc stocks, compatibility with established standards for discs and equipment, ease of handling and processing, and so on) may also affect the selection of the thickness combination of these substrates.

In this example, the substrate 350 is the same one that has been used as a stamper for adhesive layer 306 (see FIG. 3a) of this disc 300. However, it is also possible that a different substrate be used to bond to substrate 302 in forming disc 300. This different substrate may, for example, be one that has been used as a stamper and subsequently removed (as in FIG. 3a-b) during the fabrication of a different disc. Alternatively, it can simply be another substrate having a thickness about equal to that of substrate 350.

In this illustration, the structured surface 350D of substrate 350 is bonded to the top surface 302S, i.e., the blank or non-data surface, of substrate 302. Alternatively, the blank surface 350S of substrate 350 may be bonded to the substrate 302. Since the bonded composite substrate does not have to provide an optical path for a read-out beam (the structured surface 350D is not a data layer), the bonding quality is less stringent than otherwise. However, it is preferable that the respective bonding surfaces of substrates 302 and 350 be mechanically compatible, e.g., sufficiently flat and clean, to facilitate effective bonding to each other without causing potential reliability problems during subsequent processing or use. In one example, the composite or bonded substrate (from bonding substrate 302 to 350) may have a radial tilt or skew that is less than +2 degrees. The top surface of the composite substrate (e.g., 350S in FIG. 3d) may then be printed with appropriate disc label information, as needed.

A transparent cover layer 310, e.g., 75μm of a material such as a resin, and optionally, at least one protective layer 312, e.g., hard-coat lacquer, are formed over the reflective layer 308, as shown in FIG. 3d. The cover layer 310 and hard-coat layer 312 may be formed from a variety of transparent materials, including UV-curable resins, to allow transmission of a read-out beam for reading the data. These cover and hard-coat layers may be formed using conventional techniques, including spin-coating, among others.

In another example, the composite substrate, when combined with other layers and/or processing (e.g., cover and hard-coat layers, label printing), will result in a final or finished optical disc having one or more characteristics, e.g., flatness, radial tilt, among others, that comply with specifications in an applicable standard. Thus, in accordance with one BD specification, the final printed disc with its cover layer and hard-coat will have a radial tilt or skew of +0.7 degrees under normal test conditions (or +0.8 degrees under "sudden change" environmental conditions).

As previously mentioned, the standard total substrate thickness for a BD is 1.1mm, and the final disc has a thickness (e.g., including cover and hard-coat layers) of 1.2mm. In one example, the substrate 302 may have a thickness of about 0.58mm, and bonded via a 20μm adhesive layer 352 to the substrate 350 having a thickness of 0.5mm. Other embodiments may provide for different thickness combinations for substrates 302 and 350 and adhesive layer 350.

The method steps described in connection with FIGS. 3a-d are illustrative of one embodiment of the present principles, and other embodiments may involve variations of materials, thickness used in one or more steps, or method steps performed in different orders. For example, sputtering of the reflective layer 308 and the bonding of substrates 302 and 350 may also be performed in a reverse order different from that described above.

Although the above example relates to a dual-layer pre-recorded BD, the method may also be adapted to forming other types of optical discs, in which a substrate, which is otherwise discarded in conventional processing, is re-used in the manufacturing process and incorporated in the final product. For example, the method may be adapted to forming recordable dual-layer BDs, or certain types of double-sided, dual-layer DVD (different material layers may be used in these disc formats, as known to those skilled in the art). In the latter example, such as DVD- 18 format, there is a total of four data layers formed between two transparent substrates, each of which has a thickness of about 0.6mm. According to the present principles, at least one or both of these two substrates may be a composite substrate having two components, each with a thickness of about 0.3mm (or other thickness combinations adding to a total of 0.6mm). However, unlike the BD example, the composite substrate of the DVD- 18 disc should be bonded with sufficiently high quality to allow transmission of a read-out beam with reliable data access. Furthermore, during fabrication, the stripping of one of the component substrates must be done so that there is no residual adhesive on the substrate to ensure a sufficiently high quality optical path for the read-out beam. Suitable materials and bonding adhesives for use with DVD- 18 include those used in conventional DVD manufacturing processes.

FIG. 4 illustrates various steps in a method 400 for forming an optical disc according to the present principles. In step 402, a first substrate is provided with a first surface and a second surface. The first surface contains structures representing data, while the second surface is a blank surface. In step 404, a second substrate is provided with structures formed on a first surface. In one embodiment, data on the first surface of the first substrate correspond to data in a first data layer, and data on the first surface of the second substrate correspond to data in a second data layer. In step 406, the first substrate is bonded to the second substrate using at least one adhesive layer, with the first surface (i.e., structured or data-bearing surface) of the first

substrate facing the first surface (i.e., structured or data-bearing surface) of the second substrate.

In step 408, the second substrate is removed, with the structures on the second substrate being transferred to the adhesive layer remaining on the first substrate. In step 410, a third substrate, which may be the second substrate from step 408 or another substrate having substantially the same thickness as the second substrate, is bonded to the second surface of the first substrate. The second substrate (or another substrate with substantially the same thickness) may be bonded with either its first surface or second surface facing the first substrate. In another embodiment, a computer readable medium (e.g., memory, storage device, removable media, and so on) is provided with stored program instructions, such that, when executed by a processor, will perform a method, e.g., method 400, for forming one or more optical discs according to the present principles.

Although the above examples and discussion are presented for a Blu-ray disc, one or more features discussed herein may be adapted to other optical discs with different formats, including for example, other multi-layer discs, double-sided discs, pre-recorded or read-only discs, as well as recordable and hybrid discs.

While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.