Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2002-10-30
2004-09-14
Crispino, Richard (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S273700, C156S286000, C156S289000, C156S323000, C427S164000, C427S296000
Reexamination Certificate
active
06790306
ABSTRACT:
BACKGROUND OF THE INVENTION
Detailed Description of the Invention
The present invention relates to a method for forming a cover layer on a disk. In particular, the invention involves the formation of a cover layer on a disc that must meet the high accuracy requirement of new generation disks.
Ever since the development of compact disc (CD) in the early 80's, research into disks with greater capacity and better quality and related applications are continuously in progress. For instance, high-capacity Digital versatile Disc (DVD) went public in 1997, with capacity increasing from hundreds of megabytes (MB) to gigabytes (GB). For example, from a single-sided and single-layer DVD
5
(4.7 GB) to a double-sided and double-layer DVD 18 with a capacity of 18 GB. Nevertheless, this improvement on capacity is still considered inadequate for the display and recording of digital broadcasting and high density TV (HD-TV). As a result, High Density DVD (HD-DVD) having greater capacity are currently under intensive development.
Generally speaking, the method of increasing the storage capacity of discs could be achieved by either using a laser source having a shorter wavelength or by increasing the Numerical aperture(NA) of the objective lens. Currently, HD-DVD is using the methods as mentioned above to increase capacity of discs. For HD-DVD, compared to DVD, blue laser having 405 nm wavelength (650 nm for DVD) and numerical aperture is relatively increased to 0.85 (0.6 for DVD). In doing so, the storage capacity of discs could be increased to 25 GB, and even 50 GB. This conforms to the requirement of the recording capacity for the 133 min program for high display quality television.
This new generated type of discs, like the HD-DVD disc, adopts a relatively high NA, allowing increased storage density. However, it also causes adverse image aberration, such as increased spherical aberration, coma and astigmatism, In addition, the thickness of the cover layer must be reduced to compensate for the higher NA value. Using a DVR disc
10
under development as an example, basic structure is shown in
FIG. 1
, where
11
represents the substrate,
12
represents the structure of the recording layer, and
13
represents a cover layer
13
, wherein thickness of the cover layer
13
is adjusted from 0.6 mm (for DVD) to merely 0.1 mm. Furthermore, due to image aberration, the working accuracy for the surface of the cover layer must be increased as well, or a tiny thickness difference can cause image aberration and distortion.
The conventional cover protection layer for CD discs is using lacquer, the protective lacquer layer is under the printing layer of the disc. This lacquer is mostly coated onto CDs by spin coating (curtain coating or pastering could be used as well). Then the lacquer is quickly cured under ultraviolet (UV) light. The cured surface is a finished protective layer having high hardness. As various formulations for the lacquer exhibit different reaction rates, some lacquers cure instantly, and others lose their fluidity and plasticity slowly, curing gradually. This time period before curing may be a few hours or even longer, depending on various applications.
As mentioned above, there is a specific thickness requirement of the cover layer (0.1 mm) for the new generation of discs. When the cover layer is formed by spin coating, due to lacquet's viscosity, distribution of fluidity, intrinsic characteristics and curing time, the actual coating usually results in uneven distribution of thickness. The result of spin coating is shown in FIG,
2
, where two ends
211
and
212
of the uncured cover layer
21
exhibit coagulation due to surface tension. In addition, when observed on a microscopic scale, the surface of the cover layer also illustrates an uneven plain (refer to FIG.
5
). Having a surface that is not planar and the thickness not even and uniform increases the error rate when reading data. It is judged from the experimental results that it is difficult to control the cover layer thickness variation within an acceptable range of ±3% (that is 0.1+0.003 mm). In actual production, almost each disc exhibits its distinct surface distribution characteristics, thus yields of discs are seriously affected, and production costs go up.
In order to overcome the above problems, an object of the invention is to provide a more stable method for forming as a cover layer on a disc, thereby obtaining a cover layer having an even and smooth surface that meets the accuracy requirement for disc reading and recording.
Refer to
FIGS. 3
,
4
and
5
, which illustrate the formation of the cover layer according to the invention. A cover layer (e.g. uncured lacquer) is formed evenly by spin coating or any other suitable method on the recording layer
12
on the substrate
11
of a HD-DVD disc. This cover layer is mostly fluid, and does not harden before curing. For example, an uncured cover layer
31
only hardens when exposed under ultraviolet (UV) light. Average thickness of the uncured cover layer
31
must be enough to meet the required predetermined thickness after curing (or after pressing). In general situations, uncured cover layer must be greater than the predetermined laminating (press) thickness, for example, 0.1 mm. There may be some exceptions, such as when a substance that swells after reaction is used, in which the predetermined thickness of the cover layer can be relatively thinner. Then a capping board
41
is used to press against the uncured cover layer
31
, thereby maintaining the predetermined thickness, as shown in FIG.
4
. By doing so, the uncured cover layer
31
between the capping board
41
and the substrate
11
is pressed, thus the rising surface
311
moves in the direction of the arrow (FIG.
5
), and the dent portion
314
rises in the direction of the arrow simultaneously. Similarly, uneven surfaces, such as ridges
312
,
313
and dent portions
315
,
316
are squeezed by the undersurface
411
of the capping board
41
to form a smooth and flat surface. In such a way, the surface of the cover layer is able to meets the requirement of the planarity of cover layer.
As the uncured cover layer may be UV curable cationic lacquer, or free radical lacquer, which all cure after exposure to UV light. However, for cationic lacquer, the difficulty associated with the curing step is to quickly press the capping board
41
onto the cover layer
31
before the uncured cover layer cures thereby repairing and maintaining a flat and smooth surface at a microscopic scale (the tolerance for cover layer thickness variation is within ±3%). This can only be achieved by appropriate cooperation between the reactants in the cover layer and the curing time, and a suitable UV radiation and cover time must be applied.
Another way to achieve the above results is to use a capping board
41
having good transparency. The need to cure the lacquer layer makes it very important for the capping board to be transparent. Since most of the lacquer used is UV curable, the UV light must pass through the capping board and reach the lacquer layer to cure the lacquer. If the capping board absorbs UV light or scatter (diffuse?) UV is light, the curing may not be completed, causing adverse effects on the quality of HD-DVD discs. In the case where the capping board exhibits good transparency, the curing process is not as restricted by the covering time. The harden and curing effects can then be achieved by applying UV light through the capping board to the cover layer.
Another issue concerning the capping board is the stripping thereof after the cover layer is cured. Adhesion between the capping board and the lacquer layer after curing makes stripping very difficult, since the detachment may occurs not at the interface between the capping board and the cured lacquer layer, but at either the interfaces between the cured lacquer layer and recording layer
12
or between the recording layer and the substrate. It is essential to avoid detachment at interfaces other than between the ca
Chan Sing P.
Crispino Richard
Quintero Law Office
Ritek Corporation
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