Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2000-05-11
2003-01-14
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C428S064400, C369S283000
Reexamination Certificate
active
06507559
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical recording medium, in which a substrate is bonded to a light transparent cover layer smaller in thickness than the substrate, one or more recording layer is provided between the substrate and the light transparent cover layer, and light is emitted from a light source by using an object lens from the side of the light transparent cover layer so as to record and reproduce information.
BACKGROUND OF THE INVENTION
An optical information recording method has a variety of advantages including (1) recording and reproducing without making contact, (2) a recording density higher than a magnetic recording method by more than one digit, and (3) applicability to all the memory types of a Read-only type, a Write Once Read Many type, and a Rewritable type. The method also realizes a large-capacity file at low cost. Thus, a wide range of uses has been considered from industrial use to commercial use.
An optical recording medium such as an optical disk is adopted for this optical information recording method. A compact disk (CD) used as a music disk for reproduction and a mini disk (MD) for recording and reproduction have widely prevailed in the market. Such CD and MD have a light transparent substrate with thickness of 1.2 mm. One of the surfaces of the light transparent substrate includes an information recording layer and a protecting layer for protecting the information recording layer. In such CD and MD, light with a wavelength of 780 nm is emitted from the opposite side of the information recording layer to the light transparent substrate via an object lens having an NA (numerical aperture)=0.45 so as to record or reproduce information.
Incidentally, there has been demand for storing a large amount of information (data) such as moving picture information in the optical disk. Thus, a higher recording density has been considered. In the case of the optical disk, the recording density generally depends upon a spot size of a light beam on the optical disk. The spot size increases proportionally to &lgr;/NA (&lgr; represents a wavelength of a light beam, and NA represents a numerical aperture of an optical system); thus, a shorter wavelength of a light beam and a larger numerical aperture considerably contribute to a higher recording density.
However, a coma caused by the tilted optical disk increases proportionally to the cube of the NA, so that a larger numerical aperture of the optical system results in an extremely small tilt margin (allowable tilt angle) of the optical disk. Therefore, a slight tilt of the optical disk causes a blurred spot of the light beam, so that a high recording density cannot be achieved.
When the optical disk has the light transparent substrate with a thickness of t, the coma increases proportionally to t·NA
3
. Regarding a digital versatile disk (DVD), an optical system is used with a large numerical aperture of NA=0.6, and a wavelength of a light beam is shortened from 780 nm to 650 nm so as to achieve a higher recording density. A thickness of the light transparent substrate is set at 0.6 (mm), which is smaller than those of the CD and the MD, so as to obtain a sufficient tilt margin even in the case of the optical system having a large numerical aperture of NA=0.6. Such a high recording density can achieve a recording capacity of 4.7 GB, which is equivalent to no less than 2 recording hours of moving picture signals.
However, the demand for a larger capacity and a longer recording time has been further increasing. In order to respond the aforementioned problem, an optical recording medium for storing data of 8 GB per one side is disclosed in Japanese Published Unexamined Patent Publication No. 302310/1998 (Tokukaihei 10-302310).
In the invention of the publication, a numerical aperture NA of an optical system is changed from 0.6 to 0.78 so as to achieve a high recording density. However, as described above, in the case of a larger numerical aperture of the optical system, a tilt margin (allowable tilt angle) enough for suppressing a coma becomes strict (smaller). The coma is caused by tilt of the optical disk. Therefore, in this invention, a thickness of the light transparent substrate is set smaller so as to minimize the likelihood of a coma caused by a tilt angle.
In this way, when the NA is increased to about 0.8, the thickness of the light transparent substrate needs to be reduced to about 0.1 mm. However, when the light transparent substrate is smaller in thickness, rigidity cannot be maintained by the light transparent substrate alone. It is therefore necessary to provide a reinforcing substrate on the back of the light transparent substrate with reference to a light source.
Hence, the optical disk used for the optical system with a large NA has a construction in which the thick substrate with high rigidity and a thin light transparent cover layer are bonded to each other, and the optical disk further includes a recording layer between the substrate and the light transparent cover layer.
FIG. 11
shows a sectional drawing showing the construction of the optical disk. In this optical disk, a substrate
101
is used, which is made of a thermoplastic resin and includes guide concave
110
transferred upon molding. A recording layer
3
is formed on the substrate
101
by sputtering, and a light transparent cover layer
102
is bonded onto the recording layer
103
via an ultraviolet curing resin
104
.
Further, in a pickup optical system having a large numerical aperture of about NA=0.8, a focal length of a lens is shorter, resulting in a short distance of approximately 0.3 mm between the lens and a surface of an optical disk (working distance). When the working distance is short, it is necessary to consider the likelihood of an accidental collision between the optical disk and the lens and to select a material with a certain degree of hardness for the light transparent cover layer.
However, when the hard light transparent cover layer is used, the two hard layers, that differ in rigidity, are bonded to each other via an adhesive; thus, when an impact is applied to a bonding part, the light transparent cover layer is likely to be severely deformed and peeled off.
Furthermore, the bonding part of the optical disk is exposed out of an outer curved surface of the optical disk, so that the outer curved surface is likely to receive an impact upon handling the optical disk; consequently, the light transparent cover layer tends to be peeled off at a bonding surface.
Moreover, the bonding part of the optical disk is exposed at a center hole of the disk as well, so that the bonding part is likely to receive an impact upon mounting the disk into a drive; thus, the light transparent cover layer tends to be peeled off at the bonding surface.
SUMMARY OF THE INVENTION
The present invention is devised to solve the aforementioned problem. The objective is to provide an optical recording medium, in which a substrate and a light transparent layer smaller in thickness than the substrate are bonded to each other, characterized in that an outer edge and an inner edge, on which exfoliation is likely to occur, are protected so as to secure reliability against exfoliation.
In order to achieve the above objective, an optical disk of the present invention, which includes a substrate, a light transparent cover layer bonded to the substrate with a smaller thickness than the substrate, and a first recording layer formed between the substrate and the light transparent cover layer, is characterized in that the light transparent cover layer is smaller in outer diameter than the substrate.
Further, it is desirable that the substrate and the light transparent cover layer be virtually formed into disks with center holes, the light transparent cover layer be smaller in outer diameter than the substrate, and the light transparent cover layer be larger in inner radius than the substrate.
According to the above arrangement, a bonding part of the light transparent cover layer and the substrate is not e
Conlin David G.
Dick, Bronstein, Roberts & Cushman/Edwards&Angell LLP
Dinh Tan
Penny V John Joseph
Sharp Kabushiki Kaisha
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