Stamper, mold system, recording medium substrate, recording...

Details Stamper, mold system, recording medium substrate, recording... Stamper, mold system, recording medium substrate, recording...

2003-11-24

2004-12-28

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C430S270110

Reexamination Certificate

active

06835435

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a recording medium substrate, a recording medium, a stamper and a mold system for forming recording medium substrate, a substrate for optical disc, an optical disc and a production method for stampers.
2. Description of the Related Art
An optical information recording medium such as a compact disc (CD), a mini-disc (MD), and a digital versatile disc (DVD) is widely in use in audio, video, and computer systems for recording various types of information. However, recording densities of these optical information recording medium need to be further increased as the amount of information that a medium must record is expected to increase significantly.
In order to increase the recording density of recording medium, such as optical discs, it is crucial to reduce the spot diameter of focused laser beam. This requires the use of laser beam with a shorter wavelength or objective lenses with a higher numerical aperture (NA) in the laser optical system.
The laser beam is focused on a data recording surface through a light-transmitting layer formed over the data recording surface. An attempt to decrease the wavelength of the laser beam or increase the NA of the objective lens is likely to cause a significant optical aberration. For this reason, the thickness of the light-transmitting layer must be kept as small as possible so arecordduce the optical aberration.
Production of CD and MD typically involves the use of laser beam with a wavelength of 780 nm, a lens with an NA of 0.45, and a light-transmitting layer with a thickness of 1.2 mm; whereas production of DVD, a recording medium having a higher recording density than CD, requires the use of laser beam with a wavelength of 650 nm, a lens with an NA of 0.6, and a light-transmitting layer with a thickness of 0.6 mm. A substrate formed through the injection-molding process using a transparent material can be used to serve as such a light-transmitting layer.
In the pursuit of higher recording densities, even thinner light-transmitting layers are required as the wavelength of the laser beam is further decreased or the NA increased. However, reduction in the thickness of the substrate while maintaining a high precision has become ever more difficult through the conventional injection-molding process (including injection-compression). In other words, with optical discs of the conventional construction, the attempt to increase the recording density by decreasing the wavelength of laser beam or increasing the NA of the objective lens faces a technical limitation due to the difficulty in making a thinner substrate to serve as the light-transmitting layer.
To overcome such a technical limitation, some optical discs, such as that disclosed in Japanese Patent Laid-Open Publication No. Hei 10-289489, is constructed by first forming a relatively thick protection plate to serve as a substrate. This substrate is formed through injection-molding and does not need to transmit light. A film for data recording/a reflective film for data retrieval is then formed on a data recording surface of the substrate, followed by formation of a thin light-transmitting layer on top of the film. This construction not only permits formation of thin light-transmitting layers but also provides mechanical strength for the overall structure. Thus, such optical discs can readily be adapted to laser beam with a decreased wavelength or a lens with an increased NA.
The light-transmitting layer may be formed by spin-coating a UV-curing resin solution onto a substrate and curing the resin, or by placing a separately prepared transparent sheet over the substrate and applying an adhesive such as a UV-curing resin or an adhesive material to bind the sheet to the substrate. However, the approach using the transparent sheet to be bound to the substrate has strict requirements for the optical characteristics and the precision in thickness of the light-transmitting layer and thus leads to a high manufacturing cost. In comparison, the approach using the spin-coating process can achieve the required characteristics relatively easily. Specific examples of the spin-coating processes used to form light-transmitting layers are disclosed, for example, in Japanese Patent Laid-Open Publication No. Hei 10-289489, Japanese Patent Laid-Open Publication No. Hei 11-73691 and Japanese Patent Laid-Open Publication No. Hei 11-203724.
Although the recording density can be increased through the use of the optical discs with the above-described structure, which includes a protection plate to serve as a substrate, these optical discs have problems in terms of their manufacturing technologies. These problems are as described below:
1) Problem of Errors in Substrate Contruction
When injection-molded, the substrate may have a larger thickness in the area near its periphery than in the area near its center because different areas of a substrate cool down at different rates. Specifically, as shown in
FIG. 12
, a bump
2
, called as “ski jump”, may be formed on the periphery of an injection-molded plastic substrate
1
. The bump
2
may affect the incident angle and the reflection angle of the laser beam or it may collide with the laser optical system such as an objective lens. For this reason, the substrate must be post-treated to remove the bump
2
as described in, for example, Japanese Patent Laid-Open Publication No. 5-200791. This leads to an increased manufacturing cost.
2) Problem of Flash Formed on Substrates
Referring to
FIG. 13
, a mold system
100
is shown for manufacturing substrates of the type described above. The mold system
100
includes a ring mold
102
to form an outer peripheral surface of a substrate
1
, planar molds
104
and
106
arranged on opposite sides of the ring mold
102
to form top and bottom surfaces of the substrate
1
, respectively, and a pair of stampers
108
placed adjacent to the planar molds
104
and
106
. Only one stamper
108
may be placed adjacent to one of the planar molds
104
and
106
although two of them are arranged in the example shown. When only one stamper
108
is arranged adjacent to either one of the planar molds
104
and
106
, the other of the planar molds
104
and
106
may be formed integrally with the ring mold
102
.
The stamper
108
includes a transfer surface
108
A on which microscopic projections/recesses are formed to correspond to the information signals and information grooves to be record. The substrate
1
with a surface having the microscopic projections/recesses transferred from the transfer surface
108
A of the stamper
108
can be obtained by filling a cavity defined by the mold system
100
with a resin material. The area including the transferred microscopic projections/recesses serves as a data recording region.
However, as shown by an enlarged view in FIG.
14
(A), the resin material tends to seep into the gap between the ring mold
108
and the stamper, forming a flash B on the circumference of the substrate
1
. As shown in FIG.
14
(B), when the substrate
1
is removed from the mold system
100
, the flash B may be bent in a direction perpendicular to the surface of the substrate to project from the data recording region (i.e., the surface of the substrate) or it may come off the substrate
1
and stick to the data recording region. If the substrate
1
with such flash B is subjected to spin-coating to form a light-transmitting layer, the flash B causes accumulation of the coating solution. This results in non-uniform, or defective film formation. For this reason, the flash B must be cut off or removed in advance. This is also the case with the conventional optical recording medium, which employ the substrate to serve as a light-transmitting layer.
3) Problem of Damage to Molds
As described above with reference to
FIG. 14
, the stamper
108
and the ring mold
102
need to be pushed against one another by applying a substantial force in order to eliminate the gap and thereby reduce the occurrence of the flash B. This can damage edges

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