Dynamic information storage or retrieval – Storage medium structure – Layered
Reexamination Certificate
1998-12-23
2003-10-21
Korzuch, William (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Layered
C369S275200, C428S064200
Reexamination Certificate
active
06636476
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium comprising a substrate, and at least a recording layer and light-transparent layer formed on the substrate, and a method of manufacturing the optical recording medium. More particularly, the present invention concerns an optical recording medium having a multilayered substrate structure contributing to an improved strength and durability and also capable of recording data with a high density, and a method of manufacturing the optical recording medium.
2. Description of Related Art
Optical and magnetic recording media are widely used to record various kinds of information such as audio, video and other information. More particularly, they are generally classified into an optical disc in which information signals are previously written in the form of embossed pits, a phase-change optical disc in which information signals are written utilizing the phase change of its recording layer, a magnetooptic optic in which information signals are written utilizing the magneto-optical effect of its recording layer, and a magnetic disc in which information signals are magnetically written.
These disc-like recording media use a resin-made substrate in which phase pits, pre-grooves, and so forth are formed as data, tracking signals, and so forth in the form of delicate grooves and lands.
Conventionally, such a resin-made disc substrate is molded using an injection mold as shown in FIG.
1
.
FIG. 1
is a sectional view of the injection mold.
The injection mold is generally indicated with a reference
100
. It consists essentially of a fixed mold
101
a
to form one of the main surfaces of a disc substrate, a movable mold
101
b
disposed opposite to the fixed mold
101
a
to form the other main surface of the disc substrate, and a mold
112
to form a periphery of the disc substrate. The fixed mold
101
a
and movable mold
101
b
are provided each with a stamper having formed thereon grooves and lands corresponding to a desired groove/land pattern indicative of information signals, and so forth.
The movable mold
101
b
is moved toward and away from the fixed mold
101
a
by a driving mechanism (not shown). When clamped together, the fixed mold
101
a
, movable mold
101
b
and peripheral mold
112
define together a cavity
111
.
The fixed mold
101
a
has provided therein and positioned in the center of the cavity
111
a nozzle
114
to fill, by injection, the cavity
111
with a molten synthetic resin into the cavity.
In the injection mold
100
having the above configuration, the movable mold
101
b
is first moved toward the fixed mold
101
a
(clamping) by the driving mechanism not shown) to define the cavity
111
. Next, the molten synthetic resin is filled into the cavity
111
by injection through the nozzle
114
.
Then, the injected synthetic resin is cooled by a thermoregulator (not shown) to a slush state. The movable mold
101
b
has a first ejecting member
116
disposed therein. A punch
117
is thrust from the central hole of the ejecting member
116
toward the fixed mold
101
a
to make a hole which will be a center hole in a disc substrate. Thereafter, in the injection mold
100
, the injected synthetic resin is hardened by cooling by the thermoregulator (not shown).
Then, in this injection mold
100
, the movable mold
101
b
is moved away from the fixed mold
101
a
(mold opening) by the driving mechanism (not shown). Finally, the disc substrate formed in the cavity
111
is taken out by a stripping mechanism (not shown).
Thereafter, a recording layer, reflective layer, protective layer, and so forth for example, are formed on the resin-made disc substrate thus molded to produce an optical disc.
However, the above-mentioned injection molding used to produce an optical disc is disadvantageous in that at the step in which a molten resin is filled, by injection, in the injection mold, a change of the injecting pressure, change of the injecting temperature and a friction between the molten resin and mold will cause stresses in the molten resin in the cavity.
More particularly, such stresses will take place at the following steps. First, during injection of the molten resin into the injection mold
100
, the molten resin injected in the cavity
111
will flow to cause a shear stress. Next, when the injection of the molten resin into the cavity
111
is complete, a screw (not shown) for injecting the resin abruptly stops moving and also the molten resin stops flowing abruptly. Thus their respective inertia will take place as stress. Also in the process up to gate sealing for injection of the molten resin, the molten resin is pressurized to prevent the molten resin from flowing and a sink from taking place due to a volumetric shrink of the molten resin. An uneven pressure distribution in the entire disc substrate will result in a stress. Especially when cooling to harden the molten resin, an uneven temperature distribution will take place in an outer portion
120
a
(see
FIG. 2
) of the molten resin in contact with the fixed mold
101
a
and movable mold
101
b
as well as in an inner portion
120
b
(also see
FIG. 2
) not in contact with the molds, thus causing a stress.
A part of such stresses will be partially relaxed in the process until the molten resin is cooled and hardened in the mold, but the majority will reside, not relaxed, as a residual stress in the molded disc substrate.
As a result, the molded disc substrate
120
will be subject to deformations such as partial warpage
121
and sink
122
and an uneven distribution of birefringence or double refraction as shown in FIG.
2
.
The disc substrate molded from a resin by injection molding is unavoidably shrunk in the molding process, especially at the cooling step. More particularly, the shrinkage is different in the outer periphery from in the inner periphery of the disc substrate in many cases. The outer periphery of a disc substrate
130
warps, resulting in a dish-like deformation as shown in FIG.
3
.
Thus, to minimize the deformation of the disc substrates
120
and
130
, it is required for the conventional injection molding that mold clamping should be done with a reduced force and injection be made at a slower speed to reduce the packing rate, thereby reducing the pressure inside the resin. However, such measures taken in the conventional injection molding for manufacture of disc substrates are very troublesome but cannot practically attain any completion elimination of the deformation of the disc substrates
120
and
130
.
Because of such deformation of the disc substrates
120
and
130
, a predetermined land/groove pattern cannot be formed on the disc substrates
120
and
130
with a high accuracy but the disc substrate
120
incurs a poor stamping
123
of the land/groove pattern as shown in FIG.
4
. As the result, the optical disc produced using such disc substrate
120
or
130
is disadvantageous in that its signal characteristic is not satisfactory.
The recording density of an optical disc depends upon a diameter of a laser spot focused on a recording layer of the optical disc. That is, the smaller the laser spot diameter, the higher the recording density is. The laser spot diameter is proportional to a product &lgr;/NA (&lgr;: laser wavelength and NA: numerical aperture) of a reading/writing optical system. For an increased recording density in an optical disc, it is necessary that a laser having a shorter wavelength &lgr; and an objective lens having an increased numerical aperture NA should be used.
However, such an increased NA of the objective lens will raise a problem of coma aberration because the coma is proportional to ([skew angle]×NA
3
×[optical disc thickness through which a laser light passes]). To cope with this coma problem, it has been proposed to reduce the thickness of the transparent substrate for optical disc.
Normally, however, there is a relationship that the strength of an optical disc is proportional to a cube of the thickness
Akiyama Yuji
Arakawa Nobuyuki
Kashiwagi Toshiyuki
Chu Kim-Kwok
Kananen Ronald P.
Korzuch William
Sony Corporation
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