U.S. patent application number 09/841395 was filed with the patent office on 2001-09-06 for optical information recording medium and method for producing the same.
This patent application is currently assigned to Matsushita Electric Industrial Co. Ltd.. Invention is credited to Isomura, Hidemi, Moteki, Akihiro, Ohno, Eiji.
Application Number | 20010019753 09/841395 |
Document ID | / |
Family ID | 16362376 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019753 |
Kind Code |
A1 |
Ohno, Eiji ; et al. |
September 6, 2001 |
Optical information recording medium and method for producing the
same
Abstract
An optical information recording medium includes a first
substrate; at least a first dielectric layer and a recording layer
for signal recording provided on a surface of the first substrate;
and a second substrate. The first substrate and the second
substrate are assembled together in the state of being warped in
planar symmetry and flattened. The first dielectric layer and the
recording layer is interposed between the first substrate and the
second substrate.
Inventors: |
Ohno, Eiji; (Osaka, JP)
; Isomura, Hidemi; (Osaka, JP) ; Moteki,
Akihiro; (Yawata-shi, JP) |
Correspondence
Address: |
Mark D. Saralino
RENNER, OTTO, BOISSELLE & SKLAR, P.L.L.
1621 Euclid Avenue, Nineteenth Floor
Cleveland
OH
44115-2191
US
|
Assignee: |
Matsushita Electric Industrial Co.
Ltd.
|
Family ID: |
16362376 |
Appl. No.: |
09/841395 |
Filed: |
April 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09841395 |
Apr 24, 2001 |
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09436293 |
Nov 8, 1999 |
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09436293 |
Nov 8, 1999 |
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09120081 |
Jul 21, 1998 |
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6165578 |
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Current U.S.
Class: |
428/64.1 ;
G9B/7.139; G9B/7.194 |
Current CPC
Class: |
Y10S 430/146 20130101;
Y10T 428/31507 20150401; G11B 7/24 20130101; G11B 7/26 20130101;
Y10T 156/1002 20150115; Y10T 428/21 20150115; Y10T 428/31678
20150401; Y10S 428/913 20130101 |
Class at
Publication: |
428/64.1 |
International
Class: |
B32B 003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 23, 1997 |
JP |
9-196714 |
Claims
What is claimed is:
1. An optical information recording medium, comprising: a first
substrate; at least a first dielectric layer and a recording layer
for signal recording provided on a surface of the first substrate;
and a second substrate, wherein the first substrate and the second
substrate are assembled together in the state of being warped in
planar symmetry with respect to each other and flattened, the first
dielectric layer and the recording layer being interposed between
the first substrate and the second substrate.
2. An optical information recording medium according to claim 1,
further comprising a thin layer provided on a surface of the second
substrate, the surface being opposed to the first substrate.
3. An optical information recording medium according to claim 2,
wherein the thin layer includes a second dielectric layer.
4. An optical information recording medium according to claim 2,
wherein the thin layer includes a second dielectric layer formed of
an identical material as that of the first dielectric layer
provided on the first substrate.
5. An optical information recording medium according to claim 3,
wherein the thin layer includes a metal layer.
6. An optical information recording medium according to claim 2,
wherein the thin layer has such a thickness to warp the second
substrate to substantially an equal degree with the first
substrate.
7. An optical information recording medium according to claim 1,
further comprising a resin layer formed of a resin provided on a
surface of the second substrate which shrinks when being cured, the
resin layer being provided on the surface not facing the first
substrate.
8. An optical information recording medium according to claim 7,
wherein the resin layer is formed of a UV curable resin.
9. An optical information recording medium according to claim 7,
wherein the resin layer is transparent with respect to UV
light.
10. An optical information recording medium according to claim 7,
wherein the resin layer is non-transparent with respect to visible
light.
11. An optical information recording medium according to claim 10,
wherein the resin layer has graphics thereon.
12. An optical information recording medium according to claim 1,
wherein the second substrate is warped while being formed.
13. An optical information recording medium according to claim 12,
wherein the second substrate is formed by an injection method.
14. An optical information recording medium according to claim 1,
wherein at least one of the first substrate and the second
substrate has a thickness of about 0.8 mm or less.
15. An optical information recording medium, comprising: a first
substrate; at least a first dielectric layer and a recording layer
for signal recording provided on a surface of the first substrate;
a second substrate; and a thin layer on which a signal is
unrecordable, the thin layer being provided on a surface of the
second substrate, wherein the first substrate and the second
substrate are assembled together with the first dielectric layer,
the recording layer and the thin layer being interposed
therebetween.
16. An optical information recording medium according to claim 15,
wherein the thin layer includes a second dielectric layer.
17. An optical information recording medium according to claim 15,
wherein the thin layer includes a second dielectric layer formed of
an identical material as that of the first dielectric layer
provided on the first substrate.
18. An optical information recording medium according to claim 17,
wherein the first dielectric layer and the second dielectric have a
substantially equal thickness.
19. An optical information recording medium, comprising: a first
substrate; a plurality of first dielectric layers, a recording
layer for signal recording provided between the plurality of
dielectric layers, and a metal layer, which are provided on a
surface of the first substrate; a second substrate; and a thin
layer, including at least one second dielectric layer and a metal
layer, on which a signal is unrecordable, the thin layer being
provided on a surface of the second substrate, wherein the first
substrate and the second substrate are assembled together with the
plurality of first dielectric layers, the at least one second
dielectric layer, the recording layer, the metal layer, and the
thin metal layer being interposed therebetween.
20. An optical information recording medium according to claim 19,
wherein the plurality of first dielectric layers and the at least
one second dielectric layer has a substantially equal total
thickness.
21. An optical information recording medium, comprising: a first
substrate; at least a dielectric layer and a recording layer for
signal recording provided on a surface of the first substrate; a
second substrate; and a resin layer formed of a resin which shrinks
when being cured, the resin layer being provided on a surface of
the second substrate, wherein the first substrate and the second
substrate are assembled together with the dielectric layer and the
recording layer being interposed therebetween, and the resin layer
is not interposed therebetween.
22. An optical information recording medium according to claim 21,
wherein the resin layer is formed of a UV curable resin.
23. An optical information recording medium, comprising: a first
substrate; at least a dielectric layer, a recording layer for
signal recording provided on a surface of the first substrate, and
a first resin layer formed of a resin which shrinks when being
cured; and a second substrate, wherein the first substrate and the
second substrate are assembled together with the dielectric layer,
the recording layer and the first resin layer being interposed
therebetween.
24. An optical information recording medium according to claim 23,
further comprising a second resin layer having a smaller tensile
strength than that of the first resin layer, the second resin layer
being provided on a surface of the second substrate, the second
resin layer being provided on the surface facing the first
substrate.
25. An optical information recording medium according to claim 24,
wherein the second resin layer has a smaller thickness than that of
the first resin layer.
26. An optical information recording medium according to claim 24,
wherein the second resin layer is transparent with respect to UV
light.
27. An optical information recording medium according to claim 24,
wherein the second resin layer is non-transparent with respect to
visible light.
28. An optical information recording medium according to claim 24,
wherein the second resin layer has graphics thereon.
29. An optical information recording medium according to claim 23,
wherein at least one of the first substrate and the second
substrate has a thickness of about 0.8 mm or less.
30. A method for producing an optical information recording medium,
comprising the steps of: forming at least a first dielectric layer
and a recording layer for signal recording on a surface of a first
substrate; warping a second substrate; and assembling the first
substrate, which is warped, and the second substrate in planar
symmetry with respect to each other, and flattening an assembly of
the first substrate and the second substrate.
31. A method according to claim 30, wherein the step of warping the
second substrate includes the step of forming a thin layer on a
surface of the second substrate, the surface being opposed to the
first substrate.
32. A method according to claim 31, wherein the step of forming the
thin layer includes the step of forming a second dielectric
layer.
33. A method according to claim 31, wherein the step of forming the
thin layer includes the step of forming a second dielectric layer
formed of an identical material with that of the first dielectric
layer.
34. A method according to claim 32, wherein the step of forming the
thin layer includes the step of forming a metal layer.
35. A method according to claim 30, wherein the step of warping the
second substrate includes the step of forming a resin layer of a
resin which shrinks when being cured on a surface of the second
substrate, the resin layer being formed on the surface not facing
the first substrate.
36. A method according to claim 35, wherein the step of forming the
resin layer includes the step of forming a UV curable resin
layer.
37. A method according to claim 30, wherein the step of warping the
second substrate includes the step of causing a warp to the second
substrate while the second substrate is being formed.
38. An optical information recording medium according to claim 37,
wherein the second substrate is formed by an injection method.
39. A method for producing an optical information recording medium,
comprising the steps of: forming at least a dielectric layer and a
recording layer for signal recording on a surface of a first
substrate; forming a resin layer of a resin which shrinks when
being cured on the recording layer; and assembling the first
substrate and a second substrate opposed to the first substrate
with the dielectric layer and the recording layer being interposed
therebetween.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a rewritable optical
information recording medium including two substrates assembled
together and a method for producing the same.
[0003] 2. Description of the Related Art
[0004] Technologies for performing high-density recording of
information and reproduction of such information using laser beams
are known. For example, optical disks have been put into practical
use. Optical disks are roughly classified into read-only type,
write-once type, and rewritable type. The read-only type is
realized as products such as, for example, compact disks having
musical information stored therein and laser disks having image
information stored therein. The write-once type is realized as
products such as, for example, document files and still picture
files. Currently, research and development is mainly performed on
rewritable type. The rewritable type is being realized as products
such as, for example, data files for personal computers.
[0005] A general optical disk has a transparent resin substrate
having a thickness of 1.2 mm, a recording layer provided on one
surface of the substrate, and a protective layer such as, for
example, an overcoat provided on the recording layer. Another
general optical disk includes a substrate and a protective plate
formed of the same material as the substrate which are assembled
together with an adhesive.
[0006] In order to increase the recording density of optical disks,
studies have been recently performed for shortening the wavelength
of the laser light and using an objective lens having a large
numerical aperture (NA). However, as the wavelength of the laser
light is shortened and the numerical aperture is increased, the
tolerance for the angle of the disk with respect to the incident
angle of the laser light (referred to as the "tilt") is reduced.
Reducing the substrate thickness (distance from the surface of the
substrate to the recording layer) is effective in increasing the
tolerance for the tilt. For example, the substrate thickness of a
digital video disk (DVD) is 0.6 mm. Since the resin substrate
having a thickness of 0.6 mm is insufficient in mechanical
strength, two such substrates are assembled with the recording
layer interposed therebetween.
[0007] The substrates are assembled by various methods, such as,
for example, applying a hot melt resin on a surface of one
substrate and then putting the substrates into contact with each
other, assembling the two substrates with an adhesive tape
(two-sided tape) interposed therebetween, or applying a ultraviolet
(UV) curable resin on a surface of one substrate, then putting the
two substrates into contact with each other and curing the resin
with ultraviolet rays.
[0008] It has been found that a substrate having a thickness of as
small as 0.6 mm is significantly warped when provided with a thin
layer including a rewritable recording layer on a surface of the
substrate.
[0009] Such a phenomenon does not occur when a substrate having a
thickness of 0.6 mm is provided with a thin layer including a
read-only recording layer, i.e., a metal reflective layer (formed
of, for example, Al or Au) on a surface of the substrate. The
phenomenon does not occur with a substrate having a thickness of
1.2 mm regardless of a thin film, including a rewritable recording
layer or a read-only recording layer is provided.
[0010] The cause of the warp is considered to be as follows. In the
case of the rewritable type disks, a dielectric layer is included
in the thin layer in addition to the recording layer for protecting
the recording layer. Formation of the dielectric layer generates a
large stress, and the stress warps the substrate which has an
insufficient mechanical strength.
[0011] When such a significantly warped substrate and another
substrate which is not warped are assembled together to product an
optical disk capable of one-side recording and reproduction, the
resultant optical disk is also warped. Such a warped optical disk
is not usable.
[0012] When two substrates which have the same thin layers and thus
are warped at the same degree are assembled, a highly flat optical
disk is obtained because the stresses generated in both substrates
are balanced. However, use of two substrates both having the
recording layers for producing an optical disk for one-side
recording disadvantageously raises the production costs.
SUMMARY OF THE INVENTION
[0013] According to one aspect of the invention, an optical
information recording medium includes a first substrate; at least a
first dielectric layer and a recording layer for signal recording
provided on a surface of the first substrate; and a second
substrate. The first substrate and the second substrate are
assembled together in the state of being warped in planar symmetry
with respect to each other and flattened, the first dielectric
layer and the recording layer being interposed between the first
substrate and the second substrate.
[0014] In one embodiment of the invention, an optical information
recording medium further includes a thin layer provided on a
surface of the second substrate, the surface being opposed to the
first substrate.
[0015] In one embodiment of the invention, the thin layer includes
a second dielectric layer.
[0016] In one embodiment of the invention, the thin layer includes
a second dielectric layer formed of an identical material as that
of the first dielectric layer provided on the first substrate.
[0017] In one embodiment of the invention, the thin layer includes
a metal layer.
[0018] In one embodiment of the invention, the thin layer has such
a thickness to warp the second substrate to substantially an equal
degree with the first substrate.
[0019] In one embodiment of the invention, an optical information
recording medium further includes a resin layer formed of a resin
provided on a surface of the second substrate which shrinks when
being cured, the resin layer being provided on the surface not
facing the first substrate.
[0020] In one embodiment of the invention, the resin layer is
formed of a UV curable resin.
[0021] In one embodiment of the invention, the resin layer is
transparent with respect to UV light.
[0022] In one embodiment of the invention, the resin layer is
non-transparent with respect to visible light.
[0023] In one embodiment of the invention, the resin layer has
graphics thereon.
[0024] In one embodiment of the invention, the second substrate is
warped while being formed.
[0025] In one embodiment of the invention, the second substrate is
formed by an injection method.
[0026] In one embodiment of the invention, at least one of the
first substrate and the second substrate has a thickness of about
0.8 mm or less.
[0027] According to one aspect of the invention, an optical
information recording medium includes a first substrate; at least a
first dielectric layer and a recording layer for signal recording
provided on a surface of the first substrate; a second substrate;
and a thin layer on which a signal is unrecordable, the thin layer
being provided on a surface of the second substrate. The first
substrate and the second substrate are assembled together with the
first dielectric layer, the recording layer and the thin layer
being interposed therebetween.
[0028] In one embodiment of the invention, the thin layer includes
a second dielectric layer.
[0029] In one embodiment of the invention, the thin layer includes
a second dielectric layer formed of an identical material as that
of the first dielectric layer provided on the first substrate.
[0030] In one embodiment of the invention, the first dielectric
layer and the second dielectric have a substantially equal
thickness.
[0031] According to one aspect of the invention, an optical
information recording medium includes a first substrate; a
plurality of first dielectric layers, a recording layer for signal
recording provided between the plurality of dielectric layers, and
a metal layer, which are provided on a surface of the first
substrate; a second substrate; and a thin layer, including at least
one second dielectric layer and a metal layer, on which a signal is
unrecordable, the thin layer being provided on a surface of the
second substrate. The first substrate and the second substrate are
assembled together with the plurality of first dielectric layers,
the at least one second dielectric layer, the recording layer, the
metal layer, and the thin metal layer being interposed
therebetween.
[0032] In one embodiment of the invention, the plurality of first
dielectric layers and the at least one second dielectric layer has
a substantially equal total thickness.
[0033] According to one aspect of the invention, an optical
information recording medium includes a first substrate; at least a
dielectric layer and a recording layer for signal recording
provided on a surface of the first substrate; a second substrate;
and a resin layer formed of a resin which shrinks when being cured,
the resin layer being provided on a surface of the second
substrate. The first substrate and the second substrate are
assembled together with the dielectric layer and the recording
layer being interposed therebetween, and the resin layer is not
interposed therebetween.
[0034] In one embodiment of the invention, the resin layer is
formed of a UV curable resin.
[0035] According to one aspect of the invention, an optical
information recording medium includes a first substrate; at least a
dielectric layer, a recording layer for signal recording provided
on a surface of the first substrate, and a first resin layer formed
of a resin which shrinks when being cured; and a second substrate.
The first substrate and the second substrate are assembled together
with the dielectric layer, the recording layer and the first resin
layer being interposed therebetween
[0036] In one embodiment of the invention, an optical information
recording medium further includes a second resin layer having a
smaller tensile strength than that of the first resin layer, the
second resin layer being provided on a surface of the second
substrate, the second resin layer being provided on the surface
facing the first substrate.
[0037] In one embodiment of the invention, the second resin layer
has a smaller thickness than that of the first resin layer.
[0038] In one embodiment of the invention, the second resin layer
is transparent with respect to UV light.
[0039] In one embodiment of the invention, the second resin layer
is non-transparent with respect to visible light.
[0040] In one embodiment of the invention, the second resin layer
has graphics thereon.
[0041] In one embodiment of the invention, at least one of the
first substrate and the second substrate has a thickness of about
0.8 mm or less.
[0042] According to one aspect of the invention, a method for
producing an optical information recording medium includes the
steps of forming at least a first dielectric layer and a recording
layer for signal recording on a surface of a first substrate;
warping a second substrate; and assembling the first substrate,
which is warped, and the second substrate in planar symmetry with
respect to each other, and flattening an assembly of the first
substrate and the second substrate.
[0043] In one embodiment of the invention, the step of warping the
second substrate includes the step of forming a thin layer on a
surface of the second substrate, the surface being opposed to the
first substrate.
[0044] In one embodiment of the invention, the step of forming the
thin layer includes the step of forming a second dielectric
layer.
[0045] In one embodiment of the invention, the step of forming the
thin layer includes the step of forming a second dielectric layer
formed of an identical material with that of the first dielectric
layer.
[0046] In one embodiment of the invention, the step of forming the
thin layer includes the step of forming a metal layer.
[0047] In one embodiment of the invention, the step of warping the
second substrate includes the step of forming a resin layer of a
resin which shrinks when being cured on a surface of the second
substrate, the resin layer being formed on the surface not facing
the first substrate.
[0048] In one embodiment of the invention, the step of forming the
resin layer includes the step of forming a UV curable resin
layer.
[0049] In one embodiment of the invention, the step of warping the
second substrate includes the step of causing a warp to the second
substrate while the second substrate is being formed by an
injection method.
[0050] In one embodiment of the invention, the second substrate is
formed by an injection method.
[0051] According to one aspect of the invention, a method for
producing an optical information recording medium includes the
steps of forming at least a dielectric layer and a recording layer
for signal recording on a surface of a first substrate; forming a
resin layer of a resin which shrinks when being cured on the
recording layer; and assembling the first substrate and a second
substrate opposed to the first substrate with the dielectric layer
and the recording layer being interposed therebetween.
[0052] Thus, the invention described herein makes possible the
advantages of providing a flat and low-cost optical information
recording medium, for one-side recording and reproduction,
including two thin substrates assembled together, and a method for
producing the same.
[0053] These and other advantages of the present invention will
become apparent to those skilled in the art upon reading and
understanding the following detailed description with reference to
the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] FIGS. 1A through 1F are cross-sectional views illustrating a
method for producing an optical disk according to a first example
of the present invention;
[0055] FIG. 2A is a cross-sectional view of a first substrate and
layers provided thereon of the optical disk according to the first
example of the present invention;
[0056] FIG. 2B is a cross-sectional view of a second substrate and
layers provided thereon of the optical disk according to the first
example of the present invention;
[0057] FIG. 3 shows a method for measuring the warp angle of a
substrate;
[0058] FIG. 4A is a cross-sectional view of a first substrate and
layers provided thereon of an optical disk according to a second
example of the present invention;
[0059] FIG. 4B is a cross-sectional view of a second substrate and
layers provided thereon of the optical disk according to the second
example of the present invention;
[0060] FIG. 4C is a cross-sectional view of the optical disk
according to the second example of the present invention;
[0061] FIGS. 5A through 5F are cross-sectional views illustrating a
method for producing an optical disk according to a third example
of the present invention;
[0062] FIGS. 6A through 6E are cross-sectional views illustrating a
method for producing an optical disk according to a fourth example
of the present invention;
[0063] FIG. 7 is a cross-sectional view of a mold used for forming
first and second substrate of the optical disk according to the
fourth example of the present invention;
[0064] FIGS. 8A through 8F are cross-sectional views illustrating a
method for producing an optical disk according to a fifth example
of the present invention of the present invention; and
[0065] FIG. 9 is a cross-sectional view of the optical disk
according to the fifth example of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0066] The present invention will be described by way of
illustrative examples with reference to the accompanying
drawings.
[0067] First, the principle of the present invention will be
described.
[0068] A general rewritable recording medium includes a transparent
substrate formed of, for example, a polycarbonate, a plurality of
dielectric layers provided on the substrate, a recording layer
interposed between the plurality of dielectric layers, and
optionally a reflective layer. The dielectric layers are provided
for protecting the recording layer against contamination by water
and oxygen and also for protecting the substrate against the high
temperature of the recording layer caused at the time of signal
recording. Most rewritable recording mediums include a dielectric
layer as an indispensable element.
[0069] Exemplary materials usable for the dielectric layer include
oxides of metals or semi-metal materials, nitrides, chalcogenides,
fluorides, carbides, and mixtures thereof. More specifically,
exemplary materials usable for the dielectric layer include
Si.sub.02, SiO, Al.sub.2O.sub.3, GeO.sub.2, In.sub.2O.sub.3,
Ta.sub.2O.sub.5, TeO.sub.2, TiO.sub.2, MoO.sub.3, WO.sub.3,
ZrO.sub.2, Si.sub.3N.sub.4, Ge.sub.3N.sub.4, AlN, BN, TiN, ZnS,
CdS, CdSe, ZnSe, ZnTe, AgF, PbF.sub.2, MnF.sub.2, NiF.sub.2, SiC,
or mixtures thereof, a diamond thin layer and diamond-like
carbon.
[0070] Exemplary materials usable for the recording layer for an
optical disk for phase-change recording include alloys such as
GeSbTe, InSbTe, InSbTeAg, GaSb, InGaSb, GeSnTe, AgSbTe. Recording
mediums for recording information by other mechanisms are also
usable.
[0071] The dielectric layer and the recording layer are formed by,
for example, sputtering or vacuum vapor deposition. When a thin
layer including a dielectric layer is formed on a thin substrate
having a thickness of, for example, 0.6 mm, by sputtering or vacuum
vapor deposition, the substrate is significantly warped. As
described above, when such a significantly warped substrate and
another substrate which includes no thin layer and thus is not
warped are assembled to produce an optical disk for one-side
recording and reproduction, the optical disk is also warped. Such a
warped optical disk is not usable.
EXAMPLE 1
[0072] FIGS. 1A through 1F are cross-sectional views illustrating a
method for producing an optical information recording medium
(referred to simply as an optical disk) 100 according to a first
example of the present invention. FIG. 2A is a cross-sectional view
of a first substrate 1 and layers provided thereon. FIG. 2B is a
cross-sectional view of a second substrate 2 and layers provided
thereon.
[0073] The optical disk 100 in the first example includes the first
substrate 1 and the second substrate 2, each having a thickness of
0.6 mm, which are assembled together. The first substrate 1 is
provided with a laminated information rewriting layer 3 (FIG. 1C)
including at least a dielectric layer and a recording layer. By
providing the laminated information rewriting layer 3, the first
substrate 1 warps. Accordingly, the second substrate 2 is also
provided with a dielectric layer 4 so as to be warped, and the
first substrate 1 and the second substrate 2 are assembled with the
laminated information rewriting layer 3 and the dielectric layer 4
being inside. Thus, the optical disk 100 which is flat is
obtained.
[0074] The first substrate 1 shown in FIG. 1A and the second
substrate 2 shown in FIG. 1B are formed in the same step by an
injection method. The first substrate 1 and the second substrate 2
are formed of the same material and have the same size and shape.
For example, the first substrate 1 and the second substrate 2 are
formed of a polycarbonate and each have a thickness of about 0.6
mm, a diameter of about 120 mm, and a central hole diameter of
about 15 mm. The central hole of the first substrate 1 is
represented by reference numeral 1a, and the central hole of the
second substrate 2 is represented by reference numeral 2a. The
first substrate 1 and the second substrate 2 each have a guide
groove (not shown) for signal recording in the top surfaces thereof
(top surfaces in FIGS. 1A and 1B).
[0075] As shown in FIG. 1C, the laminated information rewriting
layer 3 is provided on the top surface of the first substrate 1. In
more detail, as shown in FIG. 2A, the laminated information
rewriting layer 3 includes a dielectric layer 102 formed of
ZnS-SiO.sub.2 (mixture of ZnS and SiO.sub.2), a recording layer 103
formed of GeSbTe alloy, a dielectric layer 104 formed of
ZnS-SiO.sub.2, and a metal layer 105 formed of Al as a reflective
layer, which are laminated on the top surface of the first
substrate 1 in this order.
[0076] The dielectric layer 102 is formed of ZnS-SiO.sub.2 on the
top surface of the first substrate 1 to a thickness of about 110 nm
by sputtering. Next, the recording layer 103 formed of GeSbTe alloy
on the dielectric layer 102 to a thickness of about 30 nm by
sputtering. GeSbTe alloy reversibly changes between an amorphous
state and a crystalline state in accordance with laser light
radiation. Then, the dielectric layer 104 is formed of
ZnS-SiO.sub.2 on the recording layer 103 to a thickness of about 20
nm by sputtering. The metal layer 105 is formed of Al on the
dielectric layer 104 to a thickness of about 100 nm by sputtering.
Thus, the laminated information rewriting layer 3 is formed.
[0077] As shown in FIG. 1C, the first substrate 1 is warped, by a
compressive stress of the laminated information rewriting layer 3,
at about 1.5.degree. at an outer periphery thereof with the top
surface on which the laminated information rewriting layer 3 is
projecting. The compression stress warp is mostly generated by the
dielectric layers 102 and 104 formed of ZnS-SiO.sub.2.
[0078] The warping angle is determined as shown in FIG. 3. A plate
11 corresponding to the first and second substrates 1 and 2 is held
by a support 12 so as to be horizontal at the center thereof or in
the vicinity thereof. Laser light 13 as collimated light having a
diameter of about 1 mm is incident on the plate 11 from below. The
angle of the laser light 13 and light 14 reflected by the plate 11
is set as warp angle .alpha.(.degree.).
[0079] The tolerable warp angle for an optical disk varies in
accordance with the apparatus for reproducing the information
stored in the optical disk. For example, the warp angle needs to be
about 0.7.degree. or less.
[0080] When the first substrate 1 which is warped by the provision
of the laminated information rewriting layer 3 and the second
substrate 2, which is flat, are assembled with an adhesive, the
resultant optical disk has a warp angle of about 1.degree. or more,
whether the adhesive is a hot melt resin or a UV curable resin, and
is not usable.
[0081] As shown in FIG. 2B, a dielectric layer 4 is formed of
Zns-SiO.sub.2 on a top surface of the second substrate 2 to a
thickness of about 130 nm by sputtering. Thus, the second substrate
2 has the same warp angle with that of the first substrate 1 as
shown in FIG. 1D.
[0082] As shown in FIG. 1E, a UV curable resin 7 is dropped on the
laminated information rewriting layer 3 in the vicinity of the
inner circumference of the first substrate 1 in a concentric
manner, and the first and second substrates 1 and 2 are assembled
together in the state of planar symmetry with respect to each other
with the layers thereon being inside. Next, as shown in FIG. 1F,
the assembled first and second substrates 1 and 2 are held between
glass plates 8 and 9, thereby substantially eliminating the warp of
the first and second substrates 1 and 2. Then, the assembled body
is irradiated with UV light 10 from the side of the second
substrate 2, thereby curing the UV resin 7. After removing the
glass plates 8 and 9, the optical disk 100, which is flat, is
obtained.
[0083] The ZnS-SiO.sub.2 used for the dielectric layer 4 is
substantially transparent with respect to UV light.
[0084] The tilt of the optical disk 100 produced in this manner is
about 0.5.degree. or less and thus is sufficiently usable. In the
above-described example, the first and second substrates 1 and 2
are assembled using UV light. The same effect is achieved by a hot
melt method using a hot melt resin or an adhesive tape. According
to an alternative method, a UV curable resin which is cured slowly
is applied to surfaces of the top layers on the first and second
substrates 1 and 2 by spin coating or printing. Hereinafter, such a
method will be referred to as a "slowly-effective UV radiation
method". Next, the resin is irradiated by UV light to provide the
resin with tackiness. Then, the first and second substrates 1 and 2
are pressed together, and then the resin is completely cured. The
same effect is achieved by this method. Alternatively, the glass
plates 8 and 9 for interposing the first and second substrates 1
and 2 can be replaced with plates of other materials. The warp of
the first and second substrates 1 and 2 can be eliminated by other
methods.
[0085] In the above-described example, the first and second
substrates 1 and 2 are identical substrates formed by an injection
method. The two substrates can be produced by different molds. The
present invention is not limited to the method of production of the
substrates.
[0086] The dielectric layers 102 and 104 can be formed of the same
material as the dielectric layer 4. In such a case, the dielectric
material which is isolated from the single target in the same
sputtering apparatus can be deposited on the first and second
substrates 1 and 2. Thus, the dielectric layers 102 and 4 can be
formed in the same step.
[0087] It is found that, as the thickness of the dielectric layer 4
provided on the second substrate 2 increases, the warp angle of the
second substrate 2 increases. When the thickness of the dielectric
layer 4 is about 130 nm, the warp angle of the second substrate 2
becomes almost the same as that of the first substrate 1. When the
first and second substrates 1 and 2 have substantially the same
warp angle, the stresses in the two substrates are balanced and
thus the warp of the optical disk 100 is reduced. The thickness of
the dielectric layer 4 provided on the second substrate 2 is
preferably determined to warp the second substrate 2 to about the
same degree as the first substrate 1. Specifically, in the case
where the same material is used for the dielectric layers on the
first substrate 1 and the second substrate 2, it is most preferable
that the total thickness of one or more dielectric layers on the
first substrate 1 is equal to the total thickness of one or more
dielectric layers on the second substrate 2.
[0088] A metal layer as a reflective layer formed of, for example,
Al or Au can be provided on the dielectric layer 4 on the second
substrate 2. In this case, UV light cannot be used for assembling
the first and second substrates 1 and 2. A hot melt method, a
slowly-effective UV radiation method or the like can be used.
Providing the metal layer on the dielectric layer 4 is advantageous
in improving the external appearance since, even when bubbles are
present on the surface on which the resin is applied, such bubbles
are covered by the metal layer.
[0089] The laminated information rewriting layer 3 can have a
different structure from described above. The metal layer 105 can
be eliminated, or each or both dielectric layers include a
plurality of layers formed of different materials.
EXAMPLE 2
[0090] FIGS. 4A through 4C show an optical information recording
medium (referred to simply as an optical disk) 200 according to the
second example of the present invention. FIG. 4A is a
cross-sectional view of a first substrate 5 and layers provided
thereon, and FIG. 4B is a cross-sectional view of a second
substrate 6 and layers provided thereon. FIG. 4C is a
cross-sectional view of the optical disk 200 including the first
and second substrates 5 and 6 and the layers provided thereon.
[0091] As shown in FIG. 4A, provided sequentially on the first
substrate 5 are a dielectric layer 202 formed of ZnS-SiO.sub.2, a
dielectric layer 203 formed of GeN, a recording layer 204 formed of
GeSbTe alloy, a dielectric layer 205 formed of GeN, a metal layer
206 as a reflective layer formed of Al, and an overcoat layer
207.
[0092] As shown in FIG. 4B, provided sequentially on the second
substrate 6 are a dielectric layer 212 formed of ZnS-SiO.sub.2, a
dielectric layer 213 formed of GeN, a dielectric layer 215 formed
of GeN, a metal layer 216 as a reflective layer formed of Al, and
an overcoat layer 217.
[0093] As shown in FIG. 4C, the first and second substrates 5 and 6
are assembled together with the overcoat layers 207 and 217 as
innermost layers and with a resin layer 221 interposed between the
overcoat layers 207 and 217. Hardcoat layers 201 and 211 are
provided on outer surfaces of the assembled first and second
substrates 5 and 6.
[0094] The layers provided on the second substrate 6 are the same
as the layers provided on the first substrate 5 except that the
recording layer 204 is provided above the first substrate 5. The
dielectric layers, the metal layer and the overcoat layer are
provided on both the first and second substrates 5 and 6. Each of
the dielectric layers, the metal layers and the overcoat layers on
the first and second substrates 5 and 6 respectively have the same
thicknesses.
[0095] In such a structure, substantially the same stress acts on
the first and second substrates 5 and 6, and as a result, the first
and second substrates 5 and 6 are warped to substantially the same
degree. Even when the stress acting on the first and second
substrates 5 and 6 changes in accordance with the passage of time,
the change of the stress acting on the first substrate 5 and the
change of the stress acting on the second substrate 6 are
substantially the same. Accordingly, the optical disk 200 is kept
flat for a long period of time.
[0096] When the optical disk 200 is seen from the side of the
second substrate 6, light which is incident on the second substrate
6 and reflected by the metal layer 216 is colored yellow by the
dielectric layers 213 and 215 formed of GeN. Consequently, the
unrecordable surface of the optical disk 200 looks yellow.
[0097] When the optical disk 200 is seen from the side of the first
substrate 5, light which is incident on the first substrate 5 and
reflected by the metal layer 206 is colored yellow by the
dielectric layers 203 and 205 formed of GeN and also colored blue
by the recording layer 204 formed of GeSbTe alloy. Since the
coloring degree of blue is stronger than the coloring degree of
yellow, the recordable surface of the optical disk 200 looks
blue.
[0098] Accordingly, the recordable surface and the unrecordable
surface are easily distinguishable.
EXAMPLE 3
[0099] FIGS. 5A through 5F are cross-sectional views illustrating a
method for producing an optical information recording medium
(referred to simply as an optical disk) 300 according to a third
example of the present invention.
[0100] The optical disk 300 in the third example includes a first
substrate 21 and a second substrate 22, each having a thickness of
0.6 mm, which are assembled together. The first substrate 21 is
provided with, on the top surface thereof, a laminated information
rewriting layer 23 (FIG. 5C) including at least a dielectric layer
and a recording layer. By providing the laminated information
rewriting layer 23, the first substrate 21 warps. Accordingly, the
second substrate 22 is provided with a resin layer 25 on the bottom
surface thereof so that the resin layer 25 is not opposed to the
laminated information rewriting layer 23 when the first and second
substrates 21 and 22 are assembled. The resin layer 25 is formed of
a resin which shrinks when being cured. By providing the resin
layer 25, the second substrate 22 is also warped. Then, the first
and second substrate 21 and 22 are assembled together. Thus, the
optical disk 300, which is flat, is obtained.
[0101] The first substrate 21 shown in FIG. 5A and the second
substrate 22 shown in FIG. 5B are formed in the same step by an
injection method. The first substrate 21 and the second substrate
22 are formed of the same material and have the same size and
shape. For example, the first substrate 21 and the second substrate
22 are formed of a polycarbonate and each have a thickness of about
0.6 mm, a diameter of about 120 mm, and a central hole diameter of
about 15 mm. The central hole of the first substrate 21 is
represented by reference numeral 21a, and the central hole of the
second substrate 22 is represented by reference numeral 22a. The
first substrate 21 and the second substrate 22 each have a guide
groove (not shown) for signal recording in the top surfaces thereof
(top surfaces in FIGS. 5A and 5B).
[0102] As shown in FIG. 5C, the laminated information rewriting
layer 23 is provided on the top surface of the first substrate 21.
The laminated information rewriting layer 23 has the same structure
as that of the laminated information rewriting layer 3 in the first
example. The first substrate 21 is warped at about 1.5.degree. at
an outer periphery thereof with the top surface on which the
laminated information rewriting layer 23 is projecting.
[0103] As shown in FIG. 5D, the resin layer 25 formed of a resin
which has its volume reduced when being cured is formed on the
bottom surface of the second substrate 22, i.e., the surface which
is not opposed to the first substrate 21 when the first and second
substrates 21 and 22 are assembled. Specifically, a UV curable
resin is dropped on the bottom surface of the second substrate 22,
and the second substrate 22 is spun (spin coating), thereby forming
the resin layer 25 having a uniform thickness of about 5 .mu.m.
Then, the resin layer 25 is irradiated with UV light. The UV
curable resin shrinks when being cured, and thus the volume thereof
changes by about 10% or more. As a result, tensile stress acts on
the second substrate 22. Thus, as shown in FIG. 5E, the second
substrate 22 is warped, with the surface on which the resin layer
25 is not provided projecting. In other words, the surface of the
second substrate 22 in which the guide groove is formed is
projecting.
[0104] Then, as shown in FIG. 5E, a UV curable resin 27 is dropped
on the laminated information rewriting layer 23 in the vicinity of
the inner circumference of the fist substrate 21 in a concentric
manner, and the first and second substrates 21 and 22 are
pressure-contacted to each other in the state of planar symmetry
with respect to each other with the laminated information rewriting
layer 23 being inside and the resin layer 25 outside. Next, as
shown in FIG. 5F, the assembled first and second substrates 21 and
22 are held between glass plates 28 and 29, thereby substantially
eliminating the warp of the first and second substrates 21 and 22.
Then, the assembled body is irradiated with UV light 30 from the
side of the second substrate 22, thereby curing the UV resin 27.
After removing the glass plates 28 and 29, the optical disk 300 is
obtained.
[0105] The resin layer 25 is formed of a material which is
substantially transparent with respect to UV light so as to allow
for the above-described assembly.
[0106] The tilt of the optical disk 300 produced in this manner is
about 0.5.degree. or less and thus is sufficiently usable. In the
above-described example, the first and second substrates 21 and 22
are assembled using UV light. The same effect is achieved by a hot
melt method, a method using an adhesive tape or a slowly-effective
UV radiation method.
[0107] The resin layer 25 can be formed of a material which is not
transparent with respect to visible light. In such a case, the
assembly of the first and second substrates 21 and 22 cannot be
performed by UV radiation, but can be performed by a hot melt
method, a slowly-effective UV radiation method or the like. Use of
non-transparent resin for the resin layer 25 is advantageous in
improving the external appearance since, even when bubbles are
present on the surface on which the resin is applied, such bubbles
are covered by the non-transparent resin layer 25.
[0108] In the above-described example, the resin layer 25 is formed
by spin coating. The resin layer 25 can be formed by, for example,
printing, in which case, a graphics can be provided on the resin
layer 25. In this manner, the external appearance of the optical
disk is further improved, an area in which the user records a list
of stored information can be provided.
EXAMPLE 4
[0109] FIGS. 6A through 6E are cross-sectional views illustrating a
method for producing an optical information recording medium
(referred to simply as an optical disk) 400 according to a fourth
example of the present invention. The optical disk 400 includes a
first substrate 31 and a second substrate 32 assembled together.
FIG. 7 is a cross-sectional view of a mold 60 used for forming the
first and second substrate 31 and 32.
[0110] The first substrate 31 and the second substrate 32 each have
a thickness of 0.6 mm. The first substrate 31 is provided with a
laminated information rewriting layer 33 (FIG. 6C) including at
least a dielectric layer and a recording layer. By providing the
laminated information rewriting layer 33, the first substrate 31
warps. Accordingly, the second substrate 32 is also warped, so that
the optical disk 400, which is flat, is obtained by assembling the
first substrate 31 and the second substrate 32.
[0111] The first substrate 31 shown in FIG. 6A and the second
substrate 32 shown in FIG. 6B are formed in the same step by an
injection method. The first substrate 31 and the second substrate
32 are formed of the same material and have the same size and
shape. For example, the first substrate 31 and the second substrate
32 are formed of a polycarbonate and each have a thickness of about
0.6 mm, a diameter of about 120 mm, and a central hole diameter of
about 15 mm. The central hole of the first substrate 31 is
represented by reference numeral 31a, and the central hole of the
second substrate 32 is represented by reference numeral 32a. The
first substrate 31 and the second substrate 32 each have a guide
groove (not shown) for signal recording in the top surfaces thereof
(top surfaces in FIGS. 6A and 6B).
[0112] The formation of the first and second substrates 31 and 32
will be described with reference to FIG. 7.
[0113] The mold 60 shown in FIG. 7 includes upper and lower molds
61 and 62 opposed to each other. A space 63 is formed between the
upper and lower molds 61 and 62. A resin (for example, a
polycarbonate) in a melted state is injected into the space 63,
thereby forming each of the first and second substrates 31 and 32
(FIGS. 6A through 6E). The lower mold 62 includes a stamper 64 for
forming a guide groove for signal recording in the surface of each
of the first and second substrates 31 and 32. The mold 60 is
maintained at a high temperature of, for example, about 100.degree.
C. or more, so that the injected resin is not cooled rapidly and
the groove pattern of the stamper 64 is accurately transferred to
the surface of each of the first and second substrates 31 and
32.
[0114] As shown in FIG. 6C, the laminated information rewriting
layer 33 is provided on a top surface of the first substrate 31.
The laminated information rewriting layer 33 has the same structure
as that of the laminated information rewriting layer 3 in the first
example. The first substrate 31 is warped at about 1.5.degree. at
an outer periphery thereof with the top surface on which the
laminated information rewriting layer 33 is projecting.
[0115] The second substrate 32 is formed so that the surface in
which the guide groove is formed projects. The warp angle of the
second substrate 32 is adjusted by changing the injection
conditions.
[0116] For example, when the upper and lower molds 61 and 62 (FIG.
7) are set to have the temperature in condition 1 or 2 shown in
Table 1, the second substrate 32 is warped to substantially the
same degree as that of the first substrate 31. The warp angle is
larger by condition 2 than by condition 1.
1 TABLE 1 Condition 1 Condition 2 Temperature Mold 61 125.degree.
C. 128.degree. C. Mold 62 130.degree. C. 130.degree. C.
[0117] Conditions 1 and 2 are mere examples for a specific mold.
The conditions for warping the first and second substrates 31 and
32 change in accordance with, for example, the structure of the
mold and the material of the substrate. Since the birefringence
characteristics of the substrates changes in accordance with the
temperature of the mold, the temperature of the mold needs to be
set in consideration of various characteristics of the
substrates.
[0118] As shown in FIG. 6D, a UV curable resin 37 is dropped on the
laminated information rewriting layer 33 in the vicinity of the
inner circumference of the first substrate 31 in a concentric
manner, and the first and second substrates 31 and 32 are assembled
together in the state of planar symmetry with respect to each other
with the laminated information rewriting layer 33 being inside.
Next, as shown in FIG. 6E, the assembled first and second
substrates 31 and 32 are held between glass plates 38 and 39,
thereby substantially eliminating the warp of the first and second
substrates 31 and 32. Then, the assembled body is irradiated with
UV light 40 from the side of the second substrate 32, thereby
curing the UV resin 37. After removing the glass plates 38 and 39,
the optical disk 400 is obtained.
[0119] The tilt of the optical disk 400 produced in this manner is
about 0.5.degree. or less and thus is sufficiently usable. In the
above-described example, the first and second substrates 31 and 32
are assembled using UV light. The same effect is achieved by a hot
melt method using a hot melt resin, a method using an adhesive or a
slowly-effective UV radiation method.
[0120] In this example, the second substrate 32 is warped while
being formed by an injection method. Any other method which warps
the second substrate 32 is usable.
EXAMPLE 5
[0121] FIGS. 8A through 8F are cross-sectional views illustrating a
method for producing an optical information recording medium
(referred to simply as an optical disk) 500 according to a fifth
example of the present invention. FIG. 9 is a detailed
cross-sectional view of the optical disk 500.
[0122] The optical disk 500 includes a first substrate 41 and a
second substrate 42, each having a thickness of 0.6 mm, which are
assembled together. The first substrate 41 is provided with a
laminated information rewriting layer 43 (FIG. 8C) including at
least a dielectric layer and a recording layer. By providing the
laminated information rewriting layer 43, the first substrate 41
warps. Accordingly, a resin layer 44 formed of a resin which
shrinks when being cured is further provided on the laminated
information rewriting layer 43, thereby substantially eliminating
the warp of the first substrate 41. Then, the first substrate 41
and the second substrate 42 are assembled with the resin layer 44
being inside. Thus, the optical disk 500, which is flat, is
obtained.
[0123] The first substrate 41 shown in FIG. 8A and the second
substrate 42 shown in FIG. 8B are formed in the same step by an
injection method. The first substrate 41 and the second substrate
42 are formed of the same material and have the same size and
shape. For example, the first substrate 41 and the second substrate
42 are formed of a polycarbonate and each have a thickness of about
0.6 mm, a diameter of about 120 mm, and a central hole diameter of
about 15 mm. The central hole of the first substrate 41 is
represented by reference numeral 41a, and the central hole of the
second substrate 42 is represented by reference numeral 42a. The
first substrate 41 and the second substrate 42 each have a guide
groove (not shown) for signal recording in the top surfaces thereof
(top surfaces in FIGS. 8A and 8B).
[0124] As shown in FIG. 8C, the laminated information rewriting
layer 43 is provided on the top surface of the first substrate 41.
The laminated information rewriting layer 43 has the same structure
as that of the laminated information rewriting layer 3 in the first
example. As shown in FIG. 8C, the first substrate 41 is warped at
about 1.5.degree. at an outer periphery thereof with the top
surface on which the laminated information rewriting layer 43 is
projecting.
[0125] As shown in FIG. 8D, the resin layer 44 formed of a resin
which shrinks when being cured is formed on the laminated
information rewriting layer 43. The warp of the first substrate 41
is substantially eliminated by the stress accompanying the
shrinkage of the resin layer 44. Specifically, a UV curable resin
is dropped on the bottom surface of the laminated information
rewriting layer 43, and the first substrate 41 is spun (spin
coating), thereby forming the resin layer 44 having a uniform
thickness of about 5 .mu.m. Then, the resin layer 44 is irradiated
with UV light. The UV curable resin shrinks when being cured, and
thus the volume thereof changes by about 10% or more. As a result,
tensile stress acts on the first substrate 44. Thus, as shown in
FIG. 8E, the first substrate 41 is deformed so as to reduce the
warp thereof.
[0126] Then, a UV curable resin 47 is dropped on the laminated
information rewriting layer 43 in the vicinity of the inner
circumference of the first substrate 41 in a concentric manner, and
the first and second substrates 41 and 42 are assembled together in
the state of planar symmetry with respect to each other with the
layers 43 and 44 being inside. Next, as shown in FIG. 8F, the
assembled first and second substrates 41 and 42 are held between
glass plates 48 and 49. Then, the assembled body is irradiated with
UV light 50 from the side of the second substrate 42, thereby
curing the UV resin 47. After removing the glass plates 48 and 49,
the optical disk 500 is obtained.
[0127] As shown in FIG. 9, the laminated information rewriting
layer 43 provided on the first substrate 41 includes a dielectric
layer 122, a recording layer 123, a dielectric layer 124 and a
metal layer 125. An overcoat layer 126 is provided on the laminated
information rewriting layer 43. The resin layer 44 is provided on
the second substrate 42. The first and second substrates 41 and 42
are assembled together with the layers 126 and 44 being inside.
Hardcoat layers 121 and 127 are provided on outer surfaces of the
assembled first and second substrates 41 and 42.
[0128] The tilt of the optical disk 500 produced in this manner is
about 0.5.degree. or less and thus is sufficiently usable. In the
above-described example, the first and second substrates 41 and 42
are assembled using UV light. The same effect is achieved by a hot
melt method using a hot melt resin, an adhesive tape or a
slowly-effective UV radiation method.
[0129] In the case where a resin layer is provided on the surface
of the second substrate 42 in which the guide groove is formed, the
resin layer preferably has a smaller tensile stress than that of
the resin layer 44. Thus, the tilt of the optical disk is reduced.
Specifically, a thinner layer of a resin which is the same as that
of the resin layer 44 is used, or a resin having a smaller
shrinkage ratio when being cured than that of the resin layer 44 is
used.
[0130] In the case where the resin layer is provided on the second
substrate 42, the adhesiveness between the first and second
substrate 41 and 42 is improved. Without the resin layer on the
second substrate 42, the adhesiveness between the adhesive for
bonding the first and second substrate 41 and 42 and the material
of the second substrate 42 can be insufficient. When the resin
layer 44 formed on the laminated information rewriting layer 43 is
substantially transparent with respect to UV light, the first and
second substrate 41 and 42 can be assembled using UV light as shown
in FIG. 8F.
[0131] The resin layer 44 can be formed of a material which is not
transparent with respect to visible light. In such a case, the
assembly of the first and second substrates 41 and 42 cannot be
performed by UV radiation, but can be performed by a hot melt
method, a slowly-effective UV radiation method or the like. Use of
non-transparent resin for the resin layer 44 is advantageous in
improving the external appearance since, even when bubbles are
present on the surface on which the resin is applied, such bubbles
are covered by the non-transparent resin layer 44.
[0132] In the above-described example, the resin layer 44 is formed
by spin coating. The resin layer 44 can be formed by, for example,
printing, in which case, a graphics can be provided on the resin
layer 44. In this manner, the external appearance of the optical
disk is further improved.
[0133] In the first through fifth examples, each of the first and
second substrates has a thickness of about 0.6 mm. The present
invention is applicable to substrates having other thicknesses.
[0134] In the case where the first and second substrates each have
a thickness of more than 0.8 mm, simply assembling the two
substrates in the state where the first substrate is warped by
providing a laminated information rewriting layer reduces the warp
angle of the optical disk, for example, to about 0.7.degree. or
less. Thus, the present invention is especially effective to an
optical disk produced by assembling substrates each having a
thickness of less than 0.8 mm.
[0135] In the first through fifth examples, the second substrate
has a guide groove for signal recording as well as the first
substrate. Since the recording layer is not provided on the second
substrate, the guide groove in the second substrate can be
eliminated.
[0136] As described above, an optical information recording medium
for one-side recording and reproduction including two thin
substrates assembled together according to the present invention is
flat and has a sufficiently small tilt. The optical information
recording medium according to the present invention promotes
development of and provides optical disk systems capable of higher
density recording without requiring a reduction wavelength of the
laser light or an increase in the numerical aperture of the
objective lens.
[0137] Various other modifications will be apparent to and can be
readily made by those skilled in the art without departing from the
scope and spirit of this invention. Accordingly, it is not intended
that the scope of the claims appended hereto be limited to the
description as set forth herein, but rather that the claims be
broadly construed.
* * * * *