Optical information recording medium

Details Optical information recording medium Optical information recording medium

2000-02-24

2003-12-02

Letscher, George J. (Department: 2653)

Dynamic information storage or retrieval

Storage medium structure

Layered

Reexamination Certificate

active

06657948

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical information recording medium for recording and reproducing information, and more particularly, to an optical information recording medium which can suppress warpage caused by a change in environments or variance with time.
BACKGROUND OF THE INVENTION
As an example optical information recording medium for recording and reproducing information, a thin disk type has been known. FIG.
6
(
a
) is a plan view of such an optical information recording medium and FIG.
6
(
b
) is a side elevation thereof.
FIG. 7
is a schematic cross section showing an arrangement of a conventional optical information recording medium
110
.
As shown in
FIG. 7
, the conventional optical information recording medium
110
includes a disk (see FIG.
6
(
a
)) substrate
120
made of polycarbonate or the like, on which a single- or multi-layer thin film layer
140
comprising thin films, such as dielectric films
141
and
143
(silicon nitride, etc.), a recording film
142
(TbFeCo, etc.), and a reflecting film
144
(Al, etc.), is formed by means of sputtering or the like.
Also, a thin film protecting film
150
made of a resin film or the like is formed on the thin film layer
140
, and a substrate protecting film
130
made of resin or the like is formed on the light incident surface of the substrate
120
.
The thicknesses of these films and layers are as follows: the thickness of the substrate
120
is approximately 1.2 mm; the thickness of the single- or multi-layer thin film layer
140
formed by means of sputtering or the like is 10-30 nm; the thickness of the thin film protecting film
150
is 1-30 &mgr;m; and the thickness of the substrate protecting film
130
is 1-30 &mgr;m. Thus, the substrate
120
made of polycarbonate occupies most of the optical information recording medium
110
in thickness.
The rigidity of the optical information recording medium
110
depends almost entirely on the substrate
120
, and because the substrate
120
is sufficiently thick, deformation caused by a change in environments (temperature and humidity changes) is quite small. For this reason, a balance of stress and a bending moment of each layer has not been considered generally in most of the cases.
However, there has been a demand to further increase recording and reproducing density of the optical information recording medium, and the substrate has been made thinner (for example, the thickness is now reduced to 0.6 mm from 1.2 mm) to suppress the occurrence of aberration. As a result, the rigidity of the optical information recording medium is reduced, and larger deformation occurs due to stress produced in each layer forming the optical information recording medium with a change in environments (temperature and humidity changes), thereby posing a problem that information can not be readily recorded and reproduced. Thus, there has been an increasing need for an optical information recording medium which can maintain good performance in response to a change in environments even if its rigidity is reduced by employing a thinner substrate.
Japanese Laid-open Patent Application No. 195745/1992 (Japanese Official Gazette, Tokukaihei No. 195745, published on Jul. 15, 1992) discloses a technique of suppressing deformation of the optical information recording medium, by which a warpage preventing dielectric film is provided on the back surface (the surface on which the thin film layer is not formed) of the substrate (prior art {circle around (1)}).
FIG. 8
is a cross section showing an arrangement of the above optical information recording medium (prior art {circle around (1)}). In
FIG. 8
, like components are labeled with like reference numerals with respect to
FIG. 7
for ease of explanation. As shown in
FIG. 8
, a dielectric layer
160
is provided on the light incident side of the substrate
120
made of polycarbonate, so that the same expansion coefficient is given to the recording film
142
and dielectric layer
160
which are provided respectively at the both sides of the transparent substrate
120
. Consequently, because the optical information recording medium has a symmetrical structure with respect to the substrate
120
, warpage of the optical information recording medium can be prevented.
Also, Japanese Laid-open Patent Application No. 64119/1998 (Japanese Official Gazette, Tokukaihei No. 10-64119, published on Mar. 6, 1998) discloses that, by making a thin film protecting film thicker (30-50 &mgr;m), warpage occurring with increasing temperatures can be prevented (prior art {circle around (2)}).
Further, Japanese Laid-open Patent Application No. 364248/1992 (Japanese Official Gazette, Tokukaihei No. 4-364248, published on Dec. 16, 1992) proposes an optical information recording medium which can solve problematic warpage caused by a humidity change. This optical information recording medium includes, as shown in
FIG. 9
, a thin film protecting film
150
, a thin film layer
140
, a substrate
120
, a substrate protecting film
130
, and in order to solve the problem, it additionally includes a moisture permeation preventing film
170
made of SiO
2
, AlN, etc. between the substrate
120
and substrate protecting film
130
(prior art {circle around (3)}). In
FIG. 9
, like components are labeled with like reference numerals with respect to
FIGS. 7 and 8
for ease of explanation.
However, according to the technique disclosed in Japanese Laid-open Patent Application No. 195745/1992 supra (see
FIG. 8
, prior art {circle around (1)}), the dielectric layer
160
has to be provided on the light incident side of the substrate
120
by means of sputtering or the like. In this case, the manufacturing procedure includes forming the thin film layer
140
on one surface of the substrate
120
, turning over the substrate
120
, and forming the dielectric layer
160
on the opposite surface. Thus, not only the manufacturing procedure becomes complex, but also expensive manufacturing facility is required, thereby posing a problem that the manufacturing costs are undesirably increased.
Also, the technique (prior art {circle around (2)}) of Japanese Laid-open Patent Application No. 64119/1998 supra poses a problem that the thin film protecting film is so thick that it can not be readily formed. In addition, in case that the optical information recording medium is a magneto-optical recording medium, for example, in order to turn an applied magnetic field inversely at a high speed while information is being recorded, it is preferable to approximate the thin film layer to magnetic field generating means. However, a too thick thin film protecting film can cause problematic deterioration of magnetic characteristics.
Further, the technique of Japanese Laid-open Patent Application No. 364248/1992 supra (see
FIG. 9
, prior art {circle around (3)}) demands the moisture permeation preventing film
170
made of SiO
2
, AlN, etc. to be provided on the light incident side of the substrate
120
by means of sputtering or the like. In this case, the manufacturing procedure includes forming the thin film layer
140
on one surface of the substrate
120
, turning over the substrate
120
, and forming the moisture permeation preventing film
170
on the opposite surface. Thus, not only the manufacturing procedure becomes complex, but also expensive manufacturing facility is required, thereby posing a problem that the manufacturing costs are undesirably increased.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical information recording medium which can prevent deformation (warpage) caused by temperature and humidity changes and be readily manufactured.
In order to fulfill the above and other objects, an optical information recording medium of the present invention is characterized by being furnished with:
a thin film layer, formed on a substrate, for recording and reproducing information; and
a thin film protecting film, formed on the thin film layer, for protecting the thin film layer,
a neutral plane of deformation

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