Optical information recording medium

Details Optical information recording medium Optical information recording medium

1999-04-27

2001-03-20

Evans, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S064500, C428S064600, C428S457000, C428S913000, C430S270130, C430S945000, C369S275100, C369S283000, C369S288000

Reexamination Certificate

active

06203877

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to an optical information recording medium for recording and reproducing information on being irradiated with laser beams, more particularly a phase-changing optical disk recording information on a land and groove as the recording tracks formed by a guiding groove.
2. Description of the Prior Art
An optical information recording medium, in particular an optical magnetic disk and phase-changing disk, generally has recording tracks formed by concentric or spiral guiding grooves formed on the substrate. Pitch of the tracks has been decreasing to meet demands for increased recording density. One of the techniques therefor is land-groove recording in which information is recorded on both concave (groove) and convex (land) sections for each guiding groove. However, the land-groove recording method involves problems coming from difference in recording sensitivity between the land and groove sections. One of these problems is possible defective recording resulting from insufficient phase change at the side of lower recording sensitivity, when information is recorded on both sections by the same recording power. Furthermore, there occur such a problem that it is necessary to change power for recording on the land and groove, in order to realize optimum recording on each section, which makes the recording device more complex.
The above phenomenon will be explained by referring to
FIG. 3
, which shows a sectional view of a phase-changing optical disk. The land L and groove G, formed by the guiding groove
37
on the substrate
31
, are laminated with the lower dielectric layer
33
, recording layer
34
, upper dielectric layer
35
and reflection layer
36
, in this order. The disk is irradiated with laser beams from the substrate side, which are projected onto the land L or groove G to record information by changing phase of the recording layer
34
. As shown by the sectional structure, the reflection layer
36
of high heat radiation adjoins the recording layer
34
via the groove slope on the land L, whereas the lower dielectric layer
33
of low thermal conductivity adjoins the recording layer
34
on the groove G. Therefore, thermal diffusion by heat flow H
1
from the recording layer
34
towards the reflection layer
36
is accelerated at both ends of the land L, when it is irradiated with laser beams, by which is meant that a higher recording power is needed to realize phase change at the recording layer
34
. This will result in lower recording density on the land L.
One method to solve the difference in recording sensitivity between the land and groove in an optical disk is changed thickness of the reflection, dielectric or recording layer on the land and groove, as disclosed by Japanese Patent Application Laid-Open No. 7-130006. This method changes thickness of the reflection or dielectric layer on the land and groove, to improve recording sensitivity on the land. The invention disclosed by Japanese Patent Application Laid-Open No. 7-311980 tries to enhance recording density, erasing rate and C/N ratio, and also to reduce crosstalk for recording with land-groove tracks by keeping reflectivity of the crystal sections of the land and groove in a range from 7 to 15%, inclusive. The invention disclosed by Japanese Patent Application Laid-Open No. 6-338085 tries to reduce erasing leftovers for magnetic modulation overwriting by keeping a higher recording density on the groove than on the land for a photomagnetic recording medium. The invention disclosed by Japanese Patent Application Laid-Open No. 2-108254 tries to reduce occurrence of erasing leftovers by keeping the metallic layer thicker on the groove than on the land, thereby reducing recording density on the groove to prevent formation of a magnetic domain on the groove and to allow a magnetic domain to be recorded on the land only when a tracking gap occurs.
BRIEF SUMMARY OF THE INVENTION
Object of the Invention
One of the methods proposed to solve the difference in recording sensitivity between the land and groove changes thickness of the reflection, dielectric or recording layer on the land and groove, as disclosed by Japanese Patent Application Laid-Open No. 7-130006. The invention disclosed by this specification changes thickness of the reflection or dielectric layer on the land and groove, in order to improve recording sensitivity of the land. However, increasing recording sensitivity of the land by, e.g., decreasing thickness of the reflection layer, causes a problem of decreased number of rewritable cycles, because of decreased heat radiation. A method is proposed to decrease recording sensitivity of the groove by increasing thickness of the recording layer over the groove. However, this requires etching of the recording layer on the land after the recording layer is deposited, making the film-making process more complex and causing other problems, such as additional necessity for securing uniformity of etching over the entire surface of the optical disk and contamination of the recording layer with the etchant gas.
It is an object of the present invention to provide an optical information recording medium which solves the problem of difference in recording sensitivity between the land and groove to realize increased density of recording capacity without changing any thickness of the recording or reflection layer on the land and groove.
SUMMARY OF THE INVENTION
The present invention provides an phase-changing optical disk, comprising a substrate laminated at least with a first dielectric layer, second dielectric layer, recording layer which changes its phase on being irradiated with laser beams, third dielectric layer and reflection layer in this order, wherein the first dielectric layer has a higher thermal conductivity than the second dielectric layer, and thickness of the second dielectric layer is smaller than depth of the guiding groove. The optical information recording medium of the present invention has the first dielectric layer of higher thermal conductivity adjoining the recording layer via the slope of the groove as the recording track, as shown in
FIG. 1
which is used to explain the preferred embodiments later, to accelerate thermal diffusion from the both ends of the groove toward the first dielectric layer. As a result, recording sensitivity of the groove decreases to the level for the land. There is an acceptable range for which recording power can vary, and it is not necessary to change recording power on the land and groove, when the difference in recording sensitivity between the land and groove is sufficiently small. Although the land and groove preferably have the same optimum recording power level, but no particular problem is anticipated to result from use of the same recording power for the land and groove, when the difference in optimum recording power between the land and groove is 5% or less. It is necessary to keep thickness of the second dielectric layer smaller than depth of the guiding groove; otherwise, the recording layer will no longer adjoin the first dielectric layer in the slope, preventing groove recording sensitivity from decreasing.
Furthermore, it is preferable to use ZnS—SiO
2
for the second dielectric layer for the phase-changing optical disk of the present invention, from the viewpoint of its characteristics related to erasing rate and repeatability. It is also preferable that the first dielectric layer has a thermal conductivity of 1 W/m·K or more, to accelerate thermal diffusion towards the first dielectric layer, given that ZnS—SiO
2
has a thermal conductivity of around 0.5 W/m·K. It is possible for the conventional phase-changing optical disk, laminated with a lower dielectric layer, recording layer, upper dielectric layer and reflection layer in this order, to decrease groove recording sensitivity to an extent realized by the present invention by use of a material of high thermal conductivity for the lower dielectric layer. However, erasing rate and number of repeatable cycles strongly depend

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