Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
1999-03-23
2002-11-05
Korzuch, William (Department: 2753)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S013380, C369S013400
Reexamination Certificate
active
06477135
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase-changeable optical information recording medium on which information can be recorded, reproduced and rewritten in high density and with high speed by using optical techniques such as irradiation of laser beams, and a method for recording and reproducing information thereon.
2. Description of the Prior Art An optical magnetic recording medium or a phase-changeable recording medium is known as a medium on which information can be recorded in large capacity and reproduced and rewritten at a high speed. A transportable optical recording medium is expected to be more important in a highly information-oriented society. The improvements in the function of applications and in the quality of graphic information require of the medium larger capacity and higher-speed recording or reproducing.
These optical recording media utilize a difference in the optical characteristics of the recording material created by locally irradiating the recording material with laser beams. For example, for the optical magnetic recording medium, a difference in the angle of rotation in a plane of polarization of a reflected light, which is created by a difference in the magnetized state, is utilized for recording, Furthermore, for the phase-changeable recording medium, an amount of reflected light in the crystalline state different from that in the amorphous state when light with a specific wavelength is used is utilized for recording. The phase-changeable recording medium is advantageous because the erasure of recorded information and the overwriting of information can be performed simultaneously by adjusting the output power of the laser, so that it is possible to rewrite information signals at high speed.
FIG. 7
shows a typical layer structure of a conventional optical information recording medium. A resin such as polycarbonate, polymethyl methacrylate (hereinafter, referred to as PMMA) or glass can be used for a substrate
101
. The substrate
101
includes a guide groove for guiding laser beams. Protective layers
102
and
104
will be described later. A recording layer
103
is formed of a material that has different optical characteristic states and can change between the different states reversibly. In the case of a rewritable phase-changeable optical recording material, the recording layer
103
can be formed of a so-called chalcogenide such as a material containing Te or Se as a main component, or a material containing Sb such as materials comprising Te—Sb—Ge, Te—Sn—Ge, Te—Sb—Ge—Se, Te—Sn—Ge—Au, Ag—In—Sb—Te, In—Sb—Se, In—Te—Se or the like as a main component. A reflection layer
105
generally is formed of a metal such as Au, Al, Cr or the like, or an alloy of these metals. The reflection layer
105
is provided for the purpose of radiating heat and allowing the recording thin film to absorb light effectively, but the reflection layer need not be provided. For the purpose of preventing the oxidation or the corrosion of the optical information recording medium or the attachment of dust onto the medium, an overcoat layer or a dummy substrate may be formed on the reflection layer
105
, although these are not shown in FIG.
7
. The dummy substrate may be bonded with an ultraviolet curing resin.
Furthermore, as shown in
FIG. 8
, a recording medium comprising two protective layers
102
and
106
between the substrate
101
and the recording layer
103
has been proposed. For example, Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 5-217211 discloses a recording medium comprising a recording layer containing Ag, a first protective layer formed of a nitride of SiN or AlN or a carbide of SiC in contact with the recording layer and a second protective layer formed of ZnS or a composite compound containing ZnS on the first protective layer. The first protective layer is formed to suppress a reaction between a constituent S atom of the second protective layer and a constituent Ag atom of the recording layer. Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 6-195747 discloses another example of the recording medium comprising protective layers in a two-layered structure, as shown in FIG.
8
. The recording medium includes a first and second protective layers
106
and
102
between a recording layer
103
and a substrate
101
. The first protective layer
106
in contact with the recording layer
103
is formed of Si
3
N
4
, and the second protective layer
102
in contact with the substrate
101
is formed of ZnS—SiO
2
.
The protective layers
102
,
104
and
106
serve to protect the recording layer
103
in such a manner that the material for the recording layer
103
is prevented from being oxidized, evaporated or distorted. Furthermore, it is possible to adjust the absorption of the optical information recording medium or a difference in the reflectance between a recorded portion and an erased portion by adjusting the thickness of the protective layers. Thus, the protective layers also serve to adjust the optical characteristics of the medium. Moreover, a material for the protective layers
102
,
104
and
106
is required to have good adhesiveness with a material forming the recording layer and the substrate
101
and good weather resistance so that the protective layers
102
,
104
and
106
are not cracked. When the protective layers
102
,
104
and
106
are used in contact with the recording layer
103
, the material for the protective layers is required not to impair the optical change of the material for the recording medium. Examples of the material for the protective layers
102
,
104
and
106
include a dielectric such as a sulfide such as ZnS, an oxide such as SiO
2
, Ta
2
O
5
or Al
2
O
3
, a nitride such as GeN, Si
3
N
4
or Al
3
N
4
, a nitrogen oxide such as GeON, SiON or AlON, a carbide, a fluoride or the like, or suitable combinations thereof
It is known conventionally that when information is rewritten, a marked position is dislocated slightly after rewriting, namely, a so-called overwrite distortion (distortion in the recorded mark) is caused. This distortion is caused because the temperature increase rate during laser irradiation depends on whether the recording layer was in an amorphous state or a crystalline state before rewriting, whereby a mark after rewriting has a length that does not match a predetermined length. In order to solve this problem, so-called absorption correction is performed by maintaining Ac/Aa in a predetermined range larger than 1, where Aa represents the absorptance in the amorphous portion and Ac represents the absorptance in the crystalline portion. The structure that allows such absorption correction makes the increase in the temperature in marked portions uniform so that the mark distortion is unlikely to occur at rewriting.
For example, Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 7-78354 discloses an information recording medium comprising a metal layer, a protective layer, a recording layer and a reflection layer on a substrate in this order, in which the reflectance after recording is larger than that before recording. Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 7-105574 discloses an optical information recording medium comprising an optical absorption layer formed of Ti on a substrate, in which an optical absorptance in the crystalline state in the recording layer is larger than that in the amorphous state to reduce the dislocation of recorded marks.
Especially when information is rewritten at high speed, the overwrite distortion as described above is caused readily. However, the solution of merely maintaining Ac/Aa larger than 1 cannot provide a sufficient erasure ratio. In addition, when a recording layer composition that allows a high rate of crystallization is used to raise the erasure ratio, it is difficult to obtain sufficient reliability of recorded signals.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical information recording medi
Uno Mayumi
Yamada Noboru
Chu Kim-Kwok
Korzuch William
Merchant & Gould P.C.
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