Optical information recording medium with thermal diffusion...

Details Optical information recording medium with thermal diffusion... Optical information recording medium with thermal diffusion...

1999-11-18

2002-09-10

Neyzari, Ali (Department: 2653)

Dynamic information storage or retrieval

Storage medium structure

Optical track structure

C428S064400

Reexamination Certificate

active

06449239

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical information recording medium on which information can be recorded, reproduced, erased and rewritten with high density and at high speed using an optical means, such as irradiation of a laser beam.
BACKGROUND OF THE INVENTION
In an optical information recording medium, the difference in optical characteristics caused by local irradiation of a laser beam onto a recording material is utilized as a recording state. When using a material whose optical characteristics are varied reversibly, erasing and rewriting of information is possible. As rewritable media, generally a magneto-optical recording medium and a phase change recording medium have been well known. In these optical recording media, a large volume of information can be recorded, and recording, reproduction, erasure, and rewriting can be performed at high speed. In addition, such optical recording media are excellent in portability. Therefore, it is conceivable that in a highly information-oriented society, the demands for such optical recording media will further increase, and thus it is desired to increase further their capacity and the speed in recording, reproducing, erasing or rewriting information in them.
In the phase change recording medium, with respect to light with a specific wavelength the quantity of reflected light from a portion in a crystalline state is different from that from a portion in an amorphous state, and this difference is utilized as a recording state. By modulating an output power of the laser, erasure of recorded signals and recording by overwriting can be performed at the same time. Thus, information signals can be erased and rewritten easily at high speed.
FIG. 9
shows an example of a layer structure of a conventional phase change recording medium. As shown in
FIG. 9
, the conventional phase change recording medium includes a substrate
1
, and a protective layer
2
, a recording layer
4
, a protective layer
8
, and a reflective layer
6
that are laminated sequentially on the substrate
1
. As the substrate
1
, resin such as polycarbonate or PMMA, glass, or the like is used. In the substrate
1
, a guide groove for guiding a laser beam is formed. The recording layer
4
has states different in optical characteristics and is formed of a material that can be varied reversibly between the different states. In the case of a rewritable phase change optical recording material, generally a chalcogenide-based material containing Te or Se as the main component is used as the material for the recording layer
4
. Examples of the chalcogenide—based material include a material containing 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 the main component. Generally, the reflective layer
6
is formed of metal such as Au, Al, Cr, or the like or an alloy thereof and is provided for the purpose of obtaining a heat release effect and effective optical absorption in the recording layer
4
. In addition, for the purpose of preventing oxidation and corrosion of the optical information recording medium or adhesion of dust thereon, a configuration with an overcoat layer on the reflective layer
6
or a configuration in which a dummy substrate is laminated using ultraviolet curing resin as an adhesive is used in general, although it is omitted in the figure. The protective layers
2
and
8
have functions for protecting the recording layer
4
, such as functions for preventing oxidation, evaporation, or deformation of a material of the recording layer
4
. By adjusting the thickness of the protective layers
2
and
8
, the absorptance of the recording medium or the difference in reflectance between a recording portion and an erasure portion (hereinafter referred to as “reflectance difference”) can be adjusted. Therefore, the protective layers
2
and
8
also have a function for adjusting optical characteristics of the recording medium. As conditions of the material forming the protective layers
2
and
8
, it is necessary that not only the above-mentioned purposes are attained but also excellent adhesiveness between, for example, the material of the recording layer
4
and the substrate
1
can be obtained and the protective layers
2
and
8
themselves are films with excellent weather resistance in which no cracks occur. Further, when the protective layers
2
and
8
are used while being in contact with the recording layer
4
, the material of the protective layers
2
and
8
should be one that does not hinder the optical change of the material of the recording layer
4
. As a material of the protective layers
2
and
8
, in addition to sulfide such as ZnS or the like, oxide such as SiO
2
, Ta
2
O
5
, Al
2
O
3
, or the like, nitride such as Ge—N, Si
3
N
4
, Al
3
N
4
, or the like, or nitrogen oxide such as Ge—O—N, Si—O—N, Al—O—N, or the like, dielectric such as carbide, fluoride, or the like, or suitable combinations thereof have been proposed. Generally, ZnS—SiO
2
has been used in many cases.
Conventionally, it has been known that when recorded signals are erased and rewritten, mark positions after rewriting are shifted slightly and overwriting distortion occurs. This distortion occurs because the manner of temperature rise when a laser beam is irradiated is different depending on whether the state of the recording layer
4
before rewriting is in an amorphous state or in a crystalline state, and thus rewritten marks have different lengths from predetermined lengths. In other words, when a mark is in an amorphous state, a latent heat is required for the phase change into an amorphous state at portions that were in a crystalline state before rewriting, but it is not required at portions that were in an amorphous state before rewriting. Therefore, an excess amount of heat changes a portion longer than a predetermined length in the recording layer
4
into an amorphous state. In order to solve this problem, a configuration is employed in which Ac/Aa is maintained to be larger than 1 and in a certain range, wherein Aa indicates optical absorptance of the recording layer
4
in an amorphous state and Ac represents optical absorptance of the recording layer
4
in a crystalline state. In other words, a configuration that enables so-called absorptive correction is employed. This promotes temperature increase at a crystalline portion and therefore a uniform temperature rise within a mark region after rewriting can be obtained. Thus, in this case, mark distortion does not occur easily.
As a method of attaining Ac/Aa>1, some methods have been proposed. For example, a configuration (satisfying Rc<Ra) in which the reflectance Ra of a portion in an amorphous state is set to be higher than the reflectance Rc of a portion in a crystalline state has been proposed. In this case, a high value of Ac/Aa can be obtained even when the reflectance difference Ra−Rc between the portion in the amorphous state and the portion in the crystalline state is set to be large. Concretely, for instance, Rc<Ra can be attained by providing another layer between the substrate
1
and the protective layer
2
in FIG.
7
and setting the optical constant of the another layer within a certain range.
Further, even in the case of Rc>Ra, Ac/Aa>1 can be attained. As methods for attaining this, those mainly using an optical transmission type medium and an optical absorption type medium have been known. The optical transmission type medium is used in a method in which the medium is allowed to have transmittance and a configuration satisfying 0<Tc<Ta is employed, wherein Ta indicates transmittance of the medium when its recording layer is in an amorphous state and Tc represents transmittance of the medium when its recording layer is in a crystalline state. The optical absorption type medium is used in a method in which a layer causing absorption is provided in the medium and a configuration satisfying 0<Ac2<Aa2 is employed, wherein Aa2 indicates optical absorption in this layer when the recording lay

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