Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
1999-03-12
2001-06-26
Evans, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064400, C428S064500, C428S064600, C428S457000, C428S913000, C430S270130, C430S495100, C430S945000, C369S283000, C369S288000
Reexamination Certificate
active
06251492
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium, in which writing, reading and/or erasing of information is conducted by utilizing a light beam or, more specifically, to an optical recording medium comprising a recording layer formed on a substrate in which a laser beam is applied to the recording layer from the side of the recording layer without passing through the substrate.
2. Description of the Related Art
The most optical recording media are the substrate side light incident type in which a laser beam is applied to the recording layer through a transparent substrate. Recently, however, optical recording media of the recording layer side light incident type, in which a laser beam is applied from the side of the deposited layers without passing through the substrate, have attracted attention as described in the magazine “Electronics” vol. 41, No. 5, May 1996, pp87-91.
Investigation of the recording layer side light incident type optical recording medium has begun because it has a higher recording density than the conventional substrate side light incident type optical recording medium. However, practical recording and reproducing using this type recording medium has not been reported. Accordingly, it appears that various problems must be solved before this type of medium is available for practical use.
Magneto-optical recording media and phase change-type recording media would be such possible optical recording media.
In a magneto-optical recording medium, light, particularly a laser beam, is applied to a recording layer to raise the temperature of the recording layer and reduce the coercive force of the recording layer, while the magnetization direction at the portion where light is applied changes by an external magnetic field to record or erase information. The temperature of the recording layer reaches about 200° C.
A phase change-type recording medium utilizes a reversible structural change or a phase change between amorphous and crystal states of a material made by application of light, particularly a laser beam, for recording or erasing information. The temperature of the recording layer reaches about 600° C. when recording and to about 170° C. when erasing. The phase change-type recording medium has an advantages of high speed information processing and a high recording capacity. The phase change-type recording medium also has a merit of a low cost since the structure of a driver therefor is simpler than the driver for the magneto-optical recording medium.
The typical structure of these optical recording media comprises a stack of inorganic layers deposited on a transparent disc of polycarbonate by evaporation or sputtering. Commercial discs usually have a structure of polycarbonate (PC) substrate/lower dielectric layer/recording layer/upper dielectric layer/reflecting layer/organic protecting layer. The recording layer of the magneto-optical recording medium is made of a rare earth and transitional metal alloy such as TbFeCo and the recording layer of the phase change-type recording medium is made of a carcogen alloy such as AgInSbTe or GeSbTe. The dielectric layer of the magneto-optical recording medium is made of a nitride such as Si
3
N4 and the dielectric layer of the phase change-type recording medium is made of a ZnS-based material such as ZnS•SiO
2
.
The phase change-type recording medium typically utilizes a crystal state of a recording layer as the erased state of information and an amorphous state of the recording layer, formed through fusion and rapid cooling using a high power laser beam, as the recorded state of information. The recording layer of a phase-change-type recording medium is amorphous when it is originally deposited, which is then entirely annealed to convert to a crystal state, i.e., the erased state, before it is used for recording. The entire annealing is conducted by using a laser beam, for example, with a spot of a width of 1-3 &mgr;m and a length of about 100 &mgr;m and with a power of about 1 Watt. This step of the entire annealing is called the initialization or initial crystallization.
In contrast to the above commercial typical optical discs, the recording layer side light incident type optical recording media, which are being investigated, would have a structure which has the order of the stack of the layers is reversed from that of the conventional one. That is, the recording media have a structure of PC substrate/reflecting layer/lower dielectric layer/recording layer/upper dielectric layer.
SUMMARY OF THE INVENTION
The present invention has the object of developing a recording layer side light incident type optical recording medium as mentioned above and concerns a solution to a new problem which the present inventor has discovered.
Namely, while investigating recording layer side light incident type optical recording media having a structure of PC substrate/reflecting layer/lower dielectric layer/recording layer/upper dielectric layer, the present inventor discovered that a PC substrate was thermally deformed by application of a laser beam from the recording layer side to form an orange skin-like texture.
More particularly, the above problem became significant when a laser beam with a short wavelength, a red beam, was used for the purpose of obtaining a high recording density and the thickness of the lower dielectric layer was made thinner to obtain an appropriate reflectivity therefor. Also, this problem became significant when the initial crystallization was conducted in a phase change-type optical recording medium. It is obvious that an optical recording medium having a thermally deformed substrate is not suitable or acceptable for a practical optical recording medium.
The present invention provides a recording layer side incident type optical recording medium in which the above problem discovered during an investigation into high density optical recording has been solved and which is useful in creating a practical medium.
Thus, the present invention resides in an optical recording medium, comprising in the following order:
a substrate,
a reflecting layer on said substrate,
a lower dielectric layer on said reflecting layer,
an optical recording layer on said lower dielectric layer, said optical recording layer recording information by means of a change in a physical property of the optical recording layer which is generated by an applied light beam, said change in a physical property of the optical recording layer occurring at an operating temperature, and
an upper dielectric layer on said optical recording layer,
wherein said substrate has a thermal deformation temperature lower than said operating temperature and said optical recording medium further comprises a thermally insulating layer between said substrate and said reflecting layer.
Because the inventor searched for a solution to the orange skin-like texture problem found in a recording layer side light incident type optical recording medium having a structure of PC substrate/reflecting layer/lower dielectric layer/recording layer/upper dielectric layer, the present inventor considers that the cause of the above problem is elevation of the temperature of a plastic substrate to a temperature higher than the thermal deformation temperature of the plastic substrate, which is heated by heat conduction from the recording layer heated by application of a laser beam.
Further, the present inventor discovered that significant alleviation of the problem can be obtained by inserting a thermally insulating layer of a dielectric material between the plastic substrate and the reflecting layer and a way to make the media practically useful was thus created.
REFERENCES:
patent: 5125750 (1992-06-01), Corle et al.
patent: 5479382 (1995-12-01), Nishida
patent: 5604003 (1997-02-01), Coombs
patent: 5702792 (1997-12-01), Iida
patent: 6011757 (2000-01-01), Ovshinsky
Terris, B.D., et al., “Ultra-High Density Magneto-Opitical Recording Using A SIL,” IEEE International Magnetics Conference, pp. CC-01 (Apr., 1996).
Hi
Evans Elizabeth
Teijin Limited
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