Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-12-05
2003-10-14
Angebranndt, Martin (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S945000, C428S064600, C369S275500, C369S275200, C204S192150, C204S192220, C204S192250
Reexamination Certificate
active
06632583
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical recording medium and its production method. More particularly, it relates to an optical recording medium which is capable of high-speed and high-density recording, erasing and retrieving of information by use of laser beam irradiation, and a method for producing such a recording medium.
It is known that the phase-change type optical recording media by which recording and retrieving of information are made by a phase-change, that is, a change of atomic configuration such as observed between the crystalline and the amorphous state. In the ordinary phase-change optical recording media, a protective layer comprising a dielectric material is provided on the upper and/or lower side of the recording layer where recording and retrieving of information are made by a phase-change.
Generally, in the rewritable type phase-change recording media, at least two different laser beam power units are used for generating the different crystalline states. A recording medium of this system is here illustrated. Crystallization is proceeded by heating the recording layer to a temperature well between the crystallization temperature of this layer and approx. its melting point, while formation of the amorphous state is proceeded by heating the recording layer to a temperature above the melting point thereof and then rapidly cooling it. In this case, the said protective layer functions as a heat dissiperator for providing a desired cooling rate (super-cooling rate). The protective layer is also important for suppressing deformation which may be caused by a volumetric change concomitant to fusion or phase-change of the recording layer and a thermal expansion of the protective layer itself in the amorphous mark forming process, and for preventing thermal damage of the plastic substrate or deterioration of the recording layer due to humidity.
The material used for such a protective layer is selected by taking into consideration such requirements as being optically transparent to the laser beams irradiated, high in melting point, softening point and decomposition temperature, easy to form, and possessing an appropriate degree of heat conductivity. In the course of cyclic heating and rapid cooling in the overwriting operation, a temperature variation of over several hundred ° C. takes place instantaneously, in a fraction of less than 100 nsec, in the inside of the protective layer, with its side contacting the fused section of the recording layer being elevated in temperature while the side contacting the substrate or the reflective layer being left low in temperature. Consequently, there takes place an abrupt thermal expansion-induced deformation in the direction where it acts to force out the recording layer. The protective layer, therefore, is required to be able to withstand such an abrupt thermal expansion-induced deformation of this layer itself.
As stated above, dielectric materials such as oxides and nitrides of metals have been known as the protective layer materials which are chemically stable and have sufficient heat resistance and mechanical strength even in the high temperature region.
Generally, however, because of large difference in thermal expansion coefficient and elastic properties between the dielectric film and the plastic substrate, the dielectric film tends to separate from its substrate to cause formation of pin-holes or cracking as recording and erasing are repeated. The oxides, nitrides, carbides and fluorides of silicon, tantalum, rare earth elements, etc., which are the typical amorphous dielectrics, are excellent in high temperature durability in the static high temperature condition, but they are high in hardness and fragile, so that when a sudden and local temperature change such as mentioned above takes place, a microscopic fault tends to grow into a crack to cause bursting. Also, since the plastic substrate is prone to warp due to moisture, the protective layer made of a dielectric material tends to create a stress at its interface with the substrate or the recording layer, resulting in causing layer fraction. Further, such a dielectric protective layer is poor in adhesion to the chalcogen elements which are usually used for the phase-change recording layer, so that it is more likely to separate when such elements are used.
On the other hand, in order to develop the unique properties unobtainable by use of a pure dielectric material, it has been suggested to use a composite dielectric comprising a mixture of a plural number of dielectric materials as the protective layer. As such composite dielectrics, there have been proposed various mixtures of the chalcogen element-containing compounds such as ZnS, ZnSe, PbS, CdS, etc., and the oxides, nitrides, fluorides, carbides, etc. Most notably, the use of a composite dielectric comprising a mixture of ZnS, which is the main component, with SiO
2
, Y
2
O
3
or the like has been proposed to provide a protective layer which enables over 1,000,000 times of repetitive overwriting in pit position modulation recording.
For instance, Japanese Patent Application Laid-Open (KOKAI) No. 5-174423 discloses a composite dielectric comprising ZnS and Y
2
O
3
. In an Example of this prior literature, there is a description that overwriting repeatability was improved by the use of both a protective layer comprising Y
2
O
3
alone and another protective layer comprising Y
2
O
3
substituted with ZnS by 80 mol % at most. The ZnS/SiO
2
composite dielectric has already been put to practical use as the material of the protective layers of phase-change recording media. Particularly the targets with a ZnS to SiO
2
molar ratio of 80 to 20 are widely used.
The protective layers comprising the said composite dielectrics excel those comprising purely oxide or nitride dielectrics in adhesion to the chalcogenide alloy (such as GeTeSb and AgInSbTe) films. These composite dielectric-made protective layers are also almost free of the problem of bursting resulting from generation and propagation of cracks typically seen when ZnS alone is used. Thus, by the use of these protective layers, the repetitive overwriting performance of the recording medium is improved, film separation in the accelerated test is minimized, and high overwriting reliability is obtained.
In recent years, however, request for better repetitive overwriting performance and durability of the recording has become strong. For instance, mark length modulation recording has been employed for attaining higher recording density, but when the mark length becomes shorter than about 0.5 &mgr;m, the repetition durability is remarkably reduced. This is because a slight increase of noise or a small change of reflectivity, which were overlooked in mark position recording, are not allowed in mark length modulation recording.
As a result of the present inventors' earnest study to solve the above problem, it has been found that
In the ZnS/SiO
2
(typically 80/20 in molar ratio) composite films and the said ZnS/Y
2
O
3
composite films, microscopic plastic deformations store up to cause an increase of noise or a reduction of reflectivity, and it is possible to inhibit or minimize the reduction of reflectivity and the increase of noise and to suppress the transfer of materials, by incorporating a protective layer of a specific composition such as containing an oxysulfide represented by Y
2
O
2
S.
The present invention has been attained on the basis of the above finding.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide an optical recording medium capable of preventing the drop of reflectivity and the increase of noise while suppressing the transfer of materials in use, and having excellent repetitive overwriting performance.
A second object of the present invention is to provide a method for producing the recording medium mentioned in the first object.
To attain the above aim, a first aspect of the present invention, there is provided an optical recording medium having a protective layer and a phase-change r
Kunitomo Haruo
Ohno Takashi
Alexander John B.
Angebranndt Martin
Conlin David G.
Edwards & Angell LLP
Mitsubishi Chemical Corporation
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