Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2000-02-14
2002-06-04
Evans, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064500, C428S064600, C430S270130
Reexamination Certificate
active
06399173
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a phase change optical recording medium and method for making such an optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change in reflectance by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
Most optical recording media of the phase change type used Ge—Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites.
Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb
2
or IIb-IVb-Vb
2
as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe
2
is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein a recording layer crystallizes to create an AgSbTe
2
phase.
In the case of prior art phase change type optical recording media, recording layers are formed using vacuum deposition equipment and the as-deposited recording layers remain amorphous with low reflectance. The recording layers must be crystallized by an operation generally known as initialization before the recording media can be utilized as rewritable media.
Initialization is carried out in various ways, for example, after a recording layer is formed on a substrate, by heating the substrate to the crystallization temperature of the recording layer for crystallization as disclosed in JP-A 3131/1990; illuminating a laser beam to the recording layer for crystallization, which method is called solid phase initialization, as disclosed in JP-A 366424/1992, 201734/1990 and 76027/1991; illuminating flash light to the substrate to achieve pseudo-crystallization by so-called photo-darkening, which method takes advantage of the photo characteristics of chalcogen compounds, as disclosed in JP-A 281219/1992; and high-frequency induction heating the medium. JP-A98847/1990 proposes to heat a substrate during formation of a recording layer to thereby crystallize the recording layer. JP-A 5246/1990 discloses a method involving the steps of forming a first dielectric layer, forming a recording layer thereon, heating it for crystallization, and forming a second dielectric layer thereon.
However, the initialization step by laser beam illumination takes a long time and is makes the productivity lower. Heating of the overall medium rejects the use of inexpensive resin substrates. That is, resin substrates can be distorted upon heating for initialization, causing errors in tracking. The method of illuminating flash light is also low in productivity because several shots of illumination are necessary to achieve full crystallization.
Under the circumstances, the use of a so-called bulk erasing is the only technique which is regarded commercially acceptable and currently used. The bulk eraser illuminates a beam from a high power gas or semiconductor laser through a relatively large aperture stop for crystallizing a multiplicity of tracks altogether. Since the bulk eraser permits the recording layer to be locally heated, the substrate temperature is elevated to a little extent, enabling the use of less heat resistant resins as substrates.
Initialization of an optical recording disc with a bulk eraser, however, is a time-consuming process, and it takes several minutes just to initialize the optical recording discs of 12 cm diameter. The process of initialization has been the rate-determining step in the production of the optical recording discs. Elimination or speedup of the initialization step is required for improving the production efficiency.
For the speedup of the initialization process, it is effective to decrease the crystallization temperature of the recording layer. When the recording layer has a lower crystallization temperature, the recording layer will be crystallized even if the bulk eraser was operated at a faster rate. Furthermore, if the crystallization temperature of the recording layer could be reduced to the distortion temperature of polycarbonate, polyolefin and other resins commonly used for the substrate of the optical recording medium about 120° C., the initialization can be accomplished simply by heating the medium in an oven and there will be no need to use of the expensive bulk eraser. A significant reduction in the initialization cost by the use of simple initialization process is thereby enabled. At the moment, initialization of the disc in an oven with no distortion in the resin substrate is impossible because the crystallization temperature of the phase change recording materials commonly used in the art, for example, Ag—In—Sb—Te-based materials and Ge—Sb—Te-based materials are in the range of about 170 to about 200° C.
Various processes are proposed for the purpose of reducing the crystallization temperature of the phase change recording layer. For example, JP-A 106647/1996 proposes a Ag—In—Sb—Te-based recording layer of the structure wherein AgSbTe
2
layer and In—Sb layer, or AgSbTe
2
layer, In layer, and Sb layer are separately disposed. In JP-A 106647/1996, the reduced energy for the initialization due to the adoption of the crystallized AgSbTe
2
layer is described as its merit. However, composition of the unit layers is limited in JP-A 106647/1996 since it is an object of the JP-A 106647/1996 to form a recording in the form of an artificial lattice film. As a consequence, formation of a recording layer having overall composition optimal for phase change recording is difficult. In addition, since the unit layers in the recording layer are formed as separate layers, formation of a uniform recording layer is difficult even when the recording layer is initialized by heating, and the medium suffer from insufficiently stable properties. It should be noted that the JP-A 106647/1996 is silent about the specific condition used in the initialization (linear velocity, laser power, etc.).
U.S. Pat. No. 4,889,746 proposes lamination of a single element layer such as Sb layer and a low-melting intermetallic compound layer on a heated substrate. In U.S. Pat. No. 4,889,746, composition of the layers is determined such that average composition of these layers fall within the composition of a recording layer. Also disclosed is the capability of reducing the crystallization temperature of each layer. The medium of U.S. Pat. No. 4,889,746 still suffers from insufficient uniformity of the recording layer due to the formation of the unit layers as separate layers, and therefore, from the insufficiently stable properties.
The assignee of the present invention has made proposals in JP-A 221814
Nagayama Mori
Yoshinari Jiro
Evans Elizabeth
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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