Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1999-06-28
2003-04-29
Angebranndt, Martin (Department: 1756)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192270, C430S270130, C430S945000
Reexamination Certificate
active
06554972
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to an information recording medium and its manufacturing method. More specifically, the invention relates to a phase change recording medium, which is an optical recording medium having a phase change type optical recording layer irradiated with light beams for recording and/or reproducing information and which does not need any initial crystallizing steps for crystallizing the recording layer after the recording layer is deposited.
In a phase change type optical recording medium irradiated with optical beams for recording and/or reproducing information, there are advantages in that the medium has a large capacity, high-speed accessibility and medium portability, and that it is possible to more inexpensively provide a CD interchangeable drive than competitive magneto-optical media since its reproduction principle is reflectance change type which is the same as those of CDs. In addition, there are advantages in that it is possible to easily increase the density of the medium since the medium has an excellent signal quality, and that the medium has a high recorded-data transfer rate.
The phase change recording medium is able to record information by forming a recording mark and to erase information by erasing the recording mark. The recording mark is formed by allowing a recording layer to be irradiated with a light beam of a recording level to be molten to be in a random state, and then, by cooling the recording layer in a shorter time than a recording-layer crystallizing time to quench the random state to room temperature to form an amorphous recording mark. On the other hand, the recording mark is erased by irradiating the recording layer with a light beam of an erase level to raise the temperature of the recording layer to a temperature of less than its melting point and not less than its crystallizing temperature in a longer temperature raising time than the recording-layer crystallizing time to crystallize the recording layer. In addition, the reproduction of information is carried out by utilizing the difference in reflectance between a crystal and an amorphous material.
Since the phase change recording medium can record whether its state is amorphous or crystalline before a recording operation, there is an advantage in that an overwrite operation can be carried out by one beam.
As an example of the phase change recording medium, there is an optical disc. A typical optical disc has a structure (a four-layer structure) wherein a lower dielectric layer, a recording layer, an upper dielectric layer and a reflective layer are sequentially laminated on a polycarbonate substrate, the header part of which is pre-formatted and the data part of which is pre-grooved. Moreover, a counter substrate is applied to the reflective layer via an adhesive layer, or a label is applied to the reflective layer.
As the recording layer, there is used a thin film of a chalcogen metal compound, e.g., GeSbTe, AgInSbTe or InSbTe, which suitably includes a very small amount of Cr, V, N or the like.
The dielectric layer and the reflective layer serve to prevent the oxidation of the recording layer, to prevent the deterioration of the recording layer due to accumulated overwrite, to adjust the thermal response of the recording layer during a recording operation, and to optically enhance the recording layer during a reproducing operation. In particular, with respect to the optical enhancement effect, the lower dielectric layer can increase the variation in reflectance by the multiple interference effect between the substrate and the recording layer, and the upper dielectric layer can increase the variation in reflectance by the multiple interference effect between the recording layer and the reflective layer, so that the optical enhancement effect can improve signal quality.
The phase change recording medium described above is applied to various information storage system, such as CD-RW (compact disc-rewritable) and DVD-RAM (digital versatile disc-random access memory). In future, it is expected to increase the storage capacity of the phase change recording medium, to accelerate the transfer rate thereof, and to lower the price thereof.
However, the inventor has recognized that there are various problems to be solved in phase change recording media, after having continued to make a unique study thereof. These problems will be enumerated below.
(Problem on Technique for Improving Transfer Rate)
First, the problem of transfer rates in the prior art will be described. The request for the acceleration of data transfer rates is high similar to other recording media. However, in the phase change recording media, it is required to shorten the time to crystallize a recording layer in order to improve the data transfer rate during a recording operation. Because the acceleration of the data transfer rate means the shortening of the time for an optical spot to pass. In order to shorten the crystallizing time, it has been proposed to add a very small amount of an element other than principal elements constituting the recording layer to the recording layer, and/or to provide a crystallization controlling seed layer underlying the recording layer. However, this is not sufficient for the shortening of the crystallizing time, so that the data transfer rate of the phase change recording medium is limited to tens Mbp (mega bit per second) or less.
(Problem on Reduction of Producing Costs)
Typically, a conventional method for producing a phase change recording medium comprises:
(1) Master Disc Mastering Process;
(2) Stamper Producing Process;
(3) Substrate Forming Process by Injection;
(4) Film Attaching Process by Sputtering;
(5) (Bonding process of a counter substrate if necessary);
(6) Initial Crystallizing Process; and
(7) Verifying Process.
Among a series of these processes, “(6) Initial Crystallizing Process” is a process for crystallizing an as-deposited phase change recording layer (in a state as deposited) on the whole surface of a disc. The reason why this process is provided is that the as-deposited amorphous recording layer takes a very long time required to recording unlike an amorphous mark formed by an optical recording operation. Therefore, the conventional phase change recording medium is not used as-deposited, so that it is required to crystallize the recording layer at the initial crystallizing step.
For example, in the case of an optical disc, at the “initial crystallizing step”, there is adopted a system for rotating a disc at a relatively low speed while irradiating the disc with elliptical laser beams extending in radial directions of the disc at a high power to feed the beams in radial directions at a shorter pitch than the major axes of the elliptical beams to gradually anneal the recording layer to crystallize the recording layer. Although the time required for the crystallization depends on the diameter of the disc, the linear velocity during initialization, and the feed pitch, it takes at least several minutes including the focusing time, so that the productivity is very bad. Since an actual producing line is designed so that a tact per disc is several seconds, tens initializing systems must be arranged. Therefore, there are problems in that the costs for the systems are high, that it is required to ensure the area for installing the systems, that it is required to carry out the maintenance of the systems, that the productivity of the recording medium is low, and that the producing costs increase.
(Problem on Degree of Freedom for Selection of Structure of Recording Medium)
Another problem of conventional phase change recording media is that the degree of freedom for the selection of the structure of the medium is limited. That is, although most of conventional phase change recording media are set so that Rc (the light reflectance of a crystal part) is higher than Ra (the light reflectance of an amorphous recording mark), this results from the fact that it is required to carry out the initial crystallizing step as described ab
Ashida Sumio
Ichihara Katsutaro
Nagase Toshihiko
Nakamura Naomasa
Yusu Keiichiro
Angebranndt Martin
Kabushiki Kaisha Toshiba
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