Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
1999-08-26
2002-01-01
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
06335069
ABSTRACT:
TECHNICAL FIELD
The present invention concerns a phase change optical recording medium having a recording layer of changing phase between a crystalline state and an amorphous state in accordance with the intensity of an irradiation beam and, in particular, it relates to a phase change optical recording medium capable of making an initialization process unnecessary, a manufacturing method thereof and a recording method thereon.
BACKGROUND ART
In recent years, study and development have been made vigorously on optical information recording media as means for recording, reading and erasing an enormous amount of information. Particularly, so-called phase change optical disks conducting recording/erasing of information by utilizing reversible phase change of a recording layer between two crystalline and amorphous states have been considered promising since they have an advantage that new information can be recorded simultaneously while erasing old information (hereinafter referred to as “overwriting”) by merely changing the power of a laser beam.
As the recording material for the phase change optical disk capable of overwriting, chalcogen alloys such as In—Se series alloys having low melting temperature and high absorption efficiency of laser beam (refer to Appl. Phys. Lett. Vol. 50, p 667, 1987) or In—Sb—Te (refer to Appl. Phys. Lett. Vol. 50, p 16, 1987) and Ge—Te—Sb alloy (refer to Japanese Patent Unexamined Publication Sho 62-53886) have been used mainly.
On the other hand, in a case of actually conducting recording/erasing by using the chalcogen alloys, a dielectric layer comprising at least one material selected from oxides, carbides, fluorides, sulfides and nitrides of metals or semimetals is usually disposed to either one or both of the surfaces of just beneath and just on the recording layer in order to prevent deformation of a substrate, oxidation of the recording layer, material migration along grooves or deformation of the recording layer due to heat upon recording/erasing.
Then, the phase change optical disk of a three layered or four layered structure having, on a transparent substrate, a recording layer comprising a chalcogen alloy, a dielectric layer disposed just beneath and/or just on the recording layer and a reflection layer also serving as a cooling layer (made of Al alloy or the like) disposed to the recording layer on the side opposite to the substrate is predominant, since this is desirable in view of recording/erasing characteristics.
In a usual phase change optical disk, the material of the recording layer is heated to a temperature higher than the melting point by irradiating a laser beam at a recording power to a recording layer and then quenched; so that the layer turned into an amorphous state with the result that a recording mark is formed. While the material of the recording layer is crystallized with the result that the recording mark is erased by irradiating a laser beam at an erasing power to heat the material to a temperature higher than the crystallizing temperature, followed by gradual cooling.
The phase change optical disk described above is manufactured by forming thin films constituting each of layers successively to a substrate, for example, by a sputtering method or a vapor deposition method. Since the recording layer just after the deposition is in the amorphous state, the disk is usually supplied after irradiating a laser beam to crystallize the entire surface. The process is generally called an initialization process.
The phase change optical disk of the three layered or four layered structure described above has a relation of Rc>Ra assuming the reflectivity when the recording layer is in a crystalline state as Rc and the reflectivity when the layer is in the amorphous state as Ra. The reflectivity when the recording layer is in the amorphous state is not a sufficient value for stable focusing and tracking by a usual driving apparatus. Then, a sufficient reflectivity can be obtained by turning the recording layer into the crystalline state by applying an initialization process.
However, the initialization process requires a time which it is about one minute and less than one minute in order to initialize the entire optical disk of 120 mm diameter even by a laser beam irradiation method which is most efficient, so the step is attributable to the increased cost in the manufacture of disks. That is, considering a time necessary for processing one optical disk in each of manufacturing steps for the optical disk (cycle time), the time required for the initialization process is longer compared with a step for molding a substrate or a step of depositing films. Accordingly, when the cycle time in the film deposition step is, for example, 8 sec, at least 6 to 7 sets of initialization apparatus which is extremely expensive are required in order to eliminate the time loss upon transfer to the initialization process. As a result, the manufacturing cost for the optical disks is increased by the application of the initialization process.
On the other hand, Japanese Patent Unexamined Publication Hei 7-78354 (EP 642123 A1) and Japanese Patent Unexamined Publication Hei 8-63781 disclose phase change optical disks in which a relation between the reflectivity (Rc) when the recording layer is in the crystalline state and the reflectivity (Ra) when the layer is an amorphous state is: Rc<Ra. Japanese Patent Unexamined Publication Hei 7-105574 discloses a phase change optical disk in which a relation between the light absorptivity (Ac) when the recording layer is in the crystalline state and the light absorptivity (Aa) when the layer is in the amorphous state is: Ac>Aa (that is: Rc<Ra).
Among them, Japanese Patent Unexamined Publication Hei 7-78354 and Hei 7-105574 describe that Rc<Ra or Ac>Aa can be attained by disposing a metal layer or a light absorption layer between a recording layer and a substrate. Further, Japanese Patent Unexamined Publication Hei 8-63781 describes that Rc<Ra can be attained by properly selecting the film thickness of a protection layer formed between a recording layer and a substrate.
Further, when setting the relation of the reflectivity as Rc<Ra, since the relation between the light absorptivity (Ac) when the recording layer is in the crystalline state and the light absorptivity (Aa) when the layer is in the amorphous state becomes Ac>Aa, distortion of the shape of an amorphous mark upon overwriting can be suppressed. This can decrease jitters contained in read light to obtain high recording/reading characteristic in mark edge recording capable of high density recording.
As described above, a sufficient reflectivity can be obtained even with no initialization process and high recording/reading characteristics can be obtained in the mark edge recording by a phase change optical disk with the relation of the reflectivity being Rc<Ra. However, it has been found by the study of the present inventors that when overwriting is conducted with no initialization process in this phase change optical disk, C/N (carrier to noise ratio) upon first recording is lower than C/N upon overwriting.
A subject of the present invention is to obtain high recording characteristic already from first recording even when recording is conducted without conducting the initialization process in a phase change optical disk with a relation of the reflectivity being Rc<Ra.
DISCLOSURE OF THE INVENTION
In order to solve the foregoing subject, the present invention provide a phase change optical recording medium having at least a substrate and a recording layer formed on one side thereof and changing phase between a crystalline state and an amorphous state in accordance with the intensity of an irradiation beam, and the reflectivity (of said medium) when the recording layer is in the crystalline state is made lower than the reflectivity when said layer in the amorphous state, wherein the recording layer is formed in a stable amorphous state for the entire surface at the time of this deposition.
That is, the phase change optical recor
Morimoto Isao
Nishimura Kazuhiro
Ogawa Shuichiro
Terada Masato
Asahi Kasei Kabushiki Kaisha
Evans Elizabeth
Pennie & Edmonds LLP
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