Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2001-07-11
2004-06-15
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S094000, C369S288000, C428S064200, C430S270130
Reexamination Certificate
active
06751184
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an information recording medium for optically recording, erasing, rewriting, and reproducing information, a method for producing the same, and a method for recording/reproducing information with respect to the same.
2. Description of the Related Art
In a phase-change information recording medium, information is recorded, erased and rewritten using a recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase. When this recording layer is irradiated with a high power laser beam and then is cooled rapidly, a portion thus irradiated is changed to be in an amorphous phase. Similarly, when an amorphous portion of the recording layer is irradiated with a low power laser beam and then is cooled slowly, the portion thus irradiated is changed to be in a crystal phase. Therefore, in the phase-change information recording medium, the recording layer is irradiated with laser beams having powers modulated between a high power level and a low power level, whereby the information layer can be changed freely to be in an amorphous phase or a crystal phase. In the phase-change information recording medium, information is recorded using the difference in reflectivity between an amorphous phase and a crystal phase.
In recent years, in order to enhance the recording density of an information recording medium, various techniques have been studied. For example, there are techniques of recording a smaller recording mark using a violet laser beam and of recording a smaller recording mark by making a substrate thinner on a light incident side while using a lens with a large numerical aperture. A technique of recording/reproducing information with respect to two recording layers using a laser beam incident from one side also has been studied (see JP 12(2000)-36130 A).
In order to decrease the size of a recording mark, it is necessary to shorten an irradiation time of a laser beam used for changing a phase of a recording layer. This requires that the crystallization speed of the recording layer should be high. Furthermore, in order to record/reproduce information with respect to two recording layers, it is required to use a thin recording layer on a light incident side so that sufficient light reaches a recording layer on the back side. However, when the recording layer is thinned, the number of atoms contained in the recording layer is decreased, and the movement of atoms involved in a phase change also is suppressed, which lowers the crystallization speed. Therefore, there is a demand for a material capable of forming a recording layer on which information can be recorded with reliability irrespective of its thinness.
Conventionally, as a material for a recording layer, Ge—Sb—Te system materials have been used. According to an experiment by the inventors of the present invention, it is found that, among them, a pseudo binary composition GeTe—Sb
2
Te
3
has the highest crystallization speed, and Ge
2
Sb
2
Te
5
((GeTe): (Sb
2
Te
3
)=2:1) has excellent characteristics. Furthermore, Uno et al. report a recording/reproducing experiment using a Ge—Sb—Te recording layer with a thickness of 6 nm (M. Uno, K. Nagata and N. Yamada, “Thinning Limitation of Ge—Sb—Te Recording Film for High Transmittance Media”, Proc. of PCO'99. 83-88). In this experiment, information was erased at a linear velocity of 9 m/s using a laser with a wavelength of 660 nm, and a satisfactory erasure ratio (30 dB) was obtained.
However, when the inventors conducted an experiment on a Ge—Sb—Te system material, using a violet laser with a wavelength of 405 nm, this material was found to be insufficient for use on a light incident side. Therefore, in a conventional recording layer, it was difficult to realize an information recording medium having a two-layered structure, with respect to which information is recorded/reproduced using a violet laser.
JP 2(1990)-147289 A reports that an information recording medium is obtained that has excellent repeated-recording/erasing characteristics and less change in an erasure ratio with time by adding Sb to Te—Ge—Sn of a recording layer so as to limit the content of each element. However, this is an experimental result in the case where an information recording medium includes only one recording layer, and the recording layer is thick (i.e., 30 to 100 nm). This publication does not show the effects of addition of Sn in the case where the recording layer is thinned.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a high-density recordable information recording medium having two recording layers, a method for producing the same, and a method for recording/reproducing information with respect to the same.
In order to achieve the above-mentioned object, an information recording medium of the present invention includes: a first substrate; a second substrate disposed so as to be opposed to the first substrate; a first information layer disposed between the first substrate and the second substrate; a second information layer disposed between the first information layer and the second substrate; and an intermediate layer disposed between the first information layer and the second information layer, wherein the first information layer includes a first recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase with a laser beam radiated from the first substrate side, the second information layer includes a second recording layer that is transformed in phase reversibly between a crystal phase and an amorphous phase with the laser beam, and the first recording layer contains Ge, Sn, Sb, and Te, and has a thickness of 9 nm or less.
In the above-mentioned information recording medium, the first recording layer may be made of a material represented by a composition formula: (Ge—Sn)
A
Sb
B
Te
3+A
, where 2≦A≦22 and 2≦B≦4. This composition formula represents that Ge and Sn are contained in the material by 100*A/(2A+B+3) atomic % in total. According to this constitution, even when the first recording layer is made thin, satisfactory recording/erasing characteristics are obtained with a violet laser. By setting 2≦A, an amplitude of a signal can be increased. Furthermore, by setting A ≦22, a decrease in crystallization speed can be prevented. By setting 2≦B, Te, which has a low melting point, can be prevented from being precipitated when a phase change between a crystal phase and an amorphous phase is effected. In the case of 2<B, an excess amount of Sb is added to the material represented by (Ge—Sn)
A
Sb
2
Te
3+A
. This excess amount of Sb functions to increase a crystallization temperature to enhance thermal stability of a recording mark, and suppressing the movement of a substance during repeated-recording.
In the above-mentioned information recording medium, a content of Sn in the first recording layer may be 25 atomic % or less. The content of Sn preferably is 0.1 atomic % or more. According to this constitution, even when the first recording layer is made thin, a satisfactory erasure ratio is obtained with a violet laser. Furthermore, by adjusting the content of Sn in the first recording layer and B, the crystallization speed and the crystallization temperature of the first recording layer can be controlled.
In the above-mentioned information recording medium, a transmittance Tc (%) of the first information layer in a case where the first recording layer is in a crystal phase, and a transmittance Ta (%) of the first information layer in a case where the first recording layer is in an amorphous phase may satisfy 40≦(Tc+Ta)/2 with respect to a laser beam having a wavelength in a range of 390 nm to 430 nm. According to this constitution, satisfactory recording/erasing characteristics also are obtained in the second information layer.
In the above-mentioned information recording medium, the transmittance Tc (%) and the
Kojima Rie
Nishihara Takashi
Yamada Noboru
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Tran Thang V.
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