Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2002-02-26
2004-04-20
Mulvaney, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064500, C428S064600, C430S270130
Reexamination Certificate
active
06723411
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of manufacturing an information recording medium, and in particular to a method of manufacturing an information recording medium using an optical disk.
2. Description of the Prior Art
The principle of recording information on a thin film (recording film) by irradiating with laser light is well-known. Techniques are also known which use an atomic arrangement change due to irradiation with laser light, such as a phase change or phase transformation of a film material. As this phase-change is accompanied by very little deformation of the thin film, it has the advantage that two disks can be directly glued together to make an information recording medium having a double-sided disk structure.
Usually, these information recording media comprise for example a protective layer, a recording layer such as GeSbTe, a protective layer and a reflective layer on a substrate.
However, in rewritable phase-change optical disks, such as DVD-RAM, because the protective layer penetrates into the recording film, crystallization is poor and the optimal crystallization rate is not realized, or, crystallization occurs too early and the amorphous state transformation is inadequate, so the reflectivity level after overwrite fluctuates. Hence, an interface layer having a good thermal stability was provided between the protective layer and the recording film to prevent the penetration of the protective layer into the recording film. For example, an interface layer of oxide or nitride was provided which contacts the recording film (Prevention of protective layer counter-diffusion between recording films in phase-change optical disks by oxide interface layer: Yasushi Miyauchi, Motoyasu Terao, Akemi Hirotsune, Makoto Miyamoto, Nobuhiro Tokuyado: Japan Society of Applied Physics Lectures, Vol. 3, p. 29-ZK-12 (Spring, 1998), 1127). Compared to the case of a double-sided (ZnS)
80
—(SiO
2
)
20
protective layer, the crystal nucleus growth rate and crystal growth rate are higher, and the crystallization rate is rapid. Moreover, by using a nitride on both interface layers, diffusion of the recording film material is suppressed and the crystallization rate is optimized. (Phase-change optical disks with nitrides on both sides of the recording film, Mayumi Otoba, Noboru Yamada, Hiroyuki Ota, Katsumi Kawahara, Japan Society of Applied Physics Lectures, p. 29-ZK-13 (Spring, 1998) 1128) and N. Yamada, M. Otoba, K. Kawahara, N. Miyagawa, H. Ota, N. Akahira and T. Matsunaga: Phase-change optical disk having a nitride interface layer: Jpn. J. Appl. Phys. Part 1, 37 (1998) 2104. To achieve many overwrites, diffusion into the recording film, such as diffusion of Zn, S from the upper and lower ZnS—SiO
2
protective layers into the recording film, must be prevented. For this purpose, it is effective to provide the interface layer.
In JP Hei 5-144083, the interface layer is provided on the upper and lower sides of a recording film, using TaO, CrO and MnO, for example, as the interface layer. Further, in JP Hei 6-124481, JP Hei 10-21582 and JP Hei 8-287516, TaO and other compounds (e.g., ZnS, TaS) are provided as a layer in contact with the recording film.
In JP Hei 5-144083, for example, the thickness of the interface layer is 3 nm, 150 nm, the thickness of the first protective layer is 150 nm, and (thickness of interface layer)/(thickness of interface layer+thickness of first protective layer) is 3/(150+3)=0.02.
In this specification, the term “phase-change” is used to describe not only a phase change between crystal and amorphous states, but also in the sense of a phase change such as fusion (change to liquid phase) and recrystallization, and a phase change between crystalline states. Mark edge recording means a recording method wherein the edge of a recording signal is made to correspond to “1”, and parts between marks and the inside of a mark are made to correspond to “0”. In this specification, optical disk means a disk on which information is recorded by irradiation of light, and/or a disk wherein information can be reproduced by irradiation of light.
SUMMARY OF THE INVENTION
However, in an interface layer material having good heat stability, the sputter rate is very small, and this slowed down overall production. Also, information recording media using interface layer materials of good heat stability suffered from the drawback that they were not suitable for mass production.
It is therefore an object of this invention to provide an information recording medium having good thermal stability which, due to the use of an interface layer with a high sputter rate, also has good recording/reproduction characteristics and is excellent for mass production, and to provide a method of manufacturing same.
To resolve the above problems, this invention provides an information recording medium having the following characteristics. Specifically, a Tao—O interface layer with good thermal stability and high sputter rate is used. In this way, it is possible to provide an information recording medium which has good recording/reproduction characteristics, and is excellent for mass production. In the past, materials used for the interface layer either had a low sputter rate or their thermal stability was poor, and they could not satisfy both requirements. This invention reconciles these objectives.
The interface layer is provided between a first protective layer and a recording film, which is in contact with a recording layer. The thickness of the interface layer is 0.20 or more but 0.67 or less of the total of the first protective layer and interface layer, and the composition of the interface layer contains tantalum (Ta) and oxygen (O).
Hence, rewritability is good, and the productivity is improved because the sputter rate of the interface layer is high. Herein, rewritability was determined by examining the recording waveform deterioration due to overwrites described later, or more specifically the reflectivity of the crystal level (Ic).
By arranging the thickness of the interface layer to be from 0.20 or more but 0.67 or less of the total thickness of the first protective layer and interface layer as described above, productivity is improved by approximately 170% compared to the prior art and this already gives a satisfactory improvement, but if the thickness of the interface layer is arranged to be from 0.30 or more but 0.60 or less of the total thickness of the first protective layer and interface layer, productivity is further improved and attains approximately 195% or more compared to the prior art which is very desirable.
Regarding the recording waveform deterioration due to overwrite, we examined the reflectivity variation of the crystal level (Ic).
Hereafter, the method of evaluating the reflectivity variation is described. We shall give details only for evaluating the reflectivity variation for groups, but an identical method may be used for lands.
First, the disk to be measured is set in a tester, and rotated. An optical head is then brought into the vicinity of the track to be measured. Autofocus is applied at this position, and the tracking error signal (difference signal) is monitored on an oscilloscope. The autofocus gain is controlled so that the tracking error signal amplitude in the group is maximized (AF offset control). Next, tracking is applied to the group while the autofocus is still applied. Recording is then performed by varying the laser power with a random signal. The recording power is found at which the difference (asymmetry) between the centerline of the envelope of the signal corresponding to the 3T (shortest) mark and space, and the centerline of the envelope of the signal corresponding to the longest mark and space, is +5%, and this is taken as the optimum recording power. Next, the relation between the radial (radial direction)-tilt and the jitter value after 10 overwrites (optimum power) is measured by a time interval analyzer (TIA), and the radial-tilt at which jitter is minimized, is calculated.
Andoo Keikichi
Anzai Yumiko
Hirotsune Akemi
Kurokawa Takahiro
Miyamoto Makoto
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Mulvaney Elizabeth
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