Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2000-12-18
2002-10-22
Edun, Muhammad (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S100000, C369S047100
Reexamination Certificate
active
06469977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical information recording medium that allows information signals to be recorded/reproduced with high quality by irradiating a thin film formed on a substrate with a high energy beam such as a laser beam, a method for producing the same, and a method and an apparatus for recording/reproducing information thereon.
2. Description of the Prior Art
Conventionally, it is known that it is possible to cause a phase change between an amorphous phase and a crystalline phase, which have different optical constants (refractive index n and extinction coefficient k), by irradiating a thin film made of a chalcogen material formed on a substrate with a laser light beam for local heating under different irradiation conditions. Utilizing this phenomenon, a so-called phase changeable optical information recording medium has been under development.
In the phase changeable optical information recording medium, a new signal can be recorded while erasing an existing recorded signal, using only a single laser beam, by modulating the laser output between two levels of a recording level and an erasing level in accordance with the information signal and irradiating an information track with the single laser beam. This method is advantageous for recording information for the following reasons: A magnetic circuit component as required by optical magnetic recording is not required, so that the head can be simplified; and erasure and recording can be performed simultaneously, so that a period of time required for rewriting can be shortened.
In such an optical information recording medium, the following structure is common. Dielectric layers having excellent heat resistance are provided as protective layers above and below a recording layer for the purpose of preventing the evaporation of the recording layer and the thermal deformation of the substrate that might occur during repeated use. Furthermore, a reflective layer made of a metal material is provided on the protective layer on the side opposite to the substrate for the purpose of efficiently using incident light and increasing the cooling rate so as to facilitate a change to an amorphous state. Thus, in general, at least four thin films are laminated to form the optical information recording medium.
In order to produce a phase changeable optical information recording medium with high density and large capacity, the following attempts are commonly carried out: forming a smaller mark by using a shorter wavelength of the light source or a higher NA (numerical aperture) of the object lens used for recording, and thus improving the linear density in the circumferential direction and the track density in the radial direction of the recorded mark on the substrate. Furthermore, mark edge recording in which information is defined by the length of the mark has been proposed to improve the linear density, and land & groove recording in which information is recorded both on grooves for guiding laser light formed on the substrate and lands between the guide grooves has been proposed to improve the track density, and both recording methods are adopted.
Furthermore, it was proposed that a plurality of such recordable information layers are laminated via separating layers so that the capacity is increased (e.g., JP 9-212917 A, JP10-505188 A). Moreover, layer recognition means and layer switching means for selecting one of these information layers for recording and reproduction were proposed (e.g. JP10-505188 A).
Improving not only high density but also data processing rates, namely, the velocity of recording/reproducing information, is important. For this reason, improving the linear velocity by rotating a disk at a higher revolution per minute with the radius position unchanged for recording and reproduction is under research.
In such an optical information recording medium, there are some challenges for achieving good and practical signal quality and recording/reproducing characteristics, and some measures have been taken.
First, to obtain large signal amplitudes and high C/N ratios, a large change in the reflectance between crystal and amorphous portions is required. To cause a large change, it is necessary that the absolute value of the difference in the reflectance &Dgr;R=Rc−Ra is large, where Rc is the reflectance when the recording medium is crystalline, and Ra is the reflectance when the recording medium is amorphous with respect to a laser light beam of wavelength &lgr;.
In the case of overwriting with a single beam, there is a difference in the end-point temperatures between the case where the base layer before recording is amorphous and the case where it is crystalline even if irradiated with beams of the same power level, because the crystalline portion requires a latent heat of fusion. Therefore, when overwriting a signal, the shape of the mark is distorted by the influence of a signal that has been recorded before the overwriting. This mark distortion causes a drop of the erasure ratio or an increase of errors (jitters) in the time axis direction of reproducing signals. The problem caused by this phenomenon becomes more serious as higher linear velocity and higher density are achieved (e.g., Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, Optical Data Storage '97, Proceedings of SPIE, Vol. 3109, 28 (1997)). In order to solve this problem, a method of equalizing the end-point temperatures of the amorphous and crystalline portions irradiated with beams of the same power level was proposed (e.g., JP 1-149238A). This method requires that the absorptance ratio Ac/Aa is more than 1 (Ac/Aa>1), where Ac is the absorptance of the recording layer when the recording layer is crystalline, and Aa is the absorptance of the recording layer when the recording layer is amorphous with respect to a laser light beam of wavelength &lgr;, in order to compensate the latent heat of fusion in the crystalline portion.
There are two ways of increasing the absolute value of &Dgr;R, namely, Rc>Ra, i.e., a reflectance-decrease-type in which &Dgr;R is positive, and Rc<Ra, i.e., a reflectance-increase-type in which &Dgr;R is negative. In the reflectance decrease-type, Rc can be raised easily, so that the reflectance as the base can be raised, and Ra can be substantially zero. Therefore, this is advantageous in that the contrast of signals can be large. On the other hand, as described above, either one of the following is necessary in order to increase Ac/Aa at the same time: transmitting part of the incident light or allowing light to be absorbed by a portion other than the recording layer. This is disadvantageous in efficiently utilizing the incident light and in the degree of freedom in the optical design. On the other hand, in the reflectance-increase-type, Ac/Aa can be increased at the same time when the absolute value of &Dgr;R is increased. Therefore, it is not necessary to transmit part of the incident light or to allow light to be absorbed by a portion other than the recording layer. This is advantageous in efficiently utilizing the incident light and in the degree of freedom in the optical design.
Examples of the structure of such a reflectance-increase-type recording medium are as follows: A structure is such that at least five layers of a semitransparent optical interference layer made of Au or the like, a lower protective layer, a recording layer, an upper protective layer and a reflective layer are formed in this order on a substrate, and the absolute value of &Dgr;R is increased by the reflectance-increase-type technique utilizing the interference effect of light, especially by the optical interference layer (e.g., JP 7-78354A, JP 7-105574A and JP 7-262607A).
In the recording medium provided with a plurality of information layers as described above, information is recorded on/reproduced from the second information layer positioned farther from the laser light incident side with light that has passed thro
Kitaura Hideki
Yamada Noboru
Edun Muhammad
Merchant & Gould P.C.
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