Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-07-07
2003-03-25
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C428S064200, C369S013530
Reexamination Certificate
active
06538960
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P11-196745 filed Jul. 9, 1999; which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium for an optical system having a wavelength
umerical aperture (NA) of lens of not more than about 1000 nm and a method of producing the same, more particularly a magneto-optical recording medium suppressed in temperature rise and improved in the heat characteristics without reducing the optical characteristics and a method of producing the same.
2. Description of the Related Art
An ordinary magneto-optical recording medium, for example, a magneto-optical disk is structured by, as shown in a cross-sectional view of
FIG. 1
, for example a light transmitting substrate
1
on which are successively stacked a light transmitting first dielectric layer
2
comprised of SiN of a thickness of 80 nm to 100 nm, a magneto-optical recording layer
3
comprised of for example TbFeCo, a light transmitting second dielectric layer
4
comprised of for example SiN of a thickness of 20 nm to 40 nm, and a metal reflective layer
5
comprised of Al of a thickness of 50 nm to 60 nm. In the magneto-optical recording medium of the structure shown in
FIG. 1
, the light L strikes from the substrate
1
side and the light L is mainly reflected at the interface between the second dielectric layer
4
and the metal reflective layer
5
. A reflected light of the light L is detected by a detector and converted into an electric signal.
The magneto-optical recording layer
3
is made light transmitting by setting the thickness to not more than 30 nm, for example, 15 nm to 25 nm. Since an enhancement effect of the Kerr effect can be obtained by using multiple interference between the magneto-optical recording layer
3
and the metal reflective layer
5
, the magneto-optical recording layer
3
has the advantage that the reflectance R can be lowered with the optical performance index, that is, R·&thgr;
K
(R is the reflectance, &thgr;
K
is the Kerr rotation angle) kept constant. Also, if the combination of the magneto-optical recording layer
3
and the metal reflective layer
5
is suitably selected, it is possible to prevent a temperature rise in the magneto-optical recording layer
3
and design for the optimal heat characteristics.
Below, the optical aspects and heat aspects of the above conventional magneto-optical recording medium will be explained in detail.
First, the optical aspects of the above magneto-optical recording medium will be explained. If the optical performance index is fixed when changing the film configuration, the amplitude of the signal and disk noise are kept at a constant level and the S/N does not change. In a magneto-optical disk, shot noise is predominant in a high frequency region. Also, since the shot noise level is proportional to the square root of a light quantity detected on a detector detecting the signal, the lower the reflectance, the lower the shot noise level. Therefore, the lower the reflectance can be made in the design within the range which the heat characteristics and the servo characteristics can allow, the more the SIN, especially the high-frequency characteristics, are improved.
A decrease of reflectance can be realized by coating a SiN dielectric layer and a thick magneto-optical recording layer on a substrate. However, in this case, even if the thickness of the SiN dielectric layer is changed, the reflectance will not become lower than 20%. To lower the reflectance further, a configuration making the thickness of the magneto-optical recording layer not more than 30 nm and arranging a reflective layer via the dielectric layer is effective. Particularly, when the thickness of the magneto-optical recording layer is made not more than 20 nm, the transmittance of light increases. Therefore, it becomes possible to change the reflectance in a wide range by using multiple interference between the magneto-optical recording layer and the reflective layer. As the reflective layer, a metal of a high reflectance mainly comprised of Al is usually used.
Next, the heat aspects of the above magneto-optical recording medium will be explained. From the heat aspects, the heat characteristics of the magneto-optical recording layer are controlled by suitably selecting the thicknesses of the metal reflective layer and the second dielectric layer.
However, according to the conventional magneto-optical recording medium having the above film configuration, it is difficult to sufficiently control the heat characteristics.
Along with the increasing higher density of magneto-optical recording media in these years, the spot size of light has been made smaller in diameter. Therefore, when raising the intensity of the reproducing light and repeatedly reproducing information, the recording marks are sometimes lost and information recorded on the medium erased. Especially when using a light source of a short wavelength or an optical system having an NA over 0.7, the photo-absorption concentrates at the surface of the magneto-optical recording layer. Thus the temperature of the magneto-optical recording medium including the magneto-optical recording layer rises before the heat reaches the metal reflective layer and disperses.
Generally, in a metal film, when reducing its thickness, the heat conductivity decreases due to the decrease in the mean free path of electrons. It is known that the heat conductivity drastically decreases when the thickness is made about 30 nm or less. In a magneto-optical recording layer comprised of TbFeCo or another metal layer as well, when the thickness of the magneto-optical recording layer is made 30 nm or less as described above in order to maintain the optical characteristics, the rise of temperature due to decrease in the heat conductivity becomes a problem.
As described above, in a magneto-optical recording medium, there is a problem that it is difficult to achieve both of maintenance of the optical performance index and prevention of temperature rise.
Further, when using a light source of a short wavelength of not more than 490 nm and/or an optical system having an NA over 0.7, the rise of temperature in the magneto-optical recording layer becomes remarkable. Therefore, when the second dielectric layer is formed thinly at a thickness of for example about 10 nm, the dispersion of heat from the magneto-optical recording layer becomes insufficient. Then, the problem rises in that the heat control of the magneto-optical recording layer becomes impossible.
SUMMARY OF THE INVENTION
The present invention was made in consideration of the above problem. Therefore, the present invention has as its object to provide a magneto-optical recording medium maintained in the optical performance index and improved in the heat characteristics and a method of producing the same.
To achieve the above object, the magneto-optical recording medium of the present invention is a magneto-optical recording medium for recording and reproduction of information by focusing of light of a microspot diameter, characterized by comprising a first light transmitting dielectric layer to a first surface of which the light strikes; a magneto-optical recording layer formed on a second surface opposed to the first surface of the first dielectric layer; a light transmitting metal layer formed on the magneto-optical recording layer to a predetermined thickness required for dispersion of heat from the magneto-optical recording layer and having a larger heat conductivity than the magneto-optical recording layer; a second light transmitting dielectric layer formed on the light transmitting metal layer; and a metal reflective layer formed on the second dielectric layer and reflecting light.
The magneto-optical recording medium of the present invention preferably is characterized in that the light of a microspot diameter is light from an optical system of which a ratio of (waveleng
Kawase Kimitaka
Sabi Yuichi
Dinh Tan
Sonnenschein Nath & Rosenthal
Sony Corporation
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