Optical information recording medium

Details Optical information recording medium Optical information recording medium

1998-11-16

2001-01-23

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

06177166

ABSTRACT:

The present invention relates to an optical information recording medium such as a phase-change type recording medium and a magneto-optical medium.
Along with an increasing amount of information in recent years, a recording medium capable of recording and retrieving a large amount of data at a high density and at a high speed has been demanded, and an optical disk is expected to be just suitable for such an application.
Optical disks are classified into a write-once type disk which is capable of recording only once and a rewritable type disk which is capable of recording and erasing for any number of times. As a rewritable type optical disk, a magneto-optical medium utilizing a magneto-optical effect or a phase-change medium utilizing the change in reversible change in the crystal and/or amorphous state, may be mentioned.
The phase-change medium has a merit that it is capable of recording/erasing simply by modulating the power of a laser beam without requiring an external magnetic field, and the size of a recording and retrieving device can be made small and simple.
Further, it has a merit that a high density recording can be attained by a shorter wavelength light source without any particular alteration of the material of e.g. the recording layer from the medium capable of recording/erasing with a wavelength of about 800 nm, which is currently mainly used.
As the material for the recording layer of such a phase-change medium, a thin film of a chalcogenide alloy is often used. For example, an ally of GeSbTe type, InSbTe type, GeSnTe type or AgInSbTe type may be mentioned. Its layer structure is generally a quadri-layer structure comprising a protective layer, a recording layer, a protective layer and a reflective layer.
In a rewritable phase-change type recording medium which is practically employed at present, a crystal state is an unrecorded or erased state, and an amorphous state is a recorded state.
The recording, i.e. formation of the amorphous marks, is carried out by heating the recording layer to a temperature higher than the melting point, followed by quenching. The erasing, i.e. crystallization, is carried out by heating the recording layer to a temperature higher than the crystallization temperature of the recording layer but lower than the melting point.
To prevent evaporation or deformation of the recording layer by such heating or quenching treatment, it is common to sandwich the recording layer with heat resistant and chemically stable dielectric protective layers. In the recording step, these protective layers facilitate heat dissipation from the recording layer to realize supercooled state, and thus contribute to formation of the amorphous marks, and in the erasing step, the protective layers work as heat-accumulating layers to maintain the recording layer at a high temperature suitable for solid phase crystallization.
Further, it is common that a metal reflective layer is formed on the above described sandwich structure to obtain a quadri-layer structure, whereby the heat dissipation is further facilitated so that the amorphous marks will be formed under a stabilized condition.
A phase-change type recording medium one-beam overwritable simply by modulating an intensity of one focused laser beam is erasing and re-recording steps, is noteworthy as a medium for an inexpensive recording system of high density and large capacity since a layer structure and a drive circuit structure can be simplified.
Recently, a CD (Compact Disk) or DVD (Digital Versatile Disk, or Digital Video Disk) has been developed by using such a phase-change type recording medium.
A rewritable CD (CD-Rewritable, CD-RW) does not satisfy the standard of present CD requiring a reflectance of at least 70%, but secures a compatibility of groove signals and recording signals to CD in a reflectance range of from 15 to 25%. Also, if an amplification system is applied to a regeneration system in order to cover the low reflectance, a compatibility can be secured within the scope of present CD driving techniques.
CD-RW is provided with a wobbling groove, in which recording is carried out. The wobbling frequency is one having a carrier frequency of 22.05 kHz frequency-modulated (FM) by address information. Its wobble amplitude is very small (about 30 nm) in comparison with a groove pitch (1.6 nm).
This is called as ATIP (Absolute Time In Pre-groove) signal wherein wobbling is frequency-modulated and address information of a certain track at a specific position is introduced.
ATIP signal is already used in a write once type disk (CD-recordable, CD-R) with an organic dye. By using ATIP signal, it becomes possible to control the rotational speed of an unrecorded disk, and recording can be carried out at a linear velocity of 1, 2 or as high as 4 or 6 times of the CD linear velocity (from 1.2 to 1.4 m/s).
Actually, a commercially available CD-R is generally a medium satisfactorily recordable at a linear velocity of either 2 or 4 times of the CD linear velocity.
Thus, it is demanded also with regard to phase-change recording CD-RW that is satisfactorily overwritable at a linear velocity in the range of at least 2 times (2.4-2.8 m/s) to 4 times (4.8 m/s-5.6 m/s), further in the range of 6 times (7.2-8.4 m/s) to 8 times (9.6-11.2 m/s) of the CD linear velocity.
On the other hand, a rewritable recording medium having a higher recording density, i.e. a rewritable DVD, has been developed by using such a phase-change recording technology. And also in this case, it is demanded that the rewritable DVD is satisfactorily overwritable at a linear velocity in the range of at least 2 times (7 m/s) and even to 4 times (14 m/s) of the read-only DVD linear velocity (3.5 m/s).
In such a case, in order to use an inexpensive semiconductor laser, it is desirable that the recording power is at most about 15 mW, and even if the linear velocity during recording is different, a desired or the same mark length must be recorded with high quality, simply by changing the reference clock frequency in inverse proportion to the linear velocity.
However, with a phase-change medium, if the ratio of the maximum linear velocity to the minimum linear velocity for overwriting exceeds about 2, it becomes impossible to carry out proper recording at either linear velocity, in many cases.
Usually, a recordable disk requires a different irradiation power to heat the recording layer to the same temperature for the different linear velocity. Even if the maximum temperature of the recording layer is brought to the same level by adjusting the irradiation power, if the linear velocity is different, the same heat history including temperature rising rate, cooling rate and temperature distribution may not necessarily be accomplished.
Formation of amorphous marks during recording is carried out by quenching the recording layer which has once been melted by heating, at least a specific critical cooling rate, and crystallization during erasing is carried out by relatively slowly cooling the heated recording layer. This cooling rate depends on the linear velocity when the same layer structure is employed. Namely, at a high linear velocity, the cooling rate is high, and at a low linear velocity, the cooling rate is low.
Thus, as a linear velocity during overwriting becomes higher, a cooling rate in the vicinity of a melting point becomes higher and amorphous marks are easily formed. On the contrary, as the linear velocity becomes lower, the cooling rate becomes lower and there is a fear that recrystallization during recording tends to occur.
This is proved by the following simulation results made by the present inventors.
Heat distribution simulation was carried out by solving a thermal diffusion equation when applying with recording power and erase power with regard to a disk having a protective layer (100 nm) comprising ZnS and SiO
2
, a recording layer (25 nm) comprising Ge
2
Sb
2
Te
5
, a protective layer (20 nm) comprising ZnS and SiO
2
and a reflective layer (100 nm) comprising Al alloy respectively formed on a polycarbonate substrate.
A cooling rate in

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