JPH01165048A - Optical recording medium - Google Patents

Abstract

PURPOSE:To increase the data transfer speed of an optical disk and to increase the number of repetitions of writing and rewriting by specifying the value of the right lower index of the average chemical compsn. Ge1-xBi2xTe2x+1 of an optical recording material layer sandwiched by cooling layers via protective layers. CONSTITUTION:The optical recording material layer 3 is spot-melted by laser light and thereafter, said layer is rapidly cooled to a non-crystalline state by either of the cooling layer 5 consisting of Al or the transparent cooling layer 7 via the protective layers 2, 4 consisting of SiO2, by which recording is executed, at the time of writing information to the optical disk constituted by covering the surface with the protective layer 6 consisting of an org. material and forming a substrate 1 of polycarbonate. The crystallization rate is increased and the erasing time of the recorded information is shortened and in turn, the rapid cooling is needed at the time of recording as compared to the case in which GeTe and Bi2Te3 are respectively along, if the compsn. of the layer 3 satisfies formula I. In addition, the viscosity of the material increases and the annihilation by evaporation at the time of spot melting is suppressed. The number of repetitions of the writing and erasing is thus increased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rewritable optical recording medium that can be erased at high speed and that can be repeated a large number of times.

[Conventional technology]

In recent years, there has been an increasing demand for higher density and larger capacity information storage, and research and development has been actively conducted both domestically and internationally.
In particular, optical disks that use a laser as a light source have a recording density approximately 10 to 100 times that of conventional magnetic recording media, and can record information without contact between the recording/reproducing head and the recording medium.

Because it can be played back, there is little damage to the recording medium and it has a long lifespan, so it can record a huge amount of information.
It is promising as a means of regeneration.

These optical discs can be roughly classified into three types depending on their purpose: read-only type, write-once type, and rewritable type.

The read-only type is a read-only disk from which information can only be read, and the write-once type allows information to be recorded and reproduced as needed, but the recorded information cannot be erased. On the other hand, the rewritable type is capable of recording and reproducing information, as well as erasing and rewriting recorded information, and is desired and has the greatest expectations for use as a data file for computers.

For rewritable discs, there is a magneto-optical method.

Three phase change recording methods are being developed, but each method still leaves room for improvement in terms of recording materials, writing mechanisms, etc. Among these, the phase change method generally focuses a laser beam on the recording surface of the disk, heats it, and controls the pulse output and pulse width of the laser beam, thereby changing the phase of the recording material from a crystalline state to a non-crystalline state. It causes a transition to a crystalline state or a phase transition, and records and erases information based on the difference in reflectance in each state.

An example of the main structure of this phase change type optical recording medium is shown in the schematic cross-sectional view of FIG. 3. In FIG. A Stow first protective layer 2 is formed by a method such as Stow, and a GeTe optical recording material layer 3 and a second protective layer 4 are formed thereon, and an M cooling layer 5 is formed thereon. It has a structure in which a surface protective layer 6 of an organic substance is attached.

That is, the optical recording medium of FIG. 3 is constructed by depositing each of these layers in the order of the numbers on the substrate l.

The reason why the optical recording material layer 3 is sandwiched between the two protective layers 2.4 is that the optical recording material heated by laser light to a high temperature reacts with the substrate 1 when writing and erasing signals. This is to prevent evaporation, scattering, and deterioration of the optical recording material. Although some optical recording media do not have a cooling layer 5, it is possible to provide a cooling layer 5 such as M having good thermal conductivity between the second protective layer 4 and the surface protective layer 6. , is known to be effective in increasing the cooling rate from a molten state when an optical recording material changes from a crystalline state to an amorphous state. In this case, the second protective layer 4 also serves as a heat insulating layer.

It is important to determine the optimum thickness of the second protective layer 4 as a heat insulating layer in order to ensure the characteristics of this optical recording medium, and the present inventors have determined this in Japanese Patent Application No. 62-49337. It is disclosed as follows. In other words, the thickness of the insulation layer
must satisfy the following equation.

KsTm/P<x< 2 X(Ks t/l)s
Cs)""where Ks is the thermal conductivity of the heat insulating layer, Tta is the melting point of the optical recording material, P is the input energy density of the light irradiated to this optical recording medium, t is the light irradiation time, and ρ1 is the thermal conductivity of the heat insulating layer. Density, Cs is the specific heat of the insulation layer.

Furthermore, the present inventors are currently applying for a patent for an optical recording medium in which a transparent cooling layer 7 made of AjN or the like having high thermal conductivity is interposed between the substrate 1 and the first protective layer 2.

FIG. 4 is a schematic sectional view showing the structure. In FIG. 4, parts common to those in FIG. 3 are indicated by the same symbols, and the structure is exactly the same as in FIG. 3 except that a transparent cooling layer 7 is provided between the substrate 1 and the first protective layer 2. has been done. This transparent cooling layer 7 serves to horizontally diffuse the heat diffused from the laser spot formed on the optical recording material layer 3 through the first protective layer 2 before reaching the substrate 1. Therefore, the temperature of the substrate 1 hardly increases and the quality of the substrate 1 does not change. Therefore, in this case, the first protective layer 2
The layer also serves as a heat insulating layer, and its appropriate thickness range can be determined by applying the inequality regarding X disclosed in the aforementioned Japanese Patent Application No. 62-49337.

In the optical recording medium shown in FIG. 4 which includes the transparent cooling layer 7, the transparent cooling N7 also serves to increase the cooling rate when the optical recording material layer 3 changes from the crystalline state to the amorphous state. It is also possible to omit 5.

When an optical recording medium having the above structure is used, a laser beam is normally incident on the substrate 1 from the side opposite to the side on which the optical recording material layer 3 is provided. To actually write information, the optical recording material N3 is first irradiated with light using a flash lamp to bring it into a crystalline state, and then, when recording information, a high-power, short-pulse laser beam is applied to it for 1-φ.
After condensing and irradiating the optical recording material into a spot shape of about 100mm, the laser beam irradiation is stopped and the melted spot is heated to 109 to 10"t/see by thermal conduction.
The material is rapidly cooled at a cooling rate of 200 ml to form an amorphous spot. When erasing recorded information, this amorphous spot is heated using a relatively low-power laser beam to return it to a crystalline state. The irradiation time at this time is determined based on the crystallization rate of the optical recording material.

[Problem that the invention seeks to solve]

Many materials have been proposed for optical recording materials used in phase-change optical recording media, among which GeT
e is considered to be promising because there is a large difference in reflectance between the crystalline state and the amorphous state, and the stability of recorded information is also high. However, according to the results of studies conducted by the present inventors, it takes an annealing time of at least 0.5 μsec to completely transform an amorphous GeTe thin film into a crystalline state by irradiating it with laser light. When an optical disk is manufactured using a laser beam and information is erased using a laser beam with a beam diameter of approximately 1-φ, the circumferential speed of the disk must be 2 m/sec or less, but on the other hand, the crystal state during writing must be The change to the amorphous state can be performed in 0.1 to 0.2 μsec, which is at a circumferential speed of 10 m/sec to 5 m/3
Corresponds to 6c. Based on these facts, in order to increase the peripheral speed of the optical disk and increase the data transfer rate, it is necessary to change the annealing time, that is, the erasing time, from the crystalline state to the amorphous state of GeTe, which is an optical recording material, from the crystalline state to the amorphous state. It is desirable to make the change time, that is, the same as the writing time. To do this, the crystallization rate of GeTe itself must be further increased. Also, GeTe has a high vapor pressure even in the solid state, so repeated heating and cooling Another problem is that the data gradually gets lost, and the number of repetitions of writing and erasing is small, about 1,000 times7. In addition, allTe has a high crystallization speed and light absorption coefficient.
Although s has excellent properties as an optical recording material, there is a problem that the difference in reflectance between the crystalline state and the amorphous state is small, making it difficult to obtain a high CN ratio when used in an optical disc.

The present invention has been made in view of the above points, and its purpose is to shorten the erasing time of recorded information by increasing the crystallization speed of the optical recording material, increase the data transfer speed of the optical disc, and improve the speed of the information. The purpose is to increase the number of repetitions of writing and erasing.

[Means for solving problems]

The present invention provides an optical recording medium having a structure in which the optical recording material layer is provided with a cooling layer and/or a transparent cooling layer through a protective layer. When writing information on an optical recording medium, the optical recording material layer of the optical recording medium is heated in the form of a spot by a laser beam, where it is first melted and then rapidly cooled by thermal conduction to form an amorphous state. At this time, GeTe and B11Te5
The optical recording material according to the present invention containing an appropriate amount of B11T
Since 8s acts as a nucleus for crystal growth and speeds up the change from an amorphous state to a crystalline state, if the cooling rate is not sufficiently high, crystallization will progress during cooling from the molten state and the amorphous state will occur. will not be obtained. Therefore, as long as the material composition according to the present invention is used as an optical recording material, FIGS.
Optical recording media must have at least one cooling layer or transparent cooling layer as shown in the figure; structures without these cooling layers have insufficient cooling speed, making it difficult to write information. This is because even if writing is possible, the number of repetitions will be reduced.

The composition of the optical recording material used in the optical recording medium of the present invention is basically GeTe, which is a 1=1 compound of Ge and To, and 81
It is a mixture of gTe3, which increases the crystallization rate and increases the material viscosity to reduce the vapor pressure and write,
It plays a role in suppressing the loss of material due to repeated erasure.

〔Example〕

The present invention will be explained below based on examples.

The optical recording medium of the present invention has the structure shown in FIG. 3, for example, and the optical recording materials used therein are GeTe and BitT@
It is a mixture of s. A thin film of this optical recording material can be easily produced by ordinary RF magnetron sputtering. Referring again to Figure 3, first, the thickness is 3.
wm, on a polycarbonate substrate l with a diameter of 130fi, a first protective layer 2 (Sins) + with a thickness of 0.1μ
Ge5Bi qTeq ((GeTe) h (gt
zres) t] thickness 0.07 -
an optical recording material layer 3° and a second protective layer 4 with a thickness of 0.2° (
A cooling layer 5 of M with a thickness of 0.2p is formed by sputtering in this order, and a surface protection layer 6 of an organic material with a thickness of 2n is formed as the top layer to produce an optical recording medium.

Using this optical recording medium, a laser beam having a wavelength of 830 nm and an output of 12 wW was irradiated while rotating at a circumferential speed of gm/3 eC. The diameter of the laser spot on the surface of the optical recording medium is approximately 1 μ. The optical recording material layer 3 immediately after sputtering was in an amorphous state, and its light reflectance was about 25%, but the light reflectance increased to about 57% by this laser beam irradiation. The same area was irradiated with laser light again under the same conditions, but no change was observed in the reflectance from 57%. The reason why the 0 reflectance increased from 25% to 57% was at the area where the optical recording material layer 3 was the laser spot. This is because the optical recording material changes from an amorphous state to a crystalline state during the first laser beam irradiation, and the reason why it retains its reflectance even after the second laser beam irradiation is because the optical recording material has been sufficiently crystallized by the first laser beam irradiation. This indicates that there is a

In order to confirm the above, an optical recording material film having the same composition as above was formed on a glass substrate, and its reflectance was measured while increasing the temperature at a rate of 10° C./win. The results are shown in Figure 1. Figure 1 is a diagram showing the change in reflectance with respect to temperature of the optical recording material film, and from Figure 1, the reflectance is 150.
It can be seen that the temperature rises rapidly around ℃. When the crystal morphology of the optical recording material film was examined by X-ray diffraction before and after this temperature, it was found that the optical recording material film crystallized after the reflectance increased.
The crystals are mainly GeTe and BitTes.
The reflectance value almost corresponds to the reflectance value of the optical recording medium mentioned above, and the fact that the optical recording material in the optical recording medium could be crystallized at a circumferential speed of 8 m/sec indicates the time required for crystallization. This suggests that the time is less than 0.125μsec, which is lower than the conventional 0.5μsec mentioned earlier.
This means that it is greatly improved compared to . After writing information, it was also possible to erase it at a circumferential speed of 8 m/sec. That is, frequency 1
．． When a 5 MHz pulse input was written, a CN ratio of 50 dB was obtained, but when this was erased in 9 m7 seconds, the CN ratio decreased to about 5 dB, and was almost completely erased.

This is quite natural considering that writing involves forming an amorphous spot on a crystallized optical recording material by laser heating. In this way, the optical recording medium of the present invention can increase the peripheral speed from the conventional 2 m/sec to 1 m/sec.
By increasing the data transfer speed to 3m/sec, the data transfer speed can be reduced to 0.
．． It can be increased from 24MB/sec to 0.98MB/sec.

FIG. 2 is a diagram showing the relationship between the content of BitTI3s in the optical recording material layer 3 and erasing time. As shown by the curve in FIG. 2, the erasing time becomes shorter as the BtzTes content increases. In addition, the content of 1lizTe3 and the previously mentioned 1
Table 1 shows numerical values of the relationship between the crystallization temperature and the number of crystal-amorphous cycles when the temperature is raised at a rate of 0° C./1 lin.

According to Table 1, the crystallization temperature is 100° C. when the BitTes content is 80%. This word with a low crystallization temperature means that the amorphous spot 7) written on the optical recording medium has poor thermal stability and is likely to crystallize.
This temperature is considered to be the lower limit of the crystallization temperature.

Regarding the number of repetitions, the amount of BltTel added is 1
When it exceeds 0%, a remarkable effect is observed, but when it exceeds 80%, the number of repetitions tends to decrease. Based on these facts, the amount of Bi and Te3 contained in GeTe is 80
It can be said that it is not practical to increase the value more than %. The difference in reflectance between the crystalline state and the amorphous state decreases as the content of B11Te5 increases, but even with a BitTes content of 80%, the difference in reflectance is 15%, which is sufficient.

After comprehensively examining the above results, the optimum composition range of the optical recording material layer used in the optical recording medium of the present invention is determined by the general formula Get -
When expressed as 1181gxT1112*+, 0<x≦0
It was concluded that setting the temperature to 8° is appropriate.

〔Effect of the invention〕

The optical recording material used for phase change optical recording media is GeTe.
However, it is desirable to increase the data transfer speed by increasing the information erasing time to the same level as the recording time, and to increase the number of recording-erasing repetitions. As mentioned in , the G of optical recording material
By mixing the eTe compound and the BizTes compound in an optimal range, the crystallization rate can be made faster than that of these compounds alone.At the same time, in order to take advantage of this fast crystallization rate, the optical recording material layer is sandwiched between two layers. The present invention is applied to an optical recording medium having a structure including a cooling layer that is in contact with at least one of the two protective layers to increase the cooling rate, thereby reducing the cooling rate from the molten state and the crystallization from the amorphous state at the laser-heated spot of the optical recording material. As a result, it was possible to obtain an optical recording medium that can shorten the erasing time and increase the number of repetitions.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between temperature and reflectance of the optical recording material used in the present invention, and FIG. 2 is a diagram showing the relationship between temperature and reflectance of the optical recording material used in the present invention. A diagram showing the relationship between To, content and erasing time, FIG. 3 is a schematic cross-sectional view of an optical recording medium having a cooling layer,
FIG. 4 is a schematic cross-sectional view of an optical recording medium having a cooling layer and a transparent cooling layer. 1: Substrate, 2: First protective layer, 3: Optical recording material layer, 4:
2nd protective layer, 5: cooling layer, 6: surface protective layer, 7: transparent cooling layer.ゝξ! - 1 Liao (0C) Fig. 1 Bi2Te3 content (mol 0m) Fig. 2

Claims (1)

[Claims] 1) A transparent cooling layer interposed between the substrate and the first protective layer in a laminate in which a first protective layer, an optical recording material layer, a second protective layer, and a surface protective layer are formed on a substrate. and a cooling layer interposed between the second protective layer and the surface protective layer, the optical recording medium comprising:
The average chemical composition of the optical recording material layer has the general formula Ge_1_-
It is expressed as _xBi_2_xTe_2_x_+_1, and 0
An optical recording medium characterized in that <x≦0.8.