JPH06180861A - Optical recording medium - Google Patents

Abstract

PURPOSE:To provide a rewritable optical recording medium having a higher erasion ratio from an initial erasing and a stable erasion ratio even for many times of overwriting. CONSTITUTION:A rugged area having an almost sawtooth cross section is formed in the interface 20 between a recording material layer 3 and a lower protective layer 2. Thereby, the area of the interface 20 can be increased and lots of crystalline nuclei can be easily produced. Thus, crystalline grains in erased bits are fast stabilized and a stable erasion ratio can be obtd. in an earlier stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rewritable optical recording medium which uses a change in characteristics of an optical recording material, such as a change in reflectance, a change in refractive index or a change in shape by an optical method, as a recording means, and more specifically, The present invention relates to the structure of the interface between the optical recording material layer and the protective layer.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for high density and large capacity information recording, and the research has been actively conducted in Japan and overseas. Particularly, the optical recording medium has a recording density 10 to 100 times higher than that of the conventional magnetic recording medium, and further,
Since a recording head can record and reproduce information in a non-contact state with the recording head, the recording medium is less damaged and has a long service life. Therefore, a huge amount of information is recorded and reproduced. Promising as a high-density, large-capacity recording method. This optical recording medium can be roughly classified into three types, a read-only type, a write-once type, and a rewritable type, depending on the application. The rewritable type is an optical recording medium capable of recording and reproducing information, and further erasing and rewriting recorded information, and is most promising for use as a data file for a computer.

Regarding a rewritable optical recording medium, two recording methods, a magneto-optical method and a phase change method, are being developed, and both methods still have room for improvement in terms of recording material and writing mechanism. It is left. Among the above methods, the phase change method is generally a method in which a laser beam whose pulse output and pulse width are controlled is focused on a recording surface of a medium and heated. The optical recording material on the surface of the optical recording medium undergoes a phase change due to the heating of the laser beam, causing a transition from a crystalline state to an amorphous state or a phase transition. The optical recording medium is for recording and erasing information by the difference in reflectance in each state of a phase change state, for example, a crystalline state and an amorphous state.

FIG. 4 shows the structure of a phase change type optical recording disk which is one of such optical recording media. The magneto-optical disk 10 has a structure in which Zn is formed on the surface of a substrate 1 made of polycarbonate or the like in which tracking grooves (not shown) are formed.
A lower protective layer 2 made of S or the like is formed. Then, a recording material layer 3 made of Ge ₂ Sb ₂ Te ₅ or the like is formed on the lower protective layer 2, and an upper protective layer 4 made of ZnS or the like similar to the lower protective layer 2 and a reflective layer made of Al. The cooling layer 5 is formed. In addition, the reflective cooling layer 5
An organic surface protective layer 6 that functions as a heat insulating layer together with the upper protective layer 4 is formed on the top surface of the above.

In such a phase change type optical recording medium, the recording material layer 3 is in a crystalline state in the initial state, and at the time of information recording, the laser beam is irradiated on the recording material layer 3 to melt the irradiated portion, and then it is rapidly cooled. The irradiated portion becomes amorphous and spots (recording marks) are formed. Erasing is performed by annealing this spot in the amorphous state with laser light and returning it to the crystalline state. In addition, during reproduction, laser light with an intensity that does not affect the amorphous state of the recording spot is irradiated, and the reflected light of the laser beam from the crystalline or amorphous spot is detected by the photodetector element. Playback is performed. The laser light is generally incident on the surface of the polycarbonate substrate 1 (the surface on which the recording layer 3 and the like are not formed).

As a recording / reproducing system for such a phase change type optical recording medium, there is a system for overwriting using one laser light source. This overwrite is a recording rewriting means, and as shown in FIG. 5, the power of the laser light source is modulated into two levels, high and low, with time. The recording material layer 3 is melted by the high-level (peak power 35) laser light to write a recording mark, and the recording material layer 3 is annealed by the low-level (bias power 36) laser light to erase the recording. Be seen. That is, when a high-level laser beam for writing is irradiated, regardless of whether the state before irradiation is crystalline or amorphous, the recording mark is formed by rapid cooling after melting, and the low-level laser beam is emitted. When irradiated, it is annealed to be crystallized and the data is erased. Further, during reproduction, laser light of a level (reproduction power 37) lower than the low level is irradiated to reproduce the information as described above.

[0007]

One of the problems in such an optical disk 10 is that the initial erasing rate is improved. When overwriting is repeated many times on a phase-change type optical disk, the erasing rate is initially low, but the erasing rate tends to improve as overwriting is repeated. Therefore, in order to set the erasing rate to a value equal to or higher than a certain value, it is sufficient to repeat overwriting a predetermined number of times before actually using it, but this takes time. Moreover, as the overwriting is repeated, a slight amount of stress or the like is generated in the recording material layer, which causes a writing error. Therefore, in order to improve the erasing rate, if the blank writing is repeated, the writable number of the optical disk will be reduced.

FIG. 6 shows how the erasing rate increases with the number of overwrites. This measurement was carried out on the surface of the polycarbonate substrate 1 having a diameter of 90 mm by Z
Made of ceramic consisting of a mixture of nS and SiO ₂ and thickness 1
A lower protective layer 2 having a thickness of 0 nm, an optical recording material layer 3 having a thickness of 20 nm and made of Ge ₂ Sb ₂ Te _5, an upper protective layer 4 having a thickness of 120 nm, and a reflective cooling layer 5 made of Al are sequentially formed by sputtering, and then further. As the organic protective layer 6, an optical disk spin-coated with an ultraviolet curable resin was used. The conditions of the laser light used for overwriting were that the scanning linear velocity was 9.0 m / s, the peak power as a high level for recording was 17 mW, and the bias power as a low level for erasing was 8 mW. The signal used for overwriting has a pulse width of 57.5 ns and a frequency of 5.80 MHz.

As a result, it can be seen that the erasing rate, which was initially only about 25 dB, gradually increases, and increases to about 30 dB when 10 ⁶ overwrites are repeated. At the same time, the crystal grain size in the erase bit was also measured, and as shown in FIG. 6, it can be seen that the crystal grain size becomes smaller as overwriting is repeated.

As described above, in the conventional optical disk, the initial erasing rate is small and increases as the number of rewrites increases. If such a high erasing rate can be obtained from the beginning or at an earlier stage, an optical disc with high rewriting efficiency can be obtained, and rewriting can be performed many times with a stable erasing rate. It should be possible to do so. Therefore, in the present invention, such high
It is an object to realize an optical recording medium capable of obtaining a stable erasing rate at an early stage.

[0011]

In order to solve the above problems, in the present invention, unevenness is formed to increase the area of the interface of the recording material layer. That is, a recording material layer for recording information based on optical characteristics laminated on the surface of a transparent substrate according to the present invention, and a protective layer laminated on the substrate side and the non-substrate side of the optical recording material layer. In the optical recording medium having the above, an uneven region for increasing the area of the interface is formed at the interface between the recording material layer and the protective layer on at least one of the substrate side and the non-substrate side. The concavo-convex region can be formed into various shapes, but typical examples thereof include those in which the concavo-convex region has a cross section of a substantially sawtooth shape, a substantially corrugated shape, or a substantially stepped shape.

[0012]

As described above, by increasing the area of the interface between the recording material layer and the protective layer, the number of crystal nuclei formed at the interface is sharply increased, and the increase of the erasing rate can be accelerated. Therefore, it becomes possible to obtain an optical recording medium having a high erasing rate at an early stage.

That is, as can be seen from FIG. 6, the crystal grain size decreases as the erasing rate increases.

From this fact and from a microscopic observation of erase bits and the like, the crystallized portion at the time of erasing has a difference in the crystal grain size depending on the number of rewritings or the number of times of erasing. It is considered that the erasing rate is lowered. FIG. 7 schematically shows the situation. The recording track 31, which is a groove or a land of the optical recording medium, is an initially crystallized crystalline material. Then, the portion irradiated with the high level laser light is melted and then rapidly cooled, so that it becomes an amorphous recording mark 33. When the recording mark 33 is annealed by being irradiated with a low-level laser beam, it becomes a crystalline erase bit 32. Although the erase bit 32 has the same crystal quality, the crystal grows based on crystal nuclei such as short-range regularity introduced from the interface with the protective layer at the time of rapid cooling forming the recording mark 33. The crystal grain size is smaller than that of the other recording tracks 31. Since the number of crystal nuclei increases as recording and erasing are repeated, the crystal grain size gradually decreases, and when it reaches a certain size, it stabilizes in that state. Therefore, as the number of times of rewriting increases, the crystal grain size decreases and stabilizes, so that the state of the reflected light from the erase bit stabilizes and stabilizes after the erase rate increases.

The crystal nuclei that cause such a change in crystal grain size are mainly introduced from the interface with the protective layer each time a recording mark is formed. The crystal nuclei once formed are gradually accumulated without disappearing, and eventually become a constant number. Therefore, if the area of the interface with the protective layer is increased and the number of crystal nuclei introduced at one time is increased, the crystal grain size is rapidly reduced by a few overwrites. As a result, the erasing rate sharply increases, and it becomes possible to obtain a high and stable erasing rate at an early stage.

As described above, in the optical recording medium according to the present invention in which the area of the interface is increased, a high erasing rate can be obtained by performing a small number of overwrites as initialization. Further, since the erasing rate is stable, the erasing ratio does not change even when overwriting is performed many times thereafter.

The area of the interface can be increased by introducing a concavo-convex region into the interface. As the concavo-convex region, the cross-sectional surface has a substantially sawtooth shape, a substantially corrugated shape, or a substantially stepped shape, and the interface area is increased. Various materials can be adopted as long as they can be used.

[0018]

Embodiments will be described below with reference to the drawings.

FIG. 1 schematically shows the structure of an optical disc 10 according to the present invention. The optical disc 10 of the present example is also a phase change type optical recording medium having substantially the same configuration as the conventional optical disc described above, and the common portions are denoted by the same reference numerals. That is, the optical disk 10 of this example also has a diameter of 90 m.
ZnS and S on the surface of the substrate 1 made of polycarbonate of m.
A lower protective layer 2 made of a mixture of iO ₂ and having a thickness of 10 nm, an optical recording material layer 3 made of Ge ₂ Sb ₂ Te ₅ and having a thickness of 20 nm, and a lower protective layer 2 made of ceramic and having a thickness of 120 nm Upper protective layer 4, further Al or Al
The reflective cooling layer 5 made of an alloy is sequentially formed by sputtering, and an ultraviolet curable resin is spin-coated as an organic protective layer 6.

The point to be noted in the optical disc 10 of this example is that the interface 20 between the lower protective layer 2 and the optical recording material layer 3 is an uneven region having a substantially sawtooth cross section. The interface 20 having such a shape can be easily introduced by depositing the lower protective layer 2 by a sputtering method and then performing dry etching with a mask. Then, the optical recording material layer 3 and the like can be further formed on the interface 20 having a substantially sawtooth cross section by a sputtering method as usual. The optical reflectance of the optical disk 10 of this example manufactured by such a method shows a good value of about 5% as measured after the film formation.

FIG. 2 shows changes in the erasing rate and the crystal grain size when the optical disk 10 of this example is used for overwriting. The conditions of the laser light used for the overwriting were the same as in the measurement of the conventional optical disk shown in FIG. 6, the scanning linear velocity was 9.0 m / s, the peak power was 17 mW, and the bias power was 8 mW. Similarly, the signal has a pulse width of 57.5 ns and a frequency of 5.80 M.
Hz signal.

As can be seen from FIG. 2, the optical disc of this example has an erasing rate of about 30 which is about 10 ⁶ from the beginning, that is, about 1 time of overwriting.
An erasing rate of dB can be obtained. The erasing rate is stable without much change even when overwriting is repeated about 10 ⁶ times. Further, the crystal grain size is about 20 nm from the beginning of the overwrite, and the value is substantially stable even when the overwrite is repeated about 10 ⁶ times. Therefore, it is considered that a large number of crystal nuclei can be generated by a small number of overwrites by the interface 20 including the concavo-convex region used in the optical disc of this example, and as a result, the crystal grain size can be stabilized at an early stage. To be Then, by stabilizing the crystal grain size, the contrast that causes a decrease in the erasing rate during reproduction is reduced, and a high erasing rate can be obtained.

The optical disk of the present example thus, less frequently, for example, by one overwriting, the conventional 10 ⁶
It is possible to obtain a high erasing rate obtained after overwriting about once. The erasure rate is stable thereafter, and no significant change is observed even when overwriting is repeated about 10 ⁶ times, which is considered to be a practical level. Therefore, it can be said that an optical disk capable of overwriting many times with a high and stable erasing rate could be realized.

The shape of the interface 20 including the uneven area is
The same result can be obtained not only in the case of FIG. 1 but also in the case of an interface having a substantially wavy shape as shown in FIG. Further, of course, in addition to this, various shapes such as a stepwise interface can be adopted as long as the area of the interface can be increased. Further, the concavo-convex region may be formed at the interface with the upper protective layer, and of course, the concavo-convex region may be formed at both interfaces. In addition, the thickness of the recording material layer described above,
The materials, layer structures, etc. are merely representative examples, and it goes without saying that they are changed depending on the usage conditions of the optical disc and other optical recording media, and even in such a case, the same value can be obtained by applying the present invention. It is possible to obtain an effect.

[0025]

As described above, in the optical recording medium according to the present invention, the area of the interface between the recording material layer and the protective layer is increased so that the number of crystal nuclei formed at the interface can be reduced. The number can be increased under the number of times, and at the same time, the size of the crystal grain size can be stabilized. Therefore, in the optical recording medium according to the present invention, it is possible to obtain a high and stable erasing rate at the stage of a small number of rewritings. Therefore, the time of the initial processing for obtaining a high erasing rate can be extremely shortened, and an optical recording medium having a higher erasing ratio than initially possible can be realized. Further, since the obtained high erasing rate is stable, even if the rewriting is performed a large number of times, which is said to be a practical level, 10 ⁶ times or more, the change of the erasing ratio is small, and a stable rewriting characteristic is obtained. An optical recording medium can be obtained.

Further, as described in the present invention, widening the area of such an interface can be easily realized by introducing an uneven region having a substantially sawtooth shape, a substantially wave shape, a substantially step shape or the like into the interface. It is possible to provide an optical recording medium which is inexpensive, has high performance, and has a long life.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration of an optical disc according to an embodiment of the present invention.

FIG. 2 is a graph showing changes in the erasing rate and the crystal grain size of the optical disc shown in FIG. 1 with respect to the number of overwrites.

FIG. 3 is a sectional view showing a configuration of an optical disc according to another embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a configuration of a conventional optical disc.

FIG. 5 is a graph showing a change in intensity of laser light used for overwriting.

FIG. 6 is a graph showing changes in erase rate and crystal grain size of the optical disc shown in FIG. 4 with respect to the number of overwrites.

FIG. 7 is an explanatory diagram showing a recording mark on an optical disc, a crystal structure of an erase bit, and the like.

[Explanation of symbols]

 1 ... Substrate 2 ... Protective layer 3 ... Optical recording material layer 4 ... Protective layer 5 ... Reflective cooling layer 6 ... Organic protective layer 10 ... Optical disc 20 ... Interface 31・ ・ ・ Recording track 32 ・ ・ ・ Erase bit 33 ・ ・ ・ Record mark 35 ・ ・ ・ Peak power 36 ・ ・ ・ Bias power 37 ・ ・ ・ Playback power

Front page continued (72) Inventor Toshie Nagayama 1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Fuji Electric Co., Ltd. (72) Inventor Haruo Kawakami 1-1, Tanabe-shinden, Kawasaki-ku, Kawasaki-shi, Kanagawa Electric Co., Ltd.

Claims (4)

[Claims] 1. A recording material layer for recording information based on optical characteristics laminated on the surface of a transparent substrate, and a protective layer laminated on the substrate side and the non-substrate side of the optical recording material layer. In the optical recording medium having, an uneven region for increasing an area of the interface is formed at an interface between the recording material layer and the protective layer on at least one of the substrate side and the non-substrate side. Optical recording medium. 2. The optical recording medium according to claim 1, wherein the uneven region has a substantially saw-toothed cross section. 3. The optical recording medium according to claim 1, wherein the uneven region has a substantially wavy cross section. 4. The optical recording medium according to claim 1, wherein the uneven region has a substantially stepwise cross section.