JPH06187662A - Optical recording medium - Google Patents

Abstract

PURPOSE:To contain disks equivalent to the number of recording layers in single disk by laminating two or more layers of chalcogenite materials on a transparent substrate and executing a recording and a reproducing by using two or more kinds of light beams of different wavelength corresponding to the respective recording layers. CONSTITUTION:For example, laser beams 8-10 enter from the side of a transparent substrate 1 consisting of polycarbonate substrate, transparent plastic substrate made of UV-curing resin or glass substrate. The surface of the substrate 1 has tracking grooves and pits for recording, on which a first recording layer 2 having a structure of protective film/recording film/protective film is formed. Further, a separation film 3 is formed thereon and then a second recording layer 4 is formed. The number of recording layers is same as the number of beams 8-10 so that the constitution is regarded as a constitution housing conventional three disks in one disk. This number is not limited to three but more layers may be formed. Thus, recording density is improved with the number of recording layers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium capable of high-density and high-speed recording, and more particularly to an optical recording medium having a recording layer structure which is excellent in improving the recording density of write-once and rewritable optical disks. Regarding

[0002]

2. Description of the Related Art Optical discs are classified into a read-only type and a writable type. The former is a laser disk (L
D), compact disc (CD), etc. The latter includes a write-once optical disc that can be recorded and read once, but cannot be erased and rewritten, and a rewritable optical disc that can be recorded and erased many times. Both the write-once type and the rewritable type are already widely used as a large-capacity memory for document / image files and code data. However, the current recording capacity is still insufficient to cope with the information-oriented society in the future, and a large-capacity optical disc is strongly desired. That is, it is necessary to improve the recording density.

In order to improve the recording density, there is a means of shortening the wavelength of laser light used and making the light spot size smaller. The wavelength of the semiconductor laser currently used for optical disks is 830 nm or 780 nm.
Is. The light spot diameter at each of these wavelengths is 1.6
It is about μm and 1.4 μm, and the capacity of the double-sided type with a 5-inch disk is about 600 MB to 2 GB. In order to further increase the capacity, the development of a shorter wavelength semiconductor laser is currently under active development.
Lasers on the order of 00 nm are being put to practical use. Also, II-
Development of a semiconductor laser using a group VI compound is also in progress, and it is expected that a laser on the order of 400 nm will be put to practical use in the future. In addition, second harmonic technology (SHG: Second Harmonic Generati) that halves the wavelength of laser light
The development of optical heads for 532 nm and the like based on (on) is also active. If a 532 nm optical head can be used,
The light spot diameter is about 0.9 μm, and the recording density is about twice as high. However, this level is still insufficient, and a recording density several times higher is required. If such a high recording density is achieved, the optical disc and the apparatus can be further downsized, and the amount of information that can be written on one disc is extremely large, so that the optical disc can be easily used.

The recording film of the conventional write-once type optical disc includes a Te-based film of a type in which minute holes are formed by irradiating a laser beam, and a dye film. In addition to this, a chalcogenide-based film containing at least one of S, Se, and Te as a main component, which utilizes a phase change between crystal and amorphous, and a two-layer film including a dye film and a light-reflecting film. Those utilizing thermal deformation have also been put into practical use. On the other hand, the recording layer of the rewritable optical disk uses Tb that utilizes the rotation of the deflection surface of light depending on the direction of magnetization.
CoFe-based magneto-optical films and chalcogenide-based films that cause repeated transitions between crystalline and amorphous have been put to practical use.
In each of these cases, the recording layer is a single layer, and even if the wavelength of the light is shortened, it contributes to the improvement of the recording density only by the reduction of the spot diameter.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a write-once type and rewritable type optical disc having a high recording density.

[0006]

A feature of the present invention is to write a write-once type and rewritable type optical disc having two or more recording layers with two or more kinds of laser beams having different wavelengths corresponding to the respective recording layers. , It is to improve the recording density by reading.

The configuration of the present invention will be described in more detail with reference to the drawings. As shown in FIG. 1, a polycarbonate substrate manufactured by injection molding and a UV curable resin are used (2P
Method) is used as the transparent substrate 1. Laser beam 8, 9,
10 enters from the substrate 1 side. Tracking grooves and address signal pits are transferred to the surface of the substrate 1 (not shown in FIG. 1). The first recording layer 2 is formed thereon by an evaporation method, a sputtering method, or the like. This recording layer is composed of protective film / recording film / protective film as shown in FIG. After providing the separation film 3 on the recording layer 2, the second recording layer 4 is formed. The separation film has a role of ensuring that the light beams of wavelengths λ ₂ and λ ₄ corresponding to the recording layers 2 and 4 are focused on the respective recording film surfaces. Further, it also has a function of preventing the heat generated during recording / reproduction from affecting each other. Next, after providing the separation film 5, the third recording layer 6 is formed. In this case, one disc can accommodate three conventional discs. Although FIG. 1 shows only the case where the number of recording layers is three, the number of recording layers is not limited to three, and it goes without saying that there may be more recording layers. The larger the number of recording layers, the higher the recording density. In this case, a single protective disk 7 is provided on the last recording layer 6 to form a single plate type disk, or two single single disks provided with the protective protective film 7 are bonded together with an adhesive. Can be used as a double-sided disc.

In the recording medium having such a layer structure, the following conditions are required between the recording layers. A case of three recording layers as shown in FIG. 1 will be described. Recording / reproducing is performed by using laser beams having wavelengths λ ₂ , λ ₄ , and λ ₆ corresponding to the recording layers 2, 4, and 6 arranged from the light incident side. In this case, the recording layer 2 needs to transmit light of λ ₄ and λ ₆ . Ideally, it should be 100% transparent,
It is possible to operate even at about 50%. Next, the recording layer 4 is λ
The light of ₆ needs to be transmitted, but the light of λ ₂ may or may not be transmitted. It suffices that the last recording layer 6 be capable of recording / reproducing with a laser beam of λ ₆ and may or may not absorb light of λ ₂ and λ ₄ .

Recording films that satisfy the above conditions include chalcogenide films and dye films. But,
It is difficult for the TbFeCo-based magneto-optical film to satisfy the above-described conditions for the transmittance with respect to light in the wavelength range of 400 to 850 nm that can be realized by a semiconductor laser. Also,
Among the write-once recording films, the perforated chalcogenide film and the dye film require a space above the recording film, and when other films are in direct contact with each other, there is no clean hole. Therefore, it is difficult to use a perforated recording film for the recording medium of the present invention in which a large number of recording layers are laminated. On the other hand, the write-once type and rewritable type chalcogenide film utilizing the phase change does not require such a space and is suitable for the present invention. Also, a dye film can be used as long as it is a type that is thermally deformed by a laser beam for recording, or a type that is recorded by a phase change such as crystal and amorphous.

Next, the recording film will be specifically described. The wavelength of the semiconductor laser currently used is 780 to 780.
830 nm, and a laser on the order of 600 nm is being put to practical use. Furthermore, from the 400nm range to 500n
Development of m units is underway as described above. Among chalcogenide materials, examples of materials that can be recorded / reproduced with light of the order of 400 nm to 500 nm include Sb-S series represented by Sb ₂ S ₃ and As-S series. Many of these films have a low absorptance for light in the range of 600 nm or more, regardless of whether they are in a crystalline or amorphous state. These correspond to the recording layer 2 in FIG. When a semiconductor laser with a shorter wavelength is put to practical use in the future, it is possible to select a recording film material corresponding to it. When using the above chalcogenide material as a rewritable optical disc,
It is necessary to have a layered structure as shown in FIG. A transparent protective film 11 is provided on a substrate, a chalcogenide recording film 12 is formed on the transparent protective film 11, and a transparent protective film 13 is further provided on the chalcogenide recording film 12. The protective film 11 functions as an antireflection film to increase the reproduction signal output and increase the number of times of rewriting. However, in the case of the write-once type recording layer, the number of times of rewriting is not required, so that a single layer of the recording film is sufficient without providing a protective film. When the separation film 3 functions as a protective film, the protective film that should be formed before and after the separation film may be omitted. This is not necessary when an inorganic dielectric material or an organic dielectric material having high heat resistance is used as the separation film.

Next, as a chalcogenide material capable of recording / reproducing with light of the order of 600 nm, In-- containing a large amount of Se--
There are Se, Sb-Se, Ge-Se, As-Se systems and the like. Moreover, these films have a low degree of absorbing light having a wavelength of 780 nm or more in both the crystalline state and the amorphous state.
These correspond to the recording layer 4 in FIG. Also,
A chalcogenide containing Te is a material that can be recorded and reproduced with light of 780 nm to 830 nm. Typical examples are Ge-Te, Ge-Sb-Te, In-Sb-Te, T.
There are e-Se, Sb-Te, Ga-Te systems and the like. Te
Has a role of absorbing light in this wavelength range, and the larger the amount of Te, the greater the absorption of light, which is convenient for recording and reproduction. These correspond to the recording layer 6 in FIG.

The materials for the protective films 11 and 13 are transparent inorganic dielectric materials such as SiO ₂ , SiN, ZnS and ZnS-Si.
O ₂ etc. On the other hand, as the separation films 3 and 5, in addition to the same material as the above-mentioned protective film, an organic polymer material can be used. The thickness of the separation membrane is 0.2 μm to several tens μm. For inorganic materials, sputtering method,
It can be formed by a vapor deposition method. In the case of a polymer material, it can also be formed by a spin coating method. If this separation film becomes thick, the tracking groove and the address signal pit may be buried, and normal operation may become difficult. At this time, an ultraviolet curable resin is used on a certain recording layer (2
If the groove or pit pattern is transferred again from the stamper (P method), the next recording layer can be formed thereon. Quartz or glass that transmits ultraviolet rays is suitable for the stamper material in this case. This is because when the recording layer is formed, it is highly likely that ultraviolet rays will not pass therethrough, and thus the conventional nickel stamper cannot cure the resin with ultraviolet rays. A quartz or glass stamper can be manufactured by directly forming a groove or pit on the surface of quartz or glass by reactive ion etching after a conventional mastering process.

After forming the last recording layer 6, an adhesive protection film 7 is provided. This protective film can be formed by spin-coating a polymer material solution dissolved in an organic solvent, or by spin-coating an ultraviolet curable resin and photocuring. This completes a single-sided disc. A double-sided disc is completed by laminating two single-sided discs with a hot melt adhesive or a room temperature curing adhesive.

[0014]

In the write-once and rewritable optical discs, a plurality of recording layers are provided, and recording / reproducing is performed with a plurality of laser beams having different wavelengths, whereby the recording density can be significantly increased.

[0015]

EXAMPLE Here, an optical disc having two recording layers was prepared. First, the diameter of 9cm made by injection molding
The polycarbonate substrate 1 of 1 was prepared. A fine pattern such as a tracking groove transferred from a nickel stamper is formed on one surface of this. The recording layer 2 was first formed on this. That is, the thickness of the protective film is 12
5nm of ZnS-SiO ₂ film, a thickness of 80n as a recording film
m In ₅₂ Se ₄₅ Tl ₃ film, and a ZnS—SiO ₂ film having a thickness of 0.5 μm as the separation film 3 was sequentially formed thereon by the sputtering method. ZnS-SiO ₂ here
Since the separation film can also serve as a protective film, the protection film before and after the separation film is omitted. Even if a film having such a thickness was formed, the shapes of the grooves and pits were not disturbed, and the shape was sufficient for the next recording layer. A second recording layer 4 was laminated on this separation film 3.
As the recording film, a Ge-Sb-Te film having a thickness of 80 nm was formed. Composition ratio is atomic% Ge15%, Sb30%, T
e is 55%. 0.1 μm thick as a protective film on it
Provided of ZnS-SiO ₂ film. Finally, an acrylic UV-curing resin was applied as a protective film on this and photocured to prepare a single-sided type optical disc.

A wavelength of 780 is used in the optical disk evaluation apparatus.
It was equipped with two types of optical heads of nm and 532 nm. The latter wavelength head was manufactured using SHG elements. The disc was rotated at 2,400 times per minute, and recording / reproduction was first performed on the In-Se-Tl film with an optical head of 532 nm. The recording power was about 10 mW, the pulse width was 100 ns, and continuous light of about 1 mW was used for reproduction. As a result, the C / N ratio was 50 dB. The diameter of the recording point at this time is about 0.
7 μm. Next, in order to erase the recording points, continuous light with a power of about 6 mW was irradiated, and the erase ratio was about 3
It was as good as 0 dB. The number of times recording / erasing is repeated is 10
It is ⁶ and there is no problem in practical use.

Next, a Ge-S was performed with an optical head of 780 nm.
The recording / reproducing / erasing characteristics of the b-Te film were evaluated.
The recording power, the erasing power, and the like were values almost similar to those in the case of the In-Se-Tl film in the optical head of 532 nm. The C / N ratio was about 50 dB, and the erasing ratio was about 30 dB, which was good. There was no problem in terms of the number of repetitions. Furthermore, the C / N ratio in the Ge-Sb-Te film changed only slightly regardless of what signal was recorded in the In-Se-Tl film.

At the time of reading information from the Ge-Sb-Te film, the fluctuation of the extinction coefficient due to the information being written on the Se-based material film on the light incident side hardly affects at 780 nm, but the change of the refractive index. Influences. In-Se-T
Since the change in the refractive index of the l film is small, the influence is small, but when another film is used, the separation film 3 is made thicker and Ge
An encoding method in which light is defocused on the Se-based material film when reading from the Te film, and at least the area occupied by recording points in a light spot on the Se-based material film is always constant. It is preferable to record at.

As in the above embodiment, a ZnS-SiO ₂ film was provided as the protective film 11 and an In-Se-Tl film was provided as the recording film 12 on the polycarbonate substrate 1. However, the polycarbonate substrate was a sample servo type.
Next, as the separation film 3, ZnS-SiO ₂ having a thickness of about 1 μm is used.
m formed. In this case, the shape of the pit was considerably broken, and if it remained as it was, the tracking of the next recording layer became unstable. Therefore, at the stage of providing the separation film, put an acrylic UV curable resin on this, press the glass stamper from above to spread the resin to the outer peripheral part, and then irradiate UV from the stamper side to the resin. Was cured. Next, peeling was performed at the interface between the stamper and the resin, and a new pit pattern was formed on the separation film. The thickness of the ultraviolet curable resin layer was 20 μm. This UV curable resin layer can also be regarded as a separation film. A ZnS-SiO ₂ film having a thickness of 125 nm is formed on the resin layer as a protective film.
Ge-Sb-Te film having a thickness of 90nm as a recording film, a ZnS-SiO ₂ film having a thickness of 100nm as a protective film were sequentially formed. Finally, an acrylic UV curable resin was applied onto this protective film and photocured to obtain a single-sided type optical disc.

Recording and reproducing characteristics of this disk were measured using optical heads having wavelengths of 532 nm and 780 nm. as a result,
The measured values were almost the same as the characteristic values obtained with the disk of Example 1, and it was found that the optical disk was a good one. This result shows that even if the shapes of the grooves and pits are collapsed due to the stacking of a large number of recording layers, a larger number of recording layers can be stacked by transferring the pattern from the stamper on the way as in the present embodiment. Shows. In the present embodiment, the modulation method of the information recorded on the In-Se-Tl recording film is a DC-free property, that is, an 8-10 modulation method having a property that the average recording point density in a certain range is constant, and the interlayer crossing is performed. I made the talk small.

Since sulfides such as Sb-S series have a light absorption edge on the shorter wavelength side, it is possible to further increase the recording capacity by further providing a sulfide recording layer on the light incident side.

[0022]

According to the present invention, a plurality of recording layers are provided,
By recording / reproducing with respect to each recording layer by a plurality of light beams having different wavelengths, it is possible to significantly increase the recording capacity of the write-once and rewritable optical discs.
This enables a large-capacity disk while being small.

[Brief description of drawings]

FIG. 1 is a sectional view of an optical recording medium having multiple recording layers according to an embodiment of the present invention.

FIG. 2 is a sectional view of each recording layer.

[Explanation of symbols]

1 ... Substrate, 2, 4, 6 ... Recording layer, 3, 5 ... Separation layer, 7 ...
Protective film for adhesion, 8, 9, 10 ... Light beam, 11,
13 ... Protective film, 12 ... Recording film.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsuya Nishida 1-280 Higashi Koikekubo, Kokubunji, Tokyo Metropolitan Research Center, Hitachi, Ltd.

Claims (5)

[Claims] 1. An optical recording medium comprising two or more recording layers formed on a transparent substrate, and recording / reproducing using two or more kinds of light beams having different wavelengths corresponding to each recording layer. 2. The optical recording medium according to claim 1, wherein the recording layer transmits light having a wavelength corresponding to a recording layer behind the recording layer when viewed from the incident side of the light beam. 3. The recording layer according to claim 1, wherein each of the recording layers is a single layer film of a recording film or a multilayer film having a protective film on at least one side of the recording film, and these recording layers are separated by a separation film. Optical recording media stacked together. 4. The optical recording medium according to claim 3, wherein the recording film is made of a chalcogenide material. 5. The optical recording medium according to claim 3, wherein the protective film is made of an inorganic dielectric material and the separation film is made of an inorganic dielectric material or a polymer material such as an ultraviolet curable resin.