JP2773326B2 - optical disk - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Overview] An optical disc, which may be opaque and has a purpose of being capable of performing double-sided recording using a single support substrate, includes a support substrate, a recording medium, and a protective film, The support substrate is made of a transparent or opaque material, the recording medium reaches a Curie temperature by light irradiation,
And has a magneto-optical recording characteristics in which the direction of magnetization is reversed by a magnetic field applied from an electromagnet, the protective film is a resin film transparent to the light, on the recording medium The protective film satisfies both a film thickness condition for reducing the amount of light reflected from the surface of the protective film to the photodetector and a film thickness condition for efficiently bringing the electromagnet close to the recording medium. It is configured so as to have a film thickness to be obtained.

[Industrial applications]

The present invention relates to an optical disk, and more particularly to a magneto-optical disk which may be opaque and which can perform double-sided recording using a single support substrate.

In recent years, the progress of optical disks has been remarkable, and they have begun to spread widely from information processing use to consumer use.

Because the capacity of information that can be stored is large, for example, in the field of information processing, as a large-capacity file memory,
Or, as seen on LDs (laser discs) and CDs (compact discs), huge amounts of analog information such as images and music can be digitized and packed as noise-resistant information.

Optical disks can be broadly classified into, for example, a read-only type, a write-once type, and a rewritable type, each of which has been put to practical use using a unique optical principle. Among them, a rewritable optical disk has great potential in the field of information in the future.

A well-known rewritable optical disk is a magneto-optical disk (hereinafter, a rewritable magneto-optical disk is abbreviated as an optical disk).

This optical disc has a method in which light is first irradiated through a substrate, and a magnetic field is added from a recording medium side provided on the substrate. In this case, the substrate must be transparent and Since recording cannot be performed on both sides of the substrate, the configuration is similar to double-sided recording in which two optical disks are adhered back to back.

On the other hand, there is a method in which light irradiation and application of a magnetic field are performed from the side of the recording medium provided on the substrate. In this case, the electromagnet must be provided close to the recording medium. Can not be applied.

Therefore, there is a strong demand for an optical disc capable of recording on both sides of a single substrate and capable of real double-sided recording.

[Conventional technology]

The principle of magneto-optical recording is to uniformly magnetize the magnetic film,
If a spot is irradiated with spot-shaped light until it exceeds the Curie temperature and heated, the magnetization will be lost, or if an external magnetic field that is opposite to the uniform magnetization direction of the magnetic film is applied, only the irradiated part will be in the opposite direction Or magnetized.

The magneto-optical effect is used to read the presence or absence of magnetization of the magnetized magnetic film and the direction of magnetization, and there are two types of this method.

First, a reading method that applies the so-called Faraday effect, which detects the rotation of the polarization plane of light passing through the magnetic film with respect to the magnetization direction of the magnetic film, and a method in which linearly polarized light is reflected on the surface of the magnetized magnetic film. There is a reading method in which so-called magnetic Kerr effect is applied, which becomes elliptical polarization.

Currently used optical discs perform reading mainly by applying the latter Kerr effect.

On the other hand, there are roughly two methods for writing information on an optical disk. Here, a method of modulating light according to information is called a light modulation method, and a method of modulating a magnetic field applied by an electromagnet according to information is called a magnetic field modulation method.

In the light modulation method, while a magnetic field is steadily applied to a recording medium of an optical disk by an electromagnet, light corresponding to the intensity of light is irradiated according to information. Then, the recording medium is irradiated with light and heated in a spot shape to the Curie temperature, so that the magnetization of the recording medium disappears or the magnetization direction is reversed by the influence of an external magnetic field, thereby performing the recording.

On the other hand, in the magnetic field modulation method, a magnetic field whose direction is inverted according to information is applied to a recording medium of an optical disk while constantly irradiating light. Then, since the recording medium is irradiated with light and heated in a spot shape to the Curie temperature, when the external magnetic field is reversed, the direction of the magnetization of the recording medium is also reversed and recording is performed.

FIG. 4 is a sectional view of a conventional single-sided optical disk, and FIG.
The figure is a cross-sectional view of the double-sided configuration of FIG.

In the figure, 1 is a support substrate, 2 is a recording medium, 3 is light, 4 is an electromagnet, 5 is a protective film, and 6 is an adhesive layer.

The support substrate 1 must be 1.2 mm thick and transparent. If the constituent material of the support substrate 1 is, for example, glass, tempered glass in which Na on the surface of soda glass is replaced with K, or if it is plastic, acrylic having good transparency and high mechanical strength is used. PMMA (polymethyl methacrylate) or polycarbonate is used.

The recording medium 2 has a film thickness formed by sputtering.
A magnetic thin film having a thickness of about 100 nm, such as a GdFeCo-based or TbFeCo-based film, is used, and has a thickness of about 100 nm, for example, sandwiched between Tb-SiO ₂ films to prevent oxidation.

By the way, in order to protect the recording medium 2 from dust and scratches, the light 3 is irradiated through the transparent support substrate 1.

On the other hand, the electromagnet 4 for applying a magnetic field to the recording medium 2 emits light 3
Is disposed on the side of the recording medium 2 opposite to the side of the support substrate 1 on which the light is irradiated.

Here, in the case of the light modulation method, there is nothing that comes into contact with the recording medium 2, and therefore, even if the recording medium 2 is covered with the protective film 5 against tentacles, etc. Don't worry about strength.

For the light 3, a semiconductor laser has recently been used.
Near infrared light of 830 nm is used.

Such a laser beam is directly modulated at a speed of, for example, several MHz in comparison with the presence or absence of information.

On the other hand, the electromagnet 4 is, for example, a so-called coil in which an electric wire is wound around an iron core.
A magnetic field of about 00 Oe is obtained.

In this way, the optical modulation type optical disk is irradiated with the light 3 in which the laser light is modulated sufficiently quickly while the magnetic field of a sufficient magnitude is applied by the electromagnet 4.

In FIG. 5, the double-sided optical disk has a configuration in which two support substrates 1 of the single-sided optical disk shown in FIG. 4 are completely mechanically back-to-back and bonded via an adhesive layer 6. .

Therefore, its basic operation is the same as that of the single-sided type. That is, the method of irradiating the light 3 and the method of applying the magnetic field by the electromagnet 4 remain unchanged.

However, since an external magnetic field applied by the electromagnet 4 is applied through the two support substrates 1, a stronger magnet is required. To get a stronger magnetic field,
This means that the conductance of the electromagnet 4 in which the electric wire is wound around the iron core increases.

Therefore, it is difficult to invert the magnetic pole at high speed, and the magnetic field cannot be inverted at high speed.

As a result, two rotations are required for erasure / writing, such as erasing information already recorded in the first rotation of the optical disk and recording new information in the second rotation.
Since so-called overwriting cannot be performed, the transfer rate becomes slow.

Further, the recording medium 2 must be once heated to the Curie temperature by irradiating the light 3, but the thermal inertia of the recording medium 2 cannot be neglected, and there is a limit to speeding up.

As described above, the optical modulation type optical disk has an advantage that an optical disk having a double-sided configuration can be easily formed by bonding two disks.

However, on the other hand, there are disadvantages such as a delay in the transfer rate and a limit in the recording speed.

Further, since the supporting substrate 1 is interposed, a magnetic head corresponding to an optical head, for example, a semiconductor laser or a magnetic head corresponding to the electromagnet 4 cannot be provided near the recording medium 2, so that miniaturization cannot be performed. Absent.

[Problems to be solved by the invention]

As described above, first, in the conventional optical modulation type optical disk, light irradiation is performed through the support substrate,
Light irradiation and application of a magnetic field have been performed with the support substrate interposed.

Therefore, there is a problem that the electromagnet to which the magnetic field is applied becomes large, and the transfer rate is delayed due to the influence of the large inductance of the electromagnet.

There is a magnetic field modulation type optical disk as an alternative optical disk configuration. In the case of this optical disk, if a low-inductance electromagnet such as a magnetic head for magnetic recording is used, the magnetic field can be reversed at a high speed. And the transfer rate is not delayed.

However, since the generated magnetic field is weak, it is necessary to control the floating of the magnetic head so that the electromagnet is brought close to the recording medium, for example, up to several μm. The problem was too large.

In addition, since light is irradiated through the support substrate, there is a restriction that the support substrate must be transparent, and there is a problem that an opaque material cannot be used at all.

Further, since light is irradiated to the recording medium via the support substrate, there is a problem that the so-called optical head cannot be miniaturized.

Therefore, the present invention uses a single support substrate which may be transparent or opaque, and provides a transparent protective film on the recording medium,
It is an object of the present invention to provide a double-sided magneto-optical disk capable of irradiating light through the protective film.

[Means for solving the problem]

The problem described above includes a support substrate, a recording medium, and a protective film, wherein the support substrate is made of a transparent or opaque material, and has a film of the recording medium on both surfaces, The recording medium has a magneto-optical recording characteristic in which the Curie temperature is reached by light irradiation, and the direction of magnetization is reversed by a magnetic field applied from an electromagnet, and the protective film is a resin transparent to the light. The protective film is provided on the recording medium, and the protective film has a film thickness condition for reducing the amount of incident light reflected from the surface of the protective film on the photodetector, and a recording condition of the electromagnet. This is achieved by an optical disk having a film thickness that satisfies both of the film thickness conditions for efficiently approaching the medium.

(Operation)

Instead of irradiating the recording medium with light through the support substrate in the conventional optical disk, the optical disk according to the present invention is provided with a transparent protective film on the recording medium, The light is irradiated through the light.

That is, in the present invention, recording media are provided on both surfaces of the support substrate.

Each recording medium is covered with a protective film that is transparent to the irradiating light and mechanically protects the recording medium not only with dust but also mechanically.

In this case, since light does not pass through the supporting substrate, the supporting substrate does not need to be made of transparent glass or plastic, and various opaque materials can be applied.

Further, since the thickness of the protective film is much smaller than the thickness of the support substrate, the electromagnet can be brought very close to the recording medium.

Therefore, a floating magnetic head used in a magnetic field modulation type electromagnet, for example, a magnetic disk or the like can be applied.

Then, an optical system having a light source, a lens, a photodetector and the like and an electromagnet composed of a small magnetic head can be arranged on one side of the support substrate, so that the optical head can be made compact.

Thus, a double-sided optical disk in which recording media are provided on both surfaces of a single support substrate that is not necessarily transparent can be constructed.

〔Example〕

FIG. 1 is a structural sectional view showing an embodiment of the present invention, FIG. 2 is a schematic diagram for explaining interference due to the thickness of a protective film, and FIG. 3 is an oscillogram showing the influence of interference.

In the figure, 1 is a support substrate, 2 is a recording medium, 3 is light, 4 is an electromagnet, and 5 is a protective film.

The support substrate 1 is an Al disk having a diameter of 5 inches and a thickness of 1.2 mm. Then, a head guide groove is transferred to both surfaces by a 2P method using a stamper having a head guide groove formed on a glass plate by reactive etching.

Recording medium 2, on both sides of the pre-treated support substrate 1, for example, by sputtering, a three-layer Tb-SiO ₂ / T
It is formed by providing a bFeCo / Tb-SiO _2.

The film thickness was 90 nm TbFeCo of the recording medium 2 sandwiched from above and below.
Tb-SiO ₂ is a protective film for protecting TbFeCo, which is easily oxidized in the air, and each has a thickness of 100 nm.

The protective film 5 is formed on the recording medium 2 by spin-coating, for example, an acrylic ultraviolet curing resin.

The recording characteristics of the optical disk thus manufactured were examined by a magnetic field modulation method.

In the case of the magnetic field modulation method, the direction of the magnetic field applied to the recording medium 2 of the optical disk is reversed by the electromagnet 4 while constantly irradiating the light 3.

In order to increase the reversal speed of the magnetic field, it is necessary to reduce the inductance of the electromagnet 4 as much as possible.

Therefore, the electromagnet 4 is, for example, a hollow coil without an iron core, and the light 3 can pass through the coiled hole.

Here, as shown in FIG. 2, the influence of interference due to the thickness of the protective film 5 will be examined.

In the figure, 2 is a recording medium, 31 and 32 are optical paths, 5 is a protective film, 7
Is an objective lens, and 8 is a photodetector.

The arrangement of the objective lens 7 is such that light from a light source (not shown) is focused and reflected on the recording medium 2 and is incident on the photodetector 8 again through the objective lens 7.

That is, the optical path 31 of the light reflected on the surface of the recording medium 2 is
The light is most efficiently incident on the photodetector 8 through the objective lens 7.

On the other hand, the optical path 32 of the light reflected on the surface of the protective film 5
Is located in front of the focal point of the objective lens 7 by the thickness of the protective film 5 and extends so as to surround the photodetector 8.

Then, the spread of the optical path 32 becomes narrower as the thickness of the protective film 5 becomes thinner, approaches the optical path 31, and the amount of light incident on the photodetector 8 increases, that is, noise increases and detection becomes impossible. For example, tracking becomes impossible.

On the other hand, as the thickness of the protective film 5 increases, the optical path 32 expands. That is, as the thickness of the protective film 5 increases, the amount of light incident on the photodetector 8 decreases, and the influence of noise gradually disappears.

FIG. 3 shows the influence of the interference caused by the protective film 5 on the basis of the disturbance of the tracking error signal.

As a result, the effect differs when the thickness of the protective film is about 10 μm.

That is, if the thickness of the protective film 5 is smaller than 10 μm, the tracking error signal is disturbed as shown in FIG.

On the other hand, when the film thickness of the protective film 5 is larger than 10 μm, the tracking error signal is adjusted as shown in FIG.

On the other hand, in FIG.
Is more preferably as thin as possible in order to make the recording medium 2 approach the recording medium 2 more efficiently.

The thickness of the protective film 5 satisfying both conditions is 10 μm.
That is all.

Thus, the optical disk provided with the appropriate protective film 5 is a double-sided optical disk on which both sides can be recorded and reproduced without any distinction.

Various modifications can be made to the material, shape, forming method, and the like of the support substrate, recording medium, protective film, and the like used here.

〔The invention's effect〕

As described above, in the conventional optical modulation type optical disk, instead of attaching two backing substrates back to back so as to make a configuration that can be used on both sides, the optical disk according to the present invention is used. Has a configuration in which recording media are provided on both surfaces of a single support substrate, and a protective film is further provided on each of the recording media.

Then, for example, double-sided recording can be performed by a magnetic field modulation method using a flying magnetic head.

In addition, since light irradiation and application of a magnetic field can be performed through the protective film instead of the supporting substrate, an opaque supporting substrate can be used.

Further, the optical system, the electromagnet, and the like can be reduced in size.

Therefore, the present invention largely contributes to the improvement of the performance of the optical disc and the cost reduction.

[Brief description of the drawings]

 FIG. 1 is a cross-sectional view showing a configuration of an embodiment of the present invention, FIG. 2 is a schematic diagram for explaining interference due to the thickness of a protective film, FIG. FIG. 5 is a sectional view of the double-sided structure of FIG. In the figure, 1 is a support substrate, 2 is a recording medium, 3 is light, 4 is an electromagnet, and 5 is a protective film.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Miyozo Maeda 1015 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-63-55745 (JP, A) JP-A-57- 12428 (JP, A) JP-A-55-87332 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 11/10 521

Claims (2)

(57) [Claims] 1. A recording medium comprising a support substrate, a recording medium, and a protective film, wherein the support substrate is made of a transparent or opaque material, and the recording medium reaches a Curie temperature by light irradiation, and The protective film has a magneto-optical recording characteristic in which the direction of magnetization is reversed by a magnetic field applied from an electromagnet, and the protective film is a resin film transparent to the light, and is provided on the recording medium. The protective film satisfies both a film thickness condition for reducing the amount of light incident on a photodetector of light reflected from the surface of the protective film and a film thickness condition for efficiently bringing an electromagnet close to a recording medium. An optical disc characterized by having a thickness. 2. The optical disk according to claim 1, wherein said support substrate is made of metal, plastic, or ceramic.