JPH06162587A - Optical recording and reproducing method - Google Patents

Abstract

PURPOSE:To enable magneto-optical recording with excellent reproduction characteristic at <=600nm wavelength by using Ag or an alloy or compd. consisting essentially of Ag as the reflection layer of a magneto-optical recording medium successively having a dielectric substance layer, magneto-optical recording layer and reflection layer on a substrate. CONSTITUTION:Tantalum oxide is formed as the dielectric substance layer by a reactive sputtering method on a glass substrate having, for example, 1.2mm thickness. The surface thereof is etched to the prescribed extent by high-frequency plasma, by which the surface is smoothed. The thickness of the tantalum oxide layer after the etching is initially set at about 35nm. An amorphous alloy consisting of a compsn. of Tb(20)Fe(64)Co(16)(atomic %) is formed on this tantalum oxide layer at about 20nm thickness by a DC magnetron sputtering method to form the magneto-optical recording layer. The Ag is formed thereon at about 50nm thickness by the DC magnetron sputtering method to form the reflection layer. As a result, the magneto-optical recording with the excellent reproduction characteristic is executed at <=600nm wavelength.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an optical recording / reproducing method,
The present invention relates to a method for recording / reproducing information using a magneto-optical recording medium.

[0002]

2. Description of the Related Art In recent years, along with a dramatic increase in the amount of information, there is an increasing demand for improving the recording density of information recording media. The optical disc has a high recording density and is excellent in random accessibility and portability. In particular, a magneto-optical disk (magneto-optical recording medium) can be repeatedly recorded and is excellent in reliability, so that it has already been commercialized as a recording medium for external storage devices for computers and recording devices.

The storage capacity of the magneto-optical disk currently commercialized is 300 per one side of a disk having a diameter of 5.25 inches.
It is about megabytes. In order to increase the storage capacity up to about 1 gigabyte, it has been proposed to shorten the track pitch and change the recording signal modulation method.
The prospect of practical application is emerging. However, it must be said that it is impossible to further improve the recording density (storage capacity) within the range of the current technology because the recording density is already close to the theoretical limit.

It is the size of the focused spot of the laser light used for recording / reproducing that determines the theoretical limit of the recording density in the optical disk. Therefore, in order to further increase the recording density, it is indispensable to narrow down the spot of the laser light. The spot diameter d of the laser light is expressed by the following equation by the wavelength λ of the laser light used and the numerical aperture NA of the objective lens. However, k is a constant determined by the aperture shape of the lens and the intensity distribution of the incident light beam.

D = k · λ / NA In order to reduce the spot diameter d of the laser light, the wavelength λ
It is necessary to use a light source having a short aperture and an objective lens having a large numerical aperture NA. When the numerical aperture of the lens is increased, the depth of focus becomes shallow, and the tolerance for the tilt of the disk and the unevenness of the thickness of the substrate sharply decreases, so that the servo performance of the optical head deteriorates. Therefore, the numerical aperture of the lens cannot be made much larger than the current 0.55. Therefore, in order to reduce the spot diameter of the laser beam and improve the recording density, it is essential to use a light source having a wavelength shorter than the current 825 nm and 780 nm as the light source of the optical head.

It is the reflectance and the magnetic Kerr rotation angle that dominate the quality of the reproduction signal of the magneto-optical disk, and more specifically, it is represented by the product of the square root of the reflectance and the magnetic Kerr rotation angle. This is called a performance coefficient. TbFeC is used as the recording layer of currently commercialized magneto-optical disks.
A heavy rare earth-transition metal amorphous alloy represented by o is used. The performance coefficient of these alloys shows a relatively large value at the wavelength of the laser light of the existing magneto-optical disk drive of about 800 nm, but the wavelength of the light becomes 600 nm due to the shorter wavelength.
When it becomes below, it will decrease rapidly. Further, the detection sensitivity of the photodiode of the photodetection element used for signal detection of the drive is high near 800 nm, but sharply drops below 600 nm. These facts mean that with the current technology, the intensity of the reproduction signal is extremely reduced on the short wavelength side, and stable reproduction of the recording signal becomes impossible.

As described above, in order to realize a magneto-optical disk system using a short wavelength light source aiming at an improvement in recording density, a reduction in reproduction signal strength on the short wavelength side becomes a problem. .

[0008]

As described above, in order to obtain a large reproduced signal, it is important to increase the performance coefficient of the recording medium and improve the sensitivity of the photodetector. The light is detected by the photodetector and the photodiode used in the magneto-optical disk drive.
The electrons near the junction are excited to the conduction band, and the electrons in the conduction band are pn
This is done by moving the junction and passing a current through the photodiode. Since the light absorption coefficient of the Si semiconductor that constitutes the photodiode becomes large on the short wavelength side, light having a short wavelength is absorbed near the surface of the photodiode, and the pn
It is difficult to reach the vicinity of the joint. As a result, the number of electrons excited near the pn junction decreases, and the light detection sensitivity decreases. Therefore, it is theoretically difficult to improve the detection sensitivity of the photodetector on the short wavelength side, and it is important to increase the performance coefficient of the recording medium in order to obtain a large reproduced signal on the short wavelength side.

A magneto-optical disk is generally constructed by providing a plurality of layers such as a magneto-optical recording layer, a light interference layer, a light reflecting layer, and a protective layer on a transparent substrate. The figure of merit that governs the reproduced signal strength must be considered in all of these layers. The present invention is a material constituting all layers,
Optimizing the layer thickness and layer structure, the magneto-optical disk 600
It is an object of the present invention to provide a magneto-optical disk system using a short-wavelength light of 600 nm or less by enhancing the performance coefficient at a wavelength of nm or less.

[0010]

DISCLOSURE OF THE INVENTION As a result of intensive investigations by the present inventors to provide a high recording density magneto-optical disk system, as a result, an optical disk having at least a dielectric layer, a magneto-optical recording layer and a reflective layer on a substrate in order. Ag as a reflective layer of a magnetic recording medium,
Alternatively, it has been clarified that the use of an alloy or compound containing Ag as a main component enables magneto-optical recording exhibiting excellent reproduction characteristics at a wavelength of 600 nm or less.

The present invention uses a laser beam having a wavelength of 600 nm or less for recording / reproducing on a magneto-optical recording medium having at least a dielectric layer, a magneto-optical recording layer, and a reflective layer containing Ag as a main component on a substrate. Is an optical recording / reproducing method. The present invention will be described in detail below. As the substrate of the magneto-optical recording medium used in the present invention, glass,
Examples thereof include plastics such as polycarbonate, and substrates in which a grooved resin for guiding an optical head is formed on glass. It is preferable to optimize the depth and spacing of the grooves according to the wavelength to be used, that is, the shorter the wavelength, the shallower the grooves and the finer the spacing. It is desirable that the birefringence of such a substrate be as small as possible so as not to impair the quality of the magneto-optical signal. The substrate generally has a thickness of about 1 to 2 mm.

The dielectric layer is a layer that interferes with light, is provided between the substrate and the magneto-optical recording layer, multiple-reflects light between the substrate and the magneto-optical recording layer, and causes an apparent magnetic Kerr rotation angle. Play a role of increasing the interference effect. A sufficient interference effect cannot be obtained unless its thickness is changed according to the wavelength of light used. For example, when the wavelength is about 450 nm,
When the thickness of the dielectric layer is 35 nm, the interference effect is remarkable,
When the wavelength is further shortened, the thinner the dielectric, the greater the interference effect. In addition, the dielectric layer plays a role of enhancing the adhesion between the substrate and the magneto-optical recording layer, a role of insulating the magneto-optical recording layer and the substrate, a role of protecting the magneto-optical recording layer from moisture that penetrates through the plastic substrate, and the like. Hold together.

The dielectric layer can be further provided between the magneto-optical recording layer and the reflective layer. This makes it possible to further enhance the light interference effect and increase the magnetic Kerr rotation angle. A sufficient interference effect cannot be obtained unless the thickness of one or two dielectric layers is changed according to the wavelength of light used. The thickness of the dielectric layer is 40 nm to 55 nm when the wavelength of light used is about 600 nm.
Degree (when using the primary interference point) or 180 nm
˜195 nm (when using a secondary interference point) is preferable, and 25 when the wavelength of light used is about 400 nm
It is preferably about nm to 35 nm (when using the primary interference point) or about 120 nm to 130 nm (when using the secondary interference point).

The dielectric layer provided between the magneto-optical recording layer and the reflective film plays a role of interfering light, and also has a role of insulating the magneto-optical recording layer and the reflective layer. As a material used for the dielectric layer, an amorphous thin film made of silicon nitride, tantalum oxide, silicon oxide, aluminum oxide, titanium oxide, zinc sulfide, etc., or a mixture thereof is generally used.

The magneto-optical recording layer is a layer for condensing and heating laser light to perform thermo-magnetic recording. Since the magnetization must be perpendicular to the film surface for recording, it is desirable that the magneto-optical recording layer has a large perpendicular magnetic anisotropy. When the wavelength of light is shortened and the diameter of the focused spot is reduced, the energy density of light per unit area is increased. Therefore, the temperature of the magneto-optical recording layer when the reproducing light is irradiated is about 800 nm in the conventional case. More likely to be expensive. When the temperature of the magneto-optical recording layer increases, the magnetic Kerr rotation angle decreases and the output of the reproduction signal decreases, but the rate of decrease is more remarkable as the Curie temperature of the magneto-optical recording layer is lower.

Therefore, as the material of the magneto-optical recording layer for recording / reproducing by using the light having the wavelength of 600 nm or less, the Curie temperature higher than the Curie temperature 170 to 200 ° C. of the conventional magneto-optical recording layer for the wavelength of about 800 nm is used. It is preferable to use one that has a temperature. Further, in order to obtain a large reproduced signal, it is preferable that the Kerr rotation angle is large at the wavelength of the light used, and the Curie temperature of 220 ° C. to 280 ° C. is particularly preferable.

The magneto-optical recording layer satisfying the above conditions has a Tb concentration of 15 to 25 atomic% and a Co concentration of 10 to 50.
Atomic% TbFeCo amorphous alloy, Co concentration 20
~ 50 atomic% NdTbFeCo amorphous alloy, C
An example is an o / Pt artificial lattice. It is preferable to determine the film thickness of the magneto-optical recording layer according to the wavelength to be used in consideration of the recording sensitivity to the power of laser light, the performance coefficient, etc. The film thickness can be made thinner as the wavelength of light is shorter. Is. The thickness of the magneto-optical recording layer is usually 10 nm to 40
It is about nm.

The reflective layer plays a role of reflecting the light transmitted through the magneto-optical recording layer and returning it to the magneto-optical recording layer again. As a result, the efficiency of light utilization is increased, the reflectance and the magnetic Kerr rotation angle are increased, and the coefficient of performance is increased. The reflective film also affects the thermal conductivity of the magneto-optical recording medium. While aluminum or an aluminum alloy is used as the reflective layer of a magneto-optical disk that is currently commercialized and has a wavelength of about 800 nm, in the present invention, a reflective layer mainly containing Ag is used. It is characteristic.

When conventional aluminum, aluminum alloy, or gold is used for the reflective layer, the optical constant is close to the optical constant of the recording layer at a wavelength of 600 nm or less, and a sufficient coefficient of performance cannot be obtained. It is considered that the reflecting layer maintains a difference in optical constant from the magnetic layer even at a wavelength of 600 nm or less, a high enhancement effect can be obtained, and a good performance coefficient can be obtained.

Since a simple substance of Ag is easily corroded by a sulfide gas, an element having a corrosion preventing effect, for example, Au, P
Alternatively, t, Pd, or the like may be used alone, or a plurality of types may be added to Ag to form a reflective layer. The amount of addition of other elements is less than 50 atomic%, preferably 80 atomic% or less, usually 6 in the reflective layer.
Up to about atomic%. Further, it is more preferable to provide a protective layer made of a photocurable resin or the like on the outer side of the reflective layer to protect the medium from the external environment and mechanical damage.

[0021]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples unless it exceeds the gist. Example 1 A 1.2 mm thick glass substrate was used as the substrate. A tantalum oxide thin film was formed as a dielectric layer on the substrate by the reactive sputtering method.

After that, the surface was etched by high frequency plasma for 5 minutes to smooth the surface. The initial film thickness was set so that the thickness of the tantalum oxide layer after etching was about 35 nm. Next, on this tantalum oxide layer, Tb (20)
An amorphous alloy having a composition of Fe (64) Co (16) (atomic%) was formed in a thickness of about 20 nm by a DC magnetron sputtering method to form a magneto-optical recording layer. Ag was formed thereon to a thickness of about 50 nm by a DC magnetron sputtering method to form a reflective layer.

A tantalum oxide layer having a thickness of 30 nm and an ultraviolet curable resin layer having a thickness of 3 μm are provided as protective layers on the reflective film by a reactive sputtering method and a spin coating method, respectively. And The recording characteristic and the reproducing characteristic were confirmed by measuring the magnetic Kerr hysteresis curve and the reflectance. That is, it is understood that if the squareness ratio of the hysteresis curve is 1 and the coercive force is large, good recording characteristics are exhibited, and if the product of the square root of reflectance and the magnetic Kerr rotation angle is large at the wavelength used, good reproduction is achieved. It can be seen that it shows characteristics.

The reflectance was measured through the substrate at the flat mirror surface portion of the disk where the groove was not provided.
First, the relative reflectance of the optical disk with respect to the aluminum film was measured with a spectrophotometer at an incident angle of 2 degrees, and the absolute reflectance of the aluminum film measured by another device in advance was used for calibration. In an actual drive, the light reflected on the substrate surface does not contribute to the signal. Therefore, the light reflected on the substrate surface was calculated from the optical constants of the glass substrate, and the reflectance was subtracted from the measured value.

The measurement of the magnetic Kerr hysteresis curve was carried out by a polarization plane modulation method through the substrate at the flat mirror surface portion where the groove of the disk was not provided, as in the case of the reflectance. The magnetic Kerr rotation angle was determined by comparing the hysteresis curve with the output change when the Glan-Thompson prism analyzer was rotated by a unit angle. However, in this case as well, since the light reflected by the surface of the substrate and having no rotation of the plane of polarization is included, a rotation angle smaller than the actual rotation angle can be obtained.

Therefore, the actual magnetic Kerr rotation angle was determined by correcting the obtained rotation angle in consideration of the reflected light on the substrate surface. The wavelength of measurement was selected to be 450 nm as an example. From the shape of the hysteresis curve, it was found that the squareness ratio was 1 and the coercive force was about 800 kA / m. Since this is at a level equivalent to that of a magneto-optical disk for wavelengths of about 800 nm which is currently commercialized, it can be said that it has good recording characteristics. The reflectance at 450 nm is 10.3%.
The magnetic Kerr rotation angle was 1.33 degrees, and a value of 4.3 was obtained as the coefficient of performance given by the product of the square root of reflectance and the magnetic Kerr rotation angle. It can be said that it has excellent reproduction characteristics.

Incidentally, the performance of the currently commercialized magneto-optical disk at 800 nm has a reflectance of 23.1%,
The magnetic Kerr rotation angle is 1.00 and the performance coefficient is about 4.8. Comparative Example 1 A disk was produced in the same manner as in Example 1 except that aluminum was provided as the reflective layer to a thickness of about 50 nm. The recording characteristics and the reproducing characteristics were evaluated in the same manner as in Example 1.

The squareness ratio is 1 and the coercive force is about 800 kA / m, which shows good recording characteristics. However, 450 nm
The reflectance was 16.1%, the magnetic Kerr rotation angle was 0.84 degrees, and the performance coefficient was only 3.4, which was a low value, and did not show good reproduction characteristics. Comparative Example 2 A disk was produced in the same manner as in Example 1 except that gold was provided as the reflective layer to a thickness of about 50 nm. The recording characteristics and the reproducing characteristics were evaluated in the same manner as in Example 1. Squareness ratio is 1
Then, the coercive force is about 800 kA / m, which shows good recording characteristics. However, the reflectance at 450 nm is 11.3
%, The magnetic Kerr rotation angle was 1.03 degrees, and a low coefficient of performance of 3.5 was obtained, which did not show good reproduction characteristics.

[0029]

As described in detail above, according to the present invention, there is provided a magneto-optical recording system having a high recording density, which uses light having a wavelength of 600 nm or shorter, which is shorter than the existing one.

Claims (1)

[Claims] 1. Recording / reproducing is performed on a magneto-optical recording medium having at least a dielectric layer, a magneto-optical recording layer, and a reflective layer containing Ag as a main component on a substrate in this order using laser light having a wavelength of 600 nm or less. An optical recording / reproducing method characterized by the above.