JPH0721888B2 - Method for manufacturing magneto-optical recording medium - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a method of manufacturing a magneto-optical recording medium using a rare earth-transition metal amorphous ferrimagnetic alloy thin film as a recording layer,
In particular, it relates to a method for manufacturing a magneto-optical recording medium in which the number of times of recording / erasing is increased.

[Technical background of the invention and its problems]

A magneto-optical recording medium using a rare-earth-metal amorphous ferrimagnetic alloy thin film (hereinafter referred to as RE-TM film) for a recording layer is noted as the most practical rewritable optical recording medium. ing.

By the way, the as-deposited amorphous substance has a peculiar thermal instability such that it shifts to a more stable amorphous structure or crystallizes when heat-treated. In fact, the properties of the RE-TM film, such as magnetic properties, also change due to heat treatment.
a, K.Hasegawa, K.Kawanishi, T.Tsushima: J.Appl, Phys. 49
(1978) 1759, References R. Katayama, M. Hasegawa, T. Ushikubo, T. Tsushima: The Institute of Electrical Engineers of Japan 42 (1978) 559, H. Hoffmann,
R. Winkler: J. Mag. & Mag. Mater. 13 (1979), 89, literature.
M.Kajiura, Y.Togami, K.Kobayashi: Jan.J.Appl.Phys. 20
(1981), L389, S. Yoshino, H. Takaji, S. Tsunashi.
ma, M.Masuda, S.Uchiyama: Jpn.J.Appl.Phys. 23 (1984) 1
It was reported to 88 mag.

In a magneto-optical recording medium, heating the recording layer by irradiation of a light beam is the basis of the recording / erasing operation, so such thermal instability of the RE-TM film affects the reversible number of times of recording / erasing. It is related. That is, when recording and erasing are repeated at the same location on the recording medium, that location undergoes the same change as that after heat treatment, and the magnetic characteristics such as the coercive force Hc change. When the coercive force Hc changes, the bubble domains that exist stably, that is, the size and shape of the written recording unit (bit) change, and the writing characteristics change due to the application of the laser beam and the external magnetic field. Therefore, when recording / reproducing is repeated a number of times under the same conditions, the S / N of the reproduced signal (hereinafter referred to as C / N in terms of carrier noise ratio) decreases.
In fact, the inventors recorded and recorded on an as-deposited film recording medium.
When erasing was repeated and the reproduced signal was read sequentially, the C / N of the reproduced signal decreased as the number of recording / erasing repetitions increased. This phenomenon is due to a change in magnetic characteristics of the recording layer itself due to heat.

As described above, the RE-TM film is promising as a recording layer of a magneto-optical recording medium, but there is a problem that the magnetic characteristics change due to heat when the number of times of recording and erasing is repeated many times. Was wanted.

[Object of the Invention]

The present invention has been made in order to overcome the above-mentioned problems, and a magneto-optical recording medium having a RE-TM film as a recording layer, whose magnetic characteristics are stably maintained even when recording and erasing are repeated many times. It is an object to provide a method for manufacturing a recording medium.

[Outline of Invention]

The method for manufacturing a magneto-optical recording medium according to the present invention is characterized in that after the recording layer made of the RE-TM film is formed, it is heat-treated at a temperature lower than the crystallization temperature to shift the magnetic characteristics to a stable state. To do.

That is, the inventors examined in detail the changes in the magnetic properties of the RE-TM film due to the heat treatment and the repetition of the heat treatment, and when the heat treatment was performed at a temperature at which the recording layer is considered to be reached by heating by irradiation with a light beam during recording / erasing, Changes in magnetic properties such as a decrease in magnetic force, and if heat treatment is performed once, the coercive force and the magnitude of magnetization will be reduced with repeated heat treatments.
We have found that there is no change in magnetic characteristics such as car rotation angle.

Based on such knowledge, the present invention does not form a recording layer in a state where a RE-TM film is formed as in the conventional case, but heat-treats once after forming the film to obtain the characteristics as described above. This dramatically improves the thermal stability of magnetic properties.

In the present invention, the temperature of this heat treatment is set to be lower than the crystallization temperature of the RE-TM film because if it is set to the crystallization temperature or higher, the basic characteristics of the magneto-optical recording medium are impaired. Further, the optimum value of the heat treatment temperature varies depending on the material of the recording layer, in particular, the temperature to which the RE-TM film forming the recording layer is heated at the time of recording. For example, when the RE-TM film is a so-called Curie point recording type material that heats the recording layer to the vicinity of the Curie point, the heat treatment temperature is set to the Curie point or higher. Further, although the present inventors have examined the material of the compensation temperature recording method in which the RE-TM film heats the recording layer to the compensation temperature, as a reference, which is not related to the present invention, the heat treatment temperature The coercive force of the recording layer is 500
It was confirmed that the temperature should be higher than or lower than [Oe].

〔The invention's effect〕

According to the present invention, since the magnetic characteristics of the recording layer of the magneto-optical recording medium can be brought into a thermally stable state, the decrease in C / N of the reproduced signal due to repeated recording / erasing is suppressed, It is possible to obtain the effect that the number of erasable repetitions increases.

Example of Invention

FIG. 1 shows a manufacturing process of a magneto-optical recording medium according to an embodiment of the present invention. First, as shown in FIG. 1 (a), the substrate 1,
For example, on a glass substrate or a Si substrate with a thermal oxide film, a RE-TM film (rare earth-
A transition metal amorphous ferrimagnetic alloy thin film) is formed by sputtering. Next, as shown in FIG. 1B, a Si ₃ N ₄ film having a thickness of 1000 Å, for example, is similarly formed on the recording layer 2 as a protective layer 3 for preventing oxidation by sputtering. Then, as shown in FIG. 1C, the whole is heated by using, for example, the non-inductive heater 4, and the recording layer 2 is heat-treated.

The magneto-optical recording medium manufactured in this manner is subjected to heat treatment at various temperatures, times and repetition times, and the change in the characteristics before and after the heat treatment is measured by VSM with respect to the magnetic characteristics.
The Kerr effect was used to examine the magneto-optical characteristics. The heat treatment was performed by setting the sample on the non-induction heater as described above and in a high vacuum of 10 ^-6 Torr or less.

In the following, the results of experiments conducted on a sample having a TbCo film formed as a RE-TM film will be described in detail. Figure 2 shows the coercive force Hc, Kerr rotation angle θ _K, and M for the as-deposited film that was not heat-treated when heat-treated at various temperatures for 1 hour.
This is an example of the change of 10 _⊥ / M10 _″ . Here,
M10 _⊥ is 10 [kOe] from the outside in the direction perpendicular to the film surface.
Magnetization and M10 in the case of applying a magnetic field for _"in-plane direction of the film, a magnetization when a magnetic field is applied from the outside _{10 [kOe], M10 ⊥ /} M10" perpendicular magnetic anisotropy of the film It is a standard amount of. As shown in this figure, when heat treatment is performed, θ
_{Although the K} and the magnetization in the perpendicular direction do not change, Hc decreases and the magnetization in the in-plane direction increases.

Fig. 3 shows Hc, θ _K for the as-deposited film without heat treatment after repeated heat treatment at 300 ° C for 1 hour.
It shows an example of the change of. As can be seen from this figure, it is clear that once the heat treatment is performed, Hc does not change even if the heat treatment is further repeated.

FIG. 4 shows an example of changes in Hc, θ _K with respect to the as-deposited film without heat treatment when the heat treatment time is changed. As can be seen from this figure, the amount of change in Hc, θ _K does not depend on the heat treatment time, and it is clear that after the heat treatment for 15 minutes, it is thermally stable.

In order to investigate what causes the characteristic change due to these heat treatments, the inventors investigated the composition distribution in the film thickness direction before and after the heat treatment by Auger electron spectroscopy. FIG. 6 shows an example of the results, in which the horizontal axis represents the sputtering time and the vertical axis represents the Auger peak intensity. In Fig. 5 (a)
Is the composition distribution in the film thickness direction of the film that is not heat treated,
The composition distribution in the film thickness direction of the film after the heat treatment for 1 hour is shown. The sample has the structure shown in FIG. 1, and the substrate 1 has a thermal oxide film.
A Si substrate was used. As is clear from FIG. 5, there is no significant difference in the composition of the film itself before and after the heat treatment. Therefore,
It is clear that the characteristic change due to the heat treatment is not due to the reaction between the RE-TM film as the recording layer and the substrate or the protective layer, or the oxidation of the RE-TM film.

In addition to the above, the inventors have attempted to measure the temperature change of the electric resistance in order to further investigate the thermal instability of the as-deposited RE-TM film. Measuring the change in electrical resistance with temperature is described, for example, in the literature M. Kajiura, Y. Togami, K. Kobayash.
As reported in i, T.Teranishi: Jpn, J.Appl.Phys.20 (1981) L389, it is an effective means for investigating changes such as structural relaxation of amorphous films.

In this case, the sample having the structure shown in FIG. 1 was used in the same manner as described above, and the measurement was carried out by an AC four-terminal method at 1 KHz while the sample was heated at a rate of 10 [° C./min] in an Ar stream. . FIG. 6 shows an example of the measurement result. In the figure, the line A indicates 2 from the as-deposited state of the RE-TM film (as-depo.).
It is the temperature change of the electric resistance when the temperature is raised up to 40 ° C.
Is the temperature change of the electric resistance when the cooling / heating from 240 ° C. to room temperature is repeated thereafter. As shown in this figure, there is a peculiar behavior that peak P is generated at about 190 ° C only from the as-deposited state when the film is heated, and after the first cooling from 240 ° C, the heating / cooling cycle is performed. Even if repeated, a peak like P does not occur and follows the same linear path. Therefore, it was clarified that the RE-TM film undergoes a micro-irreversible change that changes the electric resistance only when heated from the as-deposited state.

In order to investigate what kind of irreversible change is caused by these heat treatments, the inventors further investigated the amorphous structure before and after the heat treatment by X-ray diffraction and electron beam diffraction. Figure 7 shows the first
In the sample having the structure shown in the figure, it is an example of the result of X-ray diffraction when glass is used for the substrate 1, where (a) is the result in the as-deposited state before heat treatment, and (b) is The results are obtained after heat treatment at 300 ° C for 1 hour. As shown in the figure, there is no significant difference before and after the heat treatment.

In addition to this, in the sample having the structure shown in FIG.
After the sample was prepared using a Cl substrate, the substrate was removed, and a diffraction pattern by a 200 kV electron beam and a transmission electron microscope image were photographed in a transmission electron microscope, but again there was no significant difference before and after the heat treatment. From the above, it was found that the irreversible change caused by the heat treatment was not a remarkable structural change to the extent that the diffraction phenomenon of X-rays and electron beams changed significantly.

As described above, the irreversible change of the RE-TM film due to the heat treatment is
We have specifically described the Co film and described it in detail.
-The same phenomenon is observed in the TM film. For example, when the TbFe film is annealed at various temperatures for 1 hour as in Fig. 2, the coercive force H for the as-deposited film without the heat treatment is shown.
c, force one rotation angle θ _K and ratio of magnetization in the vertical direction and in-plane direction
The change in M10 _⊥ / M10 _"Hc decreases by the same heat treatment as .TbFe film also TbCo film shown in FIG. 8, it is evident that the magnetization in the plane direction is increased. This other various RE- The TM film will be described later.

Such an irreversible change due to heat of the RE-TM film usually occurs even at a temperature (150 to 350 ° C.) at which the recording medium rises when recording / erasing with a light beam. Therefore, when recording and erasing are repeated, this heat causes an irreversible change, which changes the recording and erasing characteristics. Size of written bit
There is a big problem that the shape changes. However, as shown in FIGS. 3 and 4, the inventors finally used a RE-TM film, which was heat-treated at a predetermined temperature for about 15 minutes, as a magneto-optical recording medium. For example, it was discovered that the magnetic characteristics are thermally very stable without being changed to such a degree that there is no problem as a recording medium with respect to the heat given at the time of recording or erasing.

As shown in FIG. 2, the coercive force Hc is lower when the heat treatment is performed than the RE-TM film that is not heat treated, but by adjusting the coercive force of the film before the heat treatment depending on the film forming conditions, The coercive force after that can be made a sufficient value for the magneto-optical recording medium. Also, as described above, M10 is formed by heat treatment.
_⊥ / M10 _″ decreases, that is, the perpendicular magnetic anisotropy decreases, but the Kerr rotation angle θ _K does not change as shown in FIG. 2, and if M10 _⊥ / M10 _″ is 1 or more. For example, the function as the magneto-optical recording medium is not impaired at all, and the heat treatment according to the present invention does not cause damage.

By the way, as shown in FIG. 7, the heat treatment at 300 ° C. or lower does not cause a great change in the X-ray and electron beam diffraction phenomena. However, the RE-TM film has a compositional difference of about 35
Crystallized by heat treatment at 0-500 ° C. When crystallized, the characteristics of the RE-TM film are lost and it becomes unsuitable as a magneto-optical recording medium, so the heat treatment temperature must be below the crystallization temperature. Further, although the upper limit of the heat treatment temperature for the RE-TM film in the present invention is thus lower than the crystallization temperature,
The lower limit depends on the RE-TM film material.

The present inventors conducted the following experiment in order to confirm the lower limit temperature of this heat treatment. That is, each RE-TM film material was heat-treated at two different heat-treatment temperatures for 30 minutes, and then heat-treated at the same temperature during recording (hereinafter referred to as recording temperature) for 1 hour. The coercive force change was measured. Table 1 summarizes the results.
It is Table 2. Table 1 shows the case where the RE-TM film as the recording layer is the material of the Curie point recording system, and Table 2 shows the case of the material of the compensation temperature recording system. Therefore, for the recording temperature, the Curie point is set for the Curie point recording material, and the film coercive force is 500 [Oe] for the compensation temperature recording material.
The temperature was set below.

The heat treatment temperature of Sample A was set lower than the recording temperature,
The heat treatment temperature of the sample B was set higher than the recording temperature. table 1,
In 2, Ta represents normal temperature (20 ° C to 30 ° C). Table 1
At,> Tc indicates that the compensation temperature is higher (ie, does not appear) above the Curie point. Furthermore, in Table 2,
The Curie point of the compensation temperature recording type RE-TM film material is higher than the crystallization temperature, so it is not shown.

As is clear from Table 1, in the case of the magneto-optical recording medium which was heat-treated at a temperature equal to or higher than the recording temperature after the film formation like the sample B, the coercive force does not change by the heat treatment at the recording temperature thereafter. As described above, in a magneto-optical recording medium which has been heat-treated at a lower temperature, the coercive force is lowered by the subsequent heat treatment at the recording temperature.

Therefore, in order to make the magnetic characteristics in a thermally stable state after the film formation, it is desirable to perform the heat treatment at the following temperature or higher. That is, a Curie point recording material is preferably a Curie point or higher. As shown in Table 2, it is desirable to have a compensation temperature of +100 [° C] or higher for the material of the compensation temperature recording method, but when considering a more practical magneto-optical disk system, the coercive force drops to 500 [Oe] or less. It is desirable to be above the temperature.

Based on the above examination results, the following are the experimental results for examining the characteristic changes due to repeated recording / erasing as a magneto-optical disk. FIG. 9 is a sectional view showing the structure of the magneto-optical disk used in the experiment. The substrate 11 is, for example, a glass substrate with a pre-groove having a diameter of 200 mm and a thickness of 1.5 mm, on which a first protective layer 12 is formed by sputtering to have a thickness of 1000 Å
Si ₃ N ₄ film is formed. On the first protective layer 12, a TbCo film having a thickness of 1000Å formed by sputtering is formed as a RE-TM film constituting the recording layer 13, and the first protective layer 12 is formed on the recording layer 13. A second protective layer 14 similar to the above is formed.

The magneto-optical disk having the structure shown in FIG.
I prepared a kind. The first magneto-optical disk is in a state in which the protective layers 12. and 14 and the recording layer 13 are formed on the substrate 11 by sputtering, and the second magneto-optical disk is formed on the substrate 11 at 200 ° C. after sputtering. It is based on the present invention which has been heat-treated for 13 minutes. The heat treatment was performed in an electric furnace that does not generate a magnetic field, and of course, consideration was given to the effect of oxidation.

Each of the first and second magneto-optical disks thus manufactured has an initial state of a rotation speed of 600 [rpm], an externally applied magnetic field of 300 [Oe], and a laser beam power of 3 on the surface.
Good recording / erasing characteristics were obtained at mW. Then, two types of magneto-optical disc systems are prepared, the above-mentioned two types of magneto-optical discs are loaded, and two types of recording / erasing cycles per rotation for a specific one track are simultaneously started and repeated under the above conditions. It was When the recorded signal was reproduced using a laser beam with a power of 0.7 mW every 1000 recording / erasing cycles, the C / N gradually increased in the first magneto-optical disk in the state where the sputtering film was formed. While the decrease was observed, the C / N was not decreased in the second magneto-optical disk after the heat treatment.

As described above, according to the present invention, the magneto-optical disk subjected to the heat treatment after the formation of the recording layer suppresses the decrease in the C / N of the reproduced signal due to the repeated recording and erasing as compared with the conventional one, and -It was confirmed that the number of times erasing can be repeated increases.

[Other Embodiments of the Invention]

In the above description, the case of an optical disk using a glass substrate is described, but from the viewpoint of cost and ease of forming a pregroup for tracking, it is more practical to use an organic material such as acrylic, epoxy, or polycarbonate. It is desirable to use a resin substrate. However, the organic resin substrate is deformed when heated to 200 ° C in an electric furnace. In such a case, as described below, a method of heat-treating only the recording layer made of the RE-TM film without raising the temperature of the substrate using a light beam is effective.

FIG. 10 is a block diagram of an apparatus for heat-treating a recording layer made of a RE-TM film by using a light beam. 21 is a magneto-optical disk as shown in FIG. 9, 22 is a rotation axis, 23 is a He-Ne laser, Ar
A laser light source composed of a laser or a semiconductor laser, 24 a reflecting mirror, 25 an objective lens, and 27 a magnetic field generator. 28
Is a container for sealing the space filled with the magneto-optical disk 21, 29
Is an exhaust port or a gas inlet. By adjusting the objective lens 25, a laser beam 26 having a beam diameter larger than that at the time of recording / erasing / reproducing, for example, about 3 μmφ on the surface of the disk 21
Are irradiated onto the magneto-optical disk 21.

With such a configuration, the magneto-optical disk 21 is rotated about the rotation axis 22 and the laser beam 26 is continuously irradiated to heat-treat the entire disk surface of the disk 21 for each track.
The rotation speed of the disk 21 is about 200 [rpm], and the power of the laser beam 26 for irradiation is about 12 mW on the board surface. Further, in order to ensure the heat treatment, it is desirable to irradiate the laser beam 26 continuously for several tens of revolutions per track. For example, in the case of a disk or the like having a single recording layer made of a RE-TM film and having no protective layer, oxidation during heat treatment poses a problem, so the container 28 is closed and evacuated from the exhaust port 29 to create a vacuum. Alternatively, the RE-TM film can be prevented from being oxidized by performing heat treatment in a state where an inert gas such as nitrogen gas or argon is flown through the holes 29.

As described above, when the recording layer made of the RE-TM film is heat-treated by the irradiation of the laser beam, even if the substrate is made of a material that is weak against heat, such as resin, the substrate is not heated and damaged, and the magnetic characteristics are thermally improved. A very stable magneto-optical recording medium can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a main part of a manufacturing process of a magneto-optical disk according to an embodiment of the present invention, and FIG. 2 is an experiment on a TbCo film for a change in magneto-optical characteristics with respect to a heat treatment temperature of a RE-TM film in a recording layer. FIG. 3 is a diagram showing an example of the result of the experiment, FIG. 3 is a diagram showing an example of the result of the experiment of the change in the magneto-optical characteristic of the RE-TM film depending on the number of times of heat treatment, and FIG. 4 is a diagram showing the magneto-optical characteristic of the RE-TM film. FIG. 5 (a) and FIG. 5 (b) are diagrams showing an example of the results of an experiment on the heat treatment time dependence, and FIG. 5 (a) and FIG. FIG. 6 is a diagram showing the temperature change of the electric resistance of the RE-TM film, and FIGS. 7 (a) and 7 (b) are X-ray diffraction patterns of the magneto-optical recording medium before and after the heat treatment. Figures and 8 show the changes in magneto-optical properties of the RE-TM film depending on the heat treatment temperature for the TbFe film. FIG. 9 shows an example of the results obtained, FIG. 9 is a sectional view of a magneto-optical disk for explaining another embodiment of the present invention, and FIG. 10 shows the constitution of an apparatus for heat-treating a RE-TM film with a light beam. It is a figure. 1,11 ...... Substrate, 2,13 ...... Recording layer made of rare earth-transition metal amorphous ferrimagnetic alloy film, 3,12,14 ...... Protective layer, 4 ...
… Inductive heater, 21 …… Magneto-optical disk, 23 …… Laser light source, 25 …… Objective lens, 26 …… Laser beam, 27 ……
Magnetic field generator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuro Yasuda No. 1 Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa Inside the Toshiba Research Institute, Inc. (56) Reference JP-A-59-34618 (JP, A) JP-A-61-113151 (JP, A)

Claims (1)

[Claims] 1. A method for manufacturing a magneto-optical recording medium, comprising a rare earth-transition metal amorphous ferrimagnetic alloy thin film having an easy magnetic domain in a direction perpendicular to a film surface as a recording layer, wherein the rare earth-transition metal amorphous. The ferrimagnetic alloy thin film is a Curie point recording type recording layer material, and after the recording layer is formed, the recording layer is heat-treated at a temperature below the crystallization temperature and above the Curie point without applying a magnetic field. A method for manufacturing a magneto-optical recording medium, characterized in that the magnetic property is shifted to a thermally stable state.