JP2562945B2 - Magneto-optical recording medium - Google Patents

Description

TECHNICAL FIELD The present invention relates to improving the storage stability of a magneto-optical recording medium by improving a polycarbonate substrate mainly used as a substrate of a magneto-optical recording medium.

[Prior art and its problems]

A magneto-optical recording medium having many characteristics such as high density and large capacity, no contact with a recording / reproducing head, and easy erasing and re-recording has recently been used as a recording medium for high-speed data files and video recording. Its development and practical use are being actively pursued. The general form is that a thin film recording layer mainly containing a rare earth metal and a transition metal is provided on a substrate.

As the substrate of the magneto-optical recording medium, glass or resin such as polycarbonate, polymethylmethacrylate or epoxy is used. The glass substrate has excellent optical characteristics, surface properties, and chemical stability, but has problems such as low mass-processability, fragility, difficult handling, and high cost. On the other hand, the resin substrate has advantages such as high productivity in mass production, easy handling, and relatively low price, and therefore development is currently being made mainly on the resin substrate. Among them, the polycarbonate substrates disclosed in, for example, JP-A-62-252546, JP-A-62-264463, and JP-A-63-97627 have excellent light transmittance and birefringence on the surface. It is expected to be used as a substrate for magneto-optical recording media because of its low moisture content, low hygroscopicity and low cost.

On the other hand, the information recorded by the magneto-optical recording medium is required to be stable for a long period of time, and it is usually desired that the information be erased, recorded, and reproduced so that it can be erased, recorded, and reproduced even after 10 years at 30 ° C, 90% RH. There is. In order to guarantee such reliability, an acceleration test is performed by placing the magneto-optical recording medium under high temperature and high humidity. For example, Okada et al., Optical Memory Symposium Proceedings, 60
As described on page (1986), it is said that an accelerated test at 80 ° C, 90% RH for 500 hours is equivalent to about 10 years at 30 ° C, 90% RH. When the magneto-optical recording medium is placed under such a condition and an acceleration test is performed, it is often observed that a concave defect having a diameter of several tens of μm to several hundreds of μm and a depth of several μm is formed on the polycarbonate substrate. It was found that this causes a problem such as an increase in drop out of the magneto-optical recording medium and an increase in BER (bit error rate). Therefore, it has been found that there is a problem in storage stability even when a polycarbonate substrate having the above-mentioned excellent properties is used as a substrate for a magneto-optical recording medium.

[Problems to be solved by the invention]

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a highly reliable magneto-optical recording medium by improving the storage stability of a polycarbonate substrate.

[Means for solving problems]

The above-mentioned object of the present invention is a magneto-optical recording medium in which a recording layer mainly composed of a rare earth metal and a transition metal is formed on a substrate, the substrate being a polycarbonate substrate injection-molded in a disk shape, and The amount of alkali metal present per surface area of 1 cm ² is 1 on the entire surface of the polycarbonate substrate on which the recording layer is formed.
It is achieved by a magneto-optical recording medium characterized in that there is no portion exceeding × 10 ^-6 gr.

In general, the magneto-optical recording substrate has a resin substrate for removing the release agent or the like adhering in the injection forming process in order to enhance the adhesion of the thin film, and also has dust or the like that causes drop-out. In order to remove the kimono, a surface cleaning process is performed on the recording layer and the protective layer as a pretreatment of a film forming process such as sputtering. At that time, various kinds of cleaning are used in order to increase the efficiency of cleaning, and most of the cleanings contain an alkali metal in their components. It has been found that a trace amount of alkali metal often adheres and remains on the polycarbonate substrate and the alkali metal acts as an alkaline component to accelerate the hydrolysis of the polycarbonate, which becomes a minute defect on the polycarbonate substrate. Further, even in a clean room where the dust level is sufficiently low, such a phenomenon occurs due to, for example, saliva or alkali metal contained in sweat that comes out of the human body.

In the present invention, as a result of examination based on such an idea,
After the cleaning process and before the film formation process, the amount of alkali metal present on the surface of the polycarbonate substrate is 1 × 10 ^-6 gr regardless of which 1 cm ² portion of the recording area forming the recording layer of the substrate is taken out. By the following, it became possible to reproduce information stably without causing defects on the surface of the polycarbonate substrate even when stored for a long period of time. The defect on the polycarbonate substrate becomes a problem even if it has a size of about 10 μm, which seems to cause almost no problem in other applications, but it is a size that cannot be ignored because it has a high recording density in magneto-optical recording applications.

In the present invention, in order to reduce the amount of alkali metal on the surface of the polycarbonate substrate to a certain level or less as described above, it is necessary to sufficiently control the cleaning conditions in the cleaning process of the polycarbonate substrate. In the magneto-optical recording medium of the present invention, the total amount of alkali metal stored on the entire surface of the polycarbonate is locally below a certain level, that is, a portion is 1 × 1 cm ²
If there is even one portion that exceeds 10 ^-6 gr, defects will occur, which is not preferable. Therefore, in the present invention, in determining the cleaning conditions of the polycarbonate, a sample of the polycarbonate substrate after cleaning is cut into a size of 1 cm ^{2 and} an ICP analysis (plasma emission analysis) is performed on each of them to determine the amount of alkali metal present. Is measured to determine the cleaning conditions for the polycarbonate substrate so that there is no portion where the existing amount of alkali metal per 1 cm ² of surface area exceeds 1 × 10 ⁻⁶ gr on the entire surface of the substrate.

In the present invention, after the polycarbonate substrate is prepared as described above, it is desirable to move as far as possible from the source of alkali metal such as human body to the next film forming step. In the film-saving process, a recording layer and various protective layers are formed on the polycarbonate substrate by a usual method. As the material for the recording layer, various magnetic metals and metal oxides can be used, but among them, the recording layer mainly containing a rare earth transition metal is preferable because it has excellent optical magnetic properties.

Further, in order to protect the recording layer of the present invention and to enhance its characteristics, inorganic and organic layers are also provided. Examples of the inorganic protective layer that can be used in the present invention include Si
Dielectrics such as oxides, nitrides and sulfides such as Ox, SiNx, AlNx and ZnS and mixtures thereof are preferred.

The most common method for forming these recording layer, protective layer and the like on the substrate is a sputtering method such as high frequency magnetron sputtering.

The novel features of the present invention will be described based on the following examples and comparative examples.

〔Example〕

(Sample A) A polycarbonate substrate injection-molded to have a diameter of 130 Φ and a thickness of 1.2 mm was passed through the following 7 washing baths to perform a washing treatment.

First tank: Sonoclen 277 (Kaiyo Denki) Ultrasonic cleaning tank filled with 1.0% cleaning solution.

Second tank: a cleaning tank filled with pure water.

Third tank: a cleaning tank filled with pure water.

Fourth tank: a cleaning tank filled with pure water.

Fifth tank: AD9 (manufactured by Asahi Glass Co., Ltd. containing Freon and a surfactant).

Sixth tank: A cleaning tank filled with Freon 113 (manufactured by Asahi Glass Co., Ltd.).

The seventh tank: a cleaning tank filled with Freon 113 (manufactured by Asahi Glass Co., Ltd.).

The sonocrene 277 contains alkali metal in the form of silicate, phosphate, sulfonate and the like.

After passing through the seventh tank, the sample was dried, and the polycarbonate substrate sample was cut into 1 cm ² sample pieces, and the amount of alkali metal present on all surfaces of the sample pieces was measured by ICP.

As a result, the maximum amount of alkali metal present was 6 × 10 ^-5.
It was g / cm ² .

On the substrate obtained through the above steps, the first SiN thin layer is 900 Å, the TbFeCo thin film is 1000 Å as the recording layer, and the second SiN thin layer is 1000 Å in this order by sputtering. The film was formed into a magneto-optical recording medium.

Next, put a maximum of 15 in a thermo-hygrostat under the conditions of 80 ℃ and 90% RH.
After leaving it for 00 hours, the storage time was calculated until the BER became twice the initial value. This time was taken as the life of each sample.

Four samples of the magneto-optical recording medium were measured, and the minimum value was taken as the life under the main cleaning conditions. As a result, the life under the main cleaning conditions was 150 hours.

(Sample B) Under the cleaning conditions of Sample A, the first tank was made of pure water alone without detergent, and the fourth tank was made of isopropyl alcohol. All other conditions were the same as those for sample A, and cleaning, sample preparation, and life measurement were performed.
As a result, the maximum amount of alkali metal present on the surface is 1
× 10 ⁻⁶ gr / cm ² , and the life of the magneto-optical recording medium was 1000 hours.

(Sample C) Under the washing conditions of Sample A, isopropyl alcohol was used in the fourth tank. All other conditions were the same as those for sample A, and cleaning, sample preparation, and life measurement were performed. As a result, the maximum amount of alkali metal present on the surface was 6 × 10 ^-6 gr / cm ² , and the life of the magneto-optical recording medium was
It was 300 hours.

(Sample D) Under the cleaning conditions of Sample A, a pure water tank was added as the eighth tank, and a Freon 113 tank was added as the ninth tank. All other conditions were the same as those for sample A, and cleaning, sample preparation, and life measurement were performed. As a result, the maximum amount of alkali metal present on the surface was 3 × 10 ⁻⁷ gr / cm ² , and the life of the magneto-optical recording medium was 1400 hours.

As is clear from FIG. 1 showing the results of the above-described operation, there is a correlation between the running life of the magneto-optical recording catalyst and the amount of alkali metal present on the surface of the substrate, and as in the magneto-optical recording medium of the present invention, It was found that a magneto-optical recording medium having an excellent running life can be obtained by preventing the portion where the amount of alkali metal present on the surface of the substrate exceeds 1 × 10 ^-6 gr per 1 c ² over the entire surface of the substrate.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of alkali metal present on the substrate surface and the running life of the magneto-optical recording catalyst.

Claims (1)

(57) [Claims] 1. A magneto-optical recording medium in which a recording layer mainly containing a rare earth metal and a transition metal is formed on a substrate, wherein the substrate is a polycarbonate substrate injection-molded in a disk shape, and the recording of the polycarbonate substrate is performed. 1 cm ² on the entire surface of the polycarbonate substrate on which the layer is formed
A magneto-optical recording medium characterized in that there is no portion where the amount of alkali metal present per surface area of 1 exceeds 10 × ^{6 -6} gr.