JPH0397132A - Protective layer of optical recording medium - Google Patents

Abstract

PURPOSE:To obtain a protective layer excellent in adhesion property under severe conditions such as high temp. and high humidity and to prevent peeling of the layer by constituting the protective layer of oxide, nitride or nitroxide of silicon or aluminum and specifying the refractive index of the protective layer at 830 nm wavelength to >=2.5. CONSTITUTION:The protective layers 2,4 consist of oxide, nitride or nitroxide of silicon or aluminum and the refractive index thereof at 830 nm wavelength is specified to >=2.5. Oxides, nitrides or nitroxides of silicon or aluminum are chemically stable and have excellent barrier property against water and acid and high mechanical strength in such an environment of high temp. and high humidity, however, they show poor adhesion property to a plastic substrate 1 and easily cause peeling. The refractive index of oxide, nitride or nitroxide of silicon or aluminum is 1.5 - 2.0 for a stiochiometric composition, but the protective layers are made to have refractive index >=2.5. By this method, the layers have properties of metals or semimetals and have better adhesion to the substrate 1. Thereby, the obtd. protective layers hardly peel from the substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a novel protective layer used in optical recording media. More specifically, the present invention relates to a protective layer for an optical recording medium that has excellent adhesion under harsh environments of high temperature and high humidity, and has suppressed peeling.

[Conventional technology]

In recent years, optical recording media that use light beams such as lasers have become the main recording media in the advanced information society due to their high recording density, high-speed access, high reliability, and non-contact nature. It is expected to play a key role in this field, and research is actively underway.

These recording media include read-only types such as compact disks and CDROMs, write-once types that can record and play back information such as documents and image films, and floppy disks that can record and erase information that are expected to replace them. There are three types of rewritable types that can be played back, and they are already in practical use.

In the above-mentioned write-once type and rewritable type optical recording media, a recording layer is provided on the substrate, and various types of recording layers have been developed with different principles and aspects of recording methods. For example, in the case of a write-once type, there are those using an open-hole type using organic dyes such as naphthoquinone, alloys or oxides mainly composed of chalcogen elements such as Se and Te, or those using Ga, Ge. Se, In, Sn,
In the case of the rewritable type, there are known phase-change types using alloys mainly composed of Sb, Te, Pb, Bi, etc., and magneto-optical types using alloys of rare earths and transition metals.

By the way, the materials used for these recording layers are often chemically unstable, and because they are used in thin films, they are easily oxidized by oxygen and water in the air, making recording and playback less reliable over time. There is no escape from a decline in sexuality. In order to solve these drawbacks, it is common practice to provide a protective layer above or below the recording layer or both in an optical recording medium. It is generally known that it is advantageous to use a dielectric thin film for this protective layer, which effectively prevents the intrusion of oxygen and water, has excellent barrier properties, and is also chemically stable. Examples of such materials include magnesium, silicon,
Attempts have been made to use oxides such as aluminum, nitrides, oxynitrides, fluorides, composites thereof, sulfides and selenides of metals such as zinc, and mixtures thereof.

However, protective films made of these materials have a drawback in that they have low adhesion to the substrate and are susceptible to peeling. When such peeling occurs, not only does the reflectance of the area decrease, causing errors, but also oxygen and water enter the recording layer, causing corrosion. In particular, when an optical recording medium is used in a high-temperature, high-humidity environment, differences in thermal expansion coefficients of the medium's constituent elements such as the substrate, protective layer, and recording layer, as well as distortion due to residual stress, may occur between the protective layer and the substrate. This causes problems such as the possibility of peeling between the layers, which limits the scope of use of the optical recording medium.

[Problem that the invention seeks to solve]

The present invention overcomes the drawbacks of conventional protective films used for such optical recording media, has high mechanical strength,
The purpose of this invention is to provide a protective film for an optical recording medium that can be used in harsh environments of high temperature and high humidity without causing peeling.

[Means for solving problems]

The present inventors have conducted extensive research to develop a protective film for optical recording media having the above-mentioned preferable properties. It has been found that the purpose can be achieved by using a protective layer consisting of an oxide, nitride, or oxynitride, and having a refractive index of 2.5 or more at a wavelength of 830 nm. .

The present invention will be explained in more detail below. Silicon and aluminum oxides, nitrides, and oxynitrides have excellent barrier properties against water and oxygen, are chemically stable, and have high mechanical strength even when left in high temperature and high humidity environments. However, it generally has poor adhesion to plastic substrates and is prone to peeling. On the other hand, metals are excellent materials in terms of adhesion to plastic substrates, but metals are not transparent and are therefore unsuitable for use as a protective layer directly on the substrate. The inventors of the present invention focused on this point, and found that by combining the advantages of metals, semimetals, and their oxides, nitrides, and oxynitrides, we achieved excellent properties as a protective layer that can be provided directly on the substrate of an optical recording medium. We have found a material that has

Such a material can be obtained by setting the refractive index of the protective layer to 2.5 or more at a wavelength of 830 nm. Oxides, nitrides, and oxynitrides of silicon and aluminum have a stoichiometric composition of 1. It has a refractive index of about 5 to 2.0, but if it is set to 2.5 or more, it will have the characteristics of a metal or a metalloid, and its adhesion to the substrate will improve. However, if the refractive index is too high, problems such as a decrease in the reflectance of the optical recording medium and poor transparency of the protective layer itself will occur. In this sense, it is desirable that the refractive index be 3.0 or less, more preferably 2.7 or less.

FIG. 1 is a sectional view showing an example of an optical recording medium to which the present invention is applied, and these may be of a read-only type, a write-once type, or a rewritable type. There are no particular restrictions on the recording layer in the above configuration; for example, in the case of a write-once type, it may be of an aperture type or a phase change type, or it may be one using an organic dye, or it may be of a rewritable type. In this case, it may be a magneto-optical type or a phase change type. Further, the effects of the present invention are most effectively exhibited when plastics such as acrylic resin, epoxy resin, and polycarbonate resin are used as the substrate material, but glass and the like can also be used in addition to these.

There are no particular restrictions on the method of forming the protective layer, and methods conventionally used for forming thin films, such as vapor deposition and sputtering, can be used.

The thickness of the protective layer is usually selected in the range of 10 to 200 nm. Furthermore, in the optical recording medium of the present invention, a protective layer may be provided on the recording layer to prevent oxidation and corrosion of the recording layer, if desired. The protective layer may be made of a dielectric material such as silicon oxide, silicon nitride, aluminum nitride, zinc sulfide, or a composite thereof. In the case of the protective layer used on the recording layer, unlike the protective layer of the present invention provided on the substrate, there are fewer restrictions on adhesion, so it does not necessarily have to have a refractive index of 2.5 or more.
A stoichiometric composition may be employed if desired.

〔Effect of the invention〕

According to the present invention, it is possible to obtain a protective film for an optical recording medium that has excellent barrier properties against water and oxygen, is not only chemically and mechanically stable, but also has excellent adhesion and is resistant to peeling from the substrate. .

〔Example〕

EXAMPLES Next, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples in any way.

Example 1 As shown in FIG. 1(a), a protective layer 2 made of silicon nitride with a film thickness of 80 nm and a Tb2 film with a film thickness of 80 nm were formed on a polycarbonate substrate l with a guide groove having a diameter of 130 nm. Fet
A magneto-optical disk was prepared in which a recording layer 3 made of oCO+a and a protective layer 4 made of silicon nitride with a film thickness of 80 nm were sequentially provided. Protective layers 2 and 4 are formed by reactive RF magnetron sputtering using a Si target, and recording layer 3 is formed by DC magnetron sputtering using an alloy target, and the refractive index of protective layer 2 at a wavelength of 830 nm is 2.60.
The flow rate of the reaction gas was set so that The refractive index of the protective layer 4 is 1. 9 or less, and is estimated to have a nearly stoichiometric composition. As a comparative example, a magneto-optical disk was also prepared under the same conditions except that the refractive index of the protective layer 2 was 2.3. These two magneto-optical disks were bonded together using a hot melt adhesive and left in an accelerated life test environment of 80° C. and 90% RH for 400 hours.

In the disc of the example, no change was observed from the initial state, whereas in the disc of the comparative example, peeling occurred between the substrate and the protective layer 2, causing problems in recording and reproduction. This example suggests that the occurrence of peeling of the magneto-optical disk under high temperature and high humidity conditions can be prevented by increasing the refractive index of the silicon nitride protective layer provided on the substrate.

Example 2 As shown in FIG. 1(b), a protective layer 2 made of silicon nitride with a film thickness of 80 nm and a Tbio film with a film thickness of 25 nm were placed on a polycarbonate substrate 1 with a guide groove having a diameter of 130 mm.
Recording layer 3 made of FetoCO+o and film thickness 40 nm
A protective layer 4 made of silicon oxynitride with a thickness of 50 nm
A magneto-optical disk was prepared in which a reflective layer 5 consisting of the following layers was sequentially provided. The protective layers 2 and 4 were created by reactive RF magnetron sputtering using a Si target, the recording layer 3 was created by DC magnetron sputtering using an alloy target, and the reflective layer 5 was created by RF magnetron sputtering using an AI target.
The refractive index of the protective layer 2 at a wavelength of 830 nm is 2.30.
2, 40.2. Four types of reaction gas flow rates were set so that the flow rates were 50 and 2.60. The refractive index of the protective layer 4 is 1.
9 or less, and is estimated to have a nearly stoichiometric composition. These magneto-optical disks were left under an accelerated life test environment of 80° C. and 90% RH, and their stability was tested by pit error rate (BER). The results are shown in Figure 2. As is clear from this figure, the BER is correlated with the refractive index of the protective layer 2, and when the refractive index is 2.50 or more, no change is observed for more than 800 hours, whereas when the refractive index is less than 2.50, the BER is correlated with the refractive index of the protective layer 2. Peeling occurred between layer 2 and a rapid increase in BER was observed. The results revealed that by setting the refractive index of the protective layer 2 to 2.5 or more, peeling between the substrate and the protective layer 2 can be prevented and a highly stable magneto-optical disk can be obtained.

Example 3 As shown in FIG. 1(b), a protective layer 2 made of aluminum oxynitride with a film thickness of 80 nm and a Tb2 film with a film thickness of 25 nm were placed on a polycarbonate substrate l with a guide groove having a diameter of 130 mm.
. Recording layer 3 made of FetoCO+o and film thickness 40
A magneto-optical disk was prepared in which a protective layer 4 made of silicon oxynitride with a thickness of 50 nm and a reflective layer 5 made of AI with a film thickness of 50 nm were sequentially provided. Protective layer 2 has reactivity R by AI target
F magnetron sputtering, protective layer 4 is reactive RF magnetron sputtering using a Si target, recording layer 3 is DC magnetron sputtering using an alloy target, reflective layer 5
was created by RF magnetron sputtering using an AI target, and the refractive index of the protective layer 2 at a wavelength of 830 nm was 2. 30, 2. 40, 2. 50. 2. 6
Four types of flow rates of the reaction gas were set so that the flow rate was 0. The atomic ratio of oxygen and nitrogen in the protective layer 2 was approximately 1:1. The refractive index of the protective layer 4 is 1. 9 or less, and is estimated to have a nearly stoichiometric composition.

These magneto-optical disks were left under an accelerated life test environment of 80° C. and 90% RH, and their stability was tested by BER. The results are shown in Figure 3. Even when the protective layer 2 is made of aluminum oxynitride, the BER is correlated with its refractive index, and when the refractive index is 2.50 or more, no change is observed for more than 800 hours, whereas when the refractive index is less than 2.50, it is difficult to protect the substrate. Peeling occurred between layer 2 and a rapid increase in BER was observed. The results revealed that by setting the refractive index of the protective layer 2 to 2.5 or more, peeling between the substrate and the protective layer 2 can be prevented and a highly stable magneto-optical disk can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a configuration example of the present invention, and FIGS. 2 and 3 are diagrams showing stability test results of a magneto-optical disk using the present invention. In the figure, 1 is a substrate, 2 and 4 are protective layers, 3 is a recording layer, and 5 is a reflective layer.

Claims (1)

[Claims] 1. A protective layer provided on a substrate in an optical recording medium, the protective layer comprising silicon or aluminum oxide,
Made of nitride or oxynitride, and has a wavelength of 830 nm
A protective layer for an optical recording medium, characterized in that the protective layer has a refractive index of 2.5 or more.