JPH0388144A - Optical recording medium - Google Patents

Abstract

PURPOSE:To improve the optical characteristics and sensitivity of a reflecting layer by constituting the reflecting layer of an amorphous material and preventing the increase in the noise arising from the nonuniformity of the structure. CONSTITUTION:The optical recording medium is formed by successively laminating a recording layer 2, a dielectric layer 4, and the reflecting layer 3 on a substrate 1. The dielectric layer 4 is used as a protective layer, interference layer and heat insulating layer. The optical constant is uniformized and the increase of noise is prevented if the dielectric layer 3 is constituted of the amorphous material. The optical unequalness by the nonuniformity of the structure possessed by the crystalline dielectric layer is eliminated and the optical characteristics and sensitivity are improved.

Description

[Detailed description of the invention] <Industrial application field> The present invention relates to a novel optical recording medium.

More specifically, the present invention relates to a low-noise optical recording medium that eliminates structural discontinuities that cause non-uniform light reflection.

<Background Art> In recent years, optical recording media represented by optical discs have attracted attention as a recording medium that plays a central role in an advanced information society, and research is actively underway.

There are three types of optical discs: a read-only type represented by a compact disc, a write-once type that allows information to be recorded and played back, and a rewritable type that allows information to be recorded, erased, and played back. Information is reproduced using reflected light from the surface.

For this reason, any of the above-mentioned optical discs is usually provided with some form of reflective layer.

The simplest type is a read-only type, and the reflective layer in this case only needs to have a high reflectance, and argonium, which is highly productive and inexpensive, is usually used.

On the other hand, in the case of write-once type and rewritable type, the role of the reflective layer is to increase optical contrast and adjust reflectance.

In this case as well, in terms of optical properties, the reflective layer may be made of aluminum, but since materials with high thermal conductivity reduce the sensitivity of recording and erasing, generally metals with lower thermal conductivity are used. It is more advantageous to configure

Such materials include, for example, indium, tin, antimony, tellurium, bismuth, and lead.

By the way, since the crystallization temperature of the materials listed above is around room temperature, at least immediately after formation, they tend to form non-uniform crystals with some amorphous parts remaining, and the amorphous state and crystalline state are different. Since the optical constants are significantly different, such a non-uniform structure results in uneven reflectance, which increases noise during reproduction.

In particular, the film thickness of the reflective layer used in optical recording media is between 20nW+ and 100nW+ in order to achieve both reflectance and thermal conductivity.
n+s is often selected, but this is particularly inconvenient because the size of the crystals produced in this film thickness range is comparable to the recording bit, which directly leads to an increase in noise.

<Problems to be Solved by the Invention> An object of the present invention is to provide a reflective layer that prevents an increase in noise due to the non-uniform structure as described above and has good optical properties and sensitivity.

Means for Solving the Problems> The present inventors have made extensive studies to solve the above problems, and as a means to eliminate the non-uniformity of the structure, the reflective layer is made of an amorphous material. By configuring
It has been found that the increase in noise can be prevented.

The present invention will be explained in detail below.

As long as a crystalline substance is used for the reflective layer, it is impossible to strictly avoid unevenness in optical constants due to non-uniformity in crystallinity and crystal grain size.

In contrast, the effect of the present invention is to make the reflective layer amorphous, thereby making the optical constants uniform and preventing an increase in noise.

According to this method, good reproduction characteristics can be achieved regardless of the thin film formation conditions during the formation of the reflective layer or the configuration of the optical recording medium to which it is applied.

In order to form the reflective layer in an amorphous state, there is a method of using a eutectic alloy having two or more components, or a composition close to the eutectic alloy.

Specifically, silicon, gallium, germanium, selenium, indium, tin, antimony, tellurium, thallium,
It is preferable to use an alloy having a eutectic composition of two or more elements selected from bismuth and lead, or a composition close to the eutectic composition.

For example, germanium-antimony system, gallium-antimony system, indium-antimony system, lead-antimony system, antimony-tellurium system, germanium-tellurium system,
Alloy systems such as the tin-tellurium system have advantages such as being easy to form by vapor deposition, submersion, etc., having appropriate thermal conductivity and optical constants, and being relatively inexpensive.
It is suitable as a reflective layer material for optical recording media.

Furthermore, these alloy systems also have the advantage that when it becomes necessary to adjust the crystallization temperature as described below, the crystallization temperature can be easily changed by changing the composition.

Hereinafter, the present invention will be explained in more detail by taking the germanium-antimony system as an example.

The eutectic composition of this alloy system is GetsSbs on the basis of the number of atoms.
t, and an amorphous alloy can be obtained near this composition.

If the germanium ratio is less than 5%, antimony crystals will crystallize and it will not become amorphous, and if it exceeds 35%, the reflectance will decrease, so the germanium ratio should be between 5% and 35%. .

Which composition is optimal within this range differs slightly depending on the system and structure of the optical recording medium to which it is applied.

FIG. 1 shows an example of the structure of the optical recording medium of the present invention. In the case of a read-only type in which a reflective layer is used alone as in (a), or in the case of a read-only type in which a reflective layer and a recording layer are used as in (b), If a sufficiently thick interference layer or insulation layer is inserted in between,
Since the temperature of the reflective layer does not rise when the optical recording medium is used, it is preferable to select the composition with emphasis on optical constants, and the proportion of germanium to be 5% or more and 20% or less.

On the other hand, if the interference layer or heat insulation layer is thin, or (C
), in which the reflective layer and the recording layer are in direct contact, the temperature of the reflective layer increases to the same level as the recording layer during recording (and during erasing if rewritable) due to thermal conduction.

Therefore, in order to maintain the amorphous state of the reflective layer even after recording and erasing operations, it is necessary to make its crystallization temperature higher than the operating temperature of the recording layer.

For example, in the case of magneto-optical recording media, the recording layer temperature is close to its Curie temperature, often around 100°C to 200°C, so the proportion of germanium is preferably selected to be 7% or more and 30% or less.

In the case of a phase change recording medium, the temperature rise of the reflective layer varies depending on the phase change temperature of the recording layer used. For example, if the phase change temperature is between 150°C and 200°C, the proportion of germanium should be 10% or more. By selecting 35% or less, the desired effect can be obtained.

The reflective layer of the present invention contains titanium, vanadium, chromium, cobalt, nickel, zirconium, etc. for the purpose of increasing stability.
Transition metals such as niobium, tantalum, and molybdenum may also be added.

The amount added should be such that it does not affect the optical constants or crystallization temperature of the reflective layer, preferably 5% or less based on the number of atoms.

For the recording layer, in addition to the magneto-optical method and phase change temperature method mentioned above, an aperture method, an organic dye, etc. can also be used.

In addition to plastics such as acrylic, polycarbonate, polyolefin, and epoxy, glass can also be used as the substrate material.

Furthermore, by providing a protective layer made of a dielectric material between the substrate and the recording layer or on the reflective layer, the stability of the optical recording medium can be dramatically improved.

As dielectric materials, oxides, nitrides, oxynitrides, sulfides, fluorides, etc. of magnesium, aluminum, silicon, zinc, germanium, titanium, tantalum, etc., or composites or laminates of these materials are used. good.

<Example> Hereinafter, the present invention will be explained in more detail with reference to Examples.
The present invention is not limited in any way by these examples.

Example 1 5eSb of various compositions on a microscope cover glass
An alloy thin film of 80 nm was formed by co-sputtering, and its crystallization temperature was measured.

The optical constants of the 5eSb alloy change discontinuously between the amorphous and crystalline states, so when the sample is heated,
The temperature at which the reflectance rapidly changes was defined as the crystallization temperature.

The measurement wavelength was 830 wavelengths.

FIG. 2 shows the relationship between crystallization temperature and composition of this alloy thin film.

In a thin film in which the amount of Ge is 0%, that is, only sb, the above-mentioned rapid change in reflectance does not occur.

This is considered to be because the crystallization temperature of sb is other than room temperature, so crystals have already grown immediately after film formation.

As the sample is heated, the reflectance gradually increases as the crystals grow.

This indicates that the crystals immediately after film formation are still non-uniform.

Therefore, if this thin film is employed as a reflective layer of an optical recording medium, it is expected that noise will occur due to optical unevenness.

As Ge is added to this, the crystallization temperature gradually rises from room temperature, and reaches 200° C. or higher when Ge is 35%.

As a result of X-ray diffraction, it was confirmed that samples containing 5% or more of Ge were in an amorphous state immediately after film formation.

These results suggest that by adding 5% or more of Ge to the sb reflective layer, an optically uniform reflective layer can be formed and an optical recording medium with less noise can be obtained.

Example 2 As shown in FIG.
eszGe3o alloy thin film 30nII+, GeSb alloy thin film 50no+ as reflection 1'i3, Si as protection N4
A 20 nm thick O thin film was formed by sputtering.

The recording layer is formed by a compound target with the above composition, the reflective layer is formed by co-sputtering using Ge and Sb targets, and the protective layer is formed by reactive sputtering using a Si target. Adjust the Ge ratio from 0% to 37% based on the number of atoms.
It was varied up to 5%.

In order to investigate the recording characteristics of this disc, the number of rotations was 1800.
rpI11, radial position R=30mm, frequency 1.0MH
z, I want to record a signal with a duty ratio of 50, and the recording bit length is 2.
．． 8μ11), carrier level, and noise level were measured.

Figure 4 shows the results.

In this figure, the horizontal axis represents the composition of the reflective layer in % of Ge atoms.
The vertical axis is the carrier-to-noise ratio (CNR) and the noise level.

The noise level is about -61 dB when only sb is used, but it starts to decrease from 3% to 5% when the Ge ratio is 10%.
Above that, it becomes constant at about -76 dB.

In the case of only sb, the noise is high because crystals of about several μm grow irregularly.

To prevent this, it is effective to add 5% Ge, but in the case of a phase change type recording medium like this example, the crystallization temperature of the reflective layer must be set higher than the crystallization temperature of the recording layer. It is preferable to do so in order to prevent an increase in noise due to crystallization of the reflective layer during recording.

Since the crystallization temperature of the recording layer of this example is about 160''C, considering the results of Example 1 as well, it is preferable that the amount of Ge added is 10% or more based on the number of atoms.

Due to the above effects, the CNR increases as the noise decreases, but when the amount of Ge added exceeds 25%, it begins to decrease, and particularly when it exceeds 35%, it decreases rapidly.

This is not due to an increase in noise but a decrease in carriers.

The reason for this is thought to be that the change in the optical constants of the reflective layer due to the addition of Ge became impossible to ignore, and the contrast of the recording signal decreased.

From the above results, it has become clear that it is effective to configure the reflective layer with a GeSb alloy in order to prevent an increase in noise due to crystallization of the reflective layer.

The composition of the GeSb alloy is 5% Ge to prevent noise increase.
As mentioned above, it is most appropriate to select Ge in the range of preferably 10% or more, and 35% or less in order to prevent a decrease in contrast.

Example 3 In the optical disc of Example 2, the reflective layer 3 was made of Ga, Sb.
B, InzoStlyo+ Pb+qSbst+ 5
A thin film of each bssTe+s alloy was used, and the noise level was measured when the same signal as in Example 2 was recorded.

In addition, in order to investigate the structure of each alloy thin film, only the reflective layer was formed on a microscope slide glass, and its X-ray diffraction pattern was measured.

Film formation was performed by sputtering only Ga+zSbss using a compound target, and for the others by co-sputtering using targets of each component element.

Note that the composition used here is the eutectic composition of each alloy system or a composition near it.

Table 1 shows the results.

In each alloy system, the noise level is reduced by 13 to 15 dB compared to the case where the reflective layer is formed only with sb.

This shows that the optical unevenness of each alloy is significantly lower than that of sb alone.

It was also revealed that all alloy thin films were amorphous, and that making the structure uniform was effective in reducing noise.

<Effects of the Invention> According to the present invention, it is possible to provide an optical recording medium that is free from optical unevenness due to structural non-uniformity of a crystalline reflective layer and has low noise.

[Brief explanation of drawings]

1 and 3 are diagrams showing examples of the structure of the optical recording medium of the present invention, in which 1 is a substrate, 2 is a recording layer, 3 is a reflective layer, 4 is a protective layer, an interference layer, a heat insulating layer, etc. This is the dielectric layer used. FIG. 2 is a diagram showing the crystallization temperature of the reflective layer of the optical recording medium of the present invention. FIG. 4 is a diagram showing the reproduction characteristics of the optical recording medium of the present invention, in which the solid line indicates the carrier-to-noise ratio and the - dotted line indicates the noise level.

Claims (1)

[Claims] 1. An optical recording medium having a reflective layer, characterized in that the reflective layer is made of an amorphous substance.