JPH1186340A - Optical recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease substrate surface reflected light (Rs) and to improve the recording sensitivity and the reproducing signal quality by making the refractive index in the transparent dielectric layers formed on a substrate on the side opposite to a recording layer smaller the furtherer from the substrate. SOLUTION: The light incident surface of the transparent substrate consisting of a polycarbonate resin on the side opposite to the recording layer is provided with the first and second transparent dielectric layers of the refractive index higher toward the inner side from the outer side. Silicon nitride having the refractive index (n) ranging from 1.4 to 2.0 or the like is used as the outermost layer and the film thickness thereof is specified to λ/7n to λ/3n when the wavelength of the incident light is defined as λ. The silicon nitride having a relatively high refractive index 1.6 to 2.4 or the like is used for the second transparent dielectric layer on the inner side and the film thickness thereof is specified to a range of λ/2 to λ/7. The film thickness over the entire part of the transparent dielectric layers is specified to 160 to 230 nm. If such constitution is adopted, the transparent dielectric layers function as antireflection films as well and are capable of lowering Rs to <=1%. In addition, the flatness is maintained and S/N is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical recording medium used for recording information, and more particularly to an optical recording medium having high recording sensitivity.

[0002]

2. Description of the Related Art Optical recording media have been put to practical use as high-density, low-cost information recording media. For example, CD (compact disk) for music, LD for video
So-called optical disks such as (laser disks), magneto-optical disks for data recording, and phase change disks are known. Recording and reproduction of information are performed by a change in reflectivity due to uneven pits provided on the substrate, a change in reflectivity due to crystallization and amorphization of the recording film, or a rotation of linearly polarized light due to a magnetization direction of a magnetic material.

[0003] In these media, in order to reduce the influence of dust or scratches on the surface, it is usual to use a transparent substrate to impinge light from the substrate side to record / reproduce information. In such a configuration, the beam spot has a diameter of about several mm on the light incident surface of the substrate, so that even if there is some dust or flaw on the substrate surface, the influence on the entire beam is small. The thickness of this substrate is, for example, 1.2 mm for a CD or the like. As the material of the substrate, a transparent resin substrate which is inexpensive and can be formed at a high speed by an injection molding method is used in most cases. Particularly, polycarbonates which are inexpensive and have excellent property balance are often used. The refractive index of such a resin substrate is 1.5 to 1.
This is about 6, and about 5% of the substrate surface reflected light (hereinafter, may be referred to as “Rs”) is generated at the interface between the air and the substrate. Note that Rs is approximately

[0004]

Rs = (ns−1) ² / (ns + 1) ²

[0005] Here, ns is the refractive index of the substrate. Since Rs is at most about 5%, it has been considered that it does not greatly affect the recording and reproduction of the conventional signal. However, with the recent increase in the capacity of optical discs, the demand for reproduction signal quality has become extremely strict, and small-level signal disturbances, which were not a problem in the past, have led to errors during reproduction. I have. The present inventor has analyzed the cause of such an error, and has found that one of the causes is disturbance of the light polarization plane due to unevenness of the substrate surface. For example, in the case of a magneto-optical disk, signal reproduction is performed according to the rotation direction of the light polarization plane. However, if the substrate surface has irregularities, the polarization plane of Rs is disturbed. This signal disturbance is actually observed as a low-frequency undulation of the signal. If this undulation becomes large, a reproduction error occurs.

Conventionally, it has been proposed that a light incident surface of a substrate is hard-coated with an organic substance such as an ultraviolet curable resin or an inorganic substance such as Si oxide to prevent the surface from being damaged. However, such an organic coating or inorganic coating could not remove the influence of unevenness generated before coating which occurred at the time of molding or the like.

Further, it has been found out that Rs is a component that does not contain any signal, so that even if it enters the photodetector, it only generates noise, which is a factor that lowers the SN ratio of the signal. The conventional hard coat has a refractive index of 1.
Since there is about 5 to 1.6, the influence of surface reflection was almost the same as when no hard coat was performed. In theory, when a uniform coat having a refractive index of about 1.3 is applied, R
It is possible to reduce s. However, generally, a solid solid film layer has a high refractive index, and cannot satisfy both such a low refractive index and a function as a hard coat.

[0008]

The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, in an optical recording medium having a transparent dielectric layer provided on a light incident surface of a substrate, the optical recording medium has a transparent dielectric layer. The refractive index decreases as the distance from the substrate increases,
For example, specifically, it has been found that Rs can be easily reduced by providing a substrate with a plurality of transparent dielectric layers having different refractive indexes, and the present invention has been completed. According to the present invention, it is possible to provide an optical recording medium in which the influence of substrate irregularities is small, the SN ratio is high, and the transparent dielectric layer has a sufficient function as a hard coat for preventing damage to the substrate.

By using the present invention, reflection from the substrate surface can be significantly reduced, and as a result, signal disturbance due to unevenness on the substrate surface can be suppressed. Further, since the generation of noise due to the surface reflected light is reduced, the SN ratio is also improved. As a result, a recording medium with a low error rate can be obtained. Moreover, productivity is sufficiently high. Another effect of reducing Rs is to improve recording sensitivity. That is, in a recordable optical recording medium, a part of the recording light is conventionally reduced by Rs to reduce the recording sensitivity without being changed by heat. However, in the present invention, a high recording sensitivity is obtained because Rs is reduced.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The substrate of the optical recording medium used in the present invention must be a transparent substrate. Preferably, a transparent resin substrate such as a polycarbonate resin, a polymethyl methacrylate resin, and an olefin resin is used. A polycarbonate resin substrate is particularly preferable because of its relatively low hygroscopicity and low cost.

The substrate is provided on one surface thereof with recording / reproducing pits, an organic dye layer, a recording layer containing a phase-change material, a magneto-optical material and the like as a recording layer, for recording / reproducing information. In the present invention, the transparent dielectric layer is provided on the side opposite to the side on which the recording layer is provided, that is, on the light incident surface of the substrate. In order to keep Rs low, the refractive index is the lowest on the outermost side of the dielectric (that is, the layer portion farthest from the substrate), and the refractive index increases as approaching the substrate. With this configuration, these transparent dielectric layers also serve as so-called antireflection films, and Rs can be made extremely small. In a normal medium, Rs of about 5% is obtained due to the difference in the refractive index of the resin.
However, in the configuration of the present invention, this value is, for example, 1%
It can be greatly reduced to the following.

In a preferred embodiment of the present invention, a plurality of transparent dielectric layers, each having a different refractive index, are laminated in the process of manufacturing an optical recording medium. Examples are shown in FIGS. Different materials can be used for the plurality of transparent dielectric layers, or dielectrics having the same material and different refractive indices can be used. Obtaining a different refractive index with the same substance can be easily achieved, for example, by changing the film forming pressure, the partial pressure of the reactive gas, the sputtering power, and the like in reactive sputtering.

Although the number of layers constituting the transparent dielectric layer is arbitrary, it is preferable that the number be two in order to simplify the manufacturing process. In this case, the layer (first dielectric layer) used as the outermost layer (the layer farthest from the substrate) has a condition that the refractive index is relatively low, and silicon nitride, silicon oxide, Zn
O, magnesium fluoride, aluminum oxide and the like are preferably used. In particular, silicon nitride, silicon oxide, and aluminum oxide, which have a strong film and high scratch resistance on the surface, are preferable. The refractive index n of the first dielectric layer is preferably from 1.4 to 2.0.
If the refractive index is too low, it will not serve as a hard coat. If the refractive index is too high, the effect of reducing the reflectance is reduced.

The thickness of the first dielectric layer is particularly important for reducing Rs. The thickness of the first dielectric layer is set such that light reflected at the interface with the transparent dielectric layer (second dielectric layer) inside the first dielectric layer cancels light reflected at the surface of the first dielectric layer.
Assuming that the wavelength of the incident light (in the air) is λ, it is preferable that the wavelength is approximately λ / 4n. The range in which a sufficient reflectance reduction effect can be obtained is λ / 7n to λ / 3n. Particularly preferably λ
/ 6 to λ / 3.5.

The transparent dielectric layer such as a second dielectric used on the substrate side with respect to the first dielectric layer is a transparent dielectric having a relatively high refractive index, specifically, silicon nitride, tantalum oxide, aluminum nitride. , Tantalum oxide, titanium oxide, silicon oxide, ZnS, ZnSe, ZnO and the like. Among them, silicon nitride, from which a transparent high refractive index film can be easily obtained,
Tantalum oxide, ZnS and the like are preferred. The refractive index is larger than that of the first dielectric layer, and is preferably 1.6 to 2.4. The film thickness can be arbitrarily selected from characteristics such as film stress and surface hardness described later. In particular, when it is necessary to reduce Rs, λ / 2 to λ / 2 of the incident light wavelength λ is required.
It is preferably λ / 7. .

In order to sufficiently obtain the effect of the present invention, Rs is 5
%, More preferably 4% or less, particularly preferably 3% or less. In the present invention, the thickness of the transparent dielectric layer is usually from 100 to 300 nm, especially from 150 to 300 nm, though it depends on other requirements constituting the optical recording medium.
２５０250 nm, particularly preferably 160-230 nm.

For example, in the configuration of the present invention, the incident light wavelength λ is 6
In the case of 50 nm, for example, the refractive index of the first dielectric layer is 1.5.
(Silicon oxide), the film thickness at which Rs is minimum when the refractive index of the second dielectric layer is 2.0 (silicon nitride) is
This is the case where the first dielectric layer is 90 nm and the second dielectric layer is 100 nm. At this time, Rs is 1% or less. It is preferable that the difference in the refractive index of each layer is 0.1 or more in order to increase the margin of the film thickness for reducing Rs. It is more preferably at least 0.2, particularly preferably at least 0.3.

As described above, since the transparent dielectric layer of the present invention has high scratch resistance, it has the same effect as the hard coat of the prior art. It is preferable that the hard coat is hard and that Rs is small. However, in general, a hard substance has a relatively high refractive index, resulting in a high Rs. Low refractive index inorganic coat, for example, refractive index 1.
The coat of silicon oxide 5 alone was fairly hard, but the hardness was still insufficient. On the other hand, a hard film such as silicon nitride has a problem that the reflectance Rs of the substrate surface becomes too high because the film has a high refractive index. On the other hand, in the dielectric of the present invention, a hard dielectric such as a nitride can be used, and the reflectance is very low, so that the scratch resistance can be significantly improved without impairing the recording / reproducing characteristics.

Further, since the film having a high refractive index has a relatively high stress, it has an effect of canceling out the stress of the recording film and keeping the substrate flat. With such a configuration, it is possible to suppress initial deformation and deformation of the substrate due to temperature and humidity. The rigidity of the substrate is significantly reduced as the thickness of the substrate is reduced, and the substrate is easily warped. Therefore, the effect of flattening the substrate is more remarkable as the thickness of the substrate is reduced. In the present invention, it is particularly preferable to use an optical recording medium having a substrate thickness of 0.8 mm or less because the effect becomes remarkable. Even if a layer having a low refractive index such as silicon oxide alone is used, the substrate can be flattened if the thickness of the layer is extremely large, but productivity is significantly reduced. On the other hand, if the layer has a relatively high stress such as silicon nitride, it is possible to reduce the thickness of the layer. However, since such a layer has a high refractive index, Rs is increased and the recording / reproducing characteristics are deteriorated. Has a negative effect.

In the present invention, by adjusting the thickness of the outermost first dielectric layer, the thickness of the second dielectric layer and thereafter can be selected relatively freely. Therefore, the film thickness having the highest flatness can be appropriately selected without regard to Rs. In addition, an antistatic function can be provided by making any of the layers constituting the multilayer dielectric layer a conductive film, for example, ITO, ZnO: Al, or the like. This makes it possible to suppress adhesion of dust due to static electricity.

Before forming the transparent dielectric layer of the present invention, a hard coat made of an organic material such as an ultraviolet curable resin may be applied. Since coating with an organic substance can form a film having a thickness of several μm, scratch resistance can be further improved. The organic coating may have an antistatic function. Further, as another embodiment of the present invention, the transparent dielectric layer may be a single layer, and the refractive index may be continuously changed in this layer. This can be achieved, for example, by gradually increasing the film formation pressure during film formation in reactive sputtering of silicon nitride. Other than silicon nitride, silicon oxide, aluminum nitride, tantalum oxide, or the like can be used.

Since the present invention can reduce the disorder of the polarization plane due to the unevenness of the substrate surface, it is preferably used for a magneto-optical recording medium using linearly polarized light for information reproduction. However, in addition to this, even when used for a phase-change medium or a read-only medium for reproducing uneven pits, it is possible to obtain a remarkable noise reduction effect due to a reduction in reflectance fluctuation due to unevenness, a decrease in Rs, and an improvement in recording sensitivity due to a decrease in Rs. Can be

[0023]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the present invention.

[0024]

Examples 1-5, Comparative Example 1 120 mm in diameter as substrate
φ, center hole diameter 15mmφ, substrate thickness 0.6mm,
A polycarbonate resin substrate having a groove depth of 70 nm, a track pitch of 1.2 μm, and a land portion and a groove portion having the same width and a refractive index of 1.58 was used. A mold used for molding a substrate has a depth of about 200 μm and a width of about 1 at a radius of 50 mm.
A flaw having a length of about 15 mm and a length of about 15 mm is perpendicular to the groove, and is transferred to the light incident surface of all the substrates. When the substrate is used alone, it is formed so that there is almost no warp angle.

On the side of the substrate where the groove is not provided (light incident surface), silicon nitride having a thickness of d2 = 100 nm as a second dielectric layer and silicon oxide having a thickness of d1 as a first dielectric layer are provided in this order. Was. The refractive index of silicon nitride was 2.0, and the refractive index of silicon oxide was 1.5. Film thickness d
1 was changed as shown in Table 1. A 70 nm silicon nitride and a 20 nm T
A magneto-optical recording film made of bFeCo, 15 nm of silicon nitride, and 30 nm of aluminum was formed in this order on all the disks in the same manner.

The warpage angle of these disks, 650
Rs in nm was measured. Further, a wavelength of 650 nm,
The characteristics were measured with an evaluator with NA = 0.6. Radius 35mm
At a linear velocity of 9 m / s, a recording frequency of 3 MHz, and a duty of 35
%, Narrow-band SNR (CNR) and recording start power P
th was measured. Furthermore, these discs were measured with a drive, and it was confirmed whether or not a scratch on the substrate incident surface at a radius of 50 mm would be a signal defect. The results are shown in Table 1. The warp angle indicates the maximum value in the disk, and 0 indicates a state without warpage.

[0027]

Examples 6 to 9 and Comparative Example 2 Film thickness d as first dielectric layer
Example 1 except that aluminum oxide having a refractive index of 1.7 was used in Example 1.
A disc was created in the same way as. The measurement was performed on these disks in the same manner as in Example 1. The results are shown in Table 2.

[0028]

Examples 10 to 15 and Comparative Example 3 Example 1 except that the film thickness d2 of silicon nitride as the second dielectric layer was changed and the film thickness d1 of silicon oxide as the first dielectric layer was fixed at 100 nm. A disc was created in the same way as. The warpage angle, Rs and signal characteristics of these disks were measured in the same manner as in Example 1. The results are shown in Table 3.

[0029]

Embodiment 16 A reactive sputtering of a silicon target by a mixed gas of argon and nitrogen was performed on a substrate.
Silicon nitride having a refractive index of 2.2 is used as the third dielectric layer.
The second dielectric layer was provided with 50 nm of silicon nitride having a refractive index of 2.0, and the first dielectric layer was provided with 60 nm of silicon nitride having a refractive index of 1.8. The refractive index was changed by sequentially increasing the partial pressure of nitrogen gas. The magneto-optical recording film was formed in the same manner as in Example 1. Rs and signal characteristics of this disk were measured in the same manner as in Example 1. Warpage angle is 3.2
mrad and Rs were 0.6%, narrow band SNR (CNR) was 54.8 dB, and recording start power Pth was 4.4 mW. No errors were observed due to scratches at a radius of 50 mm.

[0030]

Example 17, Comparative Examples 4 and 5 The substrate thickness was 1.2 mm,
Substrates similar to those used in Example 1 were prepared except that they were changed to 1.0 mm, 0.8 mm, and 0.6 mm.
On these substrates, silicon nitride having a refractive index of 2.0 was provided as a second dielectric layer, and silicon oxide having a refractive index of 1.5 was provided as a first dielectric layer. The thickness d1 of the first dielectric layer is 100 nm
And the thickness of the second dielectric layer was changed. Further, on the groove side of the substrate, 70 nm of silicon nitride, 30 nm of GdFeCo, 15 nm of silicon nitride, 50 nm of T
A magnetic induction super-resolution magneto-optical recording film made of bFeCo, 20 nm of silicon nitride, and 20 nm of aluminum was formed.

In these disks, if the thickness of the second dielectric layer is increased, the stress of the magneto-optical recording film on the groove side cancels out and the warp angle decreases. Table 4 shows the minimum thickness of the second dielectric layer at which the warp angle becomes 3 mrad or less. Rs was 1% or less in each case. Further, as Comparative Example 4, Example 1
Using a substrate similar to that of Example 7, a dielectric layer of silicon nitride having a refractive index of 2.0 was formed on the light incident surface side, and a dielectric layer of silicon nitride having a refractive index of 1.5 was formed on the light incident surface side as Comparative Example 5. An optical disk was formed in the same manner as in Example 17 except that the film was formed. By changing the film thickness of these silicon nitrides, the minimum film thickness at which the warp angle becomes 3 mrad or less was determined. The results are shown in Table 4. At this time, Rs was about 4% for all disks in Comparative Example 4 and about 11% for all disks in Comparative Example 5. From these examples, according to the present invention, the transparent dielectric layer can be made thinner, and at the same time, an optical recording medium having a low Rs can be obtained.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

By using the optical recording medium according to the present invention, it is possible to provide a medium satisfying all of excellent reproduction signal quality, high recording sensitivity and excellent flatness without changing the recording film. it can.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the optical recording medium of the present invention.

FIG. 2 shows another embodiment of the optical recording medium of the present invention.

FIG. 3 shows a conventional optical recording medium.

Claims (4)

[Claims] 1. An optical recording medium having a recording layer provided on a substrate and a transparent dielectric layer provided on the substrate opposite to the recording layer, wherein the refractive index of the transparent dielectric layer decreases as the distance from the substrate increases. An optical recording medium characterized by the above-mentioned. 2. The optical recording medium according to claim 1, wherein the transparent dielectric layer comprises a plurality of transparent dielectric layers having different refractive indexes. 3. The thickness of the outermost transparent dielectric layer is λ / 7n to λ.
The optical recording medium according to claim 2, wherein / 3n (where λ is the wavelength of the reproduction light in air, and n is the refractive index of the outermost dielectric). 4. The method according to claim 1, wherein the thickness of the substrate is 0.8 mm or less.
4. The optical recording medium according to any one of items 1 to 3.