JPH09153230A - Optical disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide such an optical disk that has little warpage in the environmental temp. range for use and that causes no deterioration in the signal quality when the temp. changes. SOLUTION: This optical disk to be used for recording and reproducing by laser light is obtd. by forming a recording layer and a reflecting film 3 on the one principal surface of a plastic substrate 1 and forming a transparent film 4 on the other principal surface. The refractive index n1 of the transparent film 4 and the refractive index n0 of the substrate 1 are in the relation of n1 <n0 +0.5. Moreover when a dielectric film is formed at the position nearest to the substrate 1 and on the principal surface where the recording layer and the reflecting film 3 are to be formed the difference between the film thickness d1 of the dielectric film and the film thickness d2 of the transparent film 4 is specified to <=500Å.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disc on which information is recorded and reproduced by laser light such as a semiconductor laser.

[0002]

2. Description of the Related Art In recent years, optical discs have been attracting attention as large-capacity recording media, and various types of optical discs have been put into practical use in accordance with applications.

For example, as a recordable optical disk,
A magneto-optical disk using a rare earth-transition metal (RE-TM) alloy for the recording layer, and GeSbTe, SbBiT for the recording layer.
Phase change type optical disks using phase change materials such as e, and write-once optical disks capable of recording only once have already been commercialized.

A reproduction-only optical disc is one in which signal pits are formed in advance on a substrate and an Al reflection film for reproducing by reflection of laser light is formed on the substrate. A so-called compact disc on which a musical tone signal is recorded is recorded. (CD)
, A so-called CD-ROM in which data signals are recorded are widely used. In this playback-only optical disc,
There is also proposed a device in which a transparent protective layer is formed between the substrate and the Al reflective film so that the temperature of the laser irradiation portion does not rise and the substrate is thermally damaged during reproduction.

By the way, in recent years, along with the demand for higher density of optical discs, the wavelength of the laser used for recording and reproducing has been shortened. Here, if the wavelength of the laser used is shortened,
The beam profile on the film surface of the recording layer or the reflective film is likely to be distorted due to the warp of the disk or the like, and the reproduction signal is likely to be deteriorated or the crosstalk from the adjacent signal is increased. Therefore, the warp amount of the optical disk should be as small as possible.

However, the operating environment temperature of the optical disk is 50 ° C. including the temperature rise during use inside the drive.
A considerable temperature change is conceivable. For example, in an optical disk using a substrate made of a plastic material such as polycarbonate or polymethylmethacrylate (PMMA), the amount of warpage tends to change greatly with the change in the environmental temperature. As a result, in the optical disc using plastic for the substrate, deterioration of signal quality due to temperature change is a problem.

[0007]

As for the structure of a conventional optical disk, in the case of a recordable and reproducible structure, it is general that a dielectric protective film such as SiN is formed on a plastic substrate and then a recording layer is formed. Target. As a high-density optical disk for reproduction only, there has been proposed a structure in which a dielectric protective film is formed on a substrate and then a reproduction layer having a mask layer for performing super-resolution reproduction is formed.

The problem is that the plastic substrate and the film formed in contact with it have different coefficients of thermal expansion, and the amount of expansion at the operating temperature is different, so that the disk warps.

Therefore, an object of the present invention is to provide an optical disk having a small amount of warp in a use environment temperature range, and an object of the present invention is to provide an optical disk in which signal quality is not deteriorated due to temperature change.

[0010]

The inventor of the present invention has conducted various studies to achieve the above object. as a result,
We have come to the conclusion that the problem of warpage can be solved by forming a transparent film also on the surface side of the substrate.

That is, according to the present invention, in an optical disc on which recording and reproduction are performed by laser light, a recording layer and a reflective film are formed on one main surface of a plastic substrate, and a transparent film is also formed on the other main surface. Is characterized by the formation of.

The amount of warp of the optical disk is mainly determined by the operating temperature of the disk, the thickness of each of the plastic substrate and the film formed in contact with it, the coefficient of thermal expansion, the Young's modulus, and the like. Generally, polycarbonate or the like is used for the optical disk substrate, and the first layer film formed on this substrate is S
It is a dielectric film made of iN, SiO, AlN, etc., and the difference in the thermal expansion coefficient of these materials by about two orders of magnitude is the cause of the warpage accompanying the temperature change of the disk.

Therefore, in the present invention, in order to reduce the amount of warpage due to temperature change, the surface of the substrate opposite to the surface on which the recording layer and the reflective film are formed (hereinafter referred to as the recording / reproducing layer side) ( A transparent film having the same coefficient of thermal expansion and Young's modulus as the film of the first layer is formed on the surface side) to cancel the warp.

Here, examples of the material of the transparent film that can be industrially formed include oxides and nitrides of Si, Al, Ta, Ti and the like, and the dielectric film and the transparent film are arbitrary. It may be formed on the surface side of the substrate and the recording / reproducing layer side by the combination of the above materials.

However, it is important to determine the refractive index and the thickness of the transparent film formed on the surface side of the substrate.

First, in the optical system of an actual optical disk system, the laser light is not focused on the substrate surface,
If the difference between the refractive index of the transparent film on the front side and the refractive index of the substrate itself at the wavelength of the laser light used becomes large, the recording / playback of signals cannot be performed efficiently due to the effects of reflection and absorption at the boundary surface. And it becomes a noise source of the signal. Therefore, the refractive index n ₁ of the transparent film formed on the surface side of the substrate, the refractive index of the substrate is taken as _{n 0, n 1 ≦ n 0} +
It should be 0.5.

Regarding the film thickness, the film thickness of the dielectric film on the recording / reproducing layer side is usually determined so that a good signal reproduction can be achieved thermally and optically. At this time, the film thickness of the _first dielectric film on the recording / reproducing layer side of the substrate is d ₁ ,
When the film thickness of the transparent film formed on the front surface side of the substrate is d ₂ , the film thickness d ₁ is set in order to cancel the amount of warpage due to temperature change.
And the film thickness d ₂ should be made as small as possible, and it is desirable that the difference be 500 Å or less.

On the transparent film on the front surface side, a protective film having a refractive index substantially equal to that of the substrate may be additionally formed with an ultraviolet curable resin or the like in order to protect it or to prevent charging.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION A specific configuration example of an optical disc to which the present invention is applied will be described below with reference to the drawings.

As shown in FIG. 1, the optical disk is formed by forming a recording film for recording / reproduction or a reflective film for reproduction on one surface of a plastic substrate 1 made of a plastic material such as polycarbonate. .

The magneto-optical disk will be described as an example.
A recording film having a four-layer structure is formed on one surface of the plastic substrate 1. That is, from the side closer to the substrate 1, SiN
First dielectric layer made of, for example, rare earth-transition metal (RE-
TM) etc. magneto-optical recording layer, SiN etc. second
The dielectric layer is a reflective film made of Al or the like. However, here, for convenience, the first dielectric layer is shown in the drawing as the first layer film 2 and the other three layers are shown as the recording film 3. In a read-only optical disk, the recording film 3 may be replaced with a reflective film made of Al or the like.

If desired, a protective film made of an ultraviolet curable resin or the like may be formed on the recording layer 3.

On the other hand, the recording layer 3 of the plastic substrate 1
A transparent film 4 for canceling the warp is formed on the surface opposite to the surface on which is formed.

The transparent film 4 is made of Si, Al, Ti, Ta.
A material that is optically transparent and sufficiently transmits a laser beam for recording and reproduction is selected. Specifically, it is SiN or the like.

However, if the refractive index of the transparent film 4 is significantly different from the refractive index of the plastic substrate 1, the recording / reproducing characteristics may be adversely affected. Therefore, the refractive index is n ₁ , and the refractive index of the substrate 1 is ₁ . the when the _{n 0, n 1 ≦ n 0} +
It is set to be 0.5.

The film thickness of the transparent film 4 is determined in consideration of the film thickness of the first layer film 2 from the viewpoint of eliminating warpage. That is, the difference between the film thickness d ₁ of the first layer film 2 which is a dielectric film and the film thickness d ₂ of the transparent film 4 is 500Å
It is set to be as follows.

[0027]

EXAMPLES Specific examples to which the present invention is applied will be described in detail below based on experimental results.

Example 1 The optical disk prototyped in this example has a recording layer having a four-layer structure in which a rare earth-transition metal alloy is used for the recording layer, which is sandwiched by SiN dielectric films, and an Al reflection film is further added. It is a magneto-optical disk for recording and reproduction.

To protect the recording layer, an ultraviolet curable resin was applied by spin coating to form a protective film.

The plastic substrate is a polycarbonate substrate having a thickness of 1.2 mm and a diameter of 86 mm, and its refractive index n ₀ is 1.58 for laser light having a wavelength of 680 nm.

The film thickness d ₁ of the first layer SiN dielectric film (first layer film) on the recording / reproducing layer side is 1000Å.

On the surface side of this optical disk, the film thickness d ₂ = 1
A transparent film is formed by forming a SiN film having a refractive index n ₁ = 2.02 of 000Å by a sputtering method, and an antistatic protective film made of an ultraviolet curable resin having a refractive index of 1.58 is further formed thereon by spin coating. did. Each refractive index is a value in laser light having a wavelength of 680 nm.

Example 2 The refractive index n ₁ of the transparent film on the surface side is 2.08 and the film thickness d ₂ is 90.
A sample disk was prepared in the same manner as in Example 1 except that 0 Å.

Example 3 The refractive index n ₁ of the transparent film on the surface side is 2.02, and the film thickness d ₂ is 15.
A sample disk was prepared in the same manner as in Example 1 except that the value was 00Å.

Comparative Example 1 A sample disk was prepared in the same manner as in Example 1 except that the transparent film was not provided on the surface side.

Comparative Example 2 The transparent film on the surface side has a refractive index n ₁ of 2.1 and a film thickness d ₂ of 900.
A sample disk was prepared in the same manner as in Example 1 except for the above.

Comparative Example 3 The transparent film on the surface side has a refractive index n ₁ of 2.02 and a film thickness d ₂ of 40.
A sample disk was prepared in the same manner as in Example 1 except that 0 Å.

Table 1 below shows the refractive index and film thickness of the transparent film, the film thickness difference from the SiN dielectric film and the like in each of the above Examples and Comparative Examples.

[0039]

[Table 1]

Therefore, next, the amount of warp of the substrate is measured in an environment of a temperature of 20 to 60 ° C. and a relative humidity of RH of 50% for these sample disks.
The fluctuation range of the reflectance with respect to the average reflectance within one circumference of the disk was measured under the environment of H50%. This reflectance fluctuation range is a measure of the signal amplitude fluctuation amount. The warp amount and the reflectance fluctuation width were measured at a position with a radius of 41 mm, which is the outermost circumference data area, and the reproduction laser wavelength was 680 nm.

Table 2 shows the results. In the above measurement,
The target value is a warp amount of 3 mra from the viewpoint of signal quality.
d or less and the reflectance fluctuation width is 7% or less.

[0042]

[Table 2]

From this table, it can be seen that in each of the examples, the change in the amount of warp of the disk due to the temperature change is small, the fluctuation range of the reflectance is also suppressed small, and the target value is achieved. The sign attached to the amount of warp indicates the direction of the warp, and is positive when the warp is on the recording film side.

On the other hand, in Comparative Example 1 in which the transparent film is not provided, the warp amount is remarkably large, and even in Comparative Example 3 in which the film thickness difference between the transparent film and the SiN dielectric film is large, the warpage is sufficient. The target value has not been achieved.

The refractive index of the transparent film is n _{0 of the} substrate.
In Comparative Example 2 in which the value exceeds +0.5, the amount of warp of the disk is suppressed to a small value, but the fluctuation in reflectance is large.

[0046]

As is apparent from the above description, in the optical disc of the present invention, since the transparent film is formed on the surface of the substrate opposite to the surface on which the recording film and the reflective film are formed, It is possible to suppress the warp amount of the disc and improve the signal quality. Further, at this time, since the refractive index of the transparent film is set in an appropriate range, it is possible to suppress the adverse effect due to the reflectance fluctuation and the like.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a configuration example of a magneto-optical disk to which the present invention is applied.

[Explanation of symbols]

 1 plastic substrate 2 first layer film 3 recording film 4 transparent film

Claims (3)

[Claims] 1. An optical disc for recording and / or reproducing by a laser beam, wherein a recording layer and / or a recording layer are provided on one main surface of a substrate made of plastic.
Alternatively, a reflective film is formed and a transparent film is formed on the other main surface, where n ₁ ≤n ₀ +0, where n _{1 is} the refractive index of the transparent film and n ₀ is the refractive index of the substrate. An optical disk characterized by being 0.5. 2. A dielectric film is formed at a position closest to the substrate on one main surface on which the recording layer and / or the reflection film is formed, and the film thickness d _{1 of the} dielectric film and the transparent film are The optical disc according to claim 1, wherein the difference in film thickness d ₂ is 500 Å or less. 3. The optical disc according to claim 1, wherein a protective film is formed on the transparent film.