JP2000187886A - Optical disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high performance optical disk with little warpage of the disk and little thickness unevenness of a light transmissive film layer. SOLUTION: In the optical disk 1 from which information is read out with laser light LA, a light transmissive adhesive layer 4, a reflecting layer 3 and an information recording substrate 2 are successively laminated on a light transmissive film layer 5 on the readout face side. The coefficient of linear expansion of the light transmissive film layer 5 is nearly equal to that of the information recording substrate 2 and the difference in the coefficient of linear expansion in an arbitrary direction of the light transmissive film layer 5 is less than 20%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-density optical disk, and more particularly to an optical disk suitable for reproduction using a reproduction laser beam having a wavelength in a blue region.

[0002]

2. Description of the Related Art With the spread of CDs, that is, compact disks, optical disks have become general recording media. When this recording medium starts to be generally used, its use has been expanded due to its simplicity of handling, and an optical disc having a larger capacity has been required. ,
In response to such demands, recently, DVD (Digital
A high-density optical disc called Versatile Disc has been developed.

On the other hand, in the development of optical discs, the development of optical discs with higher density than DVDs is in progress. The manufacturing methods of these optical disks are approximately the same. First, a method for manufacturing an optical disk will be described using a CD as an example. The manufacture of an optical disc starts with a step of applying a resist on a glass disk which has been polished and cleaned. This glass disk has a larger diameter than the optical disk to be manufactured. The resist diluted with a diluent is applied to the surface of the glass disk using a spinner. The thickness of the resist to be applied is the same as the depth of the pit of the finished disk,
Or make it a little thicker than this. The glass disk coated with the resist is called a resist disk.

After the resist diluent has volatilized, the resist is transported to a cutting device to perform cutting. The cutting device consists of a laser element for recording pits on the resist disk, a modulator for modulating the laser beam from the laser element, and a laser beam for exposing the pits on the resist-coated glass disk. And a turntable for rotating the glass disk. The resist disc on which the signal is exposed as an aggregate of pits on the resist is conveyed to a developing step, where the resist is developed with an alkaline solution to form pit shapes. The surface of the developed resist disk is covered with a thin conductive film such as nickel after washing and drying.

Next, the resist disc is electroformed in a plating solution. The thickness of this electroforming is a thickness suitable for a mold used at the time of molding, and is about 0.3 mm. After the completion of this electroforming, the electroformed product is peeled off from the resist plate, the surface of the electroformed product that has been in close contact with the resist is washed, and the back surface is polished so that the electroformed product has a size that can be easily mounted on a mold. The inner and outer circumferences are machined. This is generally called a stamper. As in the case of a conventional audio record, the electroformed product is called a master, and a replica of the electroformed product may be created from the master as a mother and a stamper. The stamper is mounted on an injection molding die, and is molded by an injection molding machine to have a specified disk diameter and thickness. A hole is made in the center of the disk substrate in the molding die. The pit having the same shape as the pit stamped by cutting is transferred to the information recording surface side of the disk substrate taken out from the mold of the molding machine.

In the next step, a high-reflectance metal thin film such as aluminum is coated on the information pit surface of the disk substrate to a thickness of about 50 nm by a sputtering device or the like.
Thereafter, a protective film is coated to a thickness of about 10 μm to prevent corrosion and damage of the reflective film. The protective film is made of a low-viscosity ultraviolet curable resin monomer for easy operation, and is applied to the surface of the disk substrate by a spinner. After this resin monomer has been applied to the disk substrate surface, the applied ultraviolet resin monomer is cured using an ultraviolet lamp.
After that, a label is formed on the protective film of the disk substrate by printing to complete the optical disk. This label displays information in the disc and is often formed by screen printing.

By the way, information on an optical disk is reproduced by using an optical pickup having a wavelength suitable for reproduction. A lens called an objective lens for focusing the reproduction laser light on the recording surface of the optical disk is mounted in the optical pickup. At the time of reproduction, laser light emitted from the optical pickup enters from the opposite side of the information recording surface of the optical disk, passes through the disk substrate, is focused on the information recording surface, and is modulated by pits on the disk substrate. . The reason for allowing the reproduction laser beam to pass through the disk substrate in this way is to reduce the influence on the reproduction signal with respect to dirt and scratches generated on the disk surface. By detecting the light reflected by the pits and returning to the optical pickup again, the information in the disc can be read.

At the time of reading, the read information signal is degraded due to the warpage of the disk substrate and unevenness in the thickness of the substrate. The amount of signal deterioration is related to the numerical aperture of the objective lens used in the optical pickup, as long as the wavelength of the laser beam of the pickup is the same. That is, the larger the numerical aperture of the objective lens, the smaller the pit size on the disk information surface that can be read, and the higher the recording density of the disk. It is necessary to keep it small by the cube of the numerical aperture of the lens. On the other hand, thickness unevenness with respect to the reference thickness of the disk substrate needs to be kept small by the fourth power of the numerical aperture of the objective lens. Here, assuming that the wavelength of the laser beam used for reproduction is λ, the numerical aperture of the objective lens of the optical pickup is NA, the thickness of the substrate through which the read laser beam passes is t, and the thickness unevenness is Δt, The tolerance to the thickness unevenness is defined as in the following Expressions 1 and 2.

Tolerance for warpage Δλ / (t × (NA) ³ ) Expression 1 Tolerance for uneven thickness Δλ / (Δt × (NA) ⁴ ) Expression 2 Expressions 1 and 2 expressed above The relationship 2 narrows the tolerance.
In a DVD having a higher recording density than a CD, the thickness of a substrate through which a reading laser beam passes is set to half of that of a CD in order to increase the tolerance for warpage and more stably reproduce recorded information. I have.

[0010]

As described above, in order to further increase the recording density, the thickness of the substrate through which the reading laser beam passes may be further reduced. JP-A-8-235638, JP-A-9-1996
No. 1,345,47. However, although the techniques disclosed in these publications can reduce signal degradation due to the thickness of the substrate, they have a drawback in that signal degradation due to thickness unevenness cannot be reduced.
In particular, in the future high-density discs, the wavelength of the reproduction laser light tends to be short and the numerical aperture of the objective lens tends to be large. For example, λ = 430 nm and NA = about 0.8 are common.

As a device for reducing the thickness unevenness,
According to the technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 9-13447, weirs are formed on the inner and outer peripheral portions of a disk substrate, and an ultraviolet curable resin monomer is dropped therein and rotated with a spinner to obtain a uniform film thickness. After that, it is cured with an ultraviolet lamp. However, in this case, if the thickness of the light transmitting film through which the reading laser beam is transmitted is as large as 100 μm, the cured disk substrate may be warped due to the curing shrinkage of the ultraviolet curable resin, or the light amount of the ultraviolet irradiating device at the time of curing. Due to the unevenness, the thickness unevenness after curing of the ultraviolet curable resin increases,
This is rather inconvenient for high-density discs.

On the other hand, when an experiment was conducted with a configuration in which an adhesive was applied to a polycarbonate film and bonded to a disk substrate as partially described in the above-mentioned Japanese Patent Application Laid-Open No. Hei 8-235638, Although this was improved, this disk was stuck in a room at 20 ° C.
When reproduced in a reproducing apparatus at a temperature of ° C., a large warp occurs in the disk, which is considerably inconvenient as a high-density optical disk. As described above, each of the members constituting the high-density optical disk can be practically obtained only at a level that causes a problem even if the technology disclosed in the known publication is used. The present invention focuses on the above-mentioned problems, and was created to solve the problem effectively. The purpose of the present invention is to provide an optical disc for reproducing information through a light-transmitting film layer formed on the surface of a disc. It is an object of the present invention to provide a high-performance optical disk having less warpage and less unevenness in the thickness of a light-transmitting film layer.

[0013]

The inventor of the present invention has conducted intensive studies on the causes of warpage and uneven thickness of an optical disc, and as a result,
In order to suppress these, when a sheet with small thickness unevenness is read and used as a light-transmitting film layer (protective layer) through which laser light is transmitted, the sheet used has a linear expansion coefficient within a certain range in any direction. As long as it has
Thus, the present invention has been accomplished by obtaining the above-mentioned knowledge. Of course, this film and the information substrate surface may be bonded using a generally used adhesive.

According to a first aspect of the present invention, there is provided an optical disc for reading information by using a laser beam, wherein a light-transmitting adhesive layer is provided on the light-transmitting film layer on the reading surface side.
The reflective layer and the information recording substrate are laminated in order, the linear expansion coefficient of the light transmitting film layer and the information recording substrate is substantially the same, and the linear expansion coefficient of the light transmitting film layer in any direction. The difference is made less than 20%. Further, as defined in claim 2, the maximum ratio (%) of the difference in linear expansion coefficient in an arbitrary direction of the light transmitting film layer satisfies the following expression. Maximum ratio of linear expansion coefficient difference (%) x thickness of light-transmitting film layer / thickness of information recording substrate <2

[0015] Thereby, the directional dependence of the linear expansion coefficient of the light transmissive film layer itself is reduced, and the difference in linear expansion coefficient between the light transmissive film layer and the information recording substrate is also optimized. It is possible to prevent the optical disc itself from warping or the light transmitting film layer from being uneven in thickness.
Therefore, the reproduction characteristics can be improved while maintaining high-density recording.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the optical disk according to the present invention will be described below in detail with reference to the accompanying drawings. 1 is a partially enlarged cross-sectional view showing an optical disk according to the present invention, FIG. 2 is a diagram showing a state in which a sample is cut out from a light-transmitting film layer formed in a disk shape, and FIG. 3 is a linear expansion between the samples shown in FIG. FIG. 4 is a graph showing the difference between the coefficients, FIG. 4 is a diagram showing a state in which a sample is cut out from a sheet-like light-transmitting film layer, FIG. 5 is a graph showing the temperature dependence of the elongation of the sample shown in FIG. 4, and FIG. 7 is a graph showing warping states of the example and the comparative example. As shown in FIG.
Are an information recording substrate 2, a reflective layer 3, and a light-transmitting adhesive layer 4.
And the light transmissive film layer 5 are sequentially laminated. Then, the light transmissive film layer 5 functions as a protective film, and this surface becomes the readout surface 5A. Therefore, in FIG. 1, the laser beam LA for reading information emitted from the optical pickup arrives from above, is focused by the objective lens 6, and passes through the light transmitting film layer 5 and the light transmitting adhesive layer 4. As a result, the light reaches the reflective layer 3.

The information recording substrate 2 is made of, for example, a polycarbonate resin plate, and information pits 7 are formed on the laminating surface side to form an information recording surface. The reflection layer 3 is made of, for example, a sputter-deposited aluminum film. The light-transmitting adhesive layer 4 is made of a low-viscosity monomer such as an ultraviolet-curable resin, or an adhesive sheet type (DA-8310, Nitto Denko Corporation). Further, the light-transmitting film layer 5 which is a feature of the present invention is made of, for example, a sheet made of a polycarbonate resin stretched by various molding methods. It is adhered to the information recording substrate 2 with the light transmitting adhesive layer 4 interposed. Regarding the thickness of each layer, the information recording substrate 2 is about 1.0 to 1.2 mm, the reflective layer 2 is about 60 nm, the light transmitting adhesive layer 4 is about 30 μm,
The light transmitting film layer 5 has a thickness of about 0.1 to 0.5 mm.

Accordingly, since the light-transmitting film layer 5 is a film formed in a sheet shape in advance, the thickness of the light-transmitting film layer 5 itself is very small and uniform. The light transmissive film layer 5 is set so as to have substantially the same linear expansion coefficient as that of the information recording substrate 2, and has a difference in linear expansion coefficient in an arbitrary direction of the light transmissive film layer 5. Is set to be smaller than 20%, whereby the stress generated due to the difference in shrinkage becomes extremely small, and the amount of warpage of the disk itself can be largely suppressed.

The maximum ratio (%) of the linear expansion coefficient difference in an arbitrary direction of the light transmitting film layer 5 is set so as to satisfy the following equation. Maximum ratio of linear expansion coefficient difference (%) x thickness of light-transmitting film layer / thickness of information recording substrate <2

Here, the formation of the light transmitting film layer 5 will be described in more detail. As described in the related art, in order to increase the recording density of an optical disk, it is necessary to shorten the wavelength of the reproduction laser beam and increase the numerical aperture of the objective lens. Since it is necessary to shorten the distance from the information recording surface, it is preferable to make the light transmitting film layer 5 from the reading surface side to the information recording surface thin. When the light-transmitting film layer 5 becomes thinner, the light-transmitting film layer used is covered with a low-viscosity monomer such as an ultraviolet-curable resin, as described in the above-mentioned publication, and then cured to form a film layer. And a method of attaching a thin film to form a film layer. First, a method of attaching a thin film will be described.

Here, what is called a film refers to a film having a thickness of 1 mm or less. In order to produce a film having such a thickness, there are generally a T-die molding method, an inflation molding method, and a calendar molding method. Further, in order to impart strength to the film, the film may be subjected to a stretching treatment after the molding, and this stretching treatment includes a uniaxial stretching treatment and a biaxial stretching treatment. If the thickness of the film is 0.2 mm or more, there are also a compression molding method and an injection molding method. The films manufactured by the various molding methods as described above have their respective features, and when considered to be used as the light-transmitting film layer 5 of an optical disc, there are those that can be used well and those that are extremely inconvenient. . As a result of repeating experiments using various molded films, the present inventor found that it is preferable to use the light-transmitting film layer 5 of the optical disk because the difference in linear expansion coefficient of the film depending on the direction is within a predetermined range, that is, 20%. %, Which is within a range smaller than%, and that the coefficient of linear expansion of the information recording substrate 2 needs to be substantially the same. The coefficient of linear expansion of a film greatly changes depending on the history of forming the film. The difference in the coefficient of linear expansion of the film is a substitute characteristic not only when the coefficient of linear expansion is measured but also when the Young's modulus, shrinkage, birefringence, and the like are measured. In general, the difference in the coefficient of linear expansion becomes remarkable above the glass transition point of the film.

The most suitable film is a film made of the same material as the material of the information recording substrate by the same molding technique. For example, the information recording substrate is generally formed by injection molding using a polycarbonate resin. In this case, the coefficient of linear expansion of the substrate is slightly different between the diameter direction of the substrate and the circumferential direction. Further, as a characteristic of the molding, the linear expansion coefficient having a width of about 2 mm on the outermost circumference of the substrate is also slightly different. As described above, the variation of the coefficient of linear expansion in the plane of the substrate is caused by the difference between the distribution of the coefficient of linear expansion of the light-transmitting film layer 5 laminated on the information recording surface 2 and the variation in the coefficient of warpage in the disk. Becomes When the most suitable method is used, a thin molded product sheet of about 0.1 mm to 0.5 mm must be used as the light transmitting film layer.

The fact that the light transmissive film layer 5 has a different linear expansion coefficient depending on the direction will be specifically described. FIG. 2 shows a light-transmitting film layer obtained by cutting a light-transmitting sheet into a disk shape. Eight radially elongated samples are cut out from the light-transmitting sheet so as to differ from the center by 45 °. I do. These eight samples A1
The difference between the linear expansion coefficients of A8 to A8 is measured, and the difference is determined. The result is shown in FIG. On the basis of the sample A1, the difference between the linear expansion coefficients changes like a sine curve. In the example of the light transmissive film layer 5, the difference in the linear expansion coefficient is as large as 40% at the maximum, and the magnitude of the difference is related to the magnitude of the warpage of the disc itself. As described above, a film having a difference in linear expansion coefficient of less than 20% is used as the light transmitting film layer 5 of the present invention.

The variation in the warpage of the optical disk formed by the bonding is often caused mainly by the temperature change, but may also be caused by the distribution of the amount of water absorbed in the material. For example, the substrate and the light transmitting film layer
If the film is biaxially stretched when bonded in an atmosphere of 0 ° C., the distribution of the warp of the entire optical disk may be as large as 0.5 mm at maximum in an atmosphere of 10 ° C. in a severe case. Referring to FIG. 4 and FIG. 5, the elongation of the film with respect to the temperature change will be described. As shown in FIG. 4, from the 0.1 mm thick film 8 biaxially stretched by a calender, the stretching direction is orthogonal to the stretching direction. 2 in the direction
Cut out two elongated samples, and use this as sample B1,
B2. The elongation of the two samples B1 and B2 with respect to the temperature change fluctuates greatly as shown in FIG.
It also differs considerably depending on the cutting direction. FIG.
In the description, the extension amount of the polycarbonate substrate is also described for reference.

As described above, depending on the material used as the film, the warpage also changes in proportion to the change in the temperature condition of the environment in which the disk is used. This variation is generally referred to as a disk warpage angle or surface runout. Currently commercially available CDs and the like have a UV-curable resin coated as a protective film on the substrate on which the information recording surface is formed. In this case, the protective film has a thickness of 10 μm or less and There is no problem because it is sufficiently smaller than 1.2 mm. In the structure of the optical disk according to the present invention, for example, the thickness of the information recording substrate 2 on which the information recording surface is formed is about 1 mm, while the thickness of the light transmitting film layer 5 on the reading surface side to be bonded is 0 mm. 0.5mm to 0. Since the thickness is as large as about 1 mm, the disadvantage that the amount of warpage increases as described above occurs.

The method for producing the film used here and the characteristics when used in the present invention will be described. Generally used methods include extrusion molding, inflation molding, and calender molding. Extrusion molding is a process in which a plastic material is supplied to an extruder, which is extruded from a mold and converted into a continuous body having a specific cross-sectional shape. When forming a sheet, the mold includes a fishtail die, a T-die, and the like. The thickness of the sheet can be made from 0.1 mm to several mm. When used for an optical disk, the surface needs to be smooth, so it is desirable to perform a cooling and winding step after passing through a device such as a glazing roll. The film produced in this way has a small variation in the coefficient of linear expansion in each direction of the sheet, and is suitable for an optical disk.

The inflation molding method is also referred to as hollow molding, in which a plastic is preformed in a tube shape, this is sandwiched between molds, air is blown into the inside, inflated, cooled and solidified to form a film. Since the film becomes cylindrical, it is formed into a plate shape by cutting it into two pieces. At this time, a smooth guide plate can be used for an optical disk if it is not stretched too much. The film thickness is 1
Those having a thickness of about 0 μm to 1 mm can be prepared. A film produced in this manner, for example, a film having a low tensile speed can be used as a light transmissive film layer because the coefficient of linear expansion in the film plane varies little.

A calender molding method is commonly used. The kneaded and softened plastic is rolled by a roll called a calender roll to obtain a film having a predetermined thickness. The thickness of the sheet formed by the calendering method is from about 5 μm to 1 mm, and is characterized by smoothness and no thickness unevenness. However, in this forming method, since rolling is performed by a roll, a characteristic is that a linear expansion coefficient generally differs in a length direction and a width direction of a sheet. Therefore, it is not suitable for the purpose of bonding to the disk surface as in the present invention. However, stretching is not performed much, and as a result, the sheet is processed so as not to have anisotropy in the linear expansion coefficient in the length direction and the width direction, or the sheet is near the glass transition point or higher. At a temperature, post-processing such as removing anisotropy due to the processing can be performed and it can be used. Materials processed on a sheet by various processing methods as described above can be used as the light-transmitting film layer 5 on the reading surface side of the optical disk.

As described above, there are various sheet forming methods. Regardless of the method used, the method for forming the information recording surface of the information recording substrate is generally based on the injection molding method. It is desirable that the expansion coefficient has the same distribution as that of the substrate obtained by the injection molding method. Generally, the information recording substrate 2 on which the recording surface is formed and the light-transmitting film layer 5 on the reading surface side are adhered by an adhesive composed of the light-transmitting adhesive layer 4. If the adhesive is applied at any temperature below the operating temperature of the disk, no serious problem will occur even if the adhesive is applied by any method, but if the glass transition point of the adhesive is equal to or higher than the operating temperature of the disk, it will be described in Examples. Care must be taken that the coefficient of linear expansion of the adhesive does not differ in the plane of the disk.

Here, the optical disks according to the present invention were actually manufactured as Examples 1 and 2, and will be described together with Comparative Example 1. <Example 1> An information recording substrate 2 was prepared by injection molding using a polycarbonate resin. A mold for exclusive use of an optical disk having a built-in component having a flat plate on which an information signal of an optical disk is engraved is called a stamper. The diameter of the formed information recording substrate 2 is 12 cm, and the thickness is 1.
2 mm. One surface of the substrate 2 is an information signal surface on which an information signal is formed. An aluminum reflective film was formed on the information signal surface by a sputtering method. The thickness of the reflective layer 3 was 60 nm.

On the other hand, the polycarbonate resin was molded into a sheet using a compression molding machine. The mold used was a mirror-polished one having a flat surface. The mold temperature was initially 200 ° C., then cooled to 80 ° C. with cooling water and the molded product was taken out. The thickness of this sheet was 0.2 mm. This sheet is centered around the center of the molding die,
The light-transmitting film layer 5 was formed by cutting out a circle having a diameter of 12 cm. The sheet produced here had a coefficient of linear expansion different by 2% between the length and width of the sheet. Then, an ultraviolet curable resin was applied as an adhesive on the information recording substrate 2 having the aluminum reflective layer 3 by a spinner. The application location is the middle part of the disk, the application status is width 1c
m and a thickness of about 6 mm. After applying the ultraviolet curable resin, the light-transmitting film layer 4 having a thickness of 0.2 mm cut out from the compression-molded sheet is placed thereon, and is rotated by a spinner. Was filled with the ultraviolet curable resin. Thereafter, the disk was taken out and irradiated with ultraviolet rays to cure the resin. The thickness of the adhesive layer 4, which is the UV-curable resin layer after curing, was 30 μm. The thickness on the reading surface side of the combination of the polycarbonate sheet and the adhesive layer 4 was 0.22 mm. This optical disk had little warpage of the warpage, and the change in the disk warp angle was about 0.1 degree even at a high temperature of 70 ° C.

Example 2 A sheet having a thickness of 0.2 mm obtained by extrusion was cut into a diameter of 12 cm to form a light transmitting film layer 5. This light transmissive film layer 5 was adhered to the information recording substrate 2 with the reflective layer 3 used in Example 1 using a spinner in the same manner as in Example 1 using an ultraviolet curing resin. The sheet used here had a coefficient of linear expansion of 5% different between the length and width of the sheet. The prepared optical disk had little warpage similarly to the first embodiment. Also, 70
At a high temperature of ° C., the warp angle of the optical disk was about 0.2 degrees, and it was possible to reproduce information from the optical disk.

Comparative Example 1 A sheet having a thickness of 0.2 mm produced by calendering was cut to a diameter of 12 cm to form a light-transmitting film layer, and an optical disk was produced in the same manner as in Example 2. The difference between the vertical and horizontal linear expansion coefficients of this sheet was 20%. The warpage of the disk after the optical disk was formed was small, but at a high temperature of 70 ° C., the disk had a warp angle of one degree, and information could not be reproduced. Here, in Examples 1 and 2 and Comparative Example 1, the thickness of the information recording substrate was 1.2 mm, the thickness of the light transmitting film layer was 0.2 mm, and the difference in linear expansion coefficient was 2 mm. %, 5% and 20%. When each of the above numerical values is applied to the expression defined in claim 2 as described above, Example 1 is 2 × 0.2 / 1.0 = 0.33
In Example 2, 5 × 0.2 / 1.2 = 0.83, which is smaller than 2. On the other hand, Comparative Example 1
× 0.2 / 1.2 = 3.33, which is larger than 2. Therefore, if the above expression is satisfied, a good reproduced signal can be obtained.

As described above, when the linear expansion coefficients in the respective directions of the light-transmitting film layer used on the reading surface side of the optical disc are largely different, the information on the disc cannot be reproduced in the use environment of the optical disc. Therefore, as shown in the present invention, by specifying the difference in linear expansion coefficient between the sheet-shaped light-transmitting film layers used to be smaller than 20%,
The information on the optical disk can be normally reproduced even in a high temperature environment. FIG. 6 shows the results of the changes in the warp angles of Examples 1 and 2 and Comparative Example 1 described above. Each numerical value in the present embodiment is merely an example, and the present invention is not limited to this. Further, the layer structure of the optical disk is not limited to that described in the present embodiment, and it goes without saying that the present invention can be applied even if another layer is laminated.

[0035]

As described above, according to the optical disk of the present invention, the following excellent operational effects can be exhibited. By defining the coefficient of linear expansion using a sheet-like material formed in advance as the light-transmitting film layer on the information reading surface side, thickness unevenness of the light-transmitting film layer itself and warpage of the optical disc itself can be suppressed. Therefore,
Good reproduction characteristics can be obtained while maintaining a high information recording density.

[Brief description of the drawings]

FIG. 1 is a partially enlarged sectional view showing an optical disc according to the present invention.

FIG. 2 is a diagram illustrating a state in which a sample is cut out from a light-transmitting film layer formed in a disk shape.

FIG. 3 is a graph showing a difference in linear expansion coefficient between the samples shown in FIG. 2;

FIG. 4 is a diagram showing a state in which a sample is cut out from a sheet-like light transmitting film layer.

5 is a graph showing the temperature dependence of the elongation of the sample shown in FIG.

FIG. 6 is a graph showing a warped state of an example of the present invention and a comparative example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical disk, 2 ... Information recording board, 3 ... Reflective layer, 4 ...
Light transmissive adhesive layer, 5 light transmissive film layer, 5A readout surface, LA laser light.

Claims (2)

[Claims] 1. An optical disc for reading information using a laser beam, wherein a light-transmitting adhesive layer, a reflective layer, and an information recording substrate are sequentially laminated on a light-transmitting film layer on a reading surface side, An optical disk, wherein a light-transmitting film layer and the information recording substrate have substantially the same linear expansion coefficient, and a difference in a linear expansion coefficient in an arbitrary direction of the light-transmitting film layer is smaller than 20%. 2. The optical disk according to claim 1, wherein a maximum ratio (%) of a difference in linear expansion coefficient in an arbitrary direction of the light transmitting film layer satisfies the following expression. Maximum ratio of linear expansion coefficient difference (%) x thickness of light-transmitting film layer / thickness of information recording substrate <2