JPH10302329A - Manufacture of optical recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a medium with stable and small warp irrespective of dispersion of stress in the process even when a thin plastic substrate is used and to relax restriction on the design of medium by adding such a force as to form flatness at a fixed time to settle the deflection irreversibly onto an optical recording medium. SOLUTION: A substrate 1, in a state that required layers such as a recording film, a protection coating, a coating on the optical incident plane and the like are formed on a polycarbonate base material, is held by flatening members 2 and then is pressed with a pressing force 3. The flatening members 2 are equipped with soft structure such as teflon and the like on the surface contacting the substrate 1 so that the force is uniformly added and the substrate 1 is not damaged not contaminated. The pressure is set on 1-6 kg for the substrate 1 with an outer diameter of 120 mm. In order to secure a sufficient enough flatening, pressure is applied for more than 30 minutes. In order to shorten the time of pressing, the substrate 1 is heated. Preferably, the heating temperature T is set in a range (Tg-80 deg.C)<=T<=(Tg-20 deg.C) as for the glass transmission temperature Tg of the base material plastics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical recording medium used for recording information.

[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
(Laser disk), magneto-optical disks for data recording, phase change disks, and the like 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, light is normally incident from the substrate side using a transparent substrate in order to reduce the influence of dust or scratches on the surface. 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.

With the recent increase in the capacity of optical disks, attempts have been made to increase the numerical aperture (NA) of the objective lens for recording / reproducing light so as to condense light more. The diameter of the focused spot is inversely proportional to the NA of the lens. Therefore, if the NA is increased, the diameter of the focused spot can be reduced, and finer recording and reproduction can be performed. However, with the increase in NA, there has been a problem that the allowable value for substrate warpage is significantly reduced. That is, if the substrate is warped, aberration occurs in the condensed spot, causing problems such as a decrease in the resolution of the reproduction signal and an increase in the influence (crosstalk) of the signal of the adjacent track. Since this aberration increases in proportion to the cube of NA, N
As the value of A increases, the allowable value of the warpage decreases. To solve this problem, it has been proposed to reduce the thickness of the substrate. For example, in a DVD (digital video disc) that has recently been commercialized, the thickness of the substrate is set to 0.6 mm to increase the allowable range for warpage.

However, when the substrate thickness is reduced in this way,
The rigidity of the substrate itself is significantly reduced. In the optical recording medium, the stress from the recording film, the stress due to the protective coating of the ultraviolet ray curable resin or the like on the recording film, and in some cases, the stress from the coating agent provided on the light incident side also act on the substrate. As a result, due to a decrease in the rigidity of the substrate, remarkable warpage occurs due to these stresses, and the warpage becomes larger than the increase in the allowable value of the warpage due to the reduction in the thickness of the substrate. Of course, it is possible in principle to design the substrate to be finally flat, including all of the warpage in forming the substrate, the film stress of the recording film, the stress of the coating agent, and the like. However, such a design severely limits the allowable range of the media configuration in consideration of other characteristics, and the slight variation in stress during the media manufacturing process causes
There is a concern that ultimately a large warpage variation may be caused, resulting in a significant reduction in manufacturing yield.
Such a problem may occur when two substrates are bonded to each other, but is particularly remarkable when a single substrate is used.

[0006]

The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, by applying a force for flattening the optical recording medium for a certain period of time, the stress in the previous process is reduced. It has been found that a flat shape can be formed irrespective of the variation of. The gist of the present invention is that, in a method of manufacturing an optical recording medium using a resin substrate, the method includes a step of applying a force such that the shape of an information recording area becomes flatter on the substrate after the substrate is formed (force applying step). The method for manufacturing an optical recording medium is characterized by the following.

[0007]

The substrate of the optical recording medium used in the present invention is not particularly limited as long as it is a transparent resin substrate, but polycarbonate, polymethyl methacrylate (PMMA) and the like are preferably used, and polycarbonate is particularly preferable. . The present invention is characterized in that after forming such a resin substrate, a force applying step for applying a force for flattening the shape during a manufacturing process of the optical recording medium for a certain time is provided. In these resins, deformation is fixed irreversibly by applying a certain amount of force even at a temperature below the glass transition point, so that this warping process can greatly reduce the warpage value of the medium. It is. In other words, the resin substrate originally has a structure in which molecular chains are entangled randomly.
Even when the position of the molecular chain is slightly shifted and deformed, it can be easily stabilized. Conventionally, it has been considered that such deformation should be suppressed so as not to cause substrate distortion in the optical recording medium manufacturing process. However, in the present invention, this deformation is rather positively used to correct a distorted medium into a state of small warpage.

Although the warpage of the medium is particularly problematic when the substrate is thin as described above, the present invention is more effective when used for such a thin substrate. That is, in a thin substrate, the rigidity of the substrate itself is low, and the substrate can be flattened with much smaller pressure and time than in a case where the substrate is thick. Specifically, the present invention is preferably applied when a substrate having a substrate thickness of 0.8 mm or less is used. More preferably, it is 0.7 mm or less.

Although the application of the pressing step is effective in any part of the optical recording medium manufacturing process, in order to reduce the final warpage, the recording film, the protective coating, and the light incident surface are formed on the substrate. It is preferable to perform it as a final step after forming all necessary films such as a side coat. In the following description, the case where only the substrate is used and the case where some layers are stacked on the substrate are collectively referred to as a substrate.

Various methods such as a vacuum suction method and a high-speed rotation method can be used as a method of applying a force in the force applying step in the present invention. As shown in FIG. 1, there is a method in which the substrate 1 is sandwiched between both sides by the flattening members 2 and a pressure 3 is applied. Hereinafter, the case of using this method will be described as an example, but conditions such as temperature and time can be applied to other methods as described above. Such a flattening member 2 has a relatively flexible structure such as rubber or plastic at a contact point with the substrate 1, and a force is applied to a minute area of the substrate to deform the substrate unevenly, thereby distorting the substrate. Alternatively, it is preferable to prevent the substrate from being damaged. In particular, it is preferable to use a fluorine-based resin such as Teflon in order to prevent contamination of the substrate. Also,
It is preferable that the shape of the flattening member be round at the end of the contact portion with the substrate in order to make the change in pressure applied to the substrate gentle. Of course, it is also possible to use glass, metal, or the like with the above points in mind.

Further, in order to prevent the recording surface of the substrate from being scratched or contaminated, a force is applied to a portion other than the recording region as shown in FIG. Is preferred. The application time differs depending on the temperature and the magnitude of the applied force. Generally, the lower the temperature,
In addition, the weaker the force, the longer the force is required. In order to perform sufficient flattening, it is preferable that the application time is 5 minutes or more. More preferably 10 minutes or more, particularly preferably 30 minutes
It is a force time of more than a minute. However, if the length is too long, the productivity is reduced and the substrate may be cracked. Therefore, the time is preferably 10 hours or less.

As a method of applying a force to the substrate, there is a method of fixing one or both of the inner periphery and the outer periphery by vacuum suction as described above. You may use together vacuum suction and the force by a flattening member. In the present invention, it is preferable to heat the substrate in order to reduce the time required for flattening. The substrate is heated during or before the pressurizing step. If the temperature is raised to a temperature equal to or higher than the glass transition point, the shape can be easily flattened. In this case, however, minute grooves or holes (pits) formed on the substrate surface may be deformed. Therefore, the heating temperature during the pressing step is preferably lower than the glass transition point. However, if the heating temperature is too low, the substrate is not flattened, or a significant time or force is required for flattening, which is not preferable. Therefore, the preferable maximum temperature T during the pressing step is determined with respect to the glass transition point Tg of the resin substrate.

[0013]

## EQU2 ## The range is Tg-90 ° C. ≦ T ≦ Tg−10 ° C. More preferably,

[0014]

Tg−90 ° C. ≦ T ≦ Tg−20 ° C., particularly preferably

[0015]

## EQU4 ## Tg−80 ° C. ≦ T ≦ Tg−20 ° C.

## EQU1 ## When heating the substrate, it is easy to put the substrate into a heating furnace, but there is also a method using a flattening member whose contact portion with the substrate is heated. In this case, the flattening member is preferably made of a metal having a high thermal conductivity. If the pressure is removed while the substrate temperature is still high, the substrate may warp again due to a subsequent temperature change. For this reason, it is preferable to stop the pressurization after the substrate temperature is sufficiently lowered. Specifically, it is preferable to stop pressurizing after the temperature has dropped to Tg-80 ° C or lower, and more preferably to stop pressurizing after the temperature has dropped to Tg-90 ° C or lower.

Since the substrate expands upon heating, in the case of a disk-shaped substrate having a center hole, the outer peripheral end faces outward and the inner peripheral end faces inward. If the movement of the end portion is hindered by a flattening member or the like, strong distortion may be irreversibly generated in the substrate. In this case, the occurrence of birefringence adversely affects the recording / reproducing characteristics. Therefore, it is preferable that the movement of the end is not hindered in any of the force methods. For this purpose, it is preferable that the surface of the flattening member is smooth and the edge of the substrate is not caught. In this sense, a fluororesin having excellent surface lubricity is preferable.

The force applied to the substrate is, for example, an outer diameter of 12
0.8kg or more, 7k for the whole substrate of 0φ and inner diameter 15φ
g or less. More preferably 1 kg
Not less than 6 kg. In the case of other substrates, a force in a range proportional to the area may be applied to this value. If the force is too low, the flattening effect will be reduced.
If the force is too large, the substrate is likely to be scratched or distorted, which adversely affects recording and reproduction. Further, there is a possibility that the birefringence of the substrate is increased and the recording / reproducing light is distorted.
The pressure per area is preferably 70 g / cm ² or less in order not to locally generate distortion in the substrate.
More preferably, it is 60 g / cm ² or less. The pressure may not be constant, but may change over time.
Since the substrate tends to be distorted in a high temperature state, it is preferable to increase the pressure as the temperature decreases.

Examples of the recording medium using the manufacturing method according to the present invention include a read-only medium having pits in a substrate, a write-once medium using deformation of a recording layer such as a dye, a recording medium having a phase change recording layer, Examples include a recording medium having a magneto-optical recording layer. Among them, it is particularly effective when a single substrate is used as the final form. It is particularly preferable to use it for a magneto-optical recording medium that requires the use of a single substrate. Of course, even if it is used for the production of a medium using two substrates bonded together, there is a sufficient effect. The applying step in this case may be before or after laminating.

According to the present invention, the warpage of the substrate can be considerably reduced, but cannot be completely eliminated. Therefore, it is preferable to adjust the average value of the variation of the substrate warpage to be close to zero by combining all the warpage during the molding of the substrate and the stress caused by the recording film, the resin coating, and the like even in the state without the force applying step. . In particular, it is preferable to apply an ultraviolet curable resin coat or an inorganic coat such as silicon oxide on the light incident surface side of the substrate so as to cancel the stress on the recording film side. According to the present invention, since the substrate warpage can be greatly reduced as compared with the conventional method, the restriction on the design of the medium is greatly relaxed.

[0021]

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 invention.

[0022]

[Example 1] A substrate having a diameter of 120 mmφ and a center hole diameter of 1 was used.
5mmφ, substrate thickness 0.6mm, groove depth 70nm, 1.2
A polycarbonate substrate having a land portion and a groove portion having the same width at a μm track pitch was used. The glass transition temperature Tg of this polycarbonate resin was 135 ° C.

On the substrate, 8
A 0 nm tantalum oxide was formed. Next, on the tantalum oxide, an 80 nm reproducing layer made of Gd ₂₈ (Fe ₅₀ Co ₅₀ ) _{72 and} a 40 nm recording layer made of Tb ₂₃ (Fe ₈₀ Co ₂₀ ) ₇₇ were provided. Finally, an 80 nm protective layer made of SiN was provided. On the sputtered film and on the light incident surface of the substrate, a protective coating of a 5 μm-thick ultraviolet-curable resin was provided. The maximum value of the warp angle was measured for the 20 disks thus prepared. Further, the entire surface of the disk was sandwiched between both sides by a flat Teflon plate, placed horizontally, and a weight of 2 kg was applied from above. After holding in a heating oven at 70 ° C for a predetermined time,
After cooling to room temperature over time, the weight was removed. By repeating this operation, the change in the average value and the standard deviation of the maximum warp angle of the disk with respect to the accumulated heat holding time was examined. The results are shown in FIG.

[0024]

Example 2 A disk was prepared in the same manner as in Example 1 except that the maximum value of the warp angle was set to 1 degree by adjustment during molding. Further, the entire surface of the disk was sandwiched between flat Teflon plates from both sides and placed horizontally, and a load was applied from above. 7
After being kept in a heating oven at 0 ° C. for 1 hour, and cooled to room temperature over 1 hour, the weight was removed. The change in the maximum warp angle of the disk with respect to the load at this time was examined. FIG. 4 shows the results. 8kg weight (pressure per area 7
The maximum value of birefringence was increased from 43 nm before heating to 79 nm in the disk applied with 1.9 g / cm ² ). The maximum value of birefringence increased from 42 nm to 53 nm for a disk loaded with 7 kg (pressure 62.9 g / cm ² per area). No change in birefringence was observed in the other disks.

[0025]

Example 3 A disk was prepared in the same manner as in Example 1, except that the maximum value of the warp angle was set to 1 degree by adjustment during molding. Further, the entire surface of the disk was sandwiched between both sides by a flat Teflon plate, placed horizontally, and a weight of 2 kg was applied from above. After being kept in a heating oven at a different temperature for 1 hour, and cooled to room temperature over 1 hour, the weight was removed. The change in the maximum warp angle of the disk with respect to the temperature at this time was examined. FIG. 5 shows the results. At any temperature above room temperature, a decrease in the warp angle was observed. The disk heated at 130 ° C. had a groove depth of about 20%, making signal recording and reproduction impossible. The disk heated at 120 ° C. did not change the groove shape, but the maximum value of birefringence increased from 45 nm before heating to 78 nm. At a temperature lower than that, no change was observed in both the groove shape and the birefringence.

[0026]

Example 4 The maximum value of the warp angle of 20 disks prepared up to the protective coat in the same manner as in Example 1 was measured. Further, the position of the innermost peripheral part diameter of 15 to 20 mmφ and the outermost peripheral part diameter of 59 to 60 mmφ were sandwiched from both sides by a Teflon plate, placed horizontally, and weighted from above. 70 ° C
, And then cooled to room temperature over 1 hour, and then the weight was removed. The change in the average value and the standard deviation of the warp angle of the disk with respect to the weight at this time was examined. FIG. 6 shows the results.

[0027]

Example 5 A film up to a protective coat was formed on a substrate having a thickness of 1.2 mm to 0.6 mm in the same manner as in Example 1. The maximum value of the warp angle of each disk was finally adjusted to 1 degree by adjustment during molding. Further, the position of the innermost peripheral part diameter of 15 to 20 mmφ and the outermost peripheral part diameter 5
A position of 9 to 60 mmφ was sandwiched between both sides by a Teflon plate, placed horizontally, and weighted from above. After being kept in a heating oven at 70 ° C. for 1 hour, and cooled to room temperature over 1 hour, the weight was removed. At this time, the difference in the warp angle of the disc with respect to the substrate thickness was examined. FIG. 7 shows the results.

[0028]

By using the method for manufacturing an optical recording medium according to the present invention, a medium having a small warpage can be stably manufactured even when a thin resin substrate is used. Further, since it is not necessary to completely cancel the stress by the film configuration of the medium, restrictions on the design of the medium are greatly relaxed.

[Detailed description of drawings]

FIG. 1 is an explanatory view showing one embodiment of a method for manufacturing an optical recording medium of the present invention.

FIG. 2 is an explanatory view showing another embodiment of the method for manufacturing an optical recording medium of the present invention.

FIG. 3 is a diagram illustrating changes in the average value and the standard deviation of the warp angle with respect to the pressing time when the entire surface of the substrate is pressed in Example 1.

FIG. 4 is a diagram showing a change in a warp angle with respect to a pressure in the embodiment.

FIG. 5 is a diagram showing a change in a warp angle with respect to a heating temperature in the example.

FIG. 6 is a diagram showing a change in a warp angle and a change in a standard deviation with respect to a pressurizing time when only the inner and outer peripheries are pressurized in the embodiment.

FIG. 7 is a diagram illustrating a change in a warp angle with respect to a substrate thickness in the example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Flattening member 3 Force for flattening

Claims (8)

[Claims] 1. A method for manufacturing an optical recording medium using a resin substrate, comprising a step of applying a force to the substrate so that the shape of an information recording area becomes flatter after the formation of the substrate (force applying step). Characteristic method for producing an optical recording medium. 2. The method for manufacturing an optical recording medium according to claim 1, wherein the substrate is heated to between room temperature and the glass transition point of the resin during the applying step. 3. The method according to claim 2, wherein the maximum temperature T of the substrate during the applying step is in the range of Tg−90 ° C. ≦ T ≦ Tg−10 ° C. with respect to the glass transition point Tg of the resin. A method for manufacturing an optical recording medium. 4. The method for manufacturing an optical recording medium according to claim 1, wherein said substrate has a thickness of 0.8 mm or less. 5. The method for manufacturing an optical recording medium according to claim 1, wherein at least a part of said substrate is pressed from both sides of said substrate. 6. The method of manufacturing an optical recording medium according to claim 1, wherein the inner and outer peripheral portions of at least the recording area of the substrate are pressed from both sides of the substrate. 7. The method for manufacturing an optical recording medium according to claim 1, wherein the time for applying the force is 5 minutes or more. 8. The method for manufacturing an optical recording medium according to claim 1, wherein said step is performed after forming all necessary films on said substrate.