JP2005032348A - Optical disk, and recording and reproducing system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical disk which can enlarge a usable recording area and increasing recording capacity and to provide a recording and reproducing system which can record/reproduce information to/from the optical disk whose usable recording area is enlarged and whose size is made small. <P>SOLUTION: The recording and reproducing system capable of recording/reproducing information to/from an optical disk 1 having a light incidence surface 2 on which light for recording or reproducing is made incident and a center hole 4 is provided with a clamp board 13 for clamping the optical disk 1 on the center hole 4 and/or its vicinity, a motor 11 for rotating the optical disk 1 via the clamp board 13 and a pickup part 12 for irradiating the light incidence surface 2 with light while relatively moving in the radius direction of the optical disk 1 in order to record or reproduce information. When the clamp board 13 clamps the optical disk 1, the surface 13a of the clamp board 13 on the light incidence surface side of the optical disk 1 is made nearly flat with respect to the light incidence surface 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical disc and a recording / reproducing system.
[0002]
[Prior art]
Conventionally, optical discs such as CD, DVD, BD (Blu-ray Disc) are commercially available. The optical disc has a central hole, which is used for centering the rotation center when mounted on a recording / reproducing device. The diameter is 15 mm. By aligning the diameter, various devices can record and reproduce various types of optical discs. It has become easy. In addition, the thickness and outer diameter of the optical disk are also equal, and the thickness is 1.2 mm and the outer diameter is 120 mm (80 mm is also available). As another example, MD has a central hole diameter of 11 mm, and MO disc is a mounting mechanism using a unique hub, so it is not compatible with the above-mentioned optical disc and forms a unique market. As described above, there are cases where importance is attached to the compatibility of the information recording medium, and there are cases where an original method is adopted.
[0003]
An example of a conventional optical disk recording / reproducing system will be described with reference to FIG. In the case of reproduction by the recording / reproducing system of FIG. 6, the clamp shaft C1 of the clamp base C is fitted into the center hole D1 of the optical disk D, and the clamp pressing portion H passes the optical disk D from the opposite side of the clamp base C in the vicinity of the center hole D1. The spindle motor M rotates with the clamp table C pressed by the spring S and the clamp table C fixes the optical disk D in the clamp area D2 near the center hole D1, and the optical disk D is picked up by the pickup P while rotating the optical disk D through the clamp table C. Read information from.
[0004]
By the way, in recent years, mobile products have become popular due to downsizing of devices, and some of them incorporate information recording media. The information recording medium can be a hard disk, an optical disk, a semiconductor memory, a magnetic tape, or the like, and the optical disk is useful as an inexpensive information recording medium that can be randomly accessed. However, the current optical disc has a large outer diameter and is not suitable for mobile devices. Therefore, a small-diameter optical disc is required, but by reducing the diameter, the recording area is reduced and the recording capacity is reduced, and it is impossible to cope with future increases in the amount of information. is important.
[0005]
In order to increase the recording capacity, an optical system suitable for high density is used, and the recording area is expanded. In the optical system, it is possible to respond by reducing the focal spot size by shortening the wavelength of the laser and increasing the NA of the objective lens. Also, the recording area can be expanded by increasing the area by multilayering and projecting the disc. This can be dealt with by expanding the recording area in the area. Regarding the expansion of the recording area, the expansion to the outer peripheral portion of the optical disc is close to the limit at present, the expansion to the inner peripheral portion is important, and it is naturally advantageous that the disc central hole is small. However, as shown in FIG. 6, the spindle motor M and the optical disk clamp base C are arranged on the inner periphery of the optical disk, so that it is not easy to enlarge the recording area. A configuration in which the spindle motor M and the pickup P are separately arranged on both sides of the optical disk D is also possible. However, because the thickness of the recording / reproducing apparatus increases because of the arrangement on both sides, the spindle motor M and the pickup P may be arranged on the same side. desirable.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide an optical disc capable of expanding a usable recording area and increasing a recording capacity in view of the above-described problems of the related art. It is another object of the present invention to provide a recording / reproducing system capable of recording / reproducing an optical disc with an expanded usable recording area and capable of being downsized.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, an optical disc according to the present invention has an inner peripheral portion of a substrate on a light incident surface side on which light for recording or reproduction is incident, formed into a concave portion into which a clamp table on the recording / reproducing system side can enter. And forming a clamp region in which the clamp base is clamped in the concave portion.
[0008]
According to this optical disc, a clamp area is formed in the concave portion on the inner peripheral portion of the substrate, and the clamp table on the recording / reproducing system side can enter the clamp area, so that light for recording or reproduction is irradiated. When the pickup unit on the recording / reproducing system side moves toward the inner peripheral side in the radial direction with respect to the optical disc, the position is not limited by the clamp table. For this reason, the available recording area can be expanded on the inner circumference side of the optical disc, and the recording capacity can be increased.
[0009]
In the optical disc, it is preferable to provide a convex portion on the opposite surface of the substrate so as to have a thickness corresponding to the depth of the concave portion. Thereby, the strength reduction due to the provision of the concave portion can be compensated.
[0010]
A recording / reproducing system according to the present invention is a recording / reproducing system for recording and / or reproducing an optical disc having a light incident surface on which light for recording or reproducing is incident and a center hole, wherein the optical disc is Clamping means for clamping by a clamp base at and / or in the vicinity of the center hole, rotation driving means for rotating the optical disc via the clamping means, and recording or reproduction while moving relatively in the radial direction of the optical disc Pickup means for irradiating light to the light incident surface for performing the above, and when the clamp means clamps the optical disk, the surface on the light incident surface side of the optical disk of the clamp base is relative to the light incident surface It is characterized by having a substantially flat height relationship.
[0011]
According to this recording / reproducing system, when the clamp means clamps the optical disk, the light incident surface side surface of the clamp table and the light incident surface have a substantially flat height relationship. When moving to the inner peripheral side, the position can be moved without being restricted by the clamp base. For this reason, recording / reproduction can be performed on an optical disc in which the recording area that can be used on the inner circumference side is expanded in order to increase the recording capacity. Further, since the clamp means, the rotation drive means, and the pickup means can be arranged on the light incident surface side of the optical disc, the recording / reproducing system can be miniaturized. There is no problem as long as the surface on the light incident surface side of the optical disk of the clamp table is concave than the light incident surface, but when it is convex, the protrusion amount is preferably 0.1 mm or less.
[0012]
In the recording / reproducing system, a clamp component having a diameter larger than that of the central hole is integrally fixed to the inner periphery of the surface opposite to the light incident surface with respect to the optical disc, and the clamp means clamps the clamp component. it can.
[0013]
Another recording / reproducing system according to the present invention records and / or reproduces an optical disc having a clamp area in a concave portion formed in the inner peripheral portion of the substrate on the light incident surface side on which light for recording or reproducing is incident. A recording / reproducing system for performing the above-described operation, wherein the optical disc is rotated via the clamp means, and a clamp means having a clamp base that fits in a clamp area of the optical disc and a clamp shaft that fits in a central hole of the optical disc. Rotation drive means, and pick-up means for irradiating light to the optical disk to perform recording or reproduction while moving relatively in the radial direction of the optical disk, and the clamp table is fitted in the clamp area When the surface on the light incident surface side of the optical disk is substantially flat with respect to the light incident surface of the optical disk Characterized in that it is configured such that.
[0014]
According to this recording / reproducing system, when the clamp table is fitted into the clamp area of the optical disc, the light incident surface side surface of the clamp table and the light incident surface have a substantially flat height relationship. When moving to the inner peripheral side of the optical disk in the radial direction, it is possible to move without being restricted by the clamp table. For this reason, recording / reproduction can be performed on an optical disc in which the recording area that can be used on the inner circumference side is expanded in order to increase the recording capacity. Further, since the clamp means, the rotation drive means, and the pickup means can be arranged on the light incident surface side of the optical disc, the recording / reproducing system can be miniaturized. There is no problem as long as the surface on the light incident surface side of the optical disk of the clamp table is concave than the light incident surface, but when it is convex, the protrusion amount is preferably 0.1 mm or less.
[0015]
In the recording / reproducing system, it is preferable that clamp holding means for holding the clamp means is disposed on the opposite side of the light incident surface of the optical disc.
[0016]
Another optical disc according to the present invention is an optical disc that can be recorded and / or reproduced by each of the recording / reproducing systems described above, and has an outer diameter of 60 mm or less, a center hole diameter of 10 mm or less, and a thickness of 0.8 mm or less. The cover layer has a thickness of 0.1 mm or less. According to this optical disk, the recording area that can be miniaturized and can be used can be expanded, so that it can be applied as a random accessible memory or the like of a miniaturized mobile product.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, first and second embodiments of the present invention will be described with reference to the drawings.
[0018]
<First Embodiment>
[0019]
FIG. 1 is a schematic sectional side view of an optical disc recording / reproducing system showing a first embodiment.
[0020]
As shown in FIG. 1, an optical disc 1 is provided so as to be recessed from the light incident surface 2 on a light incident surface 2 on which light is irradiated and incident for recording / reproduction, a central hole 4, and an outer peripheral side of the central hole 4. And a clamp region 6 provided on the bottom surface of the concave portion 5. The light incident surface 2 is composed of a cover layer made of a light transmissive resin, and a recording film, a reflection film, and the like are formed on the back side of the cover layer.
[0021]
The optical disk 1 is composed of a substrate made of polycarbonate resin (PC) or the like. For example, in the case of a read-only optical disk, a reflective film made of aluminum or the like is formed on a prepit formed on the substrate by sputtering or the like. In the case of the recording type, a land groove is formed on the substrate, and a recording film or the like is formed thereon.
[0022]
In the optical disc 1, the recording area 7 is formed by forming the above-described reflective film, recording film, or the like, and recording / reproduction is performed on the recording area 7 from the light incident surface 2 side. Since the recording area 7 is formed up to the inner peripheral portion 7a in the vicinity of the concave portion 5, the available recording area is enlarged on the inner peripheral side, and the recording capacity is increased. Further, the clamping region 6 on the bottom surface of the concave portion 5 is formed to be a rough surface, for example, so that a large frictional force can be obtained with the contact surface of the clamp table 13 of the recording / reproducing system.
[0023]
The recording / reproducing system in FIG. 1 irradiates light from a laser light source through an optical system such as an objective lens onto a light incident surface 2 of the optical disc 1 and a spindle motor 11 for rotationally driving the optical disc 1, and a recording area 7 during reproduction. Formed on the light incident surface 2 having a pickup portion 12 that can receive reflected light modulated by the concave and convex pits of the optical disc 1 and a clamp shaft 14 that is rotationally driven by the spindle motor 11 and fitted into the center hole 4 of the optical disc 1. A clamp base 13 that can be fitted into the recessed portion 5, a clamp pressing portion 15 that is provided on the opposite surface 3 side of the light incident surface 2 of the optical disc 1 and in the vicinity of the periphery of the center hole 4, and a clamp pressing portion 15. And a spring member 16 for urging the lower part of the figure downward.
[0024]
In the recording / reproducing system of FIG. 1, when recording or reproducing the optical disc 1, the clamp base 13 is fitted into the concave portion 5 of the light incident surface 2 of the optical disc 1 and the clamp shaft 14 is fitted into the center hole 4 and the clamp holding portion. The optical disk 1 is clamped at the clamp region 6 in the concave portion 5 and is securely fixed by pressing the disc 15 with the clamping force generated by the biasing force of the spring member 16 from the opposite surface 3 side.
[0025]
The pickup unit 12 can move in the radial direction of the optical disc 1 while being close to the light incident surface 2 of the optical disc 1, and light is incident when the optical disc 1 is rotationally driven by the spindle motor 11 via the clamp base 13. Recording or reproduction can be performed while irradiating the surface 2 with light.
[0026]
When the clamp table 13 is fitted in the concave portion 5 formed on the light incident surface 2 of the optical disc 1 and is clamped by the clamp region 6, the outer surface 13 a of the clamp table 13 becomes substantially the same height as the light incident surface 2. The outer surface 13a and the light incident surface 2 are substantially flat. For this reason, when the pickup unit 12 moves to the inner peripheral side in the radial direction of the optical disc 1 in order to perform recording or reproduction with the pickup unit 12 approaching the light incident surface 2, the clamp base 13 does not get in the way, and the pickup unit 12 The position can be moved so as to cover the inner peripheral portion 7a in the vicinity of the concave portion 5 of the recording area 7 without being restricted by the position of the clamp base 13.
[0027]
Therefore, according to the recording / reproducing system of FIG. 1, recording / reproducing can be performed on the optical disc 1 in which the recording area 7 is expanded so that the recording area can be used up to the inner peripheral portion 7a. Further, since the spindle motor 11, the pickup unit 12, and the clamp table 13 can be arranged on the same surface side of the light incident surface 2 of the optical disc 1, the recording / reproducing system can be downsized.
[0028]
If the outer surface 13a of the clamp base 13 is recessed with respect to the light incident surface 2, there is no problem in the movement of the pickup unit 12. However, if the surface 13a protrudes from the light incident surface 2, the pickup unit and the optical disc The working amount is preferably 0.1 mm or less.
[0029]
Next, a modification of the optical disc 1 in FIG. 1 will be described with reference to FIG. FIG. 2 is a schematic sectional side view of a recording / reproducing system similar to FIG.
[0030]
The optical disc 1 shown in FIG. 2 is configured in the same manner as in FIG. 1 except that a convex portion 8 is provided around the central hole 4 on the opposite surface 3 of the optical disc 1. The thickness of the convex portion 8 substantially corresponds to the depth of the concave portion 5 and can compensate for a decrease in strength due to the concave portion 5 formed on the light incident surface 2 side.
[0031]
As shown in FIG. 2, the optical disc 1 provided with the convex portion 8 can be recorded / reproduced by a recording / reproducing system similar to that shown in FIG. In this case, the clamp pressing portion 15 presses the convex portion 8 of the optical disc 1.
[0032]
Next, a modification of the recording / reproducing system of FIG. 1 will be described with reference to FIGS. FIG. 3 is a schematic sectional side view of the recording / reproducing system similar to FIG. FIG. 4 is an internal top view of the clamp shaft of FIG.
[0033]
The recording / reproducing system shown in FIG. 3 has the same configuration as that of FIG. 1 except that the mechanism for obtaining the clamping force at the clamp table is different, and the clamp shaft 14 of the clamp table 13 of FIG. A claw portion 17 is provided.
[0034]
As shown in FIG. 4, the plurality of claw portions 17 are urged by the springs 17 a provided inside the clamp shaft 14 so as to protrude outward in the radial direction toward the distal end side outer periphery 14 a, and the distal end side outer periphery 14 a. When pushed in from the side, it fits inside as shown by the broken line in the figure.
[0035]
In the recording / reproducing system of FIGS. 3 and 4, when recording or reproducing the optical disc 1, the clamp base 13 is fitted into the concave portion 5 of the light incident surface 2 of the optical disc 1, and the clamp shaft 14 is fitted into the center hole 4. When the tip of the clamp shaft 14 protrudes from the center hole 4 to the opposite surface 3 side, a plurality of claw portions 17 that have been pushed in against the urging force of the spring 17 a protrude from the outer periphery 14 a of the tip end of the clamp shaft 14. Thus, the clamping force can be obtained by the plurality of claw portions 17 urging the optical disk 1 downward from the opposite surface 3 side.
[0036]
As described above, the optical disc 1 is clamped by the clamp region 6 in the concave portion 5 and securely fixed by pressing the plurality of claws 17 with the clamping force from the opposite surface 3 side.
[0037]
According to the recording / reproducing system of FIGS. 3 and 4, the outer surface 13 a of the clamp table 13 and the light incident surface 2 of the optical disk 1 are substantially flat, as in FIGS. When moving to the inner peripheral side of the optical disk 1 in the radial direction, the clamp base 13 does not get in the way and can move so as to cover the inner peripheral portion 7a in the vicinity of the concave portion 5 of the recording area 7, thereby increasing the recording capacity. Therefore, recording / reproduction can be performed on the optical disk 1 in which the recording area 7 is enlarged so that the inner area 7a can be used.
[0038]
3 and 4 can omit the clamp holding portion 15 and the spring member 16 shown in FIGS. 1 and 2, which can contribute to further downsizing of the recording / reproducing system. In FIG. 3 as well, recording / reproduction can be performed on the optical disc 1 having the convex portion 8 as shown in FIG.
[0039]
<Second Embodiment>
[0040]
FIG. 5 is a schematic sectional side view of an optical disk recording / reproducing system showing the second embodiment.
[0041]
The optical disk 10 shown in FIG. 5 is the same as that shown in FIG. 1 except that the concave portion 5 of FIG. 1 is omitted and the center hole 9 is configured to have a larger diameter than the optical disk 1 of FIG. 1 is configured in the same manner as the optical disc 1 of FIG. In the optical disk 10, the recording area 7 is formed of a reflective film or a recording film as in FIG. 1, and the surface of the recording area 7 is the light incident surface 2. Since the recording area 7 is formed up to the inner peripheral portion 7a near the center hole 9, the available recording area is enlarged on the inner peripheral side, and the recording capacity is increased.
[0042]
The recording / reproducing system in FIG. 5 irradiates a light incident surface 2 of the optical disk 10 with a spindle motor 11 for rotationally driving the optical disk 10 from a laser light source through an optical system such as an objective lens, and a recording area 7 during reproduction. A pickup unit 12 that can receive reflected light modulated by the concave and convex pits of the optical disk 10 by a light receiving element, a clamp base 23 that is driven to rotate by the spindle motor 11 and fits in the center hole 9 of the optical disk 10, and a light incident surface 2 of the optical disk 10. And a clamp portion 25 fixed by an adhesive layer 27 in the vicinity of the periphery of the center hole 9 and a spring member 26 that urges the clamp portion 25 downward in the drawing.
[0043]
In the recording / reproducing system of FIG. 5, when the clamp table 23 is fitted into the center hole 9 of the optical disk 10 when recording or reproducing the optical disk 10, the abutting portion 24 on the upper end side of the clamp table 23 is the opposite surface of the optical disk 10. The optical disk 10 is clamped and securely fixed by abutting against the clamp portion 25 fixed to the side 3 and pressing with a clamping force by the urging force of the spring member 26 from above the clamp portion 25.
[0044]
The pickup unit 12 can move in the radial direction of the optical disk 10 while being close to the light incident surface 2 of the optical disk 10, and the optical disk 10 is rotated and driven by the spindle motor 11 via the clamp base 23. Recording or reproduction can be performed while irradiating the surface 2 with light.
[0045]
As shown in FIG. 5, when the clamp table 23 is fitted into the center hole 9 of the optical disk 10 and clamped by the clamp portion 25, the lower surface 23 a of the clamp table 13 is substantially the same height as the light incident surface 2. Thus, the surface 23a and the light incident surface 2 are substantially flat. For this reason, when the pickup unit 12 moves to the inner peripheral side in the radial direction of the optical disc 10 to perform recording or reproduction in a state where the pickup unit 12 is close to the light incident surface 2, the clamp base 23 does not get in the way, and the pickup unit 12 The position can be moved so as to cover the inner peripheral portion 7a in the vicinity of the center hole 9 of the recording area 7 without being limited by the position of the clamp base 23.
[0046]
Therefore, according to the recording / reproducing system of FIG. 5, recording / reproducing can be performed on the optical disc 10 in which the recording area 7 is expanded so that the recording area can be used up to the inner peripheral portion 7a. Further, since the spindle motor 11, the pickup unit 12, and the clamp table 23 can be arranged on the same surface side of the light incident surface 2 of the optical disc 10, the recording / reproducing system can be downsized.
[0047]
If the lower surface 23a of the clamp base 23 is recessed with respect to the light incident surface 2, there is no problem in the movement of the pickup unit 12. However, if the surface 23a protrudes from the light incident surface 2, the pickup unit In consideration of the working distance with the optical disc, the protruding amount is preferably 0.1 mm or less.
[0048]
The optical discs 1 and 10 that can be recorded / reproduced by the recording / reproducing systems of FIGS. 1 to 5 have an outer diameter of 60 mm or less, a center hole diameter of 10 mm or less, a thickness of 0.8 mm or less, and a cover layer. It can be configured such that the thickness is 0.1 mm or less and there are a plurality of recording layers. According to the optical discs 1 and 10, even when the size is reduced, the recording area that can be used on the inner circumference side can be expanded as described above. Therefore, the optical discs 1 and 10 can be applied as a randomly accessible memory or the like of a reduced size mobile product.
[0049]
As described above, the present invention has been described with reference to the embodiments. However, the present invention is not limited to these embodiments, and various modifications can be made within the scope of the technical idea of the present invention. For example, a magnet may be arranged in place of the spring members 16 and 26 in FIGS. 1, 2 and 5, and the clamping force may be obtained by this magnet. Is possible.
[0050]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the optical disk which can expand an available recording area and can increase a recording capacity can be provided. Further, it is possible to provide a recording / reproducing system capable of recording / reproducing an optical disc having an expanded usable recording area and capable of being downsized.
[Brief description of the drawings]
FIG. 1 is a schematic sectional side view of an optical disc recording / reproducing system showing a first embodiment.
2 is a schematic sectional side view of a recording / reproducing system similar to that of FIG. 1, showing a modification of the optical disc of FIG.
FIG. 3 is a schematic sectional side view of a recording / reproducing system showing a modification of FIG. 1;
4 is an internal top view of the clamp shaft of FIG. 3. FIG.
FIG. 5 is a schematic sectional side view of an optical disc recording / reproducing system showing a second embodiment;
FIG. 6 is a schematic sectional side view of a conventional optical disc recording / reproducing system.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1,10 ... Optical disk 2 ... Light-incidence surface 4,9 ... Center hole 5 ... Concave part 6 ... Clamp area | region 7 ... Recording area 7a ... Inner peripheral part 8 ... · Convex portion 11 · · · Spindle motor 12 · · · Pickup portion 13 · · · Clamp base 14 · · · Clamp shaft 15 · · · Clamp holding portion 16 · · · Spring member 17 · · · Claw portion 23 ···・ Clamp base 24 ... Abutting part 25 ... Clamp part

Claims (7)

An inner peripheral portion of the substrate on the light incident surface side on which light for recording or reproduction is incident is formed as a concave portion into which a clamp base on the recording / reproducing system side can enter, and the clamp base is clamped in the concave portion An optical disc comprising a clamp area. The optical disk according to claim 1, wherein a convex portion is provided on the opposite surface of the substrate so as to have a thickness corresponding to a depth of the concave portion. A recording / reproducing system for recording and / or reproducing an optical disc having a light incident surface on which light for recording or reproducing is incident and a central hole,
Clamping means for clamping the optical disc by a clamp base at and / or near the center hole;
A rotation drive means for rotating the optical disc through the clamp means;
Pick-up means for irradiating the light incident surface with light to perform recording or reproduction while relatively moving in the radial direction of the optical disc,
When the clamp means clamps the optical disk, the surface on the light incident surface side of the optical disk of the clamp base is configured to have a substantially flat height relationship with respect to the light incident surface. Recording / playback system. 4. The clamp component having a diameter larger than that of the center hole is integrally fixed to the inner peripheral portion of the surface opposite to the light incident surface with respect to the optical disc, and the clamp means clamps the clamp component. The recording / reproducing system described in 1. A recording / reproduction system for recording and / or reproducing an optical disc having a clamp area in a concave portion formed in an inner peripheral portion of a substrate on a light incident surface side on which light for recording or reproduction is incident,
Clamp means having a clamp base that fits in the clamp area of the optical disc and a clamp shaft that fits in the center hole of the optical disc;
A rotation drive means for rotating the optical disc through the clamp means;
Pick-up means for irradiating the optical disc with light in order to perform recording or reproduction while relatively moving in the radial direction of the optical disc,
The clamp table is configured so that a surface on the light incident surface side of the optical disc has a substantially flat height relationship with respect to the light incident surface of the optical disc when fitted into the clamp region. Recording / playback system. 6. The recording / reproducing system according to claim 5, wherein clamp pressing means for pressing the clamp means is disposed on the side opposite to the light incident surface of the optical disc. An optical disc which can be recorded and / or reproduced by the recording / reproducing system according to any one of claims 3 to 6, wherein the outer diameter is 60 mm or less, the diameter of the center hole is 10 mm or less, and the total thickness is 0. An optical disc characterized by having a thickness of 8 mm or less and a cover layer of 0.1 mm or less.

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