Title of Invention	OPTICAL INFORMATION RECORDING MEDIUM
Abstract	The invention relates to an optical information recording medium (100) has tolerance to warping. The optical information recording medium comprises at least one information recording layer (103) on a main surface of a substrate (101) and a light-transmitting layer (106). The substrate (101) comprises a clamp area (105) and an information recording area (104) in accordance with the information recording layer (103). In the light-transmitting layer (106), the average thickness of a second area (106 b) corresponding to the clamp area (105) is thinner than the average thickness of a first area (106 a) corresponding to the information recording area (104).

Title of Invention

OPTICAL INFORMATION RECORDING MEDIUM

Abstract

The invention relates to an optical information recording medium (100) has tolerance to warping. The optical information recording medium comprises at least one information recording layer (103) on a main surface of a substrate (101) and a light-transmitting layer (106). The substrate (101) comprises a clamp area (105) and an information recording area (104) in accordance with the information recording layer (103). In the light-transmitting layer (106), the average thickness of a second area (106 b) corresponding to the clamp area (105) is thinner than the average thickness of a first area (106 a) corresponding to the information recording area (104).

Full Text	OPTICAL INFORMATION RECORDING MEDIUM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium, and more particularly relates to an optical information recording medium having at least one information recording layer and a light-transmitting layer on a main surface of a substrate. 2. Description of the Prior Art Recently, research relating to various types of optical information recording has been studied in the optical information recording field. Optical information recording methods have been emerging which are applicable to a range of uses, because these methods can allow media to have higher density, can record/reproduce information by a non-contact method, and can also achieve these objectives at a low price. Currently, an optical disk has a structure produced, for example, by forming an information layer on a transparent resin layer with a thickness of 1.2 mm, and then covering and protecting the layer with an over coating, or by forming information layers on one or both sides of a transparent resin layer with an over coating, or by forming information layers on one or both sides of a transparent resin layer with a thickness of 0.6 mm, and then laminating two of the information layers. Recently, as a way to increase the recording density of optical disks, methods have been studied such as increasing the numerical aperture (NA) of an objective lens, and shortening the wavelength of the laser. In these methods, if the thickness of a recording/reproducing carrier (which is a substrate of the side on which an optical laser is incident) is thin, the influence of an aberration of the laser spot decreases, and allowance for a gradient angle (tilt) of a disk increases. From this, an idea was proposed to set the thickness of the recording/reproducing carrier to be around 0.1 mm, NA to be around 0.85, and the wavelength of a laser to be around 400 nm (see Japanese unexamined patent publication H08-235638, for example). Here, considering effects on the focus of the recording/reproducing light and risk of spherical aberration, it is preferable that the thickness variation of the recording/reproducing carrier is reduced to be within 5%. Even in an optical disk having such reduced thickness variation, the recording/reproducing carrier of which has thickness is 0.1 mm, then a thickness of the disk is preferably 1.2 mm like a conventional CD or DVD, because the disk should have compatibility with existing hardware. An optical disk having a recording/reproducing carrier with a thickness of 0.1 mm has an asymmetrical structure in the thickness direction, because the thickness of the disk is 1.2 mm. Because of the asymmetrical structure, warping of the optical disk occurs due to variations in environmental conditions such as temperature or humidity. More specifically, when placing the optical disk, which is at room temperature, in a recording/reproducing machine which has been heated to 50 degrees C by uninterrupted operations, drastic warping occurs in a short time. SUMMARY OF THE INVENTION The object of the present invention is to provide an optical information recording medium which has tolerance to warping. The information described below was discovered in an investigation by the present inventors. That is, the warping of an optical information recording medium occurs because heat conduction from a clamp area is fast. (The clamp area is a contact portion of the recording medium and a recording/reproducing machine) Therefore, an optical information recording medium of the present invention comprises a substrate, at least one information recording layer formed on a main surface of the substrate and a light-transmitting layer which covers the information recording layer. The substrate comprises a clamp area and an information recording area corresponding to the information recording layer. An average thickness of the light-transmitting layer corresponding to the clamp area is thinner than an average thickness of the light-transmitting layer corresponding to the information recording area. In the medium, even when the medium is placed in a drive which has been heated by repeated recording/reproducing, a stable recording/reproducing of the medium is possible without causing drastic warping. In addition, in the medium of the present invention, a thickness of the substrate and the thickness of the light-transmitting layer are different. The medium has an asymmetrical structure in a direction perpendicular to the film surface. Here, the warping caused by the asymmetrical structure can also be reduced. The medium of the present invention has a total thickness in the range of approximately 1.14 to 1.50 mm. In the medium of the present invention, the light-transmitting layer corresponding to the information recording area preferably has a thickness of approximately 100 µm, or approximately 75 µm. In the medium, it is preferable that the average thickness of the light-transmitting layer corresponding to the clamp area is approximately 95% or less than the average thickness of the light-transmitting layer corresponding to the information recording area. Thus, warping of the medium can be reduced. In the medium, it is also preferable that the average thickness of the light-transmitting layer corresponding to the clamp area is approximately 70% or more than the average thickness of the light-transmitting layer corresponding to the information recording area. Thus, a focusing position of an optical recording/reproducing beam is stably found when performing the recording/reproducing of the medium. A light-transmitting layer is radiation setting resin. In addition, the light-transmitting layer comprises a plurality of materials. At least one of the plurality of materials is preferably radiation setting resin. Because of this, the medium can be made inexpensively, i.e. without using expensive materials. At least one of the plurality of materials is preferably polycarbonate, and at least one of the plurality of materials is an adhesive material. The protection layer preferably has a pencil hardness of H or more. In the optical information recording medium of the present invention, there is preferably a protection layer formed on a part of or on a whole area of a recording/reproducing side of the light-transmitting layer. Thus, the light-transmitting layer is not easily scratched, and the quality of recorded/reproduced signals does not easily deteriorate. In this situation, an average thickness of the protection layer corresponding to the clamp area is thinner than an average thickness of the protection layer corresponding to the information recording area. In addition, the optical information recording medium of the present invention comprises a substrate, at least one information recording layer formed on a main surface of the substrate and a light-transmitting layer which covers the information recording layer. The substrate comprises a clamp area and an information recording area corresponding to the information recording layer. The average thickness of the light-transmitting layer corresponding to the clamp area is thinner than the average thickness of the light-transmitting layer corresponding to the information recording area. Alternatively, the protection layer may not be formed on the clamp area. In the optical information recording medium of the present invention, even when the medium is placed in a drive which has been heated by repeated recording/reproducing, a stable recording/reproducing of the medium is possible without causing drastic warping. In the medium, the thickness of the light-transmitting layer and the thickness of the substrate are different. In the medium, a total thickness of the light-transmitting layer and the protection layer is preferably approximately 100 µm, or approximately 75 urn. Here, the pencil hardness is determined by placing a sharpened pencil against the surface with a weight of 1 kg at an angle of 45 degrees, pulling the pencil under these conditions, and determining whether the surface is scratched or not. The pencil hardness is measured in accordance with JIS-K5400. With an optical information recording medium of the present invention, even when the disk is placed in a drive which has been heated by repeated recording/reproducing, a stable recording/reproducing of the medium is possible without causing warping. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 is a cross-sectional diagram showing an example of an optical information recording medium in Embodiment 1 of the present invention. Fig. 2 a cross-sectional diagram showing an example of a conventional optical information recording medium. Fig. 3 is a graph showing warp variation of a conventional optical information recording medium. Fig. 4 (a)-(b) are diagrams showing that how warping of a conventional optical information recording medium is determined. Fig. 5 is a cross-sectional diagram showing a conventional optical information recording medium which is clamped. Fig. 6 (a) is a cross-sectional diagram showing an optical information recording medium of Example 1 of the present invention, and Fig. 6 (b) is a graph showing a relation between the thickness of a light-transmitting layer of a clamp area and a maximum amount of warp variation in the optical information recording medium of Example 1. Fig. 7 is a cross-sectional diagram showing an example of a conventional optical information recording medium. Fig. 8 is a graph showing a warp variation of a conventional optical information recording medium. Fig. 9 (a) is a cross-sectional diagram showing an optical information recording medium of Example 2 of the present invention, and Fig. 9 (b) is a graph showing the relationship between the thickness of a light-transmitting layer in a clamp area and the maximum value of the amount of warp variation in the optical information recording medium of Example 2. Fig. 10 (a)-(c) are cross-sectional diagrams showing an example of deformation for an optical information recording medium of Embodiment 1 of the present invention. Fig. 11 (a)-(c) are cross-sectional diagrams showing an example of an optical information recording medium of Embodiment 2 of the present invention. Fig. 12 is a cross-sectional diagram showing an example of the optical information recording medium of Embodiment 3 of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be explained by referring to the figures. These figures are shown by way of cross-section unless otherwise specified. If the figures are symmetric, then only a portion from the axis of symmetry is shown, and the rest of the figure is omitted. Embodiment 1 Fig. 1 is an explanatory diagram showing an optical information recording medium 100 of the present invention. Fig. 1 shows a brief overview of the optical information recording medium 100 of the present invention. The medium 100 comprises an information recording layer 103 and a light-transmitting layer 106 on a main surface of a substrate 101 having a center hole 102. An information recording area 104 is an annular area on the substrate 101, where the information recording layer 103 is formed. In addition, an area formed at an inner area beyond an innermost circumference of the information recording area 104, where the medium 100 is clamped, is defined as a clamp area 105. The information recording area 104 and the clamp area 105 are neither overlapped nor contacted, and an annular area is formed between those areas. The light-transmitting layer 106 is formed to cover the information recording layer 103 at the information recording area 104 and also to cover the substrate 101 at the clamp area 105. More precisely, the light-transmitting layer 106 covers the information recording area 104 and the clamp area 105 on the substrate 101. In the light-transmitting layer 106, an area corresponding to the information recording area 104 is defined as a first area 106a, and an area corresponding to the clamp area 105 is defined as a second area 106b. Here, in the light-transmitting layer 106 of the medium 100 of the present invention, an average thickness of the second area 106b is thinner than an average thickness of the first area 106a. By employing the medium 100, even when the medium is placed in a drive which has been heated by repeated recording/reproducing, a stable recording/reproducing of the medium is possible without causing rapid, drastic warping. Note that, in this Embodiment, the light-transmitting layer 106 can have both of the structures (1) comprising a sheet substrate and an adhesive material as shown in Example 1 described below; and (2) comprising a radiation-setting resin as shown in Example 2. (Example 1) Below, structures of an optical information recording medium based on a conventional technique and of the present invention are compared. Note that, Fig. 2 through Fig. 5 show the conventional technique, and Fig. 6 shows the present invention. As shown in Fig. 6, the optical information recording medium 110 comprises a substrate 111, an information recording layer 113 and a transmitting layer 116. The substrate 111 has a thickness of approximately 1.1 mm and a diameter of approximately 120 mm, and is made of polycarbonate applied by an injection molding method. The information recording layer 113 comprises a guide-groove with a depth of approximately 20 nm formed on the substrate 111 and a recording film which contains phase-changeable materials or dielectric materials formed on the guide-groove. The transmitting layer 116 is formed on the information recording layer 113 of the substrate 111 and comprises a sheet substrate 117 with a thickness of approximately 70 µm and an adhesive material 118 with a thickness of approximately 30 µm. The light-transmitting layer 116 has a thickness of approximately 100 µm and an inside diameter of approximately 22 mm. The layer 116 comprises an adhesive material 118 and a sheet substrate 117 that are fixed together as an integrated unit. The layer 116 can be made by applying the integrated unit onto the substrate 111 with a roller. The layer 116 may also be formed by the method of superimposing both items in a vacuum. The sheet substrate 117 is made of polycarbonate applied by a casting method and the adhesive material 118 is made of a resin material including acrylate as a main component. The recording/reproducing was performed on the optical information recording medium 110 by using a laser with a wavelength of 405 nm and an objective lens with a numerical aperture of 0.85. By making the thickness of the light-transmitting layer 116 to be approximately 100 µm the medium 110 could achieve warp tolerance equivalent to that of a conventional DVD. In the light-transmitting layer 116 of the optical information recording medium 110, an average thickness of an area corresponding to a clamp area 115 is thinner than an average thickness of an area corresponding to an information recording area 114. Here, a thickness of the adhesive material 118 is constant at 30um, and the thickness of the sheet substrate 117 is different in an area 117a corresponding to the information recording area and an area 117b corresponding to the clamp area. Note that, in the structure of the conventional device shown in Fig. 2, the thickness of the sheet substrate 117 is uniform, thus the average thickness of the light-transmitting layer 116 is uniform over the whole area. Soon after placing the conventional optical information recording medium 110 shown in Fig. 2 in a drive which was heated to approximately 50 degrees C by repeated recording/reproducing, drastic warping occurred in a short time on the medium 110, as shown in Fig. 3. Fig. 3 shows the amount of warp variation at a radial position of 58 mm of the medium 110 and the time when the medium is placed in the drive is defined as 0 seconds. Here, the amount of warp is identified as an angle 131 which is defined by a laser beam reflected on the information recording layer 113 and a laser beam entering from vertically below, which is the side of recording/reproducing data on the medium 110. The direction of warp shown in Fig. 4 (a) is defined as plus and that shown in Fig. 4 (b) is defined as minus. As a reason for the occurrence of such drastic warping, it was assumed that the light-transmitting layer 116 had thermally expanded due to heat conduction from the clamp area 115. Fig. 5 shows the optical information recording medium 110 is clamped. A rotation center of the medium 110 is determined by a center cone 120, and the medium 110 is clamped at the clamp area 115 by a clamper 121 and a turn table 119. The temperature of the whole drive became approximately 50 degrees C, however, the heat conducted faster from the clamper 121 and the turn table 119 (both of which directly contacted with the clamp area 115) toward the medium 110, than from the air. This greatly affected the occurrence of the warping. As a way to reduce such warping of the optical information recording medium 110, a coefficient of thermal expansion or modulus of rigidity (Young's modulus) should have a small value. However, in order to achieve a small coefficient of thermal expansion, materials for use for the light-transmitting layer 116 are considerably restricted, and the medium 110 itself is possibly expensive. Next, in the structure of the present invention shown in Fig. 6, by changing the thickness of the light-transmitting layer 116 formed on the clamp area 115, the amount of warp of the medium 110 was measured. As shown in Fig. 6 (b), the thickness of the light-transmitting layer 116 corresponding to the clamp area 115 was changed relative to the thickness of the light-transmitting layer 116 corresponding to the information recording area 114. Here, on the layer 116 comprising the sheet substrate 117 and an adhesive material 118, the thickness of the adhesive material 118 was 30 µm, and only the thickness of the region 117b of the sheet substrate 117 corresponding to the clamp area was changed. As shown in Fig. 6 (b), the warping decreased by reducing the thickness of the light-transmitting layer 116 corresponding to the clamp area 115. This was considered to be because the thickness of the light-transmitting layer 116 corresponding to the clamp area 115 was reduced, and so the modulus of rigidity became small and the force which warped the substrate 111 also weakened. From these findings, reducing the thickness of the layer 116 corresponding to the clamp area 115 up to approximately 60 µm is extremely effective; however, it is not very effective over 60 µm. It is preferable that the amount of warp of the optical information recording medium is as small as possible. By reducing the amount of warp by approximately 80%, the recording/reproducing properties can greatly improve. In other words, the thickness of the light-transmitting layer 116 corresponding to the clamp area 115 is preferably 95 µm or less, and is preferably 95% or less than the thickness of the light-transmitting layer 116 corresponding to the information recording area 114. Here, when reducing the layer 116 corresponding to the clamp area 115, the thicknesses of the layer 116 corresponding to the clamp area 115 and the layer 116 corresponding to the information recording area 114 are different, so the surface of the medium is uneven. This may cause the problem that the focusing position of the laser when recording/reproducing may easily become unstable. Therefore, if the thickness of the clamp area 115 is different in each disk, the range of motion for a drive system which changes the focusing position of the recording/reproducing laser needs to be wide. Thus, the thickness of the light-transmitting layer 116 corresponding to the clamp area 115 is preferably close to the thickness of the layer 116 corresponding to the information recording area 114. Namely, the thickness of the light-transmitting layer 116 corresponding to the clamp area 115 is preferably 70 urn or more, and is preferably 70% or more of the thickness of the layer 116 corresponding to the information recording area 114. A light-transmitting layer formed of a sheet substrate and radiation-setting resin (UV radiation-setting resin, for example) also had almost the same results in comparison to the light-transmitting layer 116 formed of the sheet substrate 117 and the adhesive material 118 as described in the present embodiment. In addition, when the proportion of the thickness of the adhesive material or the radiation-setting resin was changed in a range between 5% and 50% of the light-transmitting layer, almost the same effect was obtained. Note that, "radiation" here includes all electromagnetic waves which can harden radiation-setting resin, for example infrared rays, visible rays, ultraviolet rays, X rays and the like. In this embodiment, an example of a rewriteable optical disk was described. In addition, the present invention can be applied to a write-once type disk which can record only once and a read-only type optical disk having A1 or Ag as main components in the reflective layer. Furthermore, the present invention can be applied not only in situations in which only one layer is formed on the information recording layer 113 as shown in this embodiment, but also in situations in which a plurality of layers are formed on the information recording layer 113, or a multi-layered optical disk which includes two or more information recording layers 113. Hereinabove, the embodiment of the present invention has been explained by using examples. Note that, the invention is not limited to the above-mentioned embodiment, and it is also possible to apply the present invention to other embodiments based on the technical idea of the present invention. In addition, the optical information recording medium 100 of the present invention can be available for a Blu-ray disc, DVD, CD, MD, MO and the like, for example. (Example 2) Below, structures of an optical information recording medium based on a conventional technique and of the present invention are compared. For descriptions which correspond to Example 1, overlapping explanations may be omitted. Note that, Fig. 7 and Fig. 8 show the conventional technique, and Fig. 9 shows an embodiment of the present invention. In Fig. 9 (a), an optical information recording medium 200 of the present invention is shown. In the medium 200 in Fig. 9, a substrate 201 was employed, which was the same as the substrate of Example 1, and a light-transmitting layer 206 was formed only of radiation-setting resin. Here, in the light-transmitting layer 206 of the medium 200 of the present invention, the average thickness of a second region 206b corresponding to the clamp area is thinner than an average thickness of a first region 206a corresponding to the information recording area. Note that, in the structure of the conventional technique shown in Fig. 7, the average thickness of the light-transmitting layer 206 is uniform over the whole area. The warping of the conventional medium 200 shown in Fig. 7 was measured when placing it in a drive which was heated to approximately 50 degrees C, the same as in Example 1. The results of the measurement are shown in Fig. 8. The denotations of the graph are the same as Fig. 3 (the example of the conventional technique) in Embodiment 1. Compared to the light-transmitting layer 116 of Embodiment 1 (see Fig. 3) formed by the sheet substrate 117 and the adhesive material 118, the amount of warp became large in the present example in which only the radiation-setting resin is used. The radiation-setting resin here including acrylate as a main component has an enormously large coefficient of thermal expansion compared to that of polycarbonate used in a general substrate. Therefore, the amount of warp becomes extremely large. Here, in the structure of the present invention shown in Fig. 9 (a), the amount of warp was measured while changing the thickness of a second area 206b of the light-transmitting layer 206 corresponding to the clamp area. The thickness of the second area 206b was reduced by polishing. The amount of warp while changing the thickness is shown in Fig. 9 (b). Because the maximum amount of warp variation was as large as 0.5 degrees when the thickness was 100 µm, reducing the thickness of the second region corresponding to the clamp area 206b was found to be very effective. The maximum amount of warp variation is preferably around 0.3 degrees. Like Example 1, in order to reduce the amount of warp by 80% or more, the thickness of the second area 206b is preferably about 95 µm, and is preferably 95% or less than the thickness the first region corresponding to the information recording area 206a. In addition, as described in Example 1, if the second region 206b of the light-transmitting layer 206 corresponding to the clamp area is too thin, then the focusing position of the recording/reproducing laser becomes unstable. More specifically, the thickness of the second region 206b is preferably 70 µm or more, and is preferably 70% or more than the first region 206a corresponding to the information recording area. (Modification example of the Embodiment 1) When forming the light-transmitting layer 206 using radiation-setting resin, the shape of the layer 206 easily changes because the resin is liquid before curing. Therefore the shape of the region of the light-transmitting layer corresponding to the clamp area can have not only the shape shown in Fig. 9, but can also easily have: the shape of the second region 216b corresponding to the clamp area shown in Fig. 10 (a) which gradually becomes thicker toward the outer circumference; the shape of the second region 226b corresponding to the clamp area shown in Fig. 10 (b) which gradually becomes thinner toward the outer circumference; or the shape of the second region 236b corresponding to the clamp area shown in Fig. 10 (c) which has a wavy shaped cross-section. Even with these shapes, the effect was almost the same as that described in Embodiment 1, and the amount of warp of the medium was found to depend on the average thickness of the second region of the light-transmitting layer corresponding to the clamp area. However, in the shapes of the second region corresponding to the clamp area as shown in Fig. 10 (a) to Fig. 10 (c), clamping the optical information recording medium becomes difficult and it may affect the recording/reproducing. Thus, the shape should be chosen not to affect the recording/reproducing. Here, although only the example of the light-transmitting layer is described, which is made of one type of radiation-setting resin, the layer may have a structure in which a plurality types of radiation-setting resin are stacked. Thus, in the light-transmitting layer comprising a plurality of layers, a similar effect can also be obtained. In this embodiment, an example of a rewriteable optical disk is described. In addition, the present invention can be applied to a write-once type disk which can record only once and a read-only type optical disk having Al or Ag as main components in the reflective layer. Furthermore, the present invention can be applied not only in situations in which only one layer is formed on the information recording layer, but also situations in which multiple layers are formed on the information recording layer, or a multi-layered optical disk that includes two or more information recording layers. Hereinabove, the embodiment of the present invention is explained by examples. Note that, the invention is not limited to the above-mentioned embodiment, and it is also possible to apply the present invention to other embodiments based on the technical idea of the present invention. Embodiment 2 An example of an optical information recording medium according to the present invention will be explained using Fig. 11. Note that, for descriptions that correspond to embodiment 1, overlapping explanations may be omitted. Note that, Fig. 11 (b) and (c) show the structure of the present invention, and Fig. 11 (a) shows the structure of the conventional technique. In a disk with high-density of which the thickness of the light-transmitting layer (as described in Embodiment 1) is approximately 100 µm, and recording/reproducing is performed under the conditions that NA is approximately 0.85 and the wavelength of the laser is approximately 400 run, the signal quality easily deteriorates compared to conventional CDs and DVDs, because of flaws or dust on the surface of the light-transmitting layer. Thus, it is necessary to form the protection layer on the surface of the light-transmitting layer of the optical information recording medium 300. The protection layer needs to have high scratch tolerance, and its pencil hardness is preferably H or more. To form the protection layer, UV setting resin is generally used. By mixing inorganic oxide particles into the resin, the hardness can further increase. When forming the protection layer using such UV setting resin, the layer usually has a large coefficient of thermal expansion and modulus of rigidity because it has a high degree of hardness after hardening. When the protection layer has a larger coefficient of thermal expansion and a higher modulus of rigidity than those of the material for the substrate, even if the layer is made to be thin, such as with a thickness of approximately 1-5 µm, the layer expands by the heat conducted from the clamper 121 or the turn table 119 as described in Fig. 5 (the conventional example) of Embodiment 1, and more drastic warping will occur as described in the conventional example of Embodiment 1. In the conventional optical information recording medium shown in Fig. 11 (a), the protection layer 307 is formed on the whole recording/reproducing side of the transmitting layer 306. The average thickness of the protection layer 307 formed on the clamp area 305 is equal to the average thickness of the protection layer 307 formed on the information recording area 304. On the contrary, in the optical information recording medium 300 of the present invention shown in Fig. 11 (b), the average thickness of the second region 317b of the protection layer 317 corresponding to the clamp area is thinner than the average thickness of the first region 317a corresponding to the information recording area. Note that, the protection layer is generally thin so that the disk can be stably clamped in most situations. It is also possible not to form a protection layer on the clamp area of an optical information recording medium. In addition, as shown in Fig. 11 (c), it is also possible that both the thickness of a light-transmitting layer 326 and a protection layer 327 of the medium 300 in the clamp area 305 is thinner than those in the information recording area 304, respectively. More precisely, in the light-transmitting layer 326, an average thickness of a second region 326b corresponding to the clamp area is thinner than an average thickness of the first region 326a corresponding to the information recording area. Furthermore, in a protection layer 327, an average thickness of the second region 327b is thinner than an average thickness of the first region 327a corresponding to the information recording area. In Fig. 11 (c), an example is shown in which the light-transmitting layer 326 is formed in a tapered shape in the clamp area 305; however, the layer 326 may have a shape like the light-transmitting layer 106 shown in Fig. 1. Hereinabove, the embodiment of the present invention is explained by examples. Note that, the invention is not limited to the above-mentioned embodiment, and it is also possible to apply the present invention to other embodiments based on the technical idea of the present invention. Embodiment 3 Here, an example of an optical information recording medium according to the present invention is explained. Note that, for descriptions which correspond to embodiments 1 or 2, overlapping explanations may be omitted. In the above-mentioned Embodiments, although a medium having only one information recording layer has been explained, the present invention is also effective for a medium having two or more information recording layers, which is called a multiple-layer disk. For example, in an optical information recording medium 400 shown in Fig. 12 having two information recording layers, which is called a double-layer disk, an intermediate layer 408 having a range of thickness between approximately 20 µm and approximately 30 µm is formed between a first information recording layer 403 and a second information recording layer 413, and a light-transmitting layer 406 is formed in a range of thickness between approximately 70 µm to approximately 80 µm. Here, in the light-transmitting layer 406, the effect of decreasing the warp as described in Embodiment 1 or 2 was also obtained by reducing the average thickness of the second region 406b corresponding to the clamp area compared to that of the first region 406a corresponding to the information recording area. Particularly, when the average thickness of the second region 406b was reduced by 95% or less, a better effect was obtained. The reason for choosing the range of thickness between 20 and 30 µm for the intermediate layers is to decrease the effect of the reflected light from the other layers when recording/reproducing each layer, and to make the recording/reproducing light capable of distinguishing the difference of the depth from the surface of the light-transmitting layer of the first information recording layer 403 and the second information recording layer 413. By setting the depth from the surface of the light-transmitting layer of the first information recording layer 403 at approximately 100 urn, the medium can achieve warp tolerance equivalent to that of a conventional DVD as described in Embodiment 1. It is possible to achieve the same effect by reducing the intermediate layer 408 corresponding to the clamp area 405 or both the light-transmitting layer 406 and the intermediate layer 408 corresponding to the clamp area 405, rather than by reducing the second region 406b of the light-transmitting layer 406 corresponding to the clamp area. Also in these situations, the total thickness of the light-transmitting layer 406 and the intermediate layer 408 corresponding to the clamp area 405 is preferably 95% or less than the total thickness of the light-transmitting layer 406 and the intermediate layer 408 corresponding to the information recording area 404. In addition, as described in Embodiment 1, in order to avoid a wide range of motion for the drive system which changes the focusing position of the recording/reproducing laser, the total thickness of the light-transmitting layer 406 and the intermediate layer 408 corresponding to the clamp area 405 is preferably 70 µm or more, and is preferably 70% or more than the total thickness of the light-transmitting layer 406 and the intermediate layer 408 corresponding to the information recording area 404. Even in such multiple-layer disk, having a protection layer as described in Embodiment 2 is preferable. Therein, reducing warping of the optical information recording medium is preferably by such method of reducing the thickness of the protection layer corresponding to the clamp area as described in Embodiment 2 or the like. Hereinabove, the embodiment of the present invention is explained by examples. Note that, the invention is not limited to the above-mentioned embodiment, and it is also possible to apply the present invention to other embodiments based on the technical idea of the present invention. An optical information recording medium of the present invention has an advantage of having tolerance to warping, and the medium will be useful for a recording medium such as Blu-ray disc. We Claim: 1. An optical information recording medium comprising: a substrate having a clamp area and an information recording area; at least one information recording layer formed on the information recording area of said substrate; and a light-transmitting layer formed on said substrate and covering said information recording layer and the clamp area, wherein the clamp area is located inward of the information recording area of said substrate, an average thickness of said light-transmitting layer adjacent to the clamp area is thinner than an average thickness of said light-transmitting layer adjacent to the information recording area, said substrate has a surface that is free of an information recording layer, and said light-transmitting layer is a monolayer of radiation setting resin. 2. The optical information recording medium as claimed in claim 1, wherein a thickness of said substrate and a thickness of said light-transmitting layer are different. 3. The optical information recording medium as claimed in claim 1, wherein the optical information recording medium has a total thickness in a range of approximately 1.14 to 1.50 mm. 4. The optical information recording medium as claimed in claim 1, wherein said light-transmitting layer adjacent to the information recording area has a thickness of approximately 100µm. 5. The optical information recording medium as claimed in claim 1, wherein said light-transmitting layer adjacent to the information recording area has a thickness of approximately 75 µm. 6. The optical information recording medium as claimed in claim 1, wherein the average thickness of said light-transmitting layer adjacent to the clamp area is approximately 95% or less than the average thickness of said light-transmitting layer adjacent to the information recording area. 7. The optical information recording medium as claimed in claim 1, further comprising a protection layer formed on at least a part of a recording/reproducing side of said light-transmitting layer. 8. The optical information recording medium as claimed in claim 7, wherein said protection layer has a pencil hardness of H or more. 9. The optical information recording medium as claimed in claim 7, wherein an average thickness of said protection layer adjacent to the clamp area is thinner than an average thickness of said protection layer adjacent to the information recording area. 10. The optical information recording medium as claimed in claim 1, wherein a surface of said light transmitting layer adjacent to the clamp area and facing away from the clamp area is recessed with respect to a surface of said light transmitting layer adjacent to the information recording area and facing away from the information recording area. 11. An optical information recording medium comprising: a substrate including a clamp area and an information recording area; at least one information recording layer formed on the information recording area of said substrate; and a light-transmitting layer formed on said substrate and covering said information recording layer and the clamp area, wherein the clamp area is located inward of the information recording area of said substrate, an average thickness of said light-transmitting layer adjacent to the clamp area is thinner than an average thickness of said light-transmitting layer adjacent to the information recording area, said substrate has a surface that is free of an information recording layer, said light-transmitting layer is radiation setting resin, and the average thickness of said light-transmitting layer adjacent to the clamp area is approximately 70% to 95% of the average thickness of said light-transmitting layer adjacent to the information recording area. 12. The optical information recording medium as claimed in claim 11, wherein a thickness of said substrate and a thickness of said light-transmitting layer are different. 13. The optical information recording medium as claimed in claim 11, wherein the optical information recording medium has a total thickness in a range of approximately 1.14 to 1.50 mm. 14. The optical information recording medium as claimed in claim 11, wherein said light-transmitting layer adjacent to the information recording area has a thickness of approximately 100 µm. 15. The optical information recording medium as claimed in claim 11, wherein said light-transmitting layer adjacent to the information recording area has a thickness of approximately 75 µm. 16. The optical information recording medium as claimed in claim 11, wherein the average thickness of said light-transmitting layer adjacent to the clamp area is approximately 95% or less than the average thickness of said light- transmitting layer adjacent to the information recording area. 17. The optical information recording medium as claimed in claim 11, further comprising a protection layer formed on at least a part of a recording/reproducing side of said light-transmitting layer. 18. The optical information recording medium as claimed in claim 17, wherein said protection layer has a pencil hardness of H or more. 19. The optical information recording medium as claimed in claim 17, wherein an average thickness of said protection layer adjacent to the clamp area is thinner than an average thickness of said protection layer adjacent to the information recording area. 20. An optical information recording medium comprising: a substrate including a clamp area and an information recording area; at least one information recording layer formed on the information recording area of said substrate; a light-transmitting layer formed on said substrate and covering said information recording layer and the clamp area; and a protection layer formed on at least a part of a recording/reproducing side of said light-transmitting layer, wherein the clamp area is located inward of the information recording area of said substrate, an average thickness of said protection layer adjacent to the clamp area is thinner than an average thickness of said protection layer adjacent to the information recording area, said substrate has a surface that is free of an information recording layer, and said light- transmitting layer is a monolayer of radiation setting resin. 21. The optical information recording medium as claimed in claim 20, wherein said protection layer has a pencil hardness of H or more. 22. The optical information recording medium as claimed in claim 20, wherein a thickness of said light-transmitting layer and a thickness of said substrate are different. 23. The optical information recording medium as claimed in claim 20, wherein a total thickness of said light-transmitting layer and said protection layer is approximately 100 µm. 24. The optical information recording medium as claimed in claim 20, wherein a total thickness of said light-transmitting layer and said protection layer is approximately 75 µm. 25. An optical information recording medium comprising: a substrate including a clamp area and an information recording area; at least one information recording layer formed on the information recording area of said substrate; and a light-transmitting layer formed on said substrate and covering said information recording layer and the clamp area, wherein the clamp area is located inward of the information recording area of said substrate, an average thickness of said light transmitting layer adjacent to the clamp area is thinner than an average thickness of said light transmitting layer adjacent to the information recording area, the average thickness of said light transmitting layer adjacent to the clamp area is greater than one half of the average thickness of said light transmitting layer adjacent to the information recording area, and said light-transmitting layer is a monolayer of radiation setting resin. 26. The optical information recording medium as claimed in claim 25, wherein said substrate has a surface that is free of an information recording layer. 27. The optical information recording medium as claimed in claim 25, wherein a surface of said light transmitting layer adjacent to the clamp area and facing away from the clamp area is recessed with respect to a surface of said light transmitting layer adjacent to the information recording area and facing away from the information recording area.

Full Text

OPTICAL INFORMATION RECORDING MEDIUM
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical information recording medium, and more
particularly relates to an optical information recording medium having at least one information
recording layer and a light-transmitting layer on a main surface of a substrate.
2. Description of the Prior Art
Recently, research relating to various types of optical information recording has been
studied in the optical information recording field. Optical information recording methods have
been emerging which are applicable to a range of uses, because these methods can allow media
to have higher density, can record/reproduce information by a non-contact method, and can
also achieve these objectives at a low price. Currently, an optical disk has a structure produced,
for example, by forming an information layer on a transparent resin layer with a thickness of
1.2 mm, and then covering and protecting the layer with an over coating, or by forming
information layers on one or both sides of a transparent resin layer with an over coating, or by
forming information layers on one or both sides of a transparent resin layer with a thickness of
0.6 mm, and then laminating two of the information layers.
Recently, as a way to increase the recording density of optical disks, methods have
been studied such as increasing the numerical aperture (NA) of an objective lens, and
shortening the wavelength of the laser. In these methods, if the thickness of a
recording/reproducing carrier (which is a substrate of the side on which an optical laser is
incident) is thin, the influence of an aberration of the laser spot decreases, and allowance for a
gradient angle (tilt) of a disk increases. From this, an idea was proposed to set the thickness of
the recording/reproducing carrier to be around 0.1 mm, NA to be around 0.85, and the

wavelength of a laser to be around 400 nm (see Japanese unexamined patent publication
H08-235638, for example). Here, considering effects on the focus of the recording/reproducing
light and risk of spherical aberration, it is preferable that the thickness variation of the
recording/reproducing carrier is reduced to be within 5%. Even in an optical disk having such
reduced thickness variation, the recording/reproducing carrier of which has thickness is 0.1
mm, then a thickness of the disk is preferably 1.2 mm like a conventional CD or DVD, because
the disk should have compatibility with existing hardware.
An optical disk having a recording/reproducing carrier with a thickness of 0.1 mm
has an asymmetrical structure in the thickness direction, because the thickness of the disk is
1.2 mm. Because of the asymmetrical structure, warping of the optical disk occurs due to
variations in environmental conditions such as temperature or humidity. More specifically,
when placing the optical disk, which is at room temperature, in a recording/reproducing
machine which has been heated to 50 degrees C by uninterrupted operations, drastic warping
occurs in a short time.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an optical information recording
medium which has tolerance to warping.
The information described below was discovered in an investigation by the present
inventors. That is, the warping of an optical information recording medium occurs because
heat conduction from a clamp area is fast. (The clamp area is a contact portion of the recording
medium and a recording/reproducing machine)
Therefore, an optical information recording medium of the present invention
comprises a substrate, at least one information recording layer formed on a main surface of the
substrate and a light-transmitting layer which covers the information recording layer. The
substrate comprises a clamp area and an information recording area corresponding to the

information recording layer. An average thickness of the light-transmitting layer corresponding
to the clamp area is thinner than an average thickness of the light-transmitting layer
corresponding to the information recording area.
In the medium, even when the medium is placed in a drive which has been heated by
repeated recording/reproducing, a stable recording/reproducing of the medium is possible
without causing drastic warping.
In addition, in the medium of the present invention, a thickness of the substrate and
the thickness of the light-transmitting layer are different.
The medium has an asymmetrical structure in a direction perpendicular to the film
surface. Here, the warping caused by the asymmetrical structure can also be reduced.
The medium of the present invention has a total thickness in the range of
approximately 1.14 to 1.50 mm.
In the medium of the present invention, the light-transmitting layer corresponding to
the information recording area preferably has a thickness of approximately 100 µm, or
approximately 75 µm.
In the medium, it is preferable that the average thickness of the light-transmitting
layer corresponding to the clamp area is approximately 95% or less than the average thickness
of the light-transmitting layer corresponding to the information recording area.
Thus, warping of the medium can be reduced.
In the medium, it is also preferable that the average thickness of the light-transmitting
layer corresponding to the clamp area is approximately 70% or more than the average thickness
of the light-transmitting layer corresponding to the information recording area.
Thus, a focusing position of an optical recording/reproducing beam is stably found
when performing the recording/reproducing of the medium.
A light-transmitting layer is radiation setting resin.
In addition, the light-transmitting layer comprises a plurality of materials. At least

one of the plurality of materials is preferably radiation setting resin.
Because of this, the medium can be made inexpensively, i.e. without using expensive
materials.
At least one of the plurality of materials is preferably polycarbonate, and at least one
of the plurality of materials is an adhesive material. The protection layer preferably has a
pencil hardness of H or more.
In the optical information recording medium of the present invention, there is
preferably a protection layer formed on a part of or on a whole area of a recording/reproducing
side of the light-transmitting layer.
Thus, the light-transmitting layer is not easily scratched, and the quality of
recorded/reproduced signals does not easily deteriorate.
In this situation, an average thickness of the protection layer corresponding to the
clamp area is thinner than an average thickness of the protection layer corresponding to the
information recording area.
In addition, the optical information recording medium of the present invention
comprises a substrate, at least one information recording layer formed on a main surface of the
substrate and a light-transmitting layer which covers the information recording layer. The
substrate comprises a clamp area and an information recording area corresponding to the
information recording layer. The average thickness of the light-transmitting layer
corresponding to the clamp area is thinner than the average thickness of the light-transmitting
layer corresponding to the information recording area. Alternatively, the protection layer may
not be formed on the clamp area.
In the optical information recording medium of the present invention, even when the
medium is placed in a drive which has been heated by repeated recording/reproducing, a stable
recording/reproducing of the medium is possible without causing drastic warping.
In the medium, the thickness of the light-transmitting layer and the thickness of the

substrate are different.
In the medium, a total thickness of the light-transmitting layer and the protection
layer is preferably approximately 100 µm, or approximately 75 urn.
Here, the pencil hardness is determined by placing a sharpened pencil against the
surface with a weight of 1 kg at an angle of 45 degrees, pulling the pencil under these
conditions, and determining whether the surface is scratched or not. The pencil hardness is
measured in accordance with JIS-K5400.
With an optical information recording medium of the present invention, even when
the disk is placed in a drive which has been heated by repeated recording/reproducing, a stable
recording/reproducing of the medium is possible without causing warping.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 is a cross-sectional diagram showing an example of an optical information
recording medium in Embodiment 1 of the present invention.
Fig. 2 a cross-sectional diagram showing an example of a conventional optical
information recording medium.
Fig. 3 is a graph showing warp variation of a conventional optical information
recording medium.
Fig. 4 (a)-(b) are diagrams showing that how warping of a conventional optical
information recording medium is determined.
Fig. 5 is a cross-sectional diagram showing a conventional optical information
recording medium which is clamped.
Fig. 6 (a) is a cross-sectional diagram showing an optical information recording
medium of Example 1 of the present invention, and
Fig. 6 (b) is a graph showing a relation between the thickness of a light-transmitting
layer of a clamp area and a maximum amount of warp variation in the optical information

recording medium of Example 1.
Fig. 7 is a cross-sectional diagram showing an example of a conventional optical
information recording medium.
Fig. 8 is a graph showing a warp variation of a conventional optical information
recording medium.
Fig. 9 (a) is a cross-sectional diagram showing an optical information recording
medium of Example 2 of the present invention, and
Fig. 9 (b) is a graph showing the relationship between the thickness of a
light-transmitting layer in a clamp area and the maximum value of the amount of warp
variation in the optical information recording medium of Example 2.
Fig. 10 (a)-(c) are cross-sectional diagrams showing an example of deformation for
an optical information recording medium of Embodiment 1 of the present invention.
Fig. 11 (a)-(c) are cross-sectional diagrams showing an example of an optical
information recording medium of Embodiment 2 of the present invention.
Fig. 12 is a cross-sectional diagram showing an example of the optical information
recording medium of Embodiment 3 of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, embodiments of the present invention will be explained by referring to
the figures. These figures are shown by way of cross-section unless otherwise specified. If the
figures are symmetric, then only a portion from the axis of symmetry is shown, and the rest of
the figure is omitted.
Embodiment 1
Fig. 1 is an explanatory diagram showing an optical information recording medium
100 of the present invention. Fig. 1 shows a brief overview of the optical information

recording medium 100 of the present invention. The medium 100 comprises an information
recording layer 103 and a light-transmitting layer 106 on a main surface of a substrate 101
having a center hole 102. An information recording area 104 is an annular area on the substrate
101, where the information recording layer 103 is formed. In addition, an area formed at an
inner area beyond an innermost circumference of the information recording area 104, where
the medium 100 is clamped, is defined as a clamp area 105. The information recording area
104 and the clamp area 105 are neither overlapped nor contacted, and an annular area is
formed between those areas. The light-transmitting layer 106 is formed to cover the
information recording layer 103 at the information recording area 104 and also to cover the
substrate 101 at the clamp area 105. More precisely, the light-transmitting layer 106 covers the
information recording area 104 and the clamp area 105 on the substrate 101. In the
light-transmitting layer 106, an area corresponding to the information recording area 104 is
defined as a first area 106a, and an area corresponding to the clamp area 105 is defined as a
second area 106b.
Here, in the light-transmitting layer 106 of the medium 100 of the present invention,
an average thickness of the second area 106b is thinner than an average thickness of the first
area 106a.
By employing the medium 100, even when the medium is placed in a drive which has
been heated by repeated recording/reproducing, a stable recording/reproducing of the medium
is possible without causing rapid, drastic warping.
Note that, in this Embodiment, the light-transmitting layer 106 can have both of the
structures (1) comprising a sheet substrate and an adhesive material as shown in Example 1
described below; and (2) comprising a radiation-setting resin as shown in Example 2.
(Example 1)
Below, structures of an optical information recording medium based on a
conventional technique and of the present invention are compared. Note that, Fig. 2 through

Fig. 5 show the conventional technique, and Fig. 6 shows the present invention.
As shown in Fig. 6, the optical information recording medium 110 comprises a
substrate 111, an information recording layer 113 and a transmitting layer 116. The substrate
111 has a thickness of approximately 1.1 mm and a diameter of approximately 120 mm, and is
made of polycarbonate applied by an injection molding method. The information recording
layer 113 comprises a guide-groove with a depth of approximately 20 nm formed on the
substrate 111 and a recording film which contains phase-changeable materials or dielectric
materials formed on the guide-groove. The transmitting layer 116 is formed on the information
recording layer 113 of the substrate 111 and comprises a sheet substrate 117 with a thickness of
approximately 70 µm and an adhesive material 118 with a thickness of approximately 30 µm.
The light-transmitting layer 116 has a thickness of approximately 100 µm and an inside
diameter of approximately 22 mm. The layer 116 comprises an adhesive material 118 and a
sheet substrate 117 that are fixed together as an integrated unit. The layer 116 can be made by
applying the integrated unit onto the substrate 111 with a roller. The layer 116 may also be
formed by the method of superimposing both items in a vacuum. The sheet substrate 117 is
made of polycarbonate applied by a casting method and the adhesive material 118 is made of a
resin material including acrylate as a main component.
The recording/reproducing was performed on the optical information recording
medium 110 by using a laser with a wavelength of 405 nm and an objective lens with a
numerical aperture of 0.85. By making the thickness of the light-transmitting layer 116 to be
approximately 100 µm the medium 110 could achieve warp tolerance equivalent to that of a
conventional DVD.
In the light-transmitting layer 116 of the optical information recording medium 110,
an average thickness of an area corresponding to a clamp area 115 is thinner than an average
thickness of an area corresponding to an information recording area 114. Here, a thickness of
the adhesive material 118 is constant at 30um, and the thickness of the sheet substrate 117 is

different in an area 117a corresponding to the information recording area and an area 117b
corresponding to the clamp area.
Note that, in the structure of the conventional device shown in Fig. 2, the thickness of
the sheet substrate 117 is uniform, thus the average thickness of the light-transmitting layer 116
is uniform over the whole area.
Soon after placing the conventional optical information recording medium 110 shown
in Fig. 2 in a drive which was heated to approximately 50 degrees C by repeated
recording/reproducing, drastic warping occurred in a short time on the medium 110, as shown
in Fig. 3. Fig. 3 shows the amount of warp variation at a radial position of 58 mm of the
medium 110 and the time when the medium is placed in the drive is defined as 0 seconds. Here,
the amount of warp is identified as an angle 131 which is defined by a laser beam reflected on
the information recording layer 113 and a laser beam entering from vertically below, which is
the side of recording/reproducing data on the medium 110. The direction of warp shown in Fig.
4 (a) is defined as plus and that shown in Fig. 4 (b) is defined as minus.
As a reason for the occurrence of such drastic warping, it was assumed that the
light-transmitting layer 116 had thermally expanded due to heat conduction from the clamp
area 115. Fig. 5 shows the optical information recording medium 110 is clamped. A rotation
center of the medium 110 is determined by a center cone 120, and the medium 110 is clamped
at the clamp area 115 by a clamper 121 and a turn table 119. The temperature of the whole
drive became approximately 50 degrees C, however, the heat conducted faster from the
clamper 121 and the turn table 119 (both of which directly contacted with the clamp area 115)
toward the medium 110, than from the air. This greatly affected the occurrence of the warping.
As a way to reduce such warping of the optical information recording medium 110, a
coefficient of thermal expansion or modulus of rigidity (Young's modulus) should have a small
value. However, in order to achieve a small coefficient of thermal expansion, materials for use
for the light-transmitting layer 116 are considerably restricted, and the medium 110 itself is

possibly expensive.
Next, in the structure of the present invention shown in Fig. 6, by changing the
thickness of the light-transmitting layer 116 formed on the clamp area 115, the amount of warp
of the medium 110 was measured.
As shown in Fig. 6 (b), the thickness of the light-transmitting layer 116
corresponding to the clamp area 115 was changed relative to the thickness of the
light-transmitting layer 116 corresponding to the information recording area 114. Here, on the
layer 116 comprising the sheet substrate 117 and an adhesive material 118, the thickness of the
adhesive material 118 was 30 µm, and only the thickness of the region 117b of the sheet
substrate 117 corresponding to the clamp area was changed.
As shown in Fig. 6 (b), the warping decreased by reducing the thickness of the
light-transmitting layer 116 corresponding to the clamp area 115. This was considered to be
because the thickness of the light-transmitting layer 116 corresponding to the clamp area 115
was reduced, and so the modulus of rigidity became small and the force which warped the
substrate 111 also weakened. From these findings, reducing the thickness of the layer 116
corresponding to the clamp area 115 up to approximately 60 µm is extremely effective;
however, it is not very effective over 60 µm. It is preferable that the amount of warp of the
optical information recording medium is as small as possible. By reducing the amount of warp
by approximately 80%, the recording/reproducing properties can greatly improve. In other
words, the thickness of the light-transmitting layer 116 corresponding to the clamp area 115 is
preferably 95 µm or less, and is preferably 95% or less than the thickness of the
light-transmitting layer 116 corresponding to the information recording area 114.
Here, when reducing the layer 116 corresponding to the clamp area 115, the
thicknesses of the layer 116 corresponding to the clamp area 115 and the layer 116
corresponding to the information recording area 114 are different, so the surface of the medium
is uneven. This may cause the problem that the focusing position of the laser when

recording/reproducing may easily become unstable. Therefore, if the thickness of the clamp
area 115 is different in each disk, the range of motion for a drive system which changes the
focusing position of the recording/reproducing laser needs to be wide. Thus, the thickness of
the light-transmitting layer 116 corresponding to the clamp area 115 is preferably close to the
thickness of the layer 116 corresponding to the information recording area 114. Namely, the
thickness of the light-transmitting layer 116 corresponding to the clamp area 115 is preferably
70 urn or more, and is preferably 70% or more of the thickness of the layer 116 corresponding
to the information recording area 114.
A light-transmitting layer formed of a sheet substrate and radiation-setting resin (UV
radiation-setting resin, for example) also had almost the same results in comparison to the
light-transmitting layer 116 formed of the sheet substrate 117 and the adhesive material 118 as
described in the present embodiment. In addition, when the proportion of the thickness of the
adhesive material or the radiation-setting resin was changed in a range between 5% and 50% of
the light-transmitting layer, almost the same effect was obtained.
Note that, "radiation" here includes all electromagnetic waves which can harden
radiation-setting resin, for example infrared rays, visible rays, ultraviolet rays, X rays and the
like.
In this embodiment, an example of a rewriteable optical disk was described. In
addition, the present invention can be applied to a write-once type disk which can record only
once and a read-only type optical disk having A1 or Ag as main components in the reflective
layer. Furthermore, the present invention can be applied not only in situations in which only
one layer is formed on the information recording layer 113 as shown in this embodiment, but
also in situations in which a plurality of layers are formed on the information recording layer
113, or a multi-layered optical disk which includes two or more information recording layers
113.
Hereinabove, the embodiment of the present invention has been explained by using

examples. Note that, the invention is not limited to the above-mentioned embodiment, and it is
also possible to apply the present invention to other embodiments based on the technical idea
of the present invention. In addition, the optical information recording medium 100 of the
present invention can be available for a Blu-ray disc, DVD, CD, MD, MO and the like, for
example.
(Example 2)
Below, structures of an optical information recording medium based on a
conventional technique and of the present invention are compared. For descriptions which
correspond to Example 1, overlapping explanations may be omitted. Note that, Fig. 7 and Fig.
8 show the conventional technique, and Fig. 9 shows an embodiment of the present invention.
In Fig. 9 (a), an optical information recording medium 200 of the present invention is
shown. In the medium 200 in Fig. 9, a substrate 201 was employed, which was the same as the
substrate of Example 1, and a light-transmitting layer 206 was formed only of radiation-setting
resin.
Here, in the light-transmitting layer 206 of the medium 200 of the present invention,
the average thickness of a second region 206b corresponding to the clamp area is thinner than
an average thickness of a first region 206a corresponding to the information recording area.
Note that, in the structure of the conventional technique shown in Fig. 7, the average
thickness of the light-transmitting layer 206 is uniform over the whole area.
The warping of the conventional medium 200 shown in Fig. 7 was measured when
placing it in a drive which was heated to approximately 50 degrees C, the same as in Example
1. The results of the measurement are shown in Fig. 8. The denotations of the graph are the
same as Fig. 3 (the example of the conventional technique) in Embodiment 1. Compared to the
light-transmitting layer 116 of Embodiment 1 (see Fig. 3) formed by the sheet substrate 117
and the adhesive material 118, the amount of warp became large in the present example in
which only the radiation-setting resin is used. The radiation-setting resin here including

acrylate as a main component has an enormously large coefficient of thermal expansion
compared to that of polycarbonate used in a general substrate. Therefore, the amount of warp
becomes extremely large.
Here, in the structure of the present invention shown in Fig. 9 (a), the amount of warp
was measured while changing the thickness of a second area 206b of the light-transmitting
layer 206 corresponding to the clamp area. The thickness of the second area 206b was reduced
by polishing. The amount of warp while changing the thickness is shown in Fig. 9 (b). Because
the maximum amount of warp variation was as large as 0.5 degrees when the thickness was
100 µm, reducing the thickness of the second region corresponding to the clamp area 206b was
found to be very effective. The maximum amount of warp variation is preferably around 0.3
degrees. Like Example 1, in order to reduce the amount of warp by 80% or more, the thickness
of the second area 206b is preferably about 95 µm, and is preferably 95% or less than the
thickness the first region corresponding to the information recording area 206a.
In addition, as described in Example 1, if the second region 206b of the
light-transmitting layer 206 corresponding to the clamp area is too thin, then the focusing
position of the recording/reproducing laser becomes unstable. More specifically, the thickness
of the second region 206b is preferably 70 µm or more, and is preferably 70% or more than the
first region 206a corresponding to the information recording area.
(Modification example of the Embodiment 1)
When forming the light-transmitting layer 206 using radiation-setting resin, the shape
of the layer 206 easily changes because the resin is liquid before curing. Therefore the shape of
the region of the light-transmitting layer corresponding to the clamp area can have not only the
shape shown in Fig. 9, but can also easily have: the shape of the second region 216b
corresponding to the clamp area shown in Fig. 10 (a) which gradually becomes thicker toward
the outer circumference; the shape of the second region 226b corresponding to the clamp area
shown in Fig. 10 (b) which gradually becomes thinner toward the outer circumference; or the

shape of the second region 236b corresponding to the clamp area shown in Fig. 10 (c) which
has a wavy shaped cross-section. Even with these shapes, the effect was almost the same as
that described in Embodiment 1, and the amount of warp of the medium was found to depend
on the average thickness of the second region of the light-transmitting layer corresponding to
the clamp area.
However, in the shapes of the second region corresponding to the clamp area as
shown in Fig. 10 (a) to Fig. 10 (c), clamping the optical information recording medium
becomes difficult and it may affect the recording/reproducing. Thus, the shape should be
chosen not to affect the recording/reproducing.
Here, although only the example of the light-transmitting layer is described, which is
made of one type of radiation-setting resin, the layer may have a structure in which a plurality
types of radiation-setting resin are stacked. Thus, in the light-transmitting layer comprising a
plurality of layers, a similar effect can also be obtained.
In this embodiment, an example of a rewriteable optical disk is described. In addition,
the present invention can be applied to a write-once type disk which can record only once and
a read-only type optical disk having Al or Ag as main components in the reflective layer.
Furthermore, the present invention can be applied not only in situations in which only one
layer is formed on the information recording layer, but also situations in which multiple layers
are formed on the information recording layer, or a multi-layered optical disk that includes two
or more information recording layers.
Hereinabove, the embodiment of the present invention is explained by examples.
Note that, the invention is not limited to the above-mentioned embodiment, and it is also
possible to apply the present invention to other embodiments based on the technical idea of the
present invention.
Embodiment 2
An example of an optical information recording medium according to the present

invention will be explained using Fig. 11. Note that, for descriptions that correspond to
embodiment 1, overlapping explanations may be omitted. Note that, Fig. 11 (b) and (c) show
the structure of the present invention, and Fig. 11 (a) shows the structure of the conventional
technique.
In a disk with high-density of which the thickness of the light-transmitting layer (as
described in Embodiment 1) is approximately 100 µm, and recording/reproducing is performed
under the conditions that NA is approximately 0.85 and the wavelength of the laser is
approximately 400 run, the signal quality easily deteriorates compared to conventional CDs
and DVDs, because of flaws or dust on the surface of the light-transmitting layer. Thus, it is
necessary to form the protection layer on the surface of the light-transmitting layer of the
optical information recording medium 300.
The protection layer needs to have high scratch tolerance, and its pencil hardness is
preferably H or more. To form the protection layer, UV setting resin is generally used. By
mixing inorganic oxide particles into the resin, the hardness can further increase. When
forming the protection layer using such UV setting resin, the layer usually has a large
coefficient of thermal expansion and modulus of rigidity because it has a high degree of
hardness after hardening.
When the protection layer has a larger coefficient of thermal expansion and a higher
modulus of rigidity than those of the material for the substrate, even if the layer is made to be
thin, such as with a thickness of approximately 1-5 µm, the layer expands by the heat
conducted from the clamper 121 or the turn table 119 as described in Fig. 5 (the conventional
example) of Embodiment 1, and more drastic warping will occur as described in the
conventional example of Embodiment 1.
In the conventional optical information recording medium shown in Fig. 11 (a), the
protection layer 307 is formed on the whole recording/reproducing side of the transmitting
layer 306. The average thickness of the protection layer 307 formed on the clamp area 305 is

equal to the average thickness of the protection layer 307 formed on the information recording
area 304.
On the contrary, in the optical information recording medium 300 of the present
invention shown in Fig. 11 (b), the average thickness of the second region 317b of the
protection layer 317 corresponding to the clamp area is thinner than the average thickness of
the first region 317a corresponding to the information recording area. Note that, the protection
layer is generally thin so that the disk can be stably clamped in most situations. It is also
possible not to form a protection layer on the clamp area of an optical information recording
medium.
In addition, as shown in Fig. 11 (c), it is also possible that both the thickness of a
light-transmitting layer 326 and a protection layer 327 of the medium 300 in the clamp area
305 is thinner than those in the information recording area 304, respectively. More precisely, in
the light-transmitting layer 326, an average thickness of a second region 326b corresponding to
the clamp area is thinner than an average thickness of the first region 326a corresponding to
the information recording area. Furthermore, in a protection layer 327, an average thickness of
the second region 327b is thinner than an average thickness of the first region 327a
corresponding to the information recording area. In Fig. 11 (c), an example is shown in which
the light-transmitting layer 326 is formed in a tapered shape in the clamp area 305; however,
the layer 326 may have a shape like the light-transmitting layer 106 shown in Fig. 1.
Hereinabove, the embodiment of the present invention is explained by examples.
Note that, the invention is not limited to the above-mentioned embodiment, and it is also
possible to apply the present invention to other embodiments based on the technical idea of the
present invention.
Embodiment 3
Here, an example of an optical information recording medium according to the
present invention is explained. Note that, for descriptions which correspond to embodiments 1

or 2, overlapping explanations may be omitted.
In the above-mentioned Embodiments, although a medium having only one
information recording layer has been explained, the present invention is also effective for a
medium having two or more information recording layers, which is called a multiple-layer
disk.
For example, in an optical information recording medium 400 shown in Fig. 12
having two information recording layers, which is called a double-layer disk, an intermediate
layer 408 having a range of thickness between approximately 20 µm and approximately 30 µm
is formed between a first information recording layer 403 and a second information recording
layer 413, and a light-transmitting layer 406 is formed in a range of thickness between
approximately 70 µm to approximately 80 µm. Here, in the light-transmitting layer 406, the
effect of decreasing the warp as described in Embodiment 1 or 2 was also obtained by reducing
the average thickness of the second region 406b corresponding to the clamp area compared to
that of the first region 406a corresponding to the information recording area. Particularly, when
the average thickness of the second region 406b was reduced by 95% or less, a better effect
was obtained.
The reason for choosing the range of thickness between 20 and 30 µm for the
intermediate layers is to decrease the effect of the reflected light from the other layers when
recording/reproducing each layer, and to make the recording/reproducing light capable of
distinguishing the difference of the depth from the surface of the light-transmitting layer of the
first information recording layer 403 and the second information recording layer 413. By
setting the depth from the surface of the light-transmitting layer of the first information
recording layer 403 at approximately 100 urn, the medium can achieve warp tolerance
equivalent to that of a conventional DVD as described in Embodiment 1.
It is possible to achieve the same effect by reducing the intermediate layer 408
corresponding to the clamp area 405 or both the light-transmitting layer 406 and the

intermediate layer 408 corresponding to the clamp area 405, rather than by reducing the second
region 406b of the light-transmitting layer 406 corresponding to the clamp area. Also in these
situations, the total thickness of the light-transmitting layer 406 and the intermediate layer 408
corresponding to the clamp area 405 is preferably 95% or less than the total thickness of the
light-transmitting layer 406 and the intermediate layer 408 corresponding to the information
recording area 404. In addition, as described in Embodiment 1, in order to avoid a wide range
of motion for the drive system which changes the focusing position of the
recording/reproducing laser, the total thickness of the light-transmitting layer 406 and the
intermediate layer 408 corresponding to the clamp area 405 is preferably 70 µm or more, and is
preferably 70% or more than the total thickness of the light-transmitting layer 406 and the
intermediate layer 408 corresponding to the information recording area 404.
Even in such multiple-layer disk, having a protection layer as described in
Embodiment 2 is preferable. Therein, reducing warping of the optical information recording
medium is preferably by such method of reducing the thickness of the protection layer
corresponding to the clamp area as described in Embodiment 2 or the like.
Hereinabove, the embodiment of the present invention is explained by examples.
Note that, the invention is not limited to the above-mentioned embodiment, and it is also
possible to apply the present invention to other embodiments based on the technical idea of the
present invention.
An optical information recording medium of the present invention has an advantage
of having tolerance to warping, and the medium will be useful for a recording medium such as
Blu-ray disc.

We Claim:
1. An optical information recording medium comprising: a substrate having a
clamp area and an information recording area;
at least one information recording layer formed on the information recording
area of said substrate; and
a light-transmitting layer formed on said substrate and covering said information
recording layer and the clamp area, wherein the clamp area is located inward of
the information recording area of said substrate, an average thickness of said
light-transmitting layer adjacent to the clamp area is thinner than an average
thickness of said light-transmitting layer adjacent to the information recording
area, said substrate has a surface that is free of an information recording layer,
and said light-transmitting layer is a monolayer of radiation setting resin.
2. The optical information recording medium as claimed in claim 1, wherein a
thickness of said substrate and a thickness of said light-transmitting layer are
different.

3. The optical information recording medium as claimed in claim 1, wherein the
optical information recording medium has a total thickness in a range of
approximately 1.14 to 1.50 mm.
4. The optical information recording medium as claimed in claim 1, wherein said
light-transmitting layer adjacent to the information recording area has a
thickness of approximately 100µm.
5. The optical information recording medium as claimed in claim 1, wherein said
light-transmitting layer adjacent to the information recording area has a
thickness of approximately 75 µm.
6. The optical information recording medium as claimed in claim 1, wherein the
average thickness of said light-transmitting layer adjacent to the clamp area is
approximately 95% or less than the average thickness of said light-transmitting
layer adjacent to the information recording area.
7. The optical information recording medium as claimed in claim 1, further
comprising a protection layer formed on at least a part of a
recording/reproducing side of said light-transmitting layer.

8. The optical information recording medium as claimed in claim 7, wherein said
protection layer has a pencil hardness of H or more.
9. The optical information recording medium as claimed in claim 7, wherein an
average thickness of said protection layer adjacent to the clamp area is thinner
than an average thickness of said protection layer adjacent to the information
recording area.

10. The optical information recording medium as claimed in claim 1, wherein a
surface of said light transmitting layer adjacent to the clamp area and facing
away from the clamp area is recessed with respect to a surface of said light
transmitting layer adjacent to the information recording area and facing away
from the information recording area.
11. An optical information recording medium comprising: a substrate including a
clamp area and an information recording area;
at least one information recording layer formed on the information recording
area of said substrate; and
a light-transmitting layer formed on said substrate and covering said information
recording layer and the clamp area, wherein the clamp area is located inward of
the information recording area of said substrate, an average thickness of said

light-transmitting layer adjacent to the clamp area is thinner than an average
thickness of said light-transmitting layer adjacent to the information recording
area, said substrate has a surface that is free of an information recording layer,
said light-transmitting layer is radiation setting resin, and the average thickness
of said light-transmitting layer adjacent to the clamp area is approximately 70%
to 95% of the average thickness of said light-transmitting layer adjacent to the
information recording area.
12. The optical information recording medium as claimed in claim 11, wherein a
thickness of said substrate and a thickness of said light-transmitting layer are
different.
13. The optical information recording medium as claimed in claim 11, wherein
the optical information recording medium has a total thickness in a range of
approximately 1.14 to 1.50 mm.
14. The optical information recording medium as claimed in claim 11, wherein
said light-transmitting layer adjacent to the information recording area has a
thickness of approximately 100 µm.

15. The optical information recording medium as claimed in claim 11, wherein
said light-transmitting layer adjacent to the information recording area has a
thickness of approximately 75 µm.
16. The optical information recording medium as claimed in claim 11, wherein
the average thickness of said light-transmitting layer adjacent to the clamp area
is approximately 95% or less than the average thickness of said light-
transmitting layer adjacent to the information recording area.
17. The optical information recording medium as claimed in claim 11, further
comprising a protection layer formed on at least a part of a
recording/reproducing side of said light-transmitting layer.
18. The optical information recording medium as claimed in claim 17, wherein
said protection layer has a pencil hardness of H or more.
19. The optical information recording medium as claimed in claim 17, wherein an
average thickness of said protection layer adjacent to the clamp area is thinner
than an average thickness of said protection layer adjacent to the information
recording area.

20. An optical information recording medium comprising: a substrate including a
clamp area and an information recording area;
at least one information recording layer formed on the information recording
area of said substrate;
a light-transmitting layer formed on said substrate and covering said information
recording layer and the clamp area; and
a protection layer formed on at least a part of a recording/reproducing side of
said light-transmitting layer, wherein the clamp area is located inward of the
information recording area of said substrate, an average thickness of said
protection layer adjacent to the clamp area is thinner than an average thickness
of said protection layer adjacent to the information recording area, said substrate
has a surface that is free of an information recording layer, and said light-
transmitting layer is a monolayer of radiation setting resin.
21. The optical information recording medium as claimed in claim 20, wherein
said protection layer has a pencil hardness of H or more.
22. The optical information recording medium as claimed in claim 20, wherein a
thickness of said light-transmitting layer and a thickness of said substrate are
different.

23. The optical information recording medium as claimed in claim 20, wherein a
total thickness of said light-transmitting layer and said protection layer is
approximately 100 µm.
24. The optical information recording medium as claimed in claim 20, wherein a
total thickness of said light-transmitting layer and said protection layer is
approximately 75 µm.
25. An optical information recording medium comprising: a substrate including a
clamp area and an information recording area;
at least one information recording layer formed on the information recording
area of said substrate; and
a light-transmitting layer formed on said substrate and covering said information
recording layer and the clamp area, wherein the clamp area is located inward of
the information recording area of said substrate, an average thickness of said
light transmitting layer adjacent to the clamp area is thinner than an average
thickness of said light transmitting layer adjacent to the information recording
area, the average thickness of said light transmitting layer adjacent to the clamp
area is greater than one half of the average thickness of said light transmitting
layer adjacent to the information recording area, and
said light-transmitting layer is a monolayer of radiation setting resin.

26. The optical information recording medium as claimed in claim 25, wherein
said substrate has a surface that is free of an information recording layer.
27. The optical information recording medium as claimed in claim 25, wherein a
surface of said light transmitting layer adjacent to the clamp area and facing
away from the clamp area is recessed with respect to a surface of said light
transmitting layer adjacent to the information recording area and facing away
from the information recording area.

Documents:

786-KOL-2004-(05-09-2011)-CORRESPONDENCE.pdf

786-KOL-2004-(20-03-2012)-CORRESPONDENCE.pdf

786-KOL-2004-(29-08-2012)-ANNEXURE TO FORM 3.pdf

786-KOL-2004-(29-08-2012)-CORRESPONDENCE-1.pdf

786-KOL-2004-(29-08-2012)-CORRESPONDENCE.pdf

786-KOL-2004-(30-11-2012)-CORRESPONDENCE.pdf

786-KOL-2004-(31-10-2012)-FORM-13.pdf

786-kol-2004-abstract.pdf

786-KOL-2004-CANCELLED PAGES.pdf

786-kol-2004-certified copies(other countries).pdf

786-kol-2004-claims.pdf

786-kol-2004-description (complete).pdf

786-kol-2004-drawings.pdf

786-kol-2004-form 1.pdf

786-kol-2004-form 13.pdf

786-kol-2004-form 18.pdf

786-kol-2004-form 2.pdf

786-kol-2004-form 3.pdf

786-kol-2004-form 5.pdf

786-kol-2004-gpa.pdf

786-KOL-2004-GRANTED-ABSTRACT.pdf

786-KOL-2004-GRANTED-CLAIMS.pdf

786-KOL-2004-GRANTED-DESCRIPTION (COMPLETE).pdf

786-KOL-2004-GRANTED-DRAWINGS.pdf

786-KOL-2004-GRANTED-FORM 1.pdf

786-KOL-2004-GRANTED-FORM 2.pdf

786-KOL-2004-GRANTED-FORM 3.pdf

786-KOL-2004-GRANTED-FORM 5.pdf

786-KOL-2004-GRANTED-SPECIFICATION-COMPLETE.pdf

786-KOL-2004-OTHERS.pdf

786-kol-2004-others1.1.pdf

786-kol-2004-pa.pdf

786-KOL-2004-PETITION UNDER RULE 137.pdf

786-kol-2004-priority document.pdf

786-KOL-2004-REPLY TO EXAMINATION REPORT.pdf

786-kol-2004-specification.pdf

786-kol-2004-translated copy of priority document.pdf

« Previous Patent

Next Patent »

Patent Number

255694

Indian Patent Application Number

786/KOL/2004

PG Journal Number

12/2013

Publication Date

22-Mar-2013

Grant Date

15-Mar-2013

Date of Filing

01-Dec-2004

Name of Patentee

PANASONIC CORPORATION

Applicant Address

1006, OAZA KADOMA, KADOMA-SHI, OSAKA

Inventors:

#	Inventor's Name	Inventor's Address
1	KAZUYA HISADA	1-2-4-401 MIDORI, TSURUMI-KU, OSAKA CITY, OSAKA 538-0054
2	KEIJI NISHIKIORI	7-32, TOUZU-OKUTANI, YAWATA CITY, KYOTO 614-0171
3	EIJI OHNO	1-12-17, KITAYAMA, HIRAKATA CITY, OSAKA 573-0171

PCT International Classification Number

G01N 33/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2003-411114	2003-12-10	Japan