Process for manufacturing a magneto-optic disc, and a magneto-optic disc manufactured by such a process

Abstract: The present disclosure provides a process for manufacturing a magneto-optic disc comprising at least a transparent substrate (11), a magneto-optic recording film (12), and a protecting film made of resin (13) for protecting the magneto-optic recording film (12), comprising the steps of forming the magneto-optic recoriding film (12) over the entire one side surface of the transparent substrate (11); providing in a portion adjacent to the peripheral edge of the magneto-optic recording film an annular groove (14) extending in depth through the magneto-optic recording film (12) to the transparent substrate (11); and forming the protecting film (13) over the surfaces of the magneto-optic recording film (12) and the annular groove (14).

Description: The present invention relates to a process for manufacturing a magneto-optic recording medium and a magneto-optic recording medium made by such a process. Development of magneto-optic discs as photo memory elements in which information can be rewritten has been actively attempted in recent years. As shown in Fig. 5, a magneto-optic disc basically comprises a transparent substrate 1 with a magneto-optic recording film 2 provided thereon, so that a laser beam or the like is irradiated in a desired magnetic recording field on the magneto-optic recording film 2 to achieve an elevated temperature and to record and erase information magnetically. Since the magneto-optic recording film 2 is usually made of an amorphous rare earth-transition metal alloy and likely to be deteriorated by contact with air, water or the like, a protecting film 3 made of resin is usually formed on the magneto-optic recording film 2, as shown in Fig.5, to protect its surface. In this case, when an outer periphery 2a of the magneto-optic recording film 2 is exposed by the protecting film 3, the magneto-optic recording film 2 is subject to deterioration from the outer periphery 2a. Hence, the outer periphery 2a of the magneto-optic recording film 2 is situated within approximately 4 - 5 mm from an outer peripheral edge 1a of the transparent substrate 1 so as to be also covered by the protecting film 3. In the case that the outer periphery 2a of the magneto-optic recording film 2 is covered by the protecting film 3, the magneto-optic recording film 2 is deposited, with the outer peripheral portion of the substrate 1 being supported by an annular holder 4, as shown in Fig.6, for example by vapour deposition or sputtering from the lower part as indicated by an arrow A, so that the magneto-optic recording film 2 is formed at the surface of the substrate 1 except that supported by the annular holder 4. Fig. 7 shows the result of measurement of reproduced signal quality C/N at specific points on radiuses of the substrate 1 of a magneto -optic disc using a polycarbonate substrate of 65mm radius. In Fig.7, each of plots indicated by a sign ○ shows a measured value at a Plain portion of the disc, and a plot indicated by a sign X shows a measured value at a groove portion of the disc. To be noted is that the plain portion is a region at an outer peripheral portion of the disc having no groove, and the groove portion is a different region provided with a groove. As clearly seen in Fig. 7, the reproduced signal quality C/N in a region at approximately 3 - 5 mm from the outer periphery of the substrate 1 largely comes down due to influence by double refraction or the like. Specific measuring conditions for obtaining data shown in Fig.7 are - rotation speed of the magneto-optic disc: 900 rpm, Laser output: 6 mW, Intensity of outside magnetic field: 300 Oe, Recording frequency: 3.14 - 3.35 MHz (bit length 0.9 µm), Pulse duty (rate of time for a practical recording per recording cycle) : 17%. As seen from the above, the magneto-optic disc using the polycarbonate substrate 1 has a lowering of reproduced signal quality C/N in a part adjacent to the outer peripheral portion. It was preferable for keeping a proper reproduced signal quality C/N that the magneto-optic recording film 2 is not provided in the region of 4 - 5 mm from the outer peripheral portion of the substrate 1. Even in case of a magneto-optic disc using a substrate 1 such as a glass substrate having no lowering of reproduced signal quality C/N in a part adjacent to the outer peripheral portion, it is required to have a supporting margin ℓ of 4 - 5 mm for supporting the substrate 1 by the holder 4 while depositing the magneto-optic recording film 2 with the method shown in Fig. 6, so that the outer periphery 2a of the magneto-optic recording film 2 was to be situated within 4 - 5 mm from the outer peripheral edge la of the substrate 1. The above magneto-optic disc has the problem of being disadvantageous in enlarging its memory capacity due to the fact that a magneto-optic recording film 2 is not formed in a rather wider range (approximately 4 - 5 mm in the radial direction) adjacent to the outer peripheral portion 1a of the substrate 1. A process for manufacturing a magneto-optic disc in which a mask is used to prevent deposition of the recording film in a region close to the outer peripheral edge of the substrate is disclosed in EP-A-0 245 833. The mask is removed before the protective film is deposited so that the edge of the recording film is covered by the protective film. This method has the disadvantages explained above in connection with Figs. 5 and 6. The same document also discloses a process in which the recording film is formed over the entire surface of one side of the substrate. A protective film formed thereafter on the recording film does not cover the edge of the recording film. The recording film is thus not sufficiently protected against corrosion. Patent Abstracts of Japan Vol. 9, No. 221 (P386) (1944) September 7, 1985 referring to JP-A-60-79540 discloses a process for manufacturing an optical recording medium in which a reflecting film is formed on one surface of a substrate except a chamfered portion at the edge of the substrate. It is an object of the present invention to provide a simple to manufacture magneto-optic recording medium having a large storing capacity and being unsusceptible to a deterioration of its properties due to corrosion. It is a further object to provide a process for manufacturing such a recording medium. These objects are met by a process set forth in claims 1 or 5 and a recording medium specified in claims 9 or 10. The features defined in the preamble of the independent claims 1,5,9 and 10 are disclosed in EP-A-0 245 833. The sub-claims are directed to preferred embodiments of the invention. Figs. 1 (a) - (c) are schematical longitudinally sectional views showing a manufacturing procedure of a magneto-optic disc in an embodiment of the present invention, Figs. 2 (a) - (c) are schematical longitudinally sectional views showing a manufacturing procedure of a magneto-optic disc in another embodiment of the present invention, Figs. 3 (a) - (c) are schematical longitudinally sectional views showing a manufacturing procedure of a magneto-optic disc in a further embodiment of the present invention, Figs. 4 (a) - (c) are schematical longitudinally sectional views showing a manufacturing procedure of a magneto-optic disc in a modified embodiment of the present invention, Figs. 5 through 7 show prior art. Fig.5 is a schematic longitudinally sectional view showing a conventional magneto-optic disc, Fig.6 is a schematic longitudinally sectional view showing a depositing process of a magneto-optic recording film for the conventional magneto-optic disc, and Fig.7 is a graph showing a dispersion of reproduced signal quality at specific points on radiuses of a polycarbonate substrate. In the process for manufacturing a magneto-optic disc of an embodiment of the present invention, a transparent substrate, a magneto-optic recording film provided over the surface on one side of the transparent substrate, and a protecting film made of resin for protecting the magneto-optic recording film may be formed with a material known in the art, respectively. In the process according to the present embodiment the magneto-optic recording film is formed on the entire surface of one side of the substrate. Thereafter, an annular groove extending in depth through the magneto-optic recording film to the substrate is provided at a portion adjacent to the peripheral edge of the magneto-optic recording film and a protecting film is then formed on the surfaces of the magneto-optic recording film and the annular groove. In this case, a portion of the magneto-optic recording film defined by the annular groove may be used for recording information, and a peripheral portion of this portion of the magneto-optic recording film is covered by the protecting film filled inside the annular groove. Also, the annular groove is formed at a position nearer the peripheral edge of the substrate, for example, in a narrow region within 1 mm from the peripheral edge of the substrate, so that the magneto-optic recording film serving as a substantial recording region can be extended quite nearer the peripheral edge of the substrate to enable an increase of memory capacity. Also, in the process according to the present invention, a magneto-optic recording film is deposited on the entire surface of one side of a substrate. Thereafter, a peripheral edge of the substrate together with a magneto-optic recording film deposited thereon is cut out to provide a tapered periphery, and a protecting film is then formed over the surfaces of the tapered periphery and of the magneto-optic recording film. In this case, the magneto-optic recording film may be left at an inward position of the tapered periphery, but. a peripheral portion of the magneto-optic recording film is covered by the protecting film on the tapered periphery. Also, in this case, the tapered periphery may be provided in a narrow range within 1 mm from the peripheral edge of the substrate to extend the magneto-optic recording film substantially serving as a recording region to a position quite near the peripheral edge of the substrate. Further, in the process according to the present invention, a tapered periphery is formed at a peripheral edge of a substrate in continuation of one side surface thereof. Thereafter, a magneto-optic recording film is formed at an entire region of at least the one side surface of the substrate, and a protecting film is then formed on the surface of the magneto-optic recording film, inclusive of the above tapered periphery. In this case, upon formation of the protecting film, resin constituting the protecting film flows down along the tapered periphery of the substrate due to weight of the resin, so that a peripheral portion of the magneto-optic recording film is covered by the protecting film. Also, in this case, the tapered periphery is sufficiently provided only in a region quite nearer the peripheral portion of the substrate, so that the magneto-optic recording film substantially serving as a recording region can be extended to a point quite nearer the peripheral portion of the substrate. In addition, in the process according to the present invention, a magneto-optic recording film is firstly formed at an entire surface of one side of a transparent substrate. Then the substrate is disposed within a bottomed tubular holder. and resin is coated on the surface of the magneto-optic recording film and filled in a gap between peripheries of the magneto-optic recording film and the substrate and the inner peripheral surface of the holder. The substrate is brought out from the holder after a predetermined time, so that a protecting film can be formed on the surface of the magneto-optic recording film and at the peripheries of the magneto-optic recording film and the substrate. According to the process, the magneto-optic recording film can be formed over the entire surface of one side of the substrate, and the periphery of the magneto-optic recording film can be covered by the protecting film. A first embodiment of the present invention will be detailed with reference to Fig. 1. As shown in Fig.1 (c), a magneto-optic disc 10 of the present embodiment comprises a transparent substrate 11, a magneto-optic recording film 12 having an axis for easy magnetization extending perpendicularly to the film surface, and a protecting film 13 made of resin for covering the magneto-optic recording film 12. At a portion adjacent to an outer peripheral edge 11a of the substrate 11, there is provided an annular groove 14 having a substantial V-like shaped cross-section and a depth extending through the recording film 12 to the substrate 11. The annular groove 14 is filled with a resin constituting the protecting film 13. Next, a manufacturing process of the magneto-optic disc 10 will be detailed. First, as shown in Fig.1(a), a magneto-optic recording film 12 is formed through vapour deposition or sputtering or the like, on an entire surface of one side of the transparent substrate 11 made of glass or resin such as polycarbonate or PMMA (polymethyl methacrylate). When for the substrate 11 such material is used as polycarbonate which has a lowering of reproduced signal quality C/N due to influence of double refraction near the outer peripheral edge, it is preferable that the substrate 11 in preparation is slightly larger in size than that of a finished product and is to be cut and removed at a portion adjacent to the outer peripheral edge. For example, in case of use of a substrate 11 of 120 mm diameter, a substrate of approximate 130mm diameter is previously formed with a magneto-optic recording film over an entire surface on one side thereof and a portion thereof adjacent to its outer peripheral portion subject to lowering of reproduced signal quality C/N is cut, so that when the substrate 11 is made of polycarbonate, the reproduced signal quality C/N can be ensured at a position adjacent to the outer peripheral portion. The substrate 11 shown in Fig.1(a) has a central round hole, but the round hole is not indispensable. After formation of the magneto-optic recording film 12. near the outer peripheral edge portion thereof is formed by use of a rotary blade an annular groove 14 which is narrow in width and has a substantial V-like shaped cross-section extending through the magneto-optic recording film 12 to the substrate 11, as shown in Fig.1(b). When the substrate 11 has a diameter, for example, of 120 mm, an interval 1₁ in the radial direction between the outer peripheral portion 11a of substrate 11 and the annular groove 14 may be set to be within approximately 1 mm. Next, as shown in Fig.1(c), the protecting film 13 is coated on the surface of the magneto-optic recording film 12 inclusive of the inside of the annular groove 14 by a coating method such as a spin coating that resin is dripped on the substrate 11 being rotated. Then, the protecting film 13 is cured. Materials for the protecting film 13 may be proper resins such as thermosetting, or ultraviolet-curing resins and can be cured by heating or irradiation of ultraviolet light. In the magneto-optic disc 10 manufactured by the above process, a portion of the magneto-optic recording film 12 defined by the annular groove 14 is used for a recording region. An outer peripheral portion 12a of the magneto-optic recording film 12 in this region is covered by the protecting film 13 filled inside the annular groove 14 to prevent deterioration of the magneto-optic recording film 12 from its outer peripheral portion 12a. In addition, since the interval 1₁ between the outer peripheral portion 11a of the substrate 11 and that 12a of the magneto-optic recording film 12 is quite short as less than approximate 1 mm, the memory capacity of the magneto-optic disc 10 is advantageously increased. A second embodiment of the present invention will be detailed with reference to Fig.2. As shown in Fig.2 (c), the magneto-optic disc 20 comprises a transparent substrate 21 having an outer peripheral portion 21a with a tapered periphery 21b at its edge. A magneto-optic recording film 22 is formed on the surface of one side of the substrate 21 excepting the tapered periphery 21b, and the tapered periphery 21b and the surface of the magneto-optic recording film 22 are covered by a protecting film 23. Next, a manufacturing process of the magneto-optic disc 20 will be detailed. First, as shown in Fig.2(a), the magneto-optic recording film 22 is formed in an entire one side surface of the substrate 21 by vapour deposition or sputtering. Then, an edge of the outer peripheral portion 21a of the substrate 21 is obliquely cut to form the tapered periphery 21b at the edge of the substrate 21. In this instance, a portion of the magneto-optic recording film 22 on the edge of the substrate 21 is removed. An interval 1₂ between an outer peripheral portion or tapered pheriphery 22a of the magneto-optic recording film 22 and the outer peripheral portion 21a of the substrate 21 is to be less than approximately 1 mm. Next, an appropriate resin is coated on the surface of the magneto-optic recording film 22 and on the tapered periphery 21b of substrate 21 by spin coating or the like to provide the protecting film 23 which is then cured. In this state, the outer peripheral portion 22a of the magneto-optic recording film 22 is covered by the protecting film 23 extending over the tapered periphery 21b. A third embodiment of the invention will be detailed with reference to Fig.3. As shown in Fig.3(c), a magneto-optic disc 30 comprises a transparent substrate 31 having an outer peripheral portion 31a with a tapered periphery 31b at its edge of the surface of one side of the substrate 31. A magneto-optic recording film 32 is formed over the surface of the substrate 31 inclusive of the tapered periphery 31b. Also, the surface of the magneto-optic recording film 32 inclusive of its outer peripheral portion or tapered periphery 32a is covered by a protecting film 33. Next, a manufacturing process of the magneto-optic disc 30 will be detailed. First, as shown in Fig.3(a), the tapered periphery 31b is formed on the edge of the outer peripheral portion 31a of the substrate 31. In this case, an interval 1₃ in the radial direction between an inner peripheral end of the tapered periphery 31b and its outer peripheral portion 31a is to be less than approximately 1 mm. Then, as shown in Fig.3(b), the magneto-optic recording film 32 is formed over the entire surface of one side of the substrate 31 inclusive of the tapered periphery 31b. Thereafter, as shown in Fig.3(c), the protecting film is coated on the surface of the magneto-optic recording film 32 by spin coating or the like and cured. Upon coating, resin serving as the protecting film 33 flows down due to its weight along the magneto-optic recording film 32 on the tapered periphery 31b and also covers the outer peripheral portion 32a of the magneto-optic recording film 32. In the magneto-optic disc 30 of this embodiment, the magneto-optic recording film 32 deposited on the tapered periphery 31b may be usable for recording information or kept free of use. Even when the magneto-optic recording film 32 on the tapered periphery 31b is not used for recording information, the device can have a sufficient memory capacity since the tapered periphery 31b merely has a width of approximately 1 mm in the radial direction of disc. <Embodiment 4> A fourth embodiment of the invention will be detailed with reference to Fig. 4. As shown in Fig.4(c), a magneto-optic disc 40 comprises a transparent substrate 41 and a magneto-optic recording film 42 formed on an entire surface of one side of the substrate 41, and the surface of the magneto-optic recording film 42 and the outer peripheral portions 42a, 41a of the magneto-optic recording film 42 and the substrate 41 are covered by a protecting film 43 made of resin. Next, a manufacturing process of the magneto-optic disc 40 will be detailed. First, as shown in Fig. 4(a), the magneto-optic recording film 42 is formed over the entire surface of one side of the substrate 41. Then, as shown in Fig.4(b), the substrate 41 is aligned coaxially with and disposed within a bottomed tubular holder 44 which has an inner diameter slightly larger than an outer diameter of the substrate 41 and a depth equal to or larger than a thickness of the substrate 41. Thereafter, resin constituting the protecting film 43 is dripped over the magneto-optic recording film 42 to be coated thereon by spin coating, so that the resin covers the surface of the magneto-optic recording film 42 and also flows in a gap between an inner peripheral surface of the holder 44 and the outer peripheral portions 42a, 41a of the magneto-optic recording film 42 and the substrate 41. In this state, when the resin is cured. the protecting film 43 is formed on the surface of the magneto-optic recording film 42 and at the outer peripheral portions 42a, 41a of the magneto-optic recording film 42 and the substrate 41. Then. the magneto-optic disc 40 is brought out from the holder 44, as shown in Fig.4(c). In the case that the holder 44 is provided at its bottom and on the inner peripheral surface with a mould release layer 44a made of such a material of favourable mould release characteristics as tetrafluoroethylene (Trademark: Teflon), the magneto-optic disc 40 can be easily brought out from the holder 44. The magneto-optic disc manufactured by this embodiment covers the outer peripheral portion of the magneto-optic recording film with the protecting film, so that deterioration of the magneto-optic recording film can be surely prevented. Also, since the magneto-optic recording film is formed on the entire surface of one side of the substrate, the memory capacity can be further enlarged. In the above embodiments, a magneto-optic disc of one-sided recording type has been referred to. For manufacturing magneto-optic disc of double-sided recording type, two magneto-optic discs manufactured by one of the above embodiments may be pasted to each other by use of adhesives, for example.

Claims: A process for manufacturing a magneto-optic recording medium comprising at least a transparent substrate (11;21), a magneto-optic recording film (12;22) on said substrate and a protective film (13;23) for protecting the magneto-optic recording film, said protective film covering an outer peripheral edge (12a;22a) of said recording film, characterised by: forming the recording film over the entire surface area on one side of the transparent substrate so that the recording film extends to the outer peripheral edge (11a;21a) of said transparent substrate; removing a portion of the recording film (12;22) at or immediately adjacent the outer peripheral edge (11a;21a) of the substrate to define a said outer peripheral edge (12a;22a) of the recording film close to the outer peripheral edge of the substrate; and forming the protective film so as to cover the defined outer peripheral edge (12a;22a) of the recording film. A process according to claim 1, wherein the removing step comprises cutting a peripheral groove (14) in the recording film (12) immediately adjacent the outer peripheral edge (11a) of the substrate. A process according to claim 2, wherein said peripheral groove (14) is V-shaped. A process according to claim 2, wherein the removing step comprises chamfering the outer peripheral edge (21a) of the substrate (21) to remove an edge portion of the substrate and a portion of the recording film (22) formed thereon. A process for manufacturing a magneto-optic recording medium comprising at least a transparent substrate (31;41), a magneto-optic recording film (32;42) on said substrate and a protective film (33;43) for protecting the magneto-optic recording film, said protective film covering an outer peripheral edge (32a;42a) of said recording film, characterised by: forming the recording film (32;42) over the entire surface area on one side of the transparent substrate (31;41) so that the recording film extends to the outer peripheral edge (31a;41a) of said transparent substrate; and forming the protective film (33;43) by depositing resin so as to cover said outer peripheral edge (32a;42a) of the recording film at said outer peripheral edge (31a;41a) of the transparent substrate. A process according to claim 5, wherein the transparent substrate (31) has a chamfered outer edge and said recording film is formed so as to cover the chamfer (31b). A process according to claim 5, wherein the step of forming the protective film (43) comprises: disposing the transparent substrate (41) having the recording film (42) formed thereon within a bottomed holder (44) whose inner dimension is slightly greater than the outer dimension of said substrate and whose depth is equal to or greater than the thickness of said substrate; and coating the surface of the recording film and filling the gap between the inner peripheral surface of said holder (44) and the outer peripheral edges (41a,42a) of the substrate and the recording film with said resin to form the protective film (43) on said surface and said edge of the recording film and said edge of the substrate. A process according to claim 7, wherein said holder (44) is provided on its inner peripheral surface with a mould release layer (44a). A magneto-optic recording medium (10) comprising at least a transparent substrate (11), a magneto-optic recording film (12) on the substrate and a protective film (13) covering the recording film, characterised in that the recording film (12) extends to the outer peripheral edge (11a) of said transparent substrate and is interrupted by a peripheral groove (14) immediately adjacent the outer peripheral edge (11a) of the substrate, said groove (14) extending down through the recording film to the substrate to define an outer peripheral edge (12a) of the recording film, and in that said protective film (13) fills said groove (14) so as to cover the defined outer peripheral edge (12a) of the recording film. A magneto-optic recording medium (30;40) comprising at least a transparent substrate (31;41) and a magneto-optic recording film (32;42) on the substrate, characterised in that the recording film (32;42) extends to the outer peripheral edge (31a;41a) of said transparent substrate, and in that a protective resin film (33;43) covers the recording film including an outer peripheral edge (32a;42a) of the recording film at the outer peripheral edge (31a;41a) of the transparent substrate. A magneto-optic recording medium (30) according to claim 10, wherein the transparent substrate (31) has a chamfered outer edge and the recording film (32) covers the chamfer (31b). A magneto-optic recording medium (40) according to claim 10, wherein the protective resin film (43) extends over the entire outer peripheral edge (41a) of the transparent substrate (41).

Inventors: KATAYAMA HIROYUKI, MIYAKE TOMOYUKI, MURAKIAMI YOSHITERU, NAKAYAMA JUNICHIRO, OHTA KENJI, KATAYAMA, HIROYUKI, MIYAKE, TOMOYUKI, MURAKAMI, YOSHITERU, NAKAYAMA, JUNICHIRO, OHTA, KENJI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G11B11
IPC codes: G11B11, F26B13