Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2001-01-10
2002-11-26
Mulvaney, Elizabeth (Department: 1772)
Stock material or miscellaneous articles
Circular sheet or circular blank
C427S539000, C438S795000
Reexamination Certificate
active
06485808
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical disk substrates for optical disk media of the type used in various optical disk devices, such as read-only type optical disks (e.g. CD-Audio, CD-I, CD-ROM, Video-CD, LD, DVD-Video, DVD-ROM, etc.), write-once (WORM) type optical disks (e.g. CD-R, DVD-R, etc.), re-writable type optical disks (e.g. DVD-RAM, DVD-RW, MO, etc.); and, the invention also relates to a method of manufacturing such optical disk substrates and to an apparatus for manufacturing such optical disk substrates.
The conventional method of duplicating an optical disk comprises the steps of: manufacturing a metal stamper having an uneven pattern (i.e. a pattern with microscopic holes, or pits, and lands on the surface of a plastic disk, hereinafter referred to as a “pits-and-lands pattern”) consisting of light spot tracking-guide grooves and/or emboss-pits, such as address pits and pits for recording information on the surface thereof, through nickel electric plating from a photoresist-coated master; injecting a plastic substrate material melted at a raised temperature into a mold in which the stamper is placed; and cooling and taking out the substrate so molded, so that a plastic substrate on whose surface the pits-and-lands pattern has been duplicated is produced. This is a common technique (injection molding method) in the manufacturing of plastic substrates for DVD-ROM, DVD-R, DVD-RAM, DVD-RW, MO, etc. as well as currently-used CD-Audio, CD-R, CD-ROM.
In accordance with conventional technology, the stamper, onto which the pits-and-lands pattern is transferred, is manufactured through nickel electric plating from the pits-and-lands pattern formed on a photoresist layer of a photoresist master that is provided in the form of a photoresist-coated glass substrate. In this case, fine irregularities (average roughness being approximately 1 to 3 nm or so) generated, irrespective of manufacturing objectives, on the surface of the photoresist film are also transferred onto the surface of the stamper together with the pits-and-lands pattern consisting of the light spot tracking-guide grooves and/or the address pits, or the emboss-pits, such as the pits for recording information etc. The pits-and-lands pattern is transferred onto the substrate surface from this stamper by the injection molding method in accordance with the conventional technology. Further, a reflective layer or a recording layer is formed on this substrate.
In general, information in the optical disk is reproduced by measuring the intensity of reflected light produced by light irradiated on the reflective layer formed on the substrate surface, or information is recorded in the optical disk and reproduced therefrom by forming a recording layer that changes its property physically or chemically in response to the irradiation of light thereon, using light irradiating a tracking-guide groove part of the recording layer as a guide. That is, in the latter case, the recording and reproduction of information is achieved in such a way that the recording layer formed on the substrate surface is irradiated by laser light to effect a change in the reluctance, thereof, etc. and the strength of the reflected light is used as a carrier of information. In this case, there is a problem in that, since the shape of the recording layer formed on the substrate surface takes a film shape that reflects fine irregularities generated in the surface, irrespective of the manufacturing objectives, it becomes a cause of noise and deteriorates the recording and reproduction characteristics.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an optical disk substrate having improved recording and reproduction characteristics by decreasing the fine irregularities which are accidentally generated on the surface of the substrate, thereby decreasing the noise of the recording medium formed through a stacking process.
The object is achieved by irradiating with ultraviolet light (hereinafter referred to as “UV-light”) the photoresist-coated master having the pits-and-lands pattern or an optical disk substrate onto which the pits-and-lands pattern was transferred.
It is generally known that the surfaces of some kinds of plastics are modified by the irradiation of UV-light having a short wavelength and hence a high energy. This is due to a mechanism whereby the UV- light irradiation cuts stable chemical bonds existing on the surface of the plastic and oxygen atoms in the air connect to disconnected bonds or the like.
Since the optical disk substrate manufactured by injection molding is made of a plastic material, the irradiation of UV-light having a short wavelength and hence a high energy cuts the chemical bonds in a thickness range from the substrate surface to the depth of only a few gm, which in turn induces an incrementing of the light absorption coefficient in a wavelength region from 280 to 400 nm and also causes the substrate surface to be flattened to a smooth surface. In this case, when the main wavelength of the irradiating UV-light is 254 nm, which generates no ozone, decomposition in the vicinity of the substrate surface and smoothing of the surface occur significantly; whereas, if UV-light having a 184 nm wavelength that generates ozone is used together with the UV-light having a 254 nm wavelength, ozone is generated by this UV-light in the vicinity of the light source, and, in turn, the UV-light having a 254 nm wavelength is absorbed by a decomposition reaction of ozone. Consequently, an increase in the optical absorption and a smoothing of the substrate surface in the vicinity of the substrate are difficult to achieve.
In the case where the optical disk substrate is polycarbonate, which is a plastic containing oxygen atoms on the main organic atomic-chain thereof, the effect produced by the UV-light irradiation becomes significant.
Therefore, in accordance with the present invention:
(1) the substrate to be used is an optical disk substrate made of polycarbonate that has a pits-and-lands pattern consisting of light spot tracking-guide grooves and/or address pits, or emboss-pits such as pits for recording information etc., characterized in that the transmittance of the substrate is not more than 50% at one wavelength in a wavelength region from 300 to 375 nm. In a substrate that has not been treated by UV-light irradiation, because the transmittance at this wavelength is much higher than that of the substrate of the present invention, whether or not the substrate has been treated with the UV-light irradiation, its irradiation conditions, etc. can be estimated by means of simple optical measurement; and, this data can be used as an index of the surface smoothing. With another method for the purpose of smoothing the surface, the change in the transmittance differs from that of the present method. By the way, the substrate thickness to be used is specified to be 0.6 mm or 1.2 mm.
(2) The substrate to be used is an optical disk substrate as described in paragraph (1), characterized in that the wavelength dependence of the transmittance of the substrate is such that the absorption starts to increase at a wavelength of 650 nm or so and increases further toward a short wavelength side; that is, the transmittance starts to decrease, and at a wavelength of 260 nm or so and thereafter the transmittance becomes virtually 0%.
(3) The substrate to be used is an optical disk substrate as described in paragraph (1), characterized in that the hardness of the substrate in the thickness range from the surface thereof to a depth of approximately 0. 5 &mgr;m is higher than that of the substrate excluding surface parts from the surface thereof to a depth of approximately 100 &mgr;m by 50 to 85%. Now, since it has been confirmed that UV-light irradiation will increase the hardness of the surface, the measured hardness was used as an index of surface smoothness, which is relatively difficult to measure. If the surface hardness is high, it brings about an effect that the recording medium is hard to deteriora
Anzai Yumiko
Miyamoto Mokoto
Nishida Tetsuya
Terao Motoyasu
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Mulvaney Elizabeth
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