Read-only optical disk

Abstract: A read-only optical disk comprising an annular transparent substrate of a synthetic resin having information pits thereon, a reflective layer comprising an Al alloy comprising Ta and a protective layer provided thereover. The read-only optical disk according to the present invention has excellent corrosion resistance and a high reliability.

Description: The present invention relates to a read-only optical disk. Read-only optical disks have generally been put to practical use as low cost and large capacity recording media. A read-only optical disk generally comprises a transparent substrate having pits (minute recesses and protrusions) corresponding to recording signals formed on one of the surfaces thereof, on which a reflective layer (usually made of Al) is provided and a protective resin coating is then further provided. However, an optical disk having this structure has a problem of corrosion resistance. For instance, peeling of the reflective layer caused by expansion and shrinkage of the substrate when undergoing an abrupt change of temperature and humidity and corrosion of the reflective layer caused by high temperature and humidity directly lead to an increased error rate. This greatly deteriorates the reliability of the optical disk. In particular, if a disk is used in a special circumstance, such as at a high or low temperature, the problem of corrosion resistance as described above becomes extremely significant. In view of the foregoing problems, the present inventors have made an earnest study to provide a read-only optical disk having high corrosion resistance. They have found that a disk with excellent corrosion resistance can be obtained by selecting a specific structure. The present invention provides a read-only optical disk comprising [etc as claim 1]. Generally the reflective layer covers the information pits. The information pits are in an intermediate area between the outer peripheral margin and inner peripheral margin of the substrate. The reflective layer covers the information pits part. The protective layer generally covers the surface of the reflective layer and the outer peripheral margin and the inner peripheral margin of the substrate. In the accompanying drawing: Fig. 1 shows a cross-sectional view of a read-only optical disk of the present invention. Reference numeral 1 represents an annular transparent substrate, reference numeral 2 represents a reflective layer, reference numeral 3 represents a protective layer and reference numeral 4 represents the information pits. The substrate used in the present invention is transparent for transmitting the reproducing light-A polycarbonate is preferably used. The polycarbonate is usually molded into a substrate by so-called injection molding wherein a resin in a molten state is injected into a molding die. In this instance, by using a die equipped therein with a stamper having an information pits pattern on the surface thereof, the information pits are formed on the surface of the substrate. In a case of a substrate with a diameter of 86 to 130 mm, the thickness of the substrate is usually 1.2 mm. Excess residual strains in the polycarbonate substrate which may occur during injection molding are not desirable in view of the optical properties thereof. Accordingly a polycarbonate having a weight average molecular weight of about 13,000 to 16,000 is preferably injection molded at a resin temperature of about 320 to 380°C. Furthermore, for avoiding corrosion or degradation of the reflective layer provided on the polycarbonate substrate, the residual amount of chlorine (halogen) as an impurity derived from the starting materials contained in the polycarbonate is preferably not more than 20 ppm and, more preferably, not more than 5 ppm. A polycarbonate is prepared by reacting a phenolic compound (bisphenol) and phosgene using a halogenated hydrocarbon as a solvent. If the chlorine compound or the halogenated hydrocarbon used in the reaction remains in the polycarbonate, it reacts with the metal constituting the reflective layer to cause corrosion of the reflective layer. To remove the halogenated hydrocarbon, there has been employed a method, for example, of adding a non-solvent for the polycarbonate to a solution of the polycarbonate and gelling the polycarbonate by spraying it into warmed water to remove the solvent. The reflective layer for reflecting reproducing light is usually formed on the substrate, and an Al alloy containing Ta is used as the material for the reflective layer in the present invention. The Al alloy containing Ta has excellent corrosion resistance to ensure high reliability of the optical disk as compared with one containing a layer consisting only of Al. The Ta content is preferably from 0.1 to 15 at% (atomic %) and, more preferably, from 0.5 to 10 at%. The thickness of the reflective layer is preferably from 0.03 to 1 µm and, more preferably, from 0.05 to 0.3 µm. The reflective layer is not provided on the outer peripheral margin and the inner peripheral margin of the substrate but only at the intermediate area, in which information pits are formed, between both of the peripheral margins of the substrate. This selective formation of the reflective layer is carried out by covering the outer peripheral margin and the inner peripheral margin of the substrate by a suitable cover upon forming the reflective layer by for example, sputtering. A surface treatment of the substrate may be employed before forming the reflective layer, thereby improving the adhesion and aging stability of the reflective layer. Methods of surface treatment are for example, plasma treatment and UV-irradiation treatment. It is sufficient to apply any of the treatments to such an extend to remove water adsorbed on the surface of the substrate or organic materials adhering to the surface of the substrate, which may cause corrosion or peeling of the reflective layer, that is, a so called water removing treatment. The extent of the treatment may differ depending on the method of treatment and, therefore it cannot be determined generally. It is however preferred that the surface of the substrate has a wettability by water of a contact angle of not greater than 60° in the case of the plasma treatment, and a wettability by water of a contact angle of not greater than 60° in the case of the UV-ray irradiation treatment. Since the surface treatment of the substrate is applied to remove the water or organic materials which may adhere to the surface of the substrate after the molding of the substrate till the formation of the reflective layer, the treatment may be omitted when the reflective layer is formed within 120 min, preferably within 30 min, from the molding of the substrate. On the reflective layer, a protective layer is further provided. As the material for the protective layer there can be preferably used, for example, a so-called curable resin such as an energy ray-curable resin mainly comprising an epoxy acrylate or urethane acrylate and a heat-curable resin comprising a silicon resin. Among these a UV-curable resin is preferred. Since the protective layers of such curable resins are usually coated directly on the reflective layer by a coating method such as spin coating and bar coating and then cured, it is necessary that they do not bring about corrosion or degradation of the Al-Ta reflective layer. Furthermore, since the substrate and the protective layer are usually in direct contact with each other at the outer peripheral margin and the inner peripheral margin of the substrate on which the reflective layer is not provided, it is also required, for example, that the protective layer has good adhesion with the substrate and does not deteriorate the substrate. Therefore, it is preferred to use a curable resin with a small content of compounds of alkali metals such as Na and K, compounds of alkaline earth metals such as Ca and compounds of halogen such as Cl, Br and F, which may cause deterioration of the reflective layer. The content of the alkali metal or alkaline earth metal compound in the curable resin is not greater than 10 ppm, preferably not greater than 5 ppm, in terms of the alkali metal or alkaline earth metal, and the content of the halogen compound is not greater than 20 ppm and, preferably, not greater than 5 ppm, in terms of the halogen. The content of the alkali metal compound, the alkaline earth metal compound and the halogen compound can be reduced to within the above-mentioned range by sufficiently washing the curable resin with, for example, water or a solvent, or by applying crystallization from a solvent. The thickness of the protective layer after curing is preferably about 1 to 20 µm. If the thickness is greater than 20 µm, the substrate may sometime be deformed, for example due to shrinkage of the protective layer, and countermeasures therefore have to be taken. On the contrary if the thickness is less than 1 µm, the durability is lowered. It is preferred that the protective layer after curing has a pencil hardness of not less than HB and a shrinkage in the curing treatment of about from 0 to 15% in view of, for example, protection against an external force, resistance to the deformation of the substrate and easy handlability. The present invention is non further described in the following Examples. Each of the following properties was measured according to the method below. 1. Content of alkali metal and alkaline earth metal The measurement was carried out by atomic-absorption spectroscopy. The content was expressed in terms of metal content and not compound of the metal. 2. Content of halogen A sample (30 mg) was oxidatively decomposed by combustion in an oxygen stream. The hydrogen halide thus produced was subjected to coulometric titration using a silver electrode. The device employed was a chlorine·sulfur analyzer TSX-10, Mitsubishi Kasei Corporation. The content was expressed based on chlorine. 3. Contact angle A face contact angle goniometer CA-S150 manufactured by Kyowa Kaimen Kagaku K.K. was used. 4. Shrinkage in curing treatment The densities before and after curing were measured and the shrinkage was calculated from the following equation: ρ₂-ρ₁ ρ₁ × 100 (%) wherein ρ₁ was the density before curing and ρ₂ was the density after curing. EXAMPLE 1 : A polycarbonate having a weight average molecular weight of 15,000 and residual chlorine content of 5 ppm was injection molded at a resin temperature of 350°C under an injection pressure of 60 kg/cm² in a molding die having a stamper therein, to obtain a polycarbonate substrate of 130 mm diameter having tracking grooves and information pits. The resultant substrate was introduced into a sputtering device and, after evacuating the inside of the device to 8 × 10 .7 Torr or lower, and after introducing Ar gas into the device, plasma treatment was applied to the substrate on the side of the information pits under conditions of 5 × 10⁻³ Torr, 100 W and 30 s, to obtain a substrate having a wettability by water of a contact angle of 58°. Then, after evacuation to 8 × 10⁻⁷ Torr or lower and subsequent introduction of Ar gas to adjust the pressure to 3 × 10⁻³ Torr, an alloy target having a composition of Al₉₇Ta₃ (at%) was DC sputtered with Ar gas on the substrate to form a reflective layer having a composition of Al₉₆Ta₄ (at%) and a thickness of 0.1 µm. The composition was analyzed by X-ray fluorometry. The reflective layer was formed in an area from 22 mm to 62 mm in the radial direction. The inner and outer peripheral margins on which the reflective layer was not formed had widths of 14.5 mm and 3 mm, respectively. A UV-ray curable epoxy acrylate resin containing 3 ppm of alkali metal and 5 ppm of halogen was spin-coated on the substrate provided with the reflective layer and cured with UV-irradiation to form a protective layer of 5 µm thickness. The protective layer after curing had a pencil hardness of H and a shrinkage of 8%. The thus obtained disk was subjected to an acceleration test under conditions of 70°C and 90% RH for 1,000 hours. When the drop-in error rate was measured over time, the increase of errors was kept at 1.6 times. EXAMPLE 2 : A disk was prepared by the same procedures as in Example 1 except for applying a UV-ray irradiation treatment instead of the plasma treatment so as to obtain a substrate having a wettability by water of a contact angle of 45°. When the acceleration test was conducted, the drop-in-error rate was kept at 1.4 times. EXAMPLES 3 to 5 : Disks were prepared by the same procedures as in Example 1 except for forming reflective layers having compositions of Al₉₉Ta₁, Al 97.5 Ta 2.5 and Al₉₅Ta₅ by placing Ta or Al chips on a target of Al₉₇Ta₃ (at%). When the acceleration test was applied, the dropin error rates were kept at 1.7, 1.4 and 1.2 times. COMPARATIVE EXAMPLE 1 : A disk was prepared by the same procedures as in Example 1 except for forming on a substrate a reflective layer consisting of Al only. The thickness of the reflective layer was 0.1 µm. When the acceleration test was carried out in the same manner as in Example 1, the error rate could not be measured due to corrosion over the entire surface.

Claims: A read-only optical disk comprising an annular transparent substrate (1) of a synthetic resin which on one side has optically readable information pits (4), a reflective layer (2) on the side of the substrate containg the pits, and a protective layer (3) on the reflective layer, characterised in that the reflective layer is an Al alloy comprising Ta. A read-only optical disk according to claim 1, wherein the reflective layer is provided on an intermediate area on which the information pits are formed between an outer peripheral margin and an inner peripheral margin of the annular substrate, and the protective layer is provided so as to protect the surface of the reflective layer, the outer peripheral margin and the inner peripheral margin of the substrate. A read-only optical disk according to claim 1 or 2, wherein the reflective layer comprises an Al alloy comprising from 0.1 to 15 atomic % of Ta. A read-only optical disk according to any one of the preceding claims, wherein the protective layer comprises a curable resin having a content of a residual alkali metal compound and/or alkaline earth metal compound of not greater than 10 ppm in terms of alkali metal and/or alkaline earth metal, and a content of a residual halogen compound of not greater that 20 ppm in terms of halogen. A read-only optical disk according to any one of the preceding claims, wherein the transparent substrate comprises a polycarbonate having a weight average molecular weight of from 13,000 to 16,000. A read-only optical disk according to claim 5, wherein the amount of chlorine remaining as an impurity in the polycarbonate is not greater than 20 ppm. A read-only optical disk according to any one of the preceding claims, wherein the surface of the transparent substrate has a wettability by water of a contact angle of not greater than 60°. A read-only optical disk according to any one of the preceding claims, wherein the pencil hardness of the protective layer after curing is not less than HB. A read-only optical disk according to any one of the preceding claims, wherein the shrinkage of the protective layer in the curing treatment is from 0 to 15%.

Inventors: HASHIMOTO TAKASHI, KAWANO TOSHIFUMI, KISAKA YOSHIYUKI, KOBAYASHI YOSHIMITSU, WAKABAYASHI MITSUGU, HASHIMOTO, TAKASHI, KAWANO, TOSHIFUMI, KISAKA, YOSHIYUKI, KOBAYASHI, YOSHIMITSU, WAKABAYASHI, MITSUGU
Applicants: MITSUBISHI CHEM IND, MITSUBISHI KASEI CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G11B7