CARDLIKE OPTICAL RECORDING MEDIUM

Abstract: A cardlike optical recording medium comprises a substrate (1), a recording layer 92) and a protective layer (4), wherein an ultra-hard film (6) containing a carbon atome is provided at least on the side of said optical recording medium on which a laser beam for writing-reproducing is irradiated.

Description: The present invention relates to an optical recording medium. Related Background Art In recent years, as information technology has spread throughout society, various optical information recording and reproducing systems have been proposed as highly efficient means for dealing with a great variety of information. The systems may use a variety of recording media including optical discs, optical cards and optical tapes. The above information recording media include those which can detect binary coded information by detecting the change in strength of a reflected light beam resulting from a change of surface shape due to the presence or absence of pits (holes), or changes in a polarized light beam due to surface magnetooptic effects. These optical information recording systems display a number of advantages such as high recording density and non-contact information recording and reproduction, resulting in the systems having a long life. In recent years, extensive research and development has been carried out in respect of cardlike optical informaiton recording systems (hereinafter referred to as optical cards) due to their large storage capacity to size ratio, ease of use and mobility. An optical recording medium basically comprises a substrate, a recording layer and a protective layer. The medium may further comprise various laminated protective layers so as to improve the resistance to heat, moisture, light, abrasion and other effects which may impair the working of the media. Scratches formed on the face of the optical recording medium which is irradiated during use, or impurities such as dust which may contaminate the said face, may cause a change in the optical characteristics, such as reflectance, resulting in an increased noise signal when the recorded signal is being reproduced. Thus there is a need to produce a high abrasion and/or scratch resistent coating on at least the aforesaid face of the optical recording medium. This is particularly so in the case of the optical card, which is used in an environment where the surface is much more likely to be scratched or otherwise damaged, than that of the optical disc. Carbon films in the form of a graphite lubricating layer are known in magnetic recording films. However, the technical problem in the field of optical recording systems is fundamentally different from that of the field of magnetic recording films in which the magnetic reading and recording head contacts the recording medium. The graphite lubricating layer thus provides for smooth travel of the reading and recording head over the recording medium and thus reduces ware on both the head and the recording medium, as detailed in US patent number 4, 411 963. In order to increase the recording density or enhance the reliability of a recording, the optical disc is generally produced in a complicated layer construction such as the air-sandwich structure, and also includes a substrate or protective layer of suitable thickness. On the other hand, the optical card has a simple layer construction in order to achieve a reduced thickness, and has a thinly formed substrate or protective layer. However, because of the very thinness of the upper layer of the recording layer, it is difficult for an optical reproducing system to defocus surface impurities, such as dust or the surface scratches, making the separation of the resulting noise signal from the desired recorded signal extremely difficult. Summary of the Invention An object of the present invention is to solve the problems inherent in conventional optical cards, and to provide an optical recording medium, particularly a cardlike optical recording medium, in which surface scratches may be reduced, at least, on the face on which the laser beam is incident when in use. We have found that an optical card of reduced thickness comprising a simple layer construction and an ultra-hard carbon film (or coating) as a protective layer on the side which is intended when in use to be irradiated, can result in an increased reliability in the reproduced signals and an improved signal to noise ratio. According to the present invention, there is provided an optical recording medium comprising a substrate a recording layer adapted to have information written thereupon and to permit recorded information to be reproduced by a laser beam, and a protective layer, characterised by an ultra-hard carbon film provided on a face of the said optical recording medium, the said face being intended when in use to be irradiated by a laser beam to record or reproduce data. BRIEF DESCRIPTION OF THE DRAWING Fig. 1, Fig. 2 and Fig. 3 are cross-sections illustrating working embodiments of the present invention; Fig. 4 and Fig. 5 are explanatory views for Comparative Examples 1 and 2; and Fig. 6 is a view showing a manner for a flexibility test. DETAILED DESCRIPTION OF THE INVENTION In the present invention, the ultra-hard film is a coating containing a diamond phase and/or an amorphous carbon phase. The cardlike optical recording medium provided with a layer of this ultra-hard film can achieve a good shape of information bits and may not cause any rewriting owing to a laser beam for reproducing. This is because diamond has so good thermal conductivity that this ultra-hard film can externally dissipate the heat accumulated in the part of the recording layer by the irradiation of the laser beam for writing-reproducing. As a result, the writing and reproducing of information can be performed with a good precision. Also, because of the hardness of the diamond, the cardlike optical recording medium of the present invention can be free from the wear of its surface, can achieve improved C/N ratio and can be free from scratches. Accordingly, no error in the reading of the auto tracking signals may be caused, and the recording of information and/or the reproducing of recorded information can be carried out with accuracy. A process for producing the ultra-hard film will be described below. This film chiefly composed of carbon can be formed by the known plasma CVD process, ion beam deposition process or ion plating process. Such a process includes, for example, the rf-plasma CVD process, the microwave CVD process and the magnetic-field-bearing plasma CVD process. Also included is the ion beam deposition process in which the film formation is performed by accelerating carbon-containing ions from an ionic source in the electric field in the form of a beam. In this process and also in the other above processes, the film can be formed by applying a voltage on the substrate. A starting carbon-containing gas component may include hydrocarbons such as methane, ethane, propane, butane, ethylene, benzene, and toluene, alcohols such as methanol, ethanol, and propanol, as well as ketones, amines and amides. There can be also used those in which the above carbon-containing organic gas has been substituted with halogen atom(s) in part or in entirety, such as CHCl₃, CCl₄, CH₂F₂ and CH₃COCCl₃. It is also possible to use the carbon-containing gas by mixing it with one or more of H₂ and rare gases. Together with the abrasion resistance and scratch resistance (hardness), the ultra-hard film is preferably required to have flexibility so that no peeling or cracking may occur even when the optical card is bent, in instances in which the cardlike optical recording medium is frequently carried around by putting it in a case or as it is. In view of this, the film chiefly composed of carbon may preferably be endowed with both the hardness as in a polycrystalline film of diamond and the flexibility as in an a-C:H film which is highly amorphous and has a rich hydrogen content. The ultra-hard film endowed with both of these properties in an appropriate state may include films having an amorphous phase containing a finely crystalline phase of diamond to the extent that the hardness may not be lowered, or films in which no mixing of crystalline phase is observed by the electron ray diffraction and which is amorphous in the long-range order and is very close to diamond in the short-range order. Also, in the films having appropriate scratch resistance and flexibility, being highly amorphous and having a small hydrogen content, desired are those having transmittance as high as possible to the laser beam for writing-reproducing. In view of this, the films may contain a graphite phase or a glassy carbon phase to the extent that the transmittance to the wavelength of the laser beam to be used may not be extremely lowered, but preferred are those in which the peak having a half-width of 50 cm⁻¹ or less originating from SP² carbon may not appear at 1550 cm⁻¹ to 1650 cm⁻¹ in the Ramans spectrum. The hydrogen content may preferably be in the range of 0.1 atom % to 40 atom %, more preferably 1 atom % to 30 atom %, taking account of the scratch resistance, i.e., the hardness, of the ultra-hard film. With regard to the flexibility, buckling stress was applied, as shown in Fig. 6, to an optical card from top side and bottom side in the state that the optical card was stood with its long side part sideways and until the middle part of the optical card was bent by 20 mm, and thereafter released to restore the optical card to the original state. Preferred optical cards are those in which no crack or wrinkle is formed on the surface of the ultra-hard film even after repeating the above operation 1,000 times. The ultra-hard film may preferably have a film thickness 0.01 to 20 µm, particularly preferably 0.1 to 20 µm, taking account of the thickness for making the optical card easy to handle and the flexibility of the ultra-hard film. Next, Fig. 1 to Fig. 3 illustrate working embodiments of the cardlike optical recording medium provided with a layer of the ultra-hard film formed in the above manner according to the present invention. There is no limitation at all in the constitution for the layers other than the ultra-hard film, constituting the optical recording medium. In the drawings, the numeral 1 denotes a transparent substrate; 2, a recording layer; 3, an adhesive layer; 4, a protective layer; 5, a support layer; 6, a ultra-hard film; and an arrow 7, an incident ray of a laser beam for writing-reproducing. Here, the ultra-hard film is provided at least on the face of the optical recording medium on which the laser beam is irradiated, in other words, on the outermost layer of the cardlike optical recording medium having the substrate, recording layer and protective layer on which the laser beam for writing-reproducing is irradiated, for example, on the substrate side or the protective layer side. Fig. 1 illustrates and example in which the ultra-hard film 6 is provided through interposition of the adhesive layer 3, on the substrate 1 corresponding to the side on which the laser beam 7 for recording-reproducing is irradiated. Fig. 2 illustrates an example in which the ultra-hard film 6 is formed on the support 5 and laminated on the transparent substrate 1 together with the support. This construction is taken when there is a problem in the adhesion between the ultra-hard film 6 and the adhesive layer 3 in the example of Fig. 1. Fig. 3 illustrates an example in which the ultra-hard film 6 is directly formed and laminated on the transparent substrate 1. This construction, having no adhesive layer and support layer unlike the example of Fig. 1 and the example of Fig. 2, may give less interfaces at which the laser beam is reflected or refracted, and makes it possible to effectively utilize the laser beam to be irradiated. However, the transparent substrate must be of material endurable to a high temperature at the time of the formation of the ultra-hard film 6. The transparent substrate 1 may preferably be of material that may have less difficulties in the optical recording-reproducing, and may be of any material so long as it has a high transmittance to the light to be used. For example, there can be used acrylic resins, polyester resins, polycarbonate resins, vinyl resins, polysulfone resins, polyimide resins, polyacetal resins, polyolefin resins, polyamide resins, cellulose derivatives, etc. As materials for the protective layer, there can be used any materials than can be usually used as materials for protective layers, specifically including polyvinyl chloride, fluorine-substituted ethylene polymers, vinyl chloride/vinyl acetate copolymers, polyvinylidene chloride, acrylic polymers such as polymethyl methacrylate, polystyrene, polyvinyl butyral, acetyl cellulose, styrene/butadiene copolymers, polyethylene, polypropylene, polycarbonate, epoxyacrylonitrile/butadiene/styrene copolymers, etc. In some cases, it is possible to use various kinds of materials depending on the use purpose, including metallic sheet made of iron, stainless steel, aluminium, tin, copper, zinc, etc,. synthetic paper, paper, fiber-reinforced plastics, composite materials such as magnetic materials comprising metallic powder and plastics, ceramics, etc. Of course, those used for the transparent substrate may be also used. The recording layer used in the present invention may comprise, for example, organic coloring matters such as cyanine type coloring matters, anthraquinone type coloring matters, merocyanine type coloring matters and transition metal complexes (for example, diamine type metal complexes, dithiol type metal complexes), or materials obtained by introducing any of these into a polymer, and also chalcogen compounds such as Bi, Te and alloys of these. The recording layer can be laminated by coating, vapor deposition, spraying or the like. As materials for the support layer 5 used in the example of Fig. 2, there can be used, for example, SiO₂, hydrocarbon polymers produced by plasma polymerization, a-C:H (amorphous carbon hydride) obtained by CVD process, glass, etc. The adhesive layer 3 may comprise adhesives of, for example, a vinyl acetate type, a vinyl acetate-acrylic type, a vinyl acetate copolymer type, a vinyl acetate emulsion type, an acrylic type, an acrylate type, an acrylic copolymer type, an ethylene type, an ethylene-vinyl acetate type, an ethylene-vinyl acetate copolymer type, a polyethylele type, a methylene chloride type, a polyamide type, a polyamide-amine type, a polyimide type, a urea type, an epoxy type, an epoxy-urethane type, an epoxy-acrylate type, an urethane-acrylate type a polyester type, a chloroprene type, a chloroprene rubber type, a nitrile type, a nitrile rubber type, a urethane type, a vinyl urethane type, a polyurethane type, an olefin type, a cyanoacrylate type, an alkyl acrylate type, a vinyl chloride type, a phenol type, an SBR (styrene butadiene rubber) type, a polyol type, a silicaalumina type, a synthetic rubber type, an emulsion type, an oligoester type, a cellulose type, a formaldehyde type, an ultraviolet-curable type, an organic solvent, a styrene-butadiene type adhesive, a Freon TA type, etc. Those requiring energy such as heat, light, electron rays or the like in effecting adhesion can be also used effectively so long as such energy may not deteriorate the function of the optical recording material. When the coloring matters and the like are used in the recording layer, it is necessary to use those which may not adversely affect the recording layer. As described in the above, in the cardlike optical recording medium according to the present invention, the ultra-hard film surface containing a carbon atom is provided on the surface on which a laser beam for writing-reproducing is irradiated, so that it is possible to improve the abrasion resistance and scratch resistance, to prevent the C/N ratio from being lowered by abrasion or the auto tracking signals from being erroneously read owing to scratches, and to obtain accurate signals for reproducing the recorded information. Moreover, since this ultra-hard film has flexibility, the cardlike optical recording medium of the present invention can prevent the ultra-hard film from being peeled or broken when it is bent. EXAMPLES The present invention will be described below by Examples, but the invention is by no means limited by these. First, prepared was a cardlike optical recording medium used in the present invention. On a polycarbonate substrate of 54 mm wide, 85.6 mm long and 0.7 mm thick as a transparent substrate, track grooves were formed by hot pressing. Subsequently, as a recording layer, a merocyanine type coloring matter was coated on the face of this track grooves by bar coating, followed by drying to form a thin film of 0.1 µm thick. The optical recording material thus obtained was laminated on a protective layer comprising polymethyl methacrylate in the manner that the optical recording layer may be in contact therewith, through interposition therebetween of an adhesive layer comprising an adhesive of an ethylene-vinyl acetate type, and the resulting laminate was contact-bonded by using a heat fixing roll heated to 90 to 150°C to produce a cardlike optical recording medium. Example 1 A parallel flat plate type RF plasma CVD device was used in producing an ultra-hard film. NaCl crystals were used as a substrate, which was set on the RF electrode side. C₂H₄ and H₂ as starting gas in the mixing ratio of 1.10 were introduced, and, keeping the pressure at 0.08 Torr., discharge was effected under RF power of 700 W at a substrate temperature of 400°C to form a ultra-hard film of 1 µm thick on the substrate. The resulting film had a hydrogen content corresponding to the hydrogen atom number percentage of 15 %, and a transmittance of 97 % to the light of 780 nm. No diamond of crystalline state was present according to electron ray diffraction, and a peak was detected at 1560 cm⁻¹ in the Raman spectrum, but with small strength and having a half width of 120 cm⁻¹. After the substrate NaCl of this film was melted and removed, the film was adhered with use of a vinyl acetate type adhesive on the substrate of the optical recording medium produced by the process described above. Example 2 A microwave CVD device was used and NaCl crystals were used as a substrate. Acetone and H₂ as starting gas in the mixing ratio of 5:95 were introduced and the pressure was set to 1 Torr. Substrate temperature was kept at 650°C and discharge was effected under 300 W. There was obtained a film of 1 µm thick, on which ring patterns of diamond were detected according to electron ray diffraction. No peak having a maximum value at at 1550 to 1650 cm⁻¹ was detected in the Raman spectrum. Hydrogen content was 5 atom %, and the transmittance at 780 nm was 95 %. In the same manner as in Example 1, the film was adhered on the optical recording medium mentioned above. Example 3 In the device same as in Example 2, a magnetic field of 1,800 Gauss at maximum was applied, and a-C:H was used as a substrate (a support layer). CCl₄ and H₂ in the mixing ratio of 1:1 were introduced and the pressure was set to 0.1 Torr. while keeping the substrate temperature at 300°C, discharge was effected under 300 W to obtain a film of 0.1 µm thick. Electron ray diffraction detected weak ring patterns, which were insufficient for being identified as diamond. No peak was detected at 1550 to 1650 cm⁻¹ in Raman spectrum. The transmittance at 780 nm was 92 %. Next, an acrylate type monomer was poured into a pregrooved mold, and a substrate bearing the diamond film previously produced was pressed down against the monomer, which was then photo-cured by irradiation from a mercury lamp of high pressure mercury. Subsequently, the mold was peeled to prepare a substrate comprising polyacrylate pregrooves made on one face of the substrate and the diamond film coated on the opposite face. A merocyanine type coloring matter was coated on the pregrooved side, and a protective layer of PMMA was adhered thereon to obtain a cardlike optical recording medium. Example 4 Using the device same as in Example 1, a substrate bias of -150 V was applied. As starting material, 100 % C₂H₄ was used, and the pressure was set to 0.05 Torr. The substrate was one in which polycarbonate was used for the substrate for use in the optical recording medium, and, keeping the substrate temperature at 120°C, discharge was effected under 500 W. The resulting film was 0.3 µm thick and was not crystalline. A peak was detected at 1570 cm⁻¹ in Raman spectrum, but the half-width was 150 cm⁻¹. Hydrogen content, 20 atom %; and the transmittance at 780 nm, 90%. On the opposite side to the side of this substrate on which the carbon film was formed, track grooves were formed by hot pressing, followed by the procedures entirely the same as those for the above-described optical recording medium, to prepare an optical recording medium. Example 5 Used here for the formation of an ultra-hard film was a Kaufmann type ion beam device. The substrate used was the same as the one used in Example 4. As starting material, 100 % CH₄ was introduced, and the pressure was kept to 1 × 10⁻⁴ Torr. Film was formed keeping the substrate temperature at 60°C, and the ion acceleration voltage at 400 V. The resulting film was 0.2 µm thick and was not crystalline. Hydrogen content was 15 atom % and Raman spectrum was very broad, having a maximum value at 1500 cm⁻¹. The transmittance at 780 nm was 90 %. Thereafter, Example 4 was repeated to prepare an optical recording medium. Comparative Example 1 On a polycarbonate substrate of 54 mm wide, 85.6 mm long and 0.7 mm thick as a transparent substrate, track grooves were formed by hot pressing. Subsequently, as a recording layer, a merocyanine type coloring matter was coated on the face of this track grooves by bar coating, followed by drying to form a thin film of about 0.1 µm thick. The optical recording material thus obtained was laminated on a protective layer comprising polymethyl methacrylate in the manner that the optical recording layer may be in contact therewith, through interposition therebetween of an adhesive layer comprising an adhesive of an ethylene-vinyl acetate type, and the resulting laminate was contact-bonded by using a heat fixing roll heated to 90 to 150°C to produce a cardlike optical recording medium (Fig. 4). Comparative Example 2 On a substrate of an optical recording medium produced in the same manner as in Comparative Example 1, a modified acrylic resin layer was provided as a hard coat 8 (Fig. 5). The flexibility, scratch resistance and abrasion resistance of the optical cards prepared in the above Examples 1 to 5 and Comparative Examples 1 and 2 were evaluated in the following manner. (1) Abrasion resistance: Carried out was abrasion resistance tests in which steel wool was reciprocated 100 times on the surface of the ultra-hard film, the hard coat or the substrate under load of 100 to 150 g/m². As a result, as shown in the table following, due to the abrasion of the surface, it became impossible to read the tracking signals on 20 times in respect of the recording medium having no coating on the substrate, and on 50 times in respect of the recording medium provided with the hard coat comprising a modified acrylic resin. In contrast thereto, in the case of the recording media of Examples 1 to 5, no difficulty in the reading of tracking signals occurred even on 100 times. (2) Scratch resistance: Next, to observe scratch resistance, the hardness of the substrate, hard coat and ultra-hard film was measured. For the measurement of this hardness, Moh's scale of hardness was used. The substrate showed a hardness of 1 or less, and the hard coat showed a hardness of 2. In contrast thereto, the ultra-hard film showed a hardness of 7 to 9. This proves that the ultra-hard film was a very high scratch resistance. (3) Flexibility: Next, in respect of the optical cards of Comparative Example 2 in which the hard coat was provided on the substrate of the side of the light-incidence face and those of Examples 1 to 5 in which the ultra-hard film was provided thereon, the flexibility of the hard coat and the respective ultra-hard films was tested. More specifically, buckling stress was applied, as shown in Fig. 6, to the optical cards from top side or bottom side in the state that the optical card is stood and until the middle part of the optical card was bent by 20 mm, and thereafter released to restore the optical card to the original state. This operation was repeated 1,000 times to observe whether cracks or wrinkles were formed on the surfaces of the hard coat and ultra-hard film. Results obtained are shown in the table following. It is seen from the above results that the optical cards according to the present invention are satisfactory for the three points, the abrasion resistance, scratch resistance and flexibility.

Claims: An optical recording medium comprising a substrate (1), a recording layer (2) adapted to have information written thereupon and to permit recorded information to be reproduced by a laser beam, and a protective layer (4), characterized by an ultra-hard carbon film (6) provided on a face of the said optical recording medium which in use of said medium is intended to be irradiated by a laser beam to record or reproduce data. An optical recording medium as claimed in claim 1, wherein the medium is a card. An optical recording medium as claimed in claims 1 or 2, wherein said ultra-hard carbon film (6) contains a diamond phase. An optical recording medium as claimed in claims 1 or 2, wherein the said ultra-hard carbon film (6) contains an amorphous carbon phase. An optical recording medium as claimed in any of the preceding claims, wherein the said ultra-hard carbon film (6) is directly formed on said substrate (1) of the optical recording medium. An optical recording medium as claimed in any of claims 1 to 4, wherein the said ultra-hard carbon film (6) is formed on the said substrate (1) with interposition of an adhesive layer (3). An optical recording medium as claimed in any of claims 1 to 4, wherein said ultra-hard carbon film (6) is formed on a support layer (5), and is provided on the said substrate (1) together with the support layer (5) and optionally with an adhesive layer (3) between the carbon film (6) and the support layer (5). An optical recording medium as claimed in any of the preceding claims, wherein the said ultra-hard carbon film (6) has a hydrogen content of 0.1 atom % to 40 atom %. An optical recording medium as claimed in any of the preceding claims, wherein the said ultra-hard carbon film (6) has a hydrogen content of 1.0 atom % to 30 atom %. An optical recording medium as claimed in any of the preceding claims, wherein the said ultra-hard carbon film (6) has film thickness of 0.01 to 20 microns. An optical recording medium as claimed in any of the preceding claims, wherein the said ultra-hard carbon film (6) has a film thickness of 0.1 to 20 microns. An optical recording medium as claimed in any of the preceding claims, wherein the said ultra-hard carbon film (6) has flexibility.

Inventors: HIRABAYASHI, KEIJI, IKOMA, KEIKO, ITO, SUSUMU, KURIHARA, NORIKO
Applicants: CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: G11B7
IPC codes: B41M5, B42D15, G11B7, G06K19