Optical recording media

Abstract: An optical recording medium (10) comprises a substrate (11) containing a random copolymer of ethylene and a cycloolefin having a specific structure and a recording layer (12) containing Te, Cr, C and H. The medium has excellent recording sensitivity and adhesion of the substrate to the recording layer.

Description: This invention relates to optical recording media of the type wherein pits corresponding to information are formed on a recording layer on a substrate by irradiation with beam of energy such as light or heat. There are two types of optical recording media, in use of one of which physically deformed portions such as holes or concavities are formed in a recording layer upon imagewise irradiation with beam of energy, and in use of the other of which portions having changed optical properties such as refractive index and reflectance are formed in a recording layer upon imagewise irradiation with beam of energy. Recording layers consisting essentially of low melting metal such as tellurium (Te) are known for the two types of optical recording media (JP-A-71195/1983 and JP-A-9234/1983). Te thin film, typical of low-melting metallic films, is capable of forming thereon desired physically deformed portions or portions having changed optical properties (hereinafter generally called "pits") by irradiating with very low energy radiation, and thus is very useful as a high sensitivity material. By sensitivity as used herein is meant the energy (mJ/cm²) required for forming pits per unit surface area. Through recording layers consisting essentially of Te are known, optical recording media comprising these recording layers laminated to conventional substrates such as a polycarbonate resin substrate can have poor sensitivity. Hence, optical recording media having improved recording sensitivity are needed. A further problem with such recording media is that the adhesion of the substrate to the recording layer is insufficient. To deal with this problem, a stage involving the removal of water contained in the polycarbonate resin to be used as the substrate is required. Further, there is sometimes required a stage for treating the surface of the polycarbonate resin substrate with plasma to improve the adhesion of the substrate to the recording layer. In EP-A-231121 we describe an optical disc structure having an air sandwich structure comprising two discs, each of which discs has a recording layer on a substrate of a copolymer of ethylene with a cycloolefin monomer. A Te film comprising C and H, and a Cr- comprising Te film have been proposed as recording media in JP-A-33320/1984 and JP-A-63084/1984, respectively. However, recording layers mainly comprising Te are not sufficiently oxidation-resistant. The present invention seeks to provide optical recording media having excellent recording sensitivity and adhesion between the substrate and the recording layer. Accordingly the invention provides an optical recording medium comprising a substrate and a recording layer formed thereon, in use of which the recording layer is irradiated with a beam of energy to form pits therein corresponding to given pieces of information and thereby record the pieces of information, wherein said recording layer is a thin film having a thickness of 10⁻⁸ to 10⁻⁶ m (100 Å to 1 µm) and consisting essentially of Te, Cr, C and H, the content of Cr in the recording layer being from 0.1 to 40 atom % based on the total atoms present in the recording layer, and said substrate comprises a cycloolefin random copolymer comprising units derived from ethylene and units derived from a cycloolefin of general formula [I] wherein n is 0 or a positive integer, preferably not more than 3, and R¹ to R¹², which may be the same or different, each represents hydrogen, halogen or a hydrocarbon group, provided that R⁹ to R¹² together may form a mono- or polycyclic hydrocarbon ring which may optionally have a double bond or bonds, or R⁹ and R¹⁰ or R¹¹ and R¹² together may form an alkylidene group. In the polymer chain of the random copolymer, the unit derived from the cycloolefins of the general formula [I] is present in the form of a recurring unit as represented by the following general formula [II]. wherein R¹ to R¹² and n are as defined above. Figure 1 is a rough sectional view of one embodiment of the optical recording medium of the present invention. Figure 2 and Figure 3 show differences of functions and effects between the optical recording media of this invention and conventional ones. Figure 4 is a rough sectional view of another embodiment of the optical recording medium of the present invention. In the Figures, numeral 10 represents optical recording medium, 11 represents substrate, 12 represents recording layer and 13 represents undercoat layer. The optical recording media of the present invention will be illustrated in more detail below. As shown in Fig. 1, an optical recording medium 10 according to the present invention comprises a substrate 11 and a recording layer 12 formed thereon. Substrate In a preferred embodiment of the present invention, the substrate 11 is composed of a cycloolefin random copolymer of ethylene and at least one cycloolefin unit of general formula [I] and having an intrinsic viscosity [η] of from 0.05 to 10 dl/g as measured at 135°C in decalin. In another preferred embodiment of the present invention, said substrate 11 is composed of a cycloolefin random copolymer composition containing a said cycloolefin random copolymer [A] having an intrinsic viscosity [η] of from 0.05 to 10 dl/g as measured at 135°C in decalin and a softening temperature (TMA) of not lower than 70°C, and a said cycloolefin random copolymer [B] having an intrinsic viscosity [η] of from 0.05 to 5 dl/g as measured at 135°C in decalin and a softening temperature (TMA) of below 70°C in a weight ratio of copolymer [A] to copolymer [B] from 100/0.1 to 100/10. When copolymer [A] is blended with copolymer [B] in the ratio specified above, the adhesion between the substrate 11 and the recording layer 12 can be further improved as compared with the case where the recording layer 12 is laminated onto a substrate 11 composed of the component [A] alone. The cycloolefin used herein include, for example the following unsaturated monomers represented by the general formula [I]. The cycloolefins represented by the general formula [I] can be easily prepared by condensation reaction of cyclopentadienes with appropriate olefins by Diels-Alder reaction. Examples of the cycloolefins represented by the general formula [I] include such compounds as exemplified in Table 1 and derivatives thereof, and in addition to 1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, such octahydronaphthalenes as 2-methyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-aoctahydronaphthalene, 2-ethyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-propyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-hexyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-stearyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2,3-dimethyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-methyl-3-ethyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-chloro-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-bromo-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-fluoro-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2,3-dichloro-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-cyclohexyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene, 2-n-butyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene and 2-isobutyl-1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene. The cycloolefin random copolymer contains as essential components the ethylene unit and the cycloolefin unit as described above. In addition to said two essential units, however, the cycloolefin random copolymer may optionally contain other copolymerizable unsaturated monomer units in such amount that they do not hinder the object of the present invention. Such unsaturated monomers which may optionally be copolymerized include α-olefins having from 3 to 20 carbon atoms, such as propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene. In the cycloolefin random copolymer which constitutes the substrate of the first preferred embodiment of the present invention, the recurring units (a) derived from ethylene are present in an amount of from 40 to 85% by mole, preferably from 50 to 75% by mole, while the recurring units (b) derived from the cycloolefin or cycloolefins are present in an amount of from 15 to 60% by mole, preferably from 25 to 50% by mole, and these recurring units (a) and (b) are arranged at random in the substantially linear chain of the copolymer. The molar percentage of the recurring units (a) and (b) were determined by ¹³C-NMR. The fact that the copolymer is completely soluble in decalin at a temperature of 135°C, indicates that the chemical structure of said copolymer is substantially linear and free from a gel-forming cross-linked structure. The copolymer has an intrinsic viscosity [η] of from 0.05 to 10 dl/g, preferably from 0.08 to 5 dl/g, as measured in decalin at a temperature of 135°C. The softening temperature (TMA) of the copolymer, as measured by a thermal mechanical analyzer is desirably at least 70°C, preferably from 90° to 250°C, and more preferably from 100° to 200°C. The softening temperature (TMA) of the copolymer [A] was determined by monitoring thermal deformation behavior of a 1 mm sheet of the copolymer using a thermomechanical analyzer supplied by Du Pont. More specifically, a quartz needle was vertically placed on the sheet under a load of 49 g and the assembly was heated at a rate of 5°C/min. The temperature at which the needle penetrated into the sheet by a depth of 0.635 mm was taken as the softening temperature of the copolymer. The copolymer has a glass transition temperature (Tg) of normally from 50° to 230°C, and preferably from 70° to 210°C. The crystallinity of the copolymer, as measured by X-ray diffractometry, is normally from 0 to 10%, preferably from 0 to 7%, and more preferably from 0 to 5%. The second preferred embodiment of optical recording medium will be described in more detail hereinafter. In the cycloolefin random copolymer [A] having a softening temperature (TMA) of not lower than 70°C, contained in the cycloolefin random copolymer composition which constitutes the substrate of the second preferred embodiment of the present invention, the recurring units (a) derived from ethylene are present in an amount of from 40 to 85 mol%, preferably from 50 to 75 mol%, while the recurring units (b) derived from the cycloolefin or cycloolefins are present in an amount of from 15 to 60 mol%, preferably 25 to 50 mol%, and these recurring units (a) and (b) are arranged at random in the substantially linear chain of the copolymer [A]. The molar percentage of the recurring units (a) and (b) were determined by ¹³C-NMR. The fact that the copolymer [A] is completely soluble in decalin at a temperature of 135°C, indicates that the chemical structure of said copolymer is substantially linear and free from a gel-forming crosslinked structure. An intrinsic viscosity [η] as measured at 135°C in decalin of the cycloolefin random copolymer [A] is in the range of from 0.05 to 10 dl/g, preferably from 0.08 to 5 dl/g. A softening temperature (TMA) as measured with a thermal mechanical analyzer of the cycloolefin random copolymer [A] is not lower than 70°C, preferably in the range of from 90 to 250°C, more preferably from 100 to 200°C. Furthermore, a glass transition temperature (Tg) of said cycloolefin random copolymer [A] is usually in the range of from 50 to 230°C, preferably from 70 to 210°C. A crystallinity index as measured by X-ray diffractometry of the cycloolefin random copolymer [A] is in the range of from 0 to 10%, preferably from 0 to 7%, more preferably from 0 to 5%. In the cycloolefin random copolymer [B] having a softening temperature of below 70°C, contained in the cycloolefin random copolymer composition which constitutes the substrate of the second preferred embodiment of the present invention, the recurring units (a) derived from ethylene are present in an amount of from 60 to 98 mol%, preferably 60 to 95 mol%, while the recurring unit (b) derived from the cycloolefin or cycloolefins are present in an amount of from 2 to 40 mol%, preferably 5 to 40 mol%, and these recurring units (a) and (b) are arranged at random in the substantially linear chain of the copolymer [B]. The molar percentage of the recurring units (a) and (b) were determined by ¹³C-NMR. The fact that the copolymer [B] is completely soluble in decalin at a temperature of 135°C, indicates that the chemical structure of said copolymer is substantially linear and free from a gel-forming crosslinked structure. The copolymer [B] has an intrinsic viscosity [η] of from 0.05 to 5 dl/g, preferably from 0.08 to 3 dl/g as measured in decalin at a temperature of 135°C. The softening temperature of the copolymer [B] as measured by a thermal mechanical analyzer is below 70°C, preferably from -10 to 60°C, more preferably from 10 to 55°C. The copolymer [B] has a glass transition temperature (Tg) of normally from -30 to 60°C, preferably from -20 to 50°C. The crystallinity of the copolymer [B] as measured by X-ray diffractometry, is normally from 0 to 10%, preferably from 0 to 7%, and more preferably from 0 to 5%. The ratio by weight of the copolymer [A] to the copolymer [B] in the cycloolefin random copolymer composition to be used as the substrate of the second preferred embodiment of the present invention is from 100/0.1 to 100/10, preferably 100/0.3 to 100/7, and more preferably 100/0.5 to 100/5. When the copolymer [A] is blended with the copolymer [B] in the ratio defined above, there can be obtained an improved adhesion between the substrate and the recording layer under severe conditions while retaining the excellent transparency and surface smoothness of the substrate itself. Further, the substrate composed of the cycloolefin random copolymer composition comprising the copolymer [A] and the copolymer [B] has such an advantage that the excellent adhesion between the substrate and the recording layer used in the present invention does not deteriorate even when the media are left to stand under high temperature and humidity conditions. The cycloolefin random copolymer and the cycloolefin random copolymers [A] and [B] of the cycloolefin random copolymer composition which constitute the substrate of the present invention may be prepared by suitably selecting the conditions under which they are prepared in accordance with the processes as proposed by the present applicant in Japanese Patent L-O-P Publns. Nos. 168708/1985, 120816/1986, 115912/1986, 115916/1986, 95905/1986, 95906/1986, 271308/1986 and 272216/1986. Furthermore, the substrates of the optical discs according to the invention may be made of polymers having recurring units of the general formula [III] resulting from ring opening of the cycloolefin monomers [I], or polymers having recurring units of the general formula [IV] resulting from hydrogenation of the units [III]. In the general formula [III] or [IV], n and R¹ to R¹² are as defined above. The cycloolefin random copolymer or the cycloolefin random copolymer compositions may incorporate heat stabilizers, weathering stabilizers, antistatic agents, slip agents, anti-blocking agents, anti-fogging agents, lubricants, dyes, pigments, natural oil, synthetic oil or wax, and the amounts of these additives may be suitably decided. For instance, the stabilisers which may be optionally incorporated include concretely phenolic antioxidants such as tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, alkyl esters of β-(3,5-di-t-butyl-4-hydroxyphenyl)propionic acid (18C or lower alkyl esters being particularly preferred) and 2,2'-oxamidobis[ethyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)]propinate, metallic salts of fatty acids, such as zinc stearate, calcium stearate and calcium 12-hydroxystearate, and fatty esters of polyhydric alcohols such as glycerin monostearate, glycerin monolaurate, glycerin distearate, pentaerythritol distearate and pentaerythritol tristearate. These compounds may be incorporated into the copolymers or the copolymer compositions either singly or in combination. For instance, there may be used such a combination of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane with zinc stearate and glycerin monostearate. In the present invention, it is preferred to use combinations of phenolic antioxidants with fatty esters of polyhydric alcohols. It is preferred that the fatty esters of the polyhydric alcohols are those wherein part of alcoholic hydroxyl groups of trihydric or polyhydric alcohols are esterified. Examples of such fatty esters of polyhydric alcohols include fatty esters of glycerin such as glycerin monostearate, glycerin monolaurate, glycerin monomyristate, glycerin monopalmitate, glycerin distearate and glycerin dilaurate, and fatty esters of pentaerythritol such as pentaerythritol monostearate, pentaerythritol monolaurate, pentaerythritol distearate, pentaerythritol dilaurate and pentaerythritol tristearate. The phenolic antioxidants are used in an amount of 0.01 to 10 parts by weight, preferably 0.05 to 3 parts by weight, more preferably 0.1 to 1 part by weight based on the amount of the cycloolefin random copolymer or the cycloolefin random copolymer composition. The fatty ester of the polyhydric alcohols are used in an amount of 0.01 to 10 parts by weight, preferably 0.05 to 3 parts by weight based on the amount of the cycloolefin random copolymer or the cycloolefin random copolymer composition. In the optical recording media of the present invention, the cycloolefin random copolymers or the cycloolefin random copolymer compositions are used as the substrates 11. The optical recording media of the present invention are superior in recording sensitivity to conventional optical recording media in which polycarbonates or poly(methyl methacrylate) are used as the substrate, though the reasons for the unexpected results so far are not known. The substrates 11 composed of the cycloolefin random copolymers or the cycloolefin random copolymer compositions are excellent in adhesion to the recording layer 12. Accordingly, the recording layer is excellent in long-term stability and can be effectively prevented from being oxidized. Accordingly, the optical recording media comprising the substrates 11 composed of the cycloolefin random copolymers or the cycloolefin random copolymer compositions and the recording layer 12 laminated thereon are excellent in recording sensitivity as well as in durability and long-term stability. Further, the optical recording media 10 of the present invention cause neither warpage nor cracking. Recording layer The recording layer 12 is a thin film consisting essentially of Te and additionally containing at least Cr, C and H. The recording layer may contain low melting elements other than Te or other components. Elements other than Te, which can be incorporated in the recording layer 12, include, for example, Ti, Mn, Ni, Zr, Nb, Ta, Al, Pt, Sb, Ge, Ag, Sm, Bi, In, Se, Pb, Co, Si, Pd, Sn and Zn. The content of Cr in the recording layer 12 is from 0.1 to 40 atom%, preferably from 0.5 to 10 atom%, especially preferably from 1 to 4 atom % based on the total atoms present in the recording layer, or from 0.1 to 40 atom%, preferably from 0.5 to 10 atom%, especially preferably from 1 to 4 atom% based on the sum of total Te and Cr atoms present in the recording layer. By incorporating Cr atom in the above-mentioned ranges into the recording layer, it becomes possible to improve the oxidation resistance of said recording layer 12 without lowering the recording sensitivity or, in some cases, with increased recording sensitivity. From the standpoint of improving recording sensitivity and duration of life of the recording layer, it is desirable that the contents of C in the recording layer 12 is less than 40 atom%, preferably from 3 to 20 atom% based on the total atoms present in the recording layer, or less than 40 atom%, preferably from 3 to 20 atom% based on the sum total of Te, Cr and C atoms present in the recording layer. By incorporating C atom in the above-mentioned ranges into the recording layer, it becomes possible to improve the recording sensitivity and recording margin of recording layer. From the standpoint of improving duration of life of the recording layer, it is desirable that the content of H in the recording layer 12 is from 1 to 40 atom%, preferably from 3 to 25 atom% based on the total atoms present in the recording layer. The amounts of elements contained in the recording layer 12, for example, metallic elements such as Te and Cr are determined by ICP emission spectroscopic analysis (induced coupled plasma emission spectrometry). The amount of C is determined by X-ray photoelectron spectrometry (ESCA) and the amount of H is determined by organic elementary analysis. In recording information to the recording layer 12 having such a composition as mentioned above, the desired recording of given pieces of information may be accomplished by irradiating the recording layer with beam of energy such as a laser beam modulated (on or off) according to the piece of information to be recorded and forming the corresponding pits on the irradiated portions of the recording layer. The pits may be those physically deformed such as holes or concavities, or may be such portions of the recording layer in which optical properties such as refractive index and reflectance have been changed by the irradiation with beam of energy. The recording layer 12 as mentioned above must be large in thickness to such an extent that sufficient light reflectance is obtained thereby and at the same time, it must be small in thickness to such an extent that no sensitivity is marred thereby. To be concrete, when physically deformed portions such as holes are formed in the recording layer 12, a film thickness of the recording layer is from 10⁻⁸ to 10⁻⁶m, preferably 10⁻⁸ to 5 x 10⁻⁷m and more preferably 1.5 x 10⁻⁸ to 7 x 10⁻⁸m (100 Å to 1 µm, preferably 100 to 5000 Å and more preferably 150 to 700 Å). When portions in which optical properties such as refractive index or reflectance have been changed are formed in the recording layer 12, a film thickness of the recording layer is from 10⁻⁸ to 10⁻⁶m, preferably 10⁻⁸ to 5 x 10⁻⁷m and more preferably 2 x 10⁻⁸ to 2 x 10⁻⁷m (100 Å to 1 µm, preferably 100 to 5000 Å and more preferably 200 to 2000 Å). The recording layer 12 may be formed on the substrate 11, for example, by the following procedure. The recording layer 12 composed of a thin Te-Cr alloy film containing C and H can be formed on the substrate 11 by using Te-Cr alloy as a target or Te and Cr respectively arranged target applying magnetron sputtering in a mixed gas of an organic gas containing C and H such as CH₄ or C₂H₂ gas and Ar gas. The recording layer 12 composed of a thin Te-Cr alloy film containing C and H can also be formed on the substrate 11 by depositing a vapor of CH₄, Te and Cr in the form of plasma without using sputtering. The recording layer 12 can also be formed on the substrate 11 by vapor phase growth or plasma vapor phase growth. Further, other methods can also be used, for example, part or all of Te, Cr, C and H atoms is ionized to beam-like state and allowed to accumlate on the substrate. The atomic content ratio of Te to Cr of the recording layer 12 composed of a thin Te-Cr alloy film containing C and H can be freely controlled by controlling the voltage applied to Te and Cr separately in the case of sputtering Te and Cr simultaneously, and by altering the alloy composition in the case of using an alloy target. The contents of C and H in the recording layer 12 composed of a thin Te-Cr alloy film can be arbitrarily controlled by the mixing ratio of CH₄ and Ar or applied power preferably high frequency. The most suitable content of H in the recording layer will be determined depending on the content of C. The content of H can be arbitrarily chosen, so long as the content of H in the recording layer is not so high that hydrogen gas (H₂) evolves. Further, since the film thickness is in proportion to sputtering time, the 'film thickness' can be easily controlled. The optical characteristics such as refractive index and extinction coefficient of the thus-formed recording layer 12 composed of a thin Te-Cr alloy film containing C and H vary depending on the contents of C and H. Accordingly, the film thickness is determined according to the required optical characteristics when the recording layer is used for the recording of information. The recording layer 12 as illustrated above has been markedly improved in oxidation resistance and recording sensitivity in comparison with recording layer formed by using a low melting metal alone such as Te alone. For instance, it has been confirmed by experiment that when the recording layer consisting essentially of Te and additionally containing Cr, C and H is stored under high temperature and humidity conditions over a long period of time, a variation in reflectance of the recording layer becomes smaller with an increase in the content of Cr and thus the recording layer of the present invention is improved in oxidation resistance in comparison with the recording layer composed of Te alone. Further, it has been confirmed by experiment that the recording layer of the present invention requires a relatively small recording energy output and has an improved recording sensitivity. Further, as shown in a curve A in Figure 2, the optical recording media of this invention shows almost no change in the C/N ratio during reproducing against minute voltage variation of a recording energy output, and has a wide recording margin, compared with conventional optical recording media comprising a Te recording layer containing C and H as shown in a curve B in Fig. 2. In addition the optical recording media of this invention shows almost no change in reflectance in the elapse of time compared with conventional optical recording media comprising a Te recording layer containing C and H as shown in a curve B in Fig. 3. Such improvement will become clear in Table 3 described hereinafter. In the present invention, after forming the recording layer 12 on the substrate 11 in the manner as mentioned above, the recording layer 12 alone or together with the substrate 11 may be heat-treated in a gas atmosphere containing inert gas, reducing gas or oxygen. The heat treatment temperature must be lower than the melting point of Te contained in the recording layer, and is preferably 70 to 300°C, particularly 90 to 150°C. The heating time is at least 5 seconds, preferably from 5 seconds to 10 hours, more preferably from 5 minutes to 2 hours. By virtue of the heat treatment of the recording layer 12 alone or together with the substrate 11 after the formation of said layer on the substrate 11 in the manner described above, the recording sensitivity in the recording layer is improved and recording margin is enlarged. This is thought to be due to the fact that the recording layer is crystallized to a certain extent by heat treatment. The present invention is not limited to the embodiment shown in Fig. 1. For instance, an undercoat layer 13 may be provided between the substrate 11 and the recording layer 12 as shown in Fig. 4. The undercoat layer includes, for example, films of fluorides such as magnesium fluoride (MgF₂), films of silicon compounds such as silicon oxide (SiO₂, SiO) or silicon nitride (Si₃N₄), metallic films composed of Ti, Ni, Cr, Al or Ni-Cr, films of fluorine-substituted hydrocarbon compounds such as polytetrafluoroethylene (PTFE) film and/or polymer films thereof, and Cr-C-H-films (films containing Cr, C and H). The undercoat layer generally has a film thickness of from 10⁻⁹ to 10⁻⁷m, preferably from 5 x 10⁻⁹ to 5 x 10⁻⁸m (10 to 1000 Å, preferably from 50 to 500 Å), though the film thickness may vary according to the material used for the undercoat layer. By virtue of the film thickness as present above, these undercoat layers exemplified above can maintain their transparency and, at the same time, can exhibit various characteristics as the undercoat layer. The undercoat layer 13 as mentioned above may be formed on the surface of the substrate 11 in the same manner as in the case of formation of the recording layer 12 by the magnetron sputtering, vapor phase growth, plasma vapor phase growth, vacuum evaporation or spincoat process. Provision of the above-mentioned undercoat layer between the substrate 11 and the recording layer 12 results in a further improvement in recording sensitivity and a further enlargement of the recording margin thereof under certain circumstances. The optical recording media obtained by laminating the recording layer 12 onto the substrate 11 in the manner described above are excellent in recording sensitivity in particular and have sometimes the enlarged recording margin. According to the present invention, moreover, a surface layer may be formed on the surface of the recording layer 12 of the optical recording medium 10 as shown in Figs. 1 and 4. The material used for forming the surface layer includes elements used for the recording layer and oxides, nitrides and elements such as Si and Ti. The surface layer has a film thickness of from 5 x 10⁻¹⁰ to 10⁻⁸m, preferably 10⁻⁹ to 5 x 10⁻⁹m (5 to 100 Å, preferably from 10 to 50 Å), though the film thickness may vary according to the material used for forming the surface layer. The water absorption value of the copolymer used as substrate material in accordance with this invention is lower than that of common resin such as polycarbonate resin, and therefore the substrate composed of said copolymer requires before sputtering only about a half of an evacuating time required by the substrate composed of polycarbonate resin. Accordingly, the productivity of the discs of this invention will be significantly increased. The present invention is illustrated below in detail with reference to Examples. Example 1 After evacuating a vacuum receptacle, Ar gas and CH₄ gas were introduced into the receptacle, and an internal pressure in the receptacle was set at 8Pa (6 x 10⁻³ Torr)(gas flow ratio of Ar/CH₄ = 9/1). In the receptacle, Te and Cr were used as targets and simultaneously sputtered, while controlling the voltage applied to each target and sputtering time, to obtain a recording layer composed of Te₈₇Cr₂C₄H₇ and having a film thickness of 2.4 x 10⁻⁸m (240 Å) on an optical disc substrate (hereinafter referred to as PO (1) substrate) composed of a non-crystalline copolymer of ethylene and 1,4,5,8-dimethano-1,2,3,4,4a,5,8,8a-octahydronaphthalene (chemical structure formula: hereinafter abbreviated to DMON), whereby an optical recording medium could be obtained. Said copolymer had an ethylene unit of 59 mol% and a DMON unit of 41 mol% as measured by ¹³C-NMR analysis, an intrinsic viscosity [η] of 0.42 dl/g as measured at 135°C in decalin and a softening temperature (TMA) of 154°C. Example 2 The procedure of Example 1 was repeated with the exception of setting the film thickness of 2.9 x 10⁻⁸m (290 Å) to obtain a recording layer composed of Te₈₇Cr₂C₄H₇ on the PO (1) substrate. Examples 3 and 4 The optical recording media obtained in Examples 1 and 2, respectively, were subjected individually to heat treatment at a temperature of 100°C in N₂ atmosphere for 20 minutes. Example 5 The procedure of Example 1 was repeated by using the gas flow ratio of Ar/CH₄ = 9/1 with the exception of using a Te₉₇Cr₃ alloy target to obtain a recording layer composed of Te₈₇Cr₂C₄H₇ and having a film thickness of 2.3 x 10⁻⁸m (230 Å) on the PO (1) substrate. Thus obtained optical recording medium was subjected to heat treatment at a temperature of 100°C in N₂ atmosphere for 20 minutes. Example 6 After evacuating a vacuum receptacle, Ar gas and CH₄ gas were introduced into the receptacle, and an internal pressure in the receptacle was set at 8Pa (6 x 10⁻³ Torr)(gas flow ratio of Ar/CH₄ = 9/1). In the receptacle, Te and Cr were used as targets and simultaneously sputtered, while controlling the voltage applied to each target and sputtering time, to obtain a recording layer containing Te, Cr, C and H on the PO (1) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te 98.8 Cr 1.2 )₉₆C₄ and having a film thickness of 2.4 x 10⁻⁸m (240 Å). Example 7 The procedure of Example 6 was repeated with the exception of setting the film thickness to 2.9 x 10⁻⁸m (290 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (1) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te₉₈Cr₂)₉₆C₄. Example 8 The procedure of Exmple 6 was repeated with the exception of setting the film thickness to 2.6 x 10⁻⁸m (260 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (1) substrate, the atomic ratio of Te, Cr, and C in said recording layer being represented by the formula (Te 96.8 Cr 3.2 )₉₆C₄. Example 9 The procedure of Example 6 was repeated with the exception of setting the film thickness to 2.3 x 10⁻⁸m (230 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (1) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te₉₂Cr₈)₉₈C₂. Example 10 The procedure of Example 6 was repeated with the exception of using a Te₉₇Cr₃ alloy target and setting the film thickness to 2.3 x 10⁻⁸m (230 Å) to obtain a recording layer containing Te, Cr, C and H on the PO(1) substrate, the atomic ratio of Te, Cr and C in the said recording layer being represented by the formula (Te 97.5 Cr 2.5 )₉₆C₄. Example 11 The procedure of Example 6 was repeated with the exception of using a Te₉₄Cr₆ alloy target and setting the film thickness to 2.5 x 10⁻⁸m (250 Å) to obtain a recording layer containing Te, C and H on the PO (1) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te 95.1 Cr 4.9 )₉₈C₂. Examples 12-17 The optical recording media obtained in Examples 6-11, respectively, were subjected individually to heat treatment at a temperature of 100°C in N₂ atmosphere for 20 minutes. Referential Example 1 Preparation of a substrate composed of a cycloolefin random copolymer composition (i) Polymerization Example 1 Synthesis of the copolymer (A) having a softening temperature of not lower than 70°C With a 2-litre glass polymerization reactor equipped with a stirring blade, there was carried out continuously a copolymerization reaction between ethylene and DMON. That is, into the polymerization reactor were continuously charged a solution of DMON in cyclohexane so that the DMON concentration in the polymerization reactor became 60 g/l, a solution of VO(OC₂H₅)Cl₂ as a catalyst in cyclohexane so that the vanadium concentration in the polymerization reactor became 0.9 mmol/l, and a solution of ethylaluminium sesquichloride (Al(C₂H₅) 1.5 Cl 1.5 ) in cyclohexane so that the aluminium concentration in the polymerization reactor became 7.2 mmol/l, while continuously withdrawing from the bottom of the polymerization reactor the polymerization liquid so that the volume of the polymerization liquid in the polymerization reactor was constantly 1 litre. Simultaneously, into the polymerization reactor from the top of the polymerization reactor ethylene was fed at a rate of 85 l/hr, hydrogen was fed at a rate of 6 l/hr and nitrogen was fed at a rate of 45 l/hr. The copolymerization reaction was carried out at 10°C by circulating a refrigerant through a jacket fitted externally to the polymerization reactor. The copolymerization was carried out under the conditions as illustrated above, whereupon a polymerization reaction mixture containing an ethylene/DMON random copolymer was obtained. The polymerization reaction was stopped by adding a small amount of isopropyl alcohol to the polymerization liquid withdrawn from the bottom of the reactor. Thereafter, the polymerization liquid was poured into a household mixer containing acetone of about three times the volume of the polymerization liquid, while rotating the mixer, thereby depositing the resulting copolymer. The deposited copolymer was collected by filtration, dispersed in acetone so that the polymer concentration became about 50 g/l, and the copolymer was treated at the boiling point of acetone for 2 hours. After the treatment as above, the copolymer was collected by filtration and dried at 120°C overnight (12 hours) under reduced pressure. The thus obtained ethylene DMON-random copolymer (A) had an ethylene unit of 59 mol% as measured by ¹³C-NMR analysis, an intrinsic viscosity [η] of 0.42 dl/g as measured at 135°C in decalin, and a softening temperature (TMA) of 154°C. (ii) Polymerization Example 2 Synthesis of the copolymer (B) having a softening temperature of below 70°C The same copolymerization reaction as in Polymerization Example (i) was carried out except that DMON, VO(OC₂H₅)Cl₂ and ethylaluminium sesquichloride were fed into the polymerization reactor so that the concentrations of DMON, VO(OC₂H₅)Cl₂ and ethylaluminium sesquichloride in the polymerization reactor became 23 g/l, 0.7 mmol/l and 5.6 mmol/l, respectively, and that ethylene, hydrogen and nitrogen were fed into the polymerization reactor at rates of 140 l/hr, 13 l/hr and 25 l/hr, respectively, and the polymerization temperature was 10°C. After the completion of the copolymerization, the resulting copolymer was deposited, collected and dried at 180°C under reduced pressure for 12 hours as in Polymerization Example (i). The thus-obtained ethylene DMON copolymer (B) had an ethylene unit of 89 mol% as measured by ¹³C-NMR analysis, an intrinsic viscosity [η] of 0.44 dl/g as measured at 135°C in decalin and a softening temperature (TMA) of 39°C. (iii) Preparation of substrate composed of a cycloolefin random copolymer composition 400 g of the copolymer (A) prepared in Polymerization Example (i) and 4 g of the copolymer (B) prepared in Polymerization Example (ii) ( A/B = 100/1 by weight) were introduced into 8 litre of cyclohexane and dissolved at about 50°C while thoroughly stirring to obtain a homogeneous solution. The thus obtained homogeneous solution was poured in 24 litres of acetone to deposit an (A)/(B) blend. The thus obtained blend was dried at 120°C under reduced pressure overnight. The (A)/(B) blend thus obtained was incorporated with 0.5% of tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane, 0.05% of zinc stearate and 0.5% of glycerin monostearate as stabilizers, each amount being based on the total weight of the copolymers (A) and (B). The resulting blend was pelletized at 23°C by using an extruder (L/D = 20) having 20mm and molded into an optical disc substrate (hereinafter referred to as PO (2) substrate) of 130 mm and 1.2 mm in thickness by using an injection molding machine. Example 18 After evacuating a vacuum receptacle, Ar gas and CH₄ gas were introduced into the receptacle, and an internal pressure was preset at 8Pa (6 x 10⁻³ Torr)(the gas flow ratio of Ar/CH₄ = 9/1). In the receptacle, Te and Cr were used as targets and simultaneously sputtered, while controlling the voltage applied to each target and the sputtering time, to obtain a recording layer composed of Te₈₇Cr₂C₄H₇ and having a film thickness of 2.4 x 10⁻⁸m (240 Å) on the PO (2) substrate composed of the cycloolefin random copolymer composition obtained in Referential Example 1, thus obtaining an optical recording medium. Example 19 The procedure of Example 18 was repeated with the exception of setting the film thickness to 2.9 x 10⁻⁸m (290 Å) to obtain a recording layer composed of Te₈₇Cr₂C₄H₇ on the PO (2) substrtate. Examples 20 and 21 The optical recording media obtained in Examples 18 and 19, respectively, were subjected individually to heat treatment at a temperature of 100°C in N₂ atmosphere for 20 minutes. Example 22 The procedure of Example 18 was repeated by using the gas flow ratio of Ar/CH₄ = 9/1 with the exception of using a Te₉₇Cr₃ alloy target to obtain a recording layer composed of Te₈₇Cr₂C₄H₇ and having a film thickness of 2.3 x 10⁻⁸m (230 Å) on the PO (2) substrtate. Examples 23 After evacuating a vacuum receptacle, Ar gas and CH₄ gas were introduced into the receptacle, and an internal pressure in the receptacle was set at 8Pa (6 x 10⁻³ Torr)(gas flow ratio of Ar/CH₄ = 9/1). In the receptacle, Te and Cr were used as targets and simultaneously sputtered, while controlling the voltage applied to each target and sputtering time, to obtain a recording layer containing Te, Cr, C and H on the PO (2) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te 98.8 Cr 1.2 )₉₆C₄ and having a film thickness of 2.4 x 10⁻⁸m (240 Å). Example 24 The procedure of Example 23 was repeated with the exception of setting the film thickness to 2.9 x 10⁻⁸m (290 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (2) substrate, the atomic ratio of Te, Cr and C atoms in said recording layer being represented by the formula (Te₉₈Cr₂)₉₆C₄. Example 25 The procedure of Example 23 was repeated with the exception of setting the film thickness to 2.6 x 10⁻⁸m (260 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (2) substrate, the atomic ratio of Te, Cr, and C in said recording layer being represented by the formula (Te 96.8 Cr 3.2 )₉₆C₄. Example 26 The procedure of Example 23 was repeated with the exception of setting the film thickness to 2.3 x 10⁻⁸m (230 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (2) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te₉₂Cr₈)₉₈C₂. Example 27 The procedure of Example 23 was repeated with the exception of using a Te₉₇Cr₃ alloy target and setting the film thickness to 2.3 x 10⁻⁸m (230 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (2) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te 97.5 Cr 2.5 )₉₆C₄. Example 28 The procedure of Example 23 was repeated with the excepton of using a Te₉₄Cr₆ alloy target and setting the film thickness to 2.5 x 10⁻⁸m (250 Å) to obtain a recording layer containing Te, Cr, C and H on the PO (2) substrate, the atomic ratio of Te, Cr and C in said recording layer being represented by the formula (Te 95.1 Cr 4.9 )₉₈C₂. Examples 29-34 The optical recording media obtained in Examples 23-28, respectively, were subjected individually to heat treatment at a temperature of 100°C in N₂ atmosphere for 20 minutes. Test results (1) A disc of optical recording medium was rotated at a rate of 1800 rpm and irradiated with laser beam at a frequency of 3.7 MHz to examine recording characteristics. C/N max as used herein is intended to show a maximum value of C/N when a laser power is changed. The recording sensitivity is intended to show a minimum value of the laser power when C/N max x 0.9 < C/N is attained, and the margin is intended to show the range of the laser power when C/N max x 0.9 < C/N is attained. The results are shown in Table 2. (2) Reflectivity R measured after allowing a disc of optical recording medium under the circumstances of 70 °C and 85 % RH was compared with the initial reflectivity R₀ to examine percentage variation in reflectance. The results are shown in Table 3. Evaluation of adhesion between the substrate and the recording layer The adhesion between the substrate and the recording layer of each of the optical recording media obtained in Examples 3 and 20 was evaluated in the following manner. The results are shown in Table 4. Adhesion test Cross-cut Adhesive test (JIS K5400) On the recording layer, there are drawn 11 parallel lines at right angles to each other in each direction of length and width at 1 mm intervals by means of a cutter knife. Cuts in the form of the squares of a checkerboard are made so as to give 100 squares per 1 cm². Evaluation on peeling is made by using cellophane tape (a product of Nichiban). Evaluation (1) Immediately after the formation of the recording layer (2) After the lapse of 100 hours under the circumstances of 80°C and 85% RH

Claims: An optical recording medium comprising a substrate and a recording layer formed thereon, in use of which the recording layer is irradiated with a beam of energy to form pits therein corresponding to given pieces of information and thereby record the pieces of information, wherein said recording layer is a thin film having a thickness of 10⁻⁸ to 10⁻⁶ m (100 Å to 1 µm) and consisting essentially of Te, Cr, C and H, the content of Cr in the recording layer being from 0.1 to 40 atom % based on the total atoms present in the recording layer, and said substrate comprises a cycloolefin random copolymer comprising units derived from ethylene and units derived from cycloolefin of general formula [I] wherein n is 0 or a positive integer and R¹ to R¹², which may be the same or different, each represents hydrogen, halogen or a hydrocarbon group, provided that R⁹ to R¹² together may form a mono- or poly-cyclic hydrocarbon ring which may optionally have a double bond or bonds, or R⁹ and R¹⁰ or R¹¹ and R¹² together may form an alkylidene group. A recording medium according to claim 1 wherein the substrate comprises a cycloolefin random copolymer having a softening temperature (TMA) of not lower than 70°C. A recording medium according to claim 1 or 2 wherein the content of C in the recording layer is less than 40 atom % based on the total atoms present in the recording layer. A recording medium according to claim 1, 2 or 3 wherein the content of H in the recording layer is from 1 to 40 atom % based on the total atoms present in the recording layer. A recording medium according to any one of the preceding claims wherein the substrate comprises a composition containing a said cycloolefin random copolymer [A] having an intrinsic viscosity [η] of from 0.05 to 10 dl/g as measured in 135°C in decalin and a softening temperature (TMA) of not lower than 70°C, and a said cycloolefin random copolymer [B] having an intrinsic viscosity [η] of from 0.05 to 5 dl/g as measured at 135°C in decalin and a softening temperature (TMA) of below 70°C in a weight ratio of copolymer [A] to copolymer [B] from 100/0.1 to 100/10.

Inventors: KUROIWA MITSUYUKI C O MITSUI, MINODA TAKESHI C O MITSUI, TODO AKIRA C O MITSUI, TOIBANA HISAHARU C O MITSUI, KUROIWA, MITSUYUKI C/O MITSUI, MINODA, TAKESHI C/O MITSUI, TODO, AKIRA C/O MITSUI, TOIBANA, HISAHARU C/O MITSUI
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1989
Main classification: G11B7
IPC codes: G11B7