Optical Recording Disk

This invention relates to an optical recording disk having a dye base recording layer, and more particularly, to an optical recording disk having an improved intermediate layer between a dye base recording layer and a reflective layer.

Optical recording disks of the write-once, rewritable and other types have been of great interest as high capacity information carrying media. Among the write-once optical recording disks, those having a recording layer predominantly comprising a dye have the advantage of low cost manufacture because the recording layer can be formed by coating.

There were commonly used optical recording disks of the air-sandwich structure having an air space on a dye base recording layer. Recently disks of the close contact type having a reflective layer in close contact with a recording layer were proposed because they could be reproduced in accordance with the compact disk (CD) standard. See Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and Proceedings SPIE--The International Society for Optical Engineering, Vol. 1078, pages 80-87, "Optical Data Storage Topical Meeting", 17-19, Jan. 1989, Los Angels. The optical recording disks of the close contact type are fabricated by forming a dye layer, a reflective layer and a protective layer on a transparent substrate in the described order so that the reflective layer is in close contact with the dye layer, meeting the disk total thickness of 1.2 mm required by the CD standard.

In order that optical recording disks having a dye base recording layer be reproduced by means of a CD player, one proposal uses a gold (Au) reflective layer because the aluminum reflective layer used in conventional CDs has insufficient reflectivity. Although gold has high reflectivity and corrosion resistance, it is expensive. It is thus contemplated to use less expensive metals having reflectivity as high as gold, for example, alloys such as Ag-Cu and Cu-Al alloys.

Disks having reflective layers of copper and copper alloys, however, are less reliable because such reflective elements can react with the dyes, causing deterioration of the dyes and a lowering of signal outputs. Especially copper is more chemically reactive with cyanine dyes typically used as recording layer material so that the cyanine dyes in the recording layer are likely to deteriorate in a high-temperature, high-humidity atmosphere. The cyanine dyes are often used as a mixture or combination with metal complex quenchers from the standpoint of light resistance, copper is also chemically reactive with such metal complex quenchers, giving rise to similar problems.

Therefore, an object of the invention is to provide a novel and improved optical recording disk having a dye base recording layer which disk is less expensive and highly reliable.

The present invention provides an optical recording disk comprising on a substrate a dye base recording layer, an intermediate layer, and a reflective layer in the described order. The reflective layer is composed of one or more metal elements, and the intermediate layer contains a compound of the one metal element or a compound of at least one of the metal elements where the reflective layer contains more than one metal element. Typically the intermediate layer is 3 to 30 nm thick.

Preferably the reflective layer contains at least one metal element selected from the group consisitng of Zn, Sn, Ag, Cu, Ti, V, Ta, Cr, Mo, W, Mn, Fe, Co, Rh, Ni, Pd, Pt, Au and Al. The metal element contained in the intermediate layer is the same as the metal element contained in the reflective layer in an amount of at least 1% by weight. The metal element is contained in the intermediate layer in an amount of at least 5% by weight. The metal element compound contained in the intermediate layer is an oxide, nitride or sulfide or a mixture thereof, especially an oxide.

The present invention provides an optical recording disk having a dye base recording layer, an intermediate layer, and a reflective layer on a substrate in a close contact arrangement and capable of recording/reproducing operation according to the CD standard. The intermediate layer disposed between the recording and reflective layers contains a compound of the same metal element as in the reflective layer. The intermediate layer is effective for mitigating the differential potential between the adjoining layers for preventing any reaction of the dye and quencher in the recording layer with the reflective layer, thus minimizing deterioration of the dye and reflective layers and a lowering of signal outputs. The intermediate layer is also effective for enhancing the adhesion between the intermediate and reflective layers. There is obtained an optical recording disk which is highly reliable.

The only figure, FIG. 1 is a fragmental cross-sectional view of an optical recording disk according to one embodiment of the invention.

The optical recording disk of the invention is an optical recording disk having a dye base recording layer, an intermediate layer, and a reflective layer in close contact relationship and capable of reproduction in accordance with the CD standard.

Referring to FIG. 1, there is schematically illustrated an optical recording disk according to one embodiment of the invention. The optical recording disk 1 shown in FIG. 1 is of the close contact type having a dye base recording layer 3 on one surface of a substrate 2, and further having an intermediate layer 4, a reflective layer 5, and an organic protective coating 6 stacked on the recording layer 3 in the described order in close arrangement.

The substrate 2 is of conventional disk shape and has commonly used dimensions, for example, a thickness of about 1.2 mm and a diameter of about 80 to 120 mm when the disk is intended for a recordable CD.

The substrate 2 is formed of a resin or glass material which is substantially transparent to recording and reading light, typically a semiconductor laser beam having a wavelength of 600 to 900 nm, especially 770 to 800 nm, most often 780 nm. The substrate material preferably has a transmittance of at least 80%, especially at least 88% so that recording and reading operation can be made through the substrate 2, that is, from the rear surface of the substrate 2 remote from the recording layer 3.

Preferably, the substrate 2 is formed of resins, typically thermoplastic resins such as polycarbonate resin, acrylic resins, amorphous polyolefins, and TPX using conventional techniques such as injection molding. On the surface of the substrate 2 where the recording layer 3 is formed, the upper surface in the illustrated embodiment, a predetermined pattern, typically a groove 21 is formed for tracking and addressing purposes. Alternatively, a resin layer (not shown) having a predetermined pattern including grooves may be formed on the substrate by 2P (photo-polymerization) method or the like.

The tracking groove 21 preferably consists of continuous spiral turns. Typically, the groove has a depth of 500 to 3,000 .ANG. and a width of 0.2 to 1.1 .mu.m, preferably 0.3 to 0.6 .mu.m (width in a radial direction with respect to the disk center). The adjoining groove turns are separated by a land having a width of 0.5 to 1.4 .mu.m, preferably 1.0 to 1.3 .mu.m. This groove configuration permits tracking signals to be obtained without reducing the reflection level of the groove. The groove may be provided with asperities for addressing signals. Where the groove is formed in the substrate surface, a provision is preferably made such that recording light may be directed to a recording layer within the groove.

The recording layer 3 is formed on the grooved substrate 2 using a dye or a mixture of compatible dyes.

For CD signal recording, the recording layer 3 preferably has a coefficient of extinction (the imaginary part of a complex index of refraction) k of from 0.03 to 0.25 at the wavelength of recording and reading light. With a coefficient of extinction k of less than 0.03, the recording layer can have a lower absorptivity so that it might become difficult to record with a commonly used power. A coefficient of extinction k of more than 0.25 can result in a drop of reflectivity to below 60%, often failing to reproduce according to the CD standard. Better results are obtained with k=0.03 to 0.20, especially 0.03 to 0.15.

The recording layer preferably has an index of refraction (the real part of a complex index of refraction) n of from 1.8 to 4.0, more preferably 2.0 to 3.0 at the wavelength of recording and reading light. With n<1.8, the reflectivity and signal modulation would be reduced, often failing to read by CD players. Few dyes have n>4.0.

The light absorbing dyes used herein preferably have maximum absorption at a wavelength in the range of from 600 to 900 nm, more preferably from 600 to 800 nm, most preferably from 650 to 750 nm. Such a light absorbing dye is preferably selected from cyanine dyes, phthalocyanine dyes, naphthalocyanine dyes, anthraquinone dyes, azo dyes, triphenylmethane dyes, pyrylium dyes, thiapyrylium dyes, squalirium dyes, chroconium dyes, and metal complex dyes alone or in admixture of two or more. Preferred cyanine dyes are cyanine dyes having an indolenine ring which may have a fused aromatic ring, especially a benzoindolenine ring.

The recording layer may also contain a quencher especially when a cyanine dye is used. The quencher is effective for improving light resistance. The quencher may be simply mixed with a light absorbing dye. Alternatively, an ionic combination or ionically bonded compound of a dye cation and a quencher anion is a useful light absorbing dye. The quencher should preferably be added in an amount of up to about 1 mol, especially about 0.05 to 0.5 mol per mol of the light absorbing dye or dyes.

Preferred quenchers used herein are metal complexes of acetylacetonates, bisdithiols such as bis(dithio-.alpha.-diketones) and bisphenyldithiols, thiocatechols, salicylaldehydeoximes, and thiobisphenolates. Also useful are amine quenchers such as amine compounds having a nitrogenous radical cation and hindered amines.

For the ionically bonded combination, cyanine dyes having indolenine rings and metal complex quenchers such as bisphenyldithiol metal complexes are preferred.

With regard to the dyes, quenchers and their combinations, reference is made to "Chemistry of Functional Dyes", CMC Publishing K.K., pages 74-76 as well as the following patent applications.

The dye used in the recording layer may be selected from the above-mentioned light absorbing dyes, dye-quencher mixtures, and bonded dye-quencher compounds as long as it has n and k in the above-defined ranges. If desired, a dye of a new design molecular structure may be synthesized.

The coefficient of extinction k of a dye with respect to recording and reading light generally varies over the range of from 0 to about 2 depending on its skeleton and substituent. In selecting a dye having a coefficient of extinction k of 0.03 to 0.25, for example, some limitations are imposed on its skeleton and substituent. Then the coating solvent is limited as the case may be. Some dyes cannot be applied to certain substrates or some cannot be deposited from a gas phase. Further, determining a new molecular design requires an increased amount of labor for design and synthesis.

Through experiments, the inventors found that a dye layer formed of a mixture of at least two dyes has a coefficient of extinction k which is determined from the coefficients of extinction k of the layers consisting of the respective dyes alone, in substantial proportion to the ratio of the dyes. Thus it is possible to form the recording layer from a compatible mixture of two or more dyes.

Most mixtures of dyes have a coefficient of extinction k substantially proportional to the mixing ratio of dyes. More particularly, a mixture of i types of dye has a coefficient of extinction k substantially equal to .SIGMA.Ciki wherein the i-th dye has a coefficient of extinction ki and a mixing fraction Ci. Thus a dye layer having k=0.03 to 0.25 may be formed by mixing dyes having different k in a controlled mixing ratio. This, in turn, means that the dyes used herein can be selected from a wider variety.

The same principle as above can be applied to an improvement in wavelength dependency. In general, a semiconductor laser produces a beam having a wavelength width of .+-.10 nm. Commercial CD players require a reflectivity of at least 70% in the wavelength range between 770 nm and 790 nm. Generally, the coefficient of extinction k of a dye is largely dependent on a wavelength. Some dyes can have an adequate coefficient k at 780 nm, but a largely deviated coefficient k at 770 or 790 nm. In this case, two distinct dyes may be mixed to form a mixture which has adequate values of k and n over the wavelength range of 780.+-.10 nm.

This eliminates the limitation of film formation such as limitation of a coating solvent and allows for the use of a readily synthesizable, inexpensive dye, a dye having excellent properties, or a sparingly soluble dye.

Where the recording layer is formed of a mixture of dyes, the dyes may be selected from those having an index of refraction n=1.6 to 6.5 and a coefficient of extinction k=0 to 2.

In determining n and k, a sample is prepared by forming a recording layer on a given transparent substrate to a thickness of about 400 to 1,000 .ANG.. Then the reflectivity of the sample is measured through the substrate or from the recording layer side. Reflectivity is measured in a specular reflection mode (of the order of 5.degree.) using light having the recording/reproducing wavelength. The transmittance of the sample is also measured. The index of refraction n and coefficient of extinction k may be calculated from these measurements according to Ishiguro Kozo, "Optics," Kyoritsu Publishing K.K., pages 168-178.

In the practice of the invention, the recording layer is often formed by spin coating, more particularly by dissolving the dye in a suitable organic solvent and applying and spreading the coating solution on a substrate while rotating the substrate. The coating is dried after spin coating, if desired. The organic solvents used herein include alcohols, ketones, esters, ethers, aromatics, and alkyl halides, with organic solvents having two or more functional groups in a molecule being preferred.

Preferably, the recording layer thus formed has a thickness of about 100 to about 300 nm although the exact thickness depends on the reflectivity or the like.

In addition to the recording layer of the coating type mentioned above, a sheet or film-shaped recording layer may also be used. In this regard, a dye film once shaped to sheet or film shape may be bonded to the substrate or used by itself as a substrate having a recording layer incorporated therein.

Closely disposed on the recording layer 3 is the intermediate layer 4. Closely disposed on the intermediate layer 4 is the reflective layer 5. The intermediate layer 4 contains a compound of at least one of the metal elements of which the reflective layer 5 is formed. Preferably the intermediate layer 4 consists of the compound of metal element.

The reflective layer is formed of at least one metal element selected from the group consisting of Zn, Sn, Ag, Cu, Ti, V, Ta, Cr, Mo, W, Mn, Fe, Co, Rh, Ni, Pd, Pt, Au, and Al. The presence of the intermediate layer eliminates the need to form the reflective layer from gold alone and allows metal elements other than gold to be used in the reflective layer. This is advantageous for cost reduction. Where the reflective layer is formed of an alloy of two or more metal elements, the intermediate layer should contain at least one of these metal elements. Alloys free of gold are advantageous for the reflective layer.

The metal compound in the intermediate layer may be any of oxides, nitrides and sulfides, with the oxide being preferred.

Preferably the metal element forming the metal compound in the intermediate layer is the one contained in the reflective layer in an amount of 1 to 100% by weight, especially 2 to 100% by weight. Preferably at least one compound of the same metal element as the metal element that is contained in the reflective layer in an amount of at least 1% by weight is contained in the intermediate layer in a total amount of 5 to 100% by weight, especially 10 to 100% by weight.

A particular composition of the reflective layer may be determined in accordance with the necessary reflectivity, color and other factors. Preferred are copper alloys such as Cu-Ag alloys and Cu-Al alloys because they have high reflectivity and similar color to gold. Cu-Zn alloys are also useful. However, since Cu-Zn alloys have somewhat low reflectivity, it is preferred that a high reflectivity film such as an Au film be additionally formed on the reflective layer to increase reflectivity. The Au film used herein may be sufficiently thin to maintain the cost reduction feature. Where these copper alloys such as Cu-Ag, Cu-Al and Cu-Zn are used in the reflective layer, the intermediate layer may be formed of a compound of copper, silver, zinc or the like such as copper oxide, silver oxide and zinc sulfide. Reflective layers of Ag, Al, and Pd are also preferred.

The intermediate layer 4 and reflective layer 5 may be formed by conventional gas phase film deposition techniques such as sputtering. In the case of sputtering, the same target may be used during deposition of both the intermediate and reflective layers. Then the intermediate and reflective layers can be continuously formed by depositing the intermediate layer through reactive sputtering with oxygen and nitrogen being introduced into the sputtering atmosphere, and resuming an argon atmosphere before deposition of the reflective layer. More particularly, in an embodiment where the reflective layer is formed of an alloy of metals A and B and the intermediate layer is formed of a compound of metal A, the intermediate and reflective layers can be continuously formed by furnishing distinct targets of metals A and B, applying activation power to only the metal A target to carry out reactive sputtering during formation of the intermediate layer, and thereafter applying activation power to both the targets during formation of the reflective layer. Of course, a compound target may be used to deposit the intermediate layer.

Where a substrate of relatively highly oxygen-permeable material is used, one simple procedure is by forming a reflective layer close to the recording layer, forming an organic protective coating thereon, and thereafter allowing the disk to stand in an oxidizing atmosphere whereupon an oxide generates for forming an intermediate layer. This is because oxygen penetrates through the substrate and recording layer and reaches the reflective layer where it is partially oxidized.

The intermediate layer 4 is preferably 3 to 30 nm thick. Too thin layers are less effective for suppressing reaction between the dye and reflective layers whereas too thick layers would detract from reflectivity. The thickness of the intermediate layer may be determined by ellipsometry, quartz crystal thickness monitor, contact step measurement, or scanning tunneling microscope.

The reflective layer 5 is preferably at least 60 nm thick. A reflective layer of less than 60 nm thick would have less strength. The upper limit of the reflective layer thickness is not critical although a thickness in excess of 200 nm is generally needless.

The reflective layer alone has a reflectivity of at least 90%, and the reflectivity of an unrecorded portion of an optical recording disk through the substrate can be at least 60%, especially at least 70%.

The organic protective coating 6 is formed on the reflective layer 5. The protective coating is preferably formed by spin coating in order to avoid any damage to the reflective layer while other coating methods including screen printing, dipping and spray coating may be used. The conditions under which the protective coating is formed may be determined by taking into account the viscosity of a coating composition, the desired coating thickness, and other factors without undue experimentation.

The organic protective coating is preferably about 1 to 20 .mu.m thick. Thinner coatings would provide less protection against corrosion of the reflective and recording layers whereas thicker coatings tend to crack due to curing shrinkage or cause disk warpage. If desired, the protective coatings may be of a multilayer structure.

The organic protective coating is preferably formed from a radiation-curable resin. More particularly, it is formed by coating a radiation-curable compound or a polymerizable composition thereof and curing the coating with radiation. The radiation-curable compounds used herein include monomers, oligomers, and polymers having incorporated in their molecule a group capable of crosslinking or polymerization upon exposure to radiation, for example, an acrylic double bond as given by acrylic acid, methacrylic acid and their esters which are sensitive to ionization energy and capable of radical polymerization, an allyl double bond as given by diallyl phthalate, and an unsaturated double bond as given by maleic acid and maleic derivatives. They are preferably polyfunctional, especially at least trifunctional and may be used alone or in admixture of two or more.

The radiation-curable monomers are usually compounds having a molecular weight of lower than 2,000 and the oligomers are those having a molecular weight of 2,000 to 10,000. Examples include acryl-containing monomers and oligomers such as styrene, ethylacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol methacrylate, 1,6-hexaneglycol diacrylate, 1,6-hexaneglycol dimethacrylate, and more preferably pentaerythritol tetraacrylate (and methacrylate), pentaerythritol triacrylate (and methacrylate), trimethylolpropane triacrylate (and methacrylate), trimethylolpropane diacrylate (and methacrylate), acryl modified products of urethane elastomers, and derivatives thereof having a functional group such as COOH incorporated therein, acrylates and methacrylates of phenol ethylene oxide adducts, and compounds including a pentaerythritol fused ring having an acryl or methacryl group or .epsilon.-caprolactone-acryl group attached thereto as disclosed in Japanese Patent Application No. 72888/1987, and specialacrylates as disclosed in Japanese Patent Application No. 72888/1987. Other radiation-curable oligomers include oligoester acrylates, acryl modified urethane elastomers and their derivatives having a functional group such as COOH incorporated therein.

In addition to or instead of the above-mentioned compounds, radiation-curable compounds obtained by modifying thermoplastic resins so as to be radiation sensitive may be used. Examples of such radiation-curable resins include thermoplastic resins having incorporated in their molecule a group capable of crosslinking or polymerization upon exposure to radiation, for example, an acrylic double bond as given by acrylic acid, methacrylic acid and their esters having a radically polymerizable unsaturated double bond, an allyl double bond as given by diallyl phthalate, and an unsaturated bond as given by maleic acid and maleic derivatives. Examples of the thermoplastic resins which can be modified to be radiation curable include vinyl chloride copolymers, saturated polyester resins, polyvinyl alcohol resins, epoxy resins, phenoxy resins, and cellulosic derivatives. Other resins which can be modified to be radiation curable include polyfunctional polyesters, polyether ester resins, polyvinyl pyrrolidone resins and derivatives thereof (e.g., PVP olefin copolymers), polyamide resins, polyimide resins, phenolic resins, spiroacetal resins, and acrylic resins containing at least one acryl ester and methacryl ester containing a hydroxyl group as a polymerizing component.

The radiation to which the coating is exposed may be ultraviolet (UV) radiation, electron radiation, or the like, with UV being preferred. For UV curing, photo-polymerization initiators or sensitizers are generally added to the radiation-curable compounds. Any desired photo-polymerization initiator or sensitizer may be used, for example, acetophenones, benzoins, benzophenones, and thioxanthones. A mixture of photo-polymerization initiators may be used. The polymerizable composition generally contains about 0.5 to 5% by weight of photo-polymerization initiator. Such a polymerizable composition may be synthesized by conventional methods or prepared by mixing commercially available compounds.

Another preferred composition containing a radiation-curable compound of which the organic protective coating is formed is a composition containing an epoxy resin and a cationic photo-polymerization catalyst. The epoxy resins used herein are preferably alicyclic epoxy resins, especially those having at least two epoxy groups in a molecule. Examples of the alicyclic epoxy resin include 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl) adipate, bis(3,4-epoxycyclohexyl) adipate, 2-(3,4-epoxycyclohexyl- 5,5-spiro-3,4-epoxy)cyclohexane-meta-dioxane, bis(2,3-epoxycyclopentyl) ether, and vinylcyclohexene dioxide. The alicyclic epoxy resin preferably has an epoxy equivalent of 60 to 300, especially 100 to 200 for better curing though not limited thereto.

Well-known cationic photo-polymerization catalysts may be used. Examples include metal fluoroborate salts, boron trifluoride complexes, bis(perfluoroalkyl sulfonyl)methane metal salts, aryldiazonium compounds, aromatic onium salts of Group 6A elements, aromatic onium salts of Group 5A elements, dicarbonyl chelates of Group 3A to 5A elements, thiopyrilium salts, Group 6A elements having a MF.sub.6 anion wherein M is P, As. or Sb, triarylsulfonium complex salts, aromatic iodonium complex salts, and aromatic sulfonium complex salts, with the polyarylsulfonium complex salts, aromatic sulfonium and iodonium complex salts of halogen-containing complex ions, and aromatic onium salts of Group 3A, 5A and 6A elements being preferred.

Also useful are cationic photo-polymerization catalysts containing an organometallic compound and a photo-decomposable organic silicon compound. These cationic photo-polymerization catalysts are non-strong acid systems. The organometallic compounds include complex compounds having alkoxy, phenoxy, .beta.-diketonato and other groups coordinated to a metal atom such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al and Zr. Preferred among these are organic aluminum compounds, for example, trismethoxy aluminum, trispropionato aluminum, tristrifluoroacetyl aluminum, and trisethylacetoacetonato aluminum. The photodecomposable organic silicon compounds yield silanol upon exposure to light such as ultraviolet radiation and include silicon compounds having a peroxysilano, o-nitrobenzyl or .alpha.-ketosilyl group.

Preferably the composition contains about 0.05 to 0.7 parts, especially about 0.1 to 0.5 parts by weight of the cationic photo-polymerization catalyst per 100 parts by weight of the epoxy resin.

More preferably, a composition containing an acryl-containing compound as a radiation curable compound and a photo-polymerization initiator or sensitizer is coated and cured with radiation, typically UV radiation.

In forming a protective coating, a coating of a polymerizable composition as defined above is exposed to UV radiation, or heated and then exposed to UV radiation if desired. Electron radiation may be used instead of UV radiation. Typically, coatings are exposed to UV radiation at an intensity of at least about 50 mW/cm.sup.2 and a dose of about 500 to 2,000 mJ/cm.sup.2. The UV radiation source may be any of conventional ones such as mercury lamps. On UV exposure, the compounds undergo radical polymerization.

Various pigment particles may be contained in the protective coating, if desired.

On the organic protective coating, there may be formed a labeling layer in the form of at least one printed layer. The labeling layer is preferably formed by printing a radiation-curable resin composition containing a pigment and curing it. Alternatively, a thermosetting ink composition may be used.

Recording or additional recording may be carried out on the optical recording disk 1 of the above-described construction by directing recording light having a wavelength of 780 nm, for example, in pulse form to the recording layer 3 in the groove 21 through the substrate 2 to form an irradiated or recorded spot whose optical reflectivity has changed. The recording layer 3 absorbs light so that it is heated while the substrate 2 is heated at the same time. As a result, the recording material, typically dye melts or decomposes near the interface between the substrate 2 and the recording layer 3, probably applying a pressure to the interface to deform the bottom and side walls of the groove 21.

Examples of the present invention are given below by way of illustration and not by way of limitation.

An optical recording disk sample as shown in FIG. 1 was prepared by starting with a polycarbonate resin substrate having a continuous spiral groove and a diameter of 120 mm and a thickness of 1.2 mm.

A recording layer was formed on the substrate by spin coating. The coating solution used in forming the recording layer contained a dye and an organic solvent. The dye was a mixture of 60% by weight of dye A1, 30% by weight of dye A2, and 10% by weight of singlet oxygen quencher Q1 (copper complex) or Q2 (nickel complex), all defined below. The organic solvent used was diacetone alcohol. The coating solution contained 5% by weight of the dye. The recording layer was 200 nm thick. ##STR1##

An intermediate layer of copper oxide, chromium oxide or silver oxide was formed by sputtering a compound target.

A reflective layer was formed from Cu-Ag alloy, Ag-Cr alloy or silver by sputtering.

A protective coating was formed by spin coating a UV-curable resin available as SD-17 from Dai-Nippon Ink K.K. and curing it with UV radiation. It was 5 .mu.m thick at the end of curing.

For comparison purposes, a sample free of the intermediate layer was fabricated (sample No. 2).

The compositions of the respective layers of several samples are shown in Table 1.

Using a commercially available CD player for reading, the disk samples were measured for block error rate (BLER) both at the initial and after 100 hour storage in a 80.degree. C./RH 80% environment. BLER was counted for 10 minutes and an average value per second was calculated. The CD standard prescribes a BLER of up to 220 counts/sec.

The results are shown in Table 1. The effectiveness of the invention is evident from Table 1.