An optical recording medium

Abstract: An optical recording medium includes a substrate (10) having pregrooves, and a metal recording thin film (20), a buffering layer (30) and a reflecting layer (40) which are stacked in sequence on the substrate. The buffering layer (30) is formed of one or more dyes having a refractive index (n) of 1.4 or more and an absorption coefficient (k) of 1.6 or less at a wavelength of 650nm, and a refractive index (n) of 1.8 or more and an absorption coefficient (k) of 0.001 or more at a wavelength of 780nm. The optical recording medium is reproducible using a CD player or a DVD player.

Description: The present invention relates to an optical recording medium, and more particularly, to an optical recording medium which has a high reflectivity at wavelength range of 630∼790nm, and which information can be reproduced from by a digital versatile disc player (DVDP) and a compact disc player (CDP). The recording area per recording unit of an optical recording medium is smaller than that of a conventional magnetic recording medium, so that the optical recording medium has been used as a high density recording medium. The optical recording medium is classified into read only memory (ROM) type which only reproduces already recorded information, write once read many (WORM) type which is recordable just once and readable many times, and an erasable type which is erasable and rewritable after recording. The information recorded in the optical recording medium should be reproduced by a player for the ROM type medium. To this end, the optical recording medium must satisfy specification of a conventional standardization (ORANGE BOOK), so that reflectivity of 65% or more and carrier-to-noise ratio (CNR) of 47dB or more are required. In a recordable optical recording medium, the recorded information can be reproduced based on a change in the reflectivity caused by a physical modification, phase change or change of magnetic properties in a recording layer before and after recording. Also, for making the optical recording medium compatible with a CD, a long-term data stability, a high recording sensitivity capable of supporting the high-speed recording rate, and economical advantages are required as well as the above high reflectivity and CNR characteristic. In order to improve the characteristic of the optical recording medium and make the manufacturing process easy, various optical recording media being formed of various materials have been suggested, and some of which are in practical use. In response to demands for a high density optical recording medium, DVD has been developed. reproducing data by 650nm diode laser and having 4.7GB capacity. A DVD player can also play discs of CD format, but unfortunately cannot play discs of CD-R format, which are recordable and readable by CD equipment. For playback by DVD equipment, media should have more than 30% reflectivity at 650nm. CD-R discs have less than 15% reflectivity at 650nm due to high absorption of the recording layer at 600∼750nm, which prevents their use in DVD players. As a conventional optical recording medium. Japanese Laid-open Patent Publication No. showha 63-268142 discloses a recording medium having a structure in which a sensitizing layer formed of gelatin, casein or polyvinyl alcohol (PVA), and a metal thin film formed of chromium (Cr), nickel (Ni) or gold (Au) are sequentially stacked on a substrate. According to the optical recording principle of the recording medium, the metal thin film absorbs heat of an irradiated laser beam, and thus the sensitizing layer and the metal thin film are deformed to form a recording pit. However, the recording pit of this recording medium is exposed so that it is difficult to store the recorded information on a long-term basis. U.S. Patent No. 4.983,440 discloses a recording medium having a structure in which two metal thin films as a recording layer, and a protecting layer for protecting the recording layer are sequentially stacked on a substrate. However, this recording medium has a very low reflectivity of 20% or less, so the recording medium is not compatible with conventional CDS. According to U.S. Patent No. 4.990.388, a recording medium has a light absorbing layer of a thickness of 20∼500nm, containing cyanine dye, a light reflecting layer formed of gold (Au), silver (Ag), copper (Cu), aluminum (Al) or alloys thereof, and a protecting layer are sequentially stacked on a substrate, and an intermediate layer having a thickness of 2nm ∼ 500µm is formed between the substrate and the light absorbing layer. The surface layer (the intermediate layer acts as a surface layer when the intermediate layer is thick) existing between the substrate and the light absorbing layer is deformed by heat generated when the light absorbing layer absorbs laser beams to form a pit for optical recording. U.S. Patent No. 5,213,955 discloses an optical recording medium in which a light absorbing layer having a thickness of 250∼410nm, containing dye, a light reflecting layer formed of Au, Ag, Cu, Al or alloys thereof, and a protecting layer are sequentially stacked on a substrate. In the optical recording medium, the side of substrate contacting the light absorbing layer is deformed by the laser beam to achieve optical recording. As described above, in the conventional optical recording medium, a recording layer generates heat by absorbing laser beams to deform the substrate, thereby forming a recording pit. To this end, the recording layer contains a dye as a light absorbing material. However, because such dye is expensive, the manufacturing costs increase if the optical recording medium contains a dye. Thus, making a dye containing layer thin has been attempted, however, the recording characteristics thereof have been unfavorable. Also, U.S. Patent No. 5,318,882 discloses an optical recording medium having an improved stability to light and heat, which adopts a recording layer which contains a light-vulnerable organic dye, such as a cyanine dye, and a light stabilizer for complementing the dye, such as nitrosodiphenylamine and derivatives thereof. However, the optical recording medium has poor reliability and may lower the recording sensitivity, and a 3T signal is easily damaged. Also, the optical recording medium cannot be compatible with a CD player and a DVD player. Japanese Laid-open Patent Publication No. Heisei 3-215466 discloses a recording medium having a structure in which a recording layer and a metal reflecting layer are sequentially formed on a substrate, and the recording layer contains phthalocyanine as a near infrared ray absorbent. However, the wavelength of a reading laser beam is limited to 790±20nm, and a reflectivity at 650nm is lower than a predetermined level, so the recording medium cannot be used for a DVD player. WO 97/21216 discloses an optical recording medium having a grooved transparent substrate. A recording layer is provided on the substrate, the recording layer including a partial mirror, buffer layer and thick reflective layer to provide a Fabry-Perot etalon. WO 98/10416 discloses an optical recording medium, having a recording layer comprising a partially transparent layer, a transparent buffer layer and a thick layer forming a Fabry-Perot etalon. Two further documents, not published at the priority date of this invention, also disclose relevant information. EP 836179 discloses a buffer layer having light absorption of less than or equal to 1.0 at 650nm and less than or equal to 0.1 at 780nm. WO 99/41742 discloses an optical recording medium having a buffer layer of refractive index between 1.45 and 3 and an extinction coefficient between 0 and 0.3, both measured in the solid state at 770-790nm. According to the present invention, there is provided an optical recording medium comprising a substrate having pregrooves, and a metal recording thin film, a buffering layer and a reflecting layer which are stacked in sequence on the substrate, wherein the buffering layer is formed of one or more dyes having a refractive index (n) of 1.4 or more and an absorption coefficient (k) of 1.6 or less at a wavelength of 650nm, and a refractive index (n) of 1.8 or more and an absorption coefficient (k) of 0.001 or more at a wavelength of 780nm, the pregroove of the substrate (10) has a depth of 100∼300nm, a width of 100∼350nm at the bottom, and a width of 200∼550nm at the middle, wherein the buffering layer (30) further includes an additive which has a decomposition or sublimation temperature of 100∼200°C, and weight of which rapidly decreases during decomposition. Preferably, the decomposition temperature of the dye is 80∼250°C. Preferably, the thickness of the buffering layer, measured at a region corresponding to the pregrooves of the substrate, ranges between 50∼200nm in groove area. Preferably, the buffering layer further includes an organic polymer, and the content of the organic polymer is 30wt% or less based on the total weight of the buffering layer. Preferably, the dye is at least one selected from the group consisting of anthraquinone, dioxadine, tripheno-dithiazine, phenanthrene, cyanine, phthalocyanine, merocyanine, pyrylium, xanthine, triphenylmethane, croconium, azo, indigoid, methine, azulene, squarium, sulfide and methal dithiolate. Also, the organic polymer may be at least one selected from the group consisting of vinylalcohol resin, vinylacetate resin, acrylate resin, polyester resin, polyether resin, polystyrene resin, polyurethane resin, cellulose resin and fatty acid resin. Also, the content of the additive may be 1∼20wt% based on the total weight of the buffering layer. Preferably, the additive has a rate of weight decrease of 20wt% or more based on the normal weight of the additive, when the weight decrease rate is measured at a temperature which is 50°C higher than the decomposition or sublimation temperature of the additive. Preferably, the additive is at least one selected from the group consisting of metallocene expressed by a formula M(C 5 H 5 ) 2' , where M is titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), osmium (Os) or palladium (Pd), tetraethyllead complex, peroxide, azo compound and derivatives thereof. An example of the present invention will now be described in detail with reference to the accompanying drawings in, which: FIG. 1 is a view illustrating the structure of an optical disc and recording/reproducing principle according to a preferred embodiment of the present invention; FIG. 2 is a graph showing the change in reflectivity according to the thickness of a buffering layer of the optical disc shown in FIG. 1; FIGs. 3A and 3B are graphs showing the results of thermal analysis and weight decrease on an additive used for forming a buffering layer of an optical disc according to the present invention; FIG. 4 is a graph showing the change in the carrier-to-noise ratio (CNR) according to the recording power of an optical disc according to a preferred embodiment of the present invention; and, FIG. 5 shows change in reflectivity according to the wavelength of a reproducing light in the optical disc of the present invention and a conventional optical disc (Dye CD-R). As shown in FIG. 1, an optical recording medium according to a preferred embodiment of the present invention includes a substrate 10 having pregrooves for guiding light for recording, and a metal recording thin film 20, a buffering layer 30, a reflecting layer 40 and a protecting layer 50 which are sequentially stacked on the substrate 10. In the optical pickup according to the present invention, having the above structure, the metal recording thin film 20 is heated by a laser beam during optical recording, and the heat is transferred to the substrate 10 and the buffering layer 30. Accordingly, a region of the substrate 10. adjacent to the heated region of the metal recording thin film 20, is expanded and deformed, and the heated region of the metal recording thin film 20 swells out toward the buffering layer 30 by the expanding force of the substrate 10. As described above, in the recorded portion, the substrate swells up and the buffering layer is deformed by heat, so the reflecting layer may also be deformed. Here, the degree of deformation is proportional to the heat generated from the metal recording layer and dependent on the components and thickness of the metal recording thin film and the buffering layer. In the optical recording medium of the present invention, the reflectivity of the recorded portion is lower than that of the non-recorded portion, causing a difference in the reflectivity between the recorded portion and the non-recorded portion, wherein the reflectivity difference makes recording and playback possible. Here, the reflectivity of the recorded portion is lower than that of the non-recorded portion due to the following reasons. First, the lower reflectivity in the recorded portion is due to a Fabry-Perot effect by the change in thickness of the buffering layer. That is, if the substrate 10 is swollen by a laser, the thickness of the buffering layer 30 is reduced to d2 after recording from d1 before recording. Here, the reflectivity which is varied according to the thickness of the buffering layer is decreased in the recorded portion as shown in FIG. 2. That is, for the reproducing process, incident light is reflected as a reflective light Rg in the metal recording thin film and a reflective light Rr in the reflecting layer, and the interference between the reflective lights is changed according to the thickness and refractive index of the buffering layer and the thickness of the metal recording thin film, so the reflectivity is changed according to the thickness of the buffering layer as shown in FIG. 2. In FIG. 2 which shows change of reflectivity depending on the thickness of the buffering layer, the buffering layer has a refractive index (n) of 2.3 and an absorption coefficient (k) of 0.02 at 780nm, and k of 1.5 at 650nm. Second, optical changes occur in the buffering layer. That is, if a portion of the metal recording thin film 20 is heated by absorbing the irradiated recording light, the temperature of the portion sharply increases. And in some cases, the buffer layer can absorb the writing laser beam and generate heat that depends on the k value at 780nm. As a result, the material of the buffering layer 30 is damaged by the heat and the optical characteristics thereof are changed, causing differences in the optical paths between the non-recorded portion and the recorded portion. Thus, the reflectivity of the recorded portion decreases. Hereinafter, characteristics required for each constituent layer of the optical recording medium according to the present invention will be described in detail. In the optical recording medium of the present invention, grooves for guiding the light for recording or reproducing are formed in one side of the substrate 10, and the shape of pregrooves acts as an important factor on the reflectivity and tracking characteristics. In order to make a sufficient recording signal from a thin buffering layer having good reflectivity and tracking characteristics, preferably, the pregrooves formed in the substrate have a depth of 100∼300nm, a bottom width of 100∼350nm, a middle width of 200∼550nm, and a "U" shape. If the depth of the pregroove is less than 100nm, the reflectivity greatly increases by the expansion of the substrate, so that much noise is included in a recorded signal. On the contrary, if the depth of the pregoove exceeds 300nm, the groove image of the buffer layer is too deep, thereby decreasing the reflectivity. Also, if the middle and bottom widths of the pregroove are less than the above range, amplitude of a push-pull signal is decreased while the reflectivity increases, so the tracking characteristics are lowered. If the middle and bottom widths of the pregroove exceed the above range, the push-pull signal is improved while reflectivity is decreased. In the present invention, the substrate 10 is formed of a material which is transparent to a laser beam and capable of being easily expanded and deformed by heat, and has excellent impact intensity. As a material for satisfying the above conditions, polycarbonate, polymethylmethacrylate (PMMA), epoxy resin, polyester or amorphous polyolefin is used. Preferably, the thermal deformation temperature of the resins is 80∼200°C, particularly, 100∼200°C. The metal recording thin film 20 of the optical recording medium according to the present invention acts as a heat generating layer by absorbing a laser beam and a partial mirror for providing a contrast difference before and after recording, so a predetermined absorbance and retlectivity are required. Preferably, the metal recording thin film 20 has a thickness of 10 ∼ 300Å, a transmittance of 95∼5% and a reflectivity of 5∼95%. If the thickness is 10Å or less. amount of heat generated by the light absorption for the recording is not sufficient to deform the substrate, and the reflectivity from the metal recording thin film 20 is small, thereby reducing the reflectivity. On the contrary, if the thickness thereof exceeds 300Å, the reflectivity at the metal recording thin film 20 increases, so scattering of the light is increased at the side of the pregrooves, thereby lowering the reflectivity of the optical recording medium. Also, expansion of the substrate for recording is hindered by the metal recording thin film and thermal conductivity of the metal recording thin film is increased. Accordingly, heat is easily lost during recording to lower the recording sensitivity. Preferably, the thermal conductivity of the metal recording thin film 20 is 4W/cm· °C or less. If the thermal conductivity of the metal recording thin film exceeds 4W/cm·°C, it is difficult for the thin film itself to be heated to a predetermined temperature since the heat of the recording thin film, generated by absorbing a laser beam, is rapidly diffused into its surroundings. Even if the recording thin film is heated to the intended temperature, adjacent tracks may be deformed since the size of the recording pit is increased. Also, it is preferable that the thermal linear expansion coefficient of the metal recording thin film is equal to or greater than 3 × 10 -6 /°C. If the linear expansion coefficient is less than the level, the metal recording layer cracks by the expansion of the substrate during recording, so that a uniform recorded signal value cannot be obtained. Preferably, the metal recording thin film is formed of a metal such as gold (Au), aluminum (Al), chromium (Cr), Al/Ti, copper (Cu), Cu/Al, nickel (Ni), platinum (Pt), silver (Ag), tantalum (Ta), iron (Fe) and titanium (Ti), alloys thereof or a non-metallic inorganic substance including silicon, wherein the metal has a coefficient k, greater than 0.01, of the imaginary part of a complex refractive index. If the k is less than 0.01, the deformation of the recorded portion is decreased as light absorption for recording is decreased, thereby lowering the recording sensitivity. The metal recording thin film may be formed using a general method such as a vacuum deposition, an electron beam (E-beam) and a sputtering method. The buffering layer absorbs the expansion of the substrate during recording, increases interference of the reflective lights at the metal recording thin film and the reflective film, caused by a change in its own thickness by the expansion of the substrate, or causes difference of optical paths between the non-recorded portion and recorded portion by its own deformation or decomposition by the heat, to make a recording signal. To this end, preferably, the buffering layer is formed of a material having a large refractive index, and capable of being easily decomposed or deformed. Also, it is preferably to use a material which is easy to spin coat, thus a material having a high solubility to an organic solvent is used. Preferably, the buffering layer has a thickness of 50∼200nm at a portion corresponding to the pregroove of the substrate, which is less than a conventional dye layer, as a recording layer, having a thickness of 150∼300nm, thereby lowering the manufacturing costs of the optical recording medium. Also. even though the buffering layer is thin, the recording signal is strengthened to a sufficient level by a signal by the metal recording thin film. That is, because the recording signal is made by the interference between the reflective lights at the metal recording thin film and the reflecting layer, the recording signal is increased. If the buffering layer is formed to be thin, the effect of the absorption spectrum for an organic substance decreases relative to a thick buffering layer. The present invention uses a material having an absorption coefficient (k) of 1.6 or less and a refractive index (n) of 1.4 or more at 650nm, and the absorption coefficient (k) of 0.001 or more and refractive index (n) of 1.8 or more at 780nm, that is, a material having the maximum absorption peak at 680∼750nm, preferably, one or more dye, as the major component of the buffering layer. Thus, a reflectivity of 30% or more can be maintained at 650nm, so a DVD player can reproduce the information stored on the optical recording medium of the present invention. Preferably, the decomposition temperature of the dye as the major component of the buffering layer is 80∼250°C. Preferably, the dye of the present invention is anthraquinone, dioxadine, tripheno-dithiazine, phenanthrene, cyanine, phthalocyanine, merocyanine, pyrylium, xanthine, triphenylmethane, croconium, azo, indigoid, methine. azulene, squarium, sulfide or methal dithiolate. A small amount of organic polymer may be mixed with the dye. Preferably, the organic polymer includes vinylalcohol resin, vinylacetate resin, acrylate resin, polyester resin, polyether resin, polystyrene resin, polyurethane resin, cellulose resin and fatty acid resin. Here. the content of the organic polymer is 30wt% or less based on the total weight of the buffering layer. Preferably, the buffering layer further includes an additive which decomposes or sublimates at 100∼200°C, and whose weight rapidly decreases during decomposition or sublimation. Assuming that the rate of weight decrease is measured at a temperature which is 50°C higher than the decomposition or sublimation temperature of the additive, the rate of weight decrease of the additive during decomposition is 20wt% or more based on the normal weight of the additive. The additive may include metallocene, tetraethyllead complex, peroxide, azo compound and derivatives thereof. Here, metallocene, tetraethyllead complex and derivatives thereof absorb heat, and peroxide, azo compound and derivatives thereof emit heat when the temperature increased to the decomposition temperature or more by a recording light or heat generated from the metal recording thin film. Preferably, the azo compound is 2,2'-azobisisobutylonitrile (AIBN) or 1,1'-azodicarboxyldipiperidine. That is, preferably, when the buffering layer contains a dye which emits heat due to the recording light or the heat of the metal recording thin film, an additive, which absorbs heat during decomposition, is used. Meanwhile, it is preferable that an additive which emits heat during decomposition is used when the dye of the buffering layer absorbs heat due to the recording light or the heat of the metal recording thin film. The additives increase recording sensitivity, improves optical reliability in a case, and controls and maintains the degree of deformation of the buffering layer, to improve BLER and jitter after recording. The content of the additive is preferably 1∼20wt% based on the total weight of the buffering layer. If the content of the additive exceeds 20wt%, the recording characteristics are damaged. On the contrary, if the content thereof is less than 1wt%, the effect of the additive is negligible. Metallocene. as one of the additives, is a general term referring to a molecule having a sandwich structure such as a non-electrolyte complex of biscyclopentadienyl compound. The metallocene is expressed by a formula M(C 5 H 5 ) 2 , where M is titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), osmium (Os) or palladium (Pd), and which mostly have a sublimation property of losing weight rapidly. FIG. 3A is a graph showing rate of weight decrease ("a") and thermal analysis ("b") of ferrocene, which is metallocene when M is Fe. As shown in FIG. 3A, rapid weight loss occurs near 100°C. The metallocene is indirectly heated when the metal recording thin film or the dye of the buffering layer emits heat by absorbing laser beams during recording, so decomposed or sublimated by adsorbing heat. If the dye, as the major component of the buffering layer, emits heat during decomposition, the area of a region in which the dye is decomposed can be controlled by adding an additive such as metallocene which absorbs heat during decomposition. That is, the dye is completely decomposed only in the recorded portion. Meanwhile, the azo compound or peroxide compound is indirectly decomposed by the metal recording thin film or dye during recording, emitting heat. Because the azo compound or peroxide compound is decomposed at a low temperature, the decomposition temperature of the buffering layer is lowered. Thus, if the dye of the buffering layer emits heat during decomposition, the area of a region in which the dye is decomposed can be controlled by adding the azo compound or peroxide compound which emits heat during decomposition, so the complete dye decomposition occurs only in the recorded portion. FIG. 3B is a graph showing rate of weight decrease ("a") and thermal analysis ("b") of AIBN which loses weight below 150°C. As described above, as the additives rapidly lose weight during decomposition or sublimation due to the above-described properties, the boundary of the recorded portion and non-recorded portion becomes distinct, so the recording characteristics (BLER, jitter, etc.) of the recording signal are improved. Also, the additive acts as a removing agent for removing singlet oxygen generated when a disc is exposed to light, thereby improving optical reliability. In the present invention, the buffering layer is formed of the above-described materials by a general method. For example, after dissolving the above materials for the buffering layer in an organic solvent, the resultant solution is spin-coated on the metal recording thin film 20, resulting in the buffering layer. Here, the organic solvent is not limited as long as if the organic solvent does not damage the substrate, and can be easily dissolved in an organic solvent. The reflecting layer 40, for a high reflectivity during recording or reproducing, is formed of a metal having a high thermal conductivity and high reflectivity to prevent easy deformation. Thus, the reflecting layer 40 is formed of Au, Al, Cu, Cr, Ag, Ti or alloys thereof, to a thickness of 500∼2,500Å by a vacuum deposition or sputtering method. The protecting layer 50 protects other constituent layers of the optical recording medium. The protecting layer 50 is formed by a general method. For example, the protecting layer 50 is formed by spin-coating on the reflecting layer 40 an epoxyacrylate resin or acrylate resin which is transparent and curable by ultraviolet rays, and has a great impact intensity, and then curing the spin-coated product by irradiating ultraviolet rays. In the optical recording medium of the present invention, having the above-described structure, the dye layer, e.g., the buffering layer, is formed to be very thin, to reduce the amount of use of the dye, thereby lowering the manufacturing costs of the optical recording medium. Also, because the reflectivity is 60% or more at 770∼790nm and 30% or more at 630∼660nm, reproduction by a DVD player is possible, and the optical reliability as well as the recording sensitivity are improved. Hereinafter, the present invention will be described in detail with reference to the following examples. In these examples, rate of weight decrease is defined as the weight is measured at a temperature which is 50°C higher than the decomposition or sublimation temperature of the additive with respect to the normal weight of the additive. The following description refers to dyes as L-04 and S-04, using the designation of the Japanese Institute for Photosensitising Dyes. L-04 is 2- [5-1,3-dihydro-1,1-dimethyl-3-butyl-2H-benzeneindol-2-ylidene] -1,3-pentadienyl-1,1-dimethyl-3-butyl-1H-benzene indolinium perchlorate. S-04 is 2-[5-1,3-dihydro-3,3-dimethyl-3-propyl-2H-indol-2-ylidene]-1,3-pentadienyl-1,1-dimethyl-3-butyl-1H-indolinium perclorate. Example 1 A nickel (Ni) recording thin film was vacuum-deposited to a thickness of 8nm on a polycarbonate (PC) substrate with a thickness of 1.2mm having pregrooves with a depth of 260nm, a width of 250nm at the bottom thereof, a width of 477nm at the middle thereof and a track pitch of 1.6µm. Then, a coating solution obtained by dissolving a dye mixture of 0.262g (L-04:S-04=1:3, where L-04 (Japanese Institute for Photosensitizing Dyes) has refractive index (n) of 2.3, absorption (k) of 0.02 at 780nm, and absorption (k) of 1.5 at 650nm, and decomposition temperature of 238°C, and S-04 (Japanese Institute for Photosensitizing Dyes) has refractive index (n) of 2.8, absorption (k) of 0.001 at 780nm, and refractive index (n) of 1.5 and absorption (k) of 1.5 at 650nm, and decomposition temperature of 231°C) and ferrocene of 0.026g (sublimation temperature of 110°C and rate of weight decrease of 100wt%; ferrocene having a melting point of 174°C and a decomposition temperature of 400°C starts to sublimate at 100°C by heating, and completely sublimates at 150°C without any residue) in 10ml diacetone alcohol (DAA) was spin-coated on the Ni recording thin film at 4,000 rpm to form a buffering layer. Here, the thickness of the buffering layer corresponding to the pregroove portion, measured by a scanning electron microscope (SEM), was about 90nm. After drying the resultant structure at 40°C for 4 hours in a vacuum oven, Ag was vacuum-deposited to a thickness of about 1,000Å to form a reflecting layer. Then, an ultraviolet (UV)-curable epoxyacrylate resin was spin-coated on the reflecting layer and then cured to form a protecting layer, resulting in a disc. According to the results of the evaluation on the obtained disc, using a tester adopting a laser beam of 780nm, reflectivity before recording was 70%, and the maximum reflectivity R top was 66% and the CNR was 63dB using a 0.7mW recording laser power with respect to a disc which was recorded at a recording speed of 1.3m/sec using 8mW, 720kHz recording power. Under the above recording conditions, a recorded signal of 47dB or more CNR was reproduced with a recording power of 5mW or more as shown in FIG. 4. Also, after recording an audio signal in the disc at 2X and 4X speeds using a CD-R recorder (400tx, Yamaha, Japan), the recorded audio signal could be reproduced using a CD player (Mark II, Philips). According to the recording characteristics of the disc, evaluated by a CD-CATS (Audio Development Co., Sweden), all items evaluated satisfied the standard of the CD (Orange Book Part II). Also, the reflectivity of the disc, measured by a UV spectrometer (UV-310PC, Shimadzu, Japan), was 30% or higher at a wavelength of 650nm, as shown in FIG. 5 (graph "a"). Thus, it is evident that information recorded in the disc could be reproduced by a digital versatile disc player (DVDP) and actually by a DVD player (SD-3000, Toshiba, Japan). For comparison, the reflectivity of a Dye CD-R which is a conventional optical recording medium, having a thick dye layer, as a recording layer, was measured by the same method, and the result is shown as graph "b" in FIG. 5. As shown in FIG. 5, the CD-R has a high reflectivity only at 780nm while having a reflectivity lower than 30% near 650nm. Thus, it is evident that the conventional dye type optical disc could be reproduced by only a CD player, not a DVD player Example 2 A tantalum (Ta) recording thin film was vacuum-deposited to 8nm thickness on a polycarbonate (PC) substrate with a thickness of 1.2mm having pregrooves with a depth of 230nm, a width of 250nm at the bottom thereof, a width of 410nm at the middle thereof and a track pitch of 1.6µm. Then, a coating solution obtained by dissolving 0.3g of dye mixture (L-04:Luxol Fast Blue=1:1, where L-04 (Japanese Institute for Photosensitizing Dyes) has refractive index (n) of 2.3, absorption (k) of 0.02 at 780nm, and absorption (k) of 1.5 at 650nm, and decomposition temperature of 238°C, and Luxol Fast Blue (Aldrich Co.) has refractive index (n) of 1.8, absorption (k) of 0.003 at 780nm, and refractive index (n) of 1.6 and absorption (k) of 0.4 at 650nm, and decomposition temperature of 250°C) and 0.3g of AIBN (decomposition temperature of 110°C and rate of weight decrease of 100wt%; AIBN is completely volatilized at its original decomposition temperature) in 10ml diacetone alcohol (DAA) was spin-coated on the Ta recording thin film at 4,000 rpm to form a buffering layer having a thickness of 100nm. Hereinafter, the same steps as those of Example 1 were performed to manufacture an optical disc, and the performance of the disc was evaluated. The reflectivity before recording, measured using a laser beam of 780nm, was 69.7%, and the maximum reflectivity R top was 65% and the CNR was 65dB after recording. Also, after recording an audio signal in the disc using a recorder (RPD-1000, Pioneer. Japan), the recorded audio signal could be reproduced using a CD player (Mark II, Philips). According to the recording characteristics of the disc, evaluated by a CD-CATS (Audio Development Co., Sweden), all items evaluated satisfied the standard of the CD (Orange Book Part II). Also, the reflectivity of the disc was 42% at a wavelength of 650nm. Thus, it is evident that information recorded in the disc could be reproduced by a DVDP. Example 3 An optical disc having a buffering layer with a thickness of 85nm was manufactured by the same method as that of Example 1, except that a substrate having pregrooves with a depth of 230nm, a width of 250nm at the bottom thereof, a width of 470nm at the middle thereof was used, and the performance of the disc was evaluated. According to the results of the performance evaluation, the reflectivity at 780nm before recording, measured using a laser beam of 780nm, was 72%, and the maximum reflectivity R top was 68% and the CNR was 62dB after recording. The recording characteristics of the disc. evaluated after recording an audio signal in the disc at a velocity of 2X and 4X using a CD-R recorder (400tx. Yamaha, Japan), satisfied the standard of the CD. Also. the reflectivity at 650nm was 44%, so the recorded audio signal could be reproduced using a DVDP. Example 4 An optical disc having a buffering layer with a thickness of 90nm was manufactured by the same method as that of Example 1, except that a metal recording thin film was formed of Al., and the performance of the disc was evaluated. According to the results of the performance evaluation, the reflectivity before recording, measured using a laser beam of 780nm, was 71%, and the maximum reflectivity R top was 65% and the CNR of a recording signal was 62dB after recording at a rate of 1.3m/sec. Also, the reflectivity at 650nm was 32%. That is, after recording an audio signal in the disc using a CD recorder, the recorded audio signal could be reproduced using a CDP and DVDP. Example 5 An optical disc having a buffering layer with a thickness of 95nm was manufactured by the same method as that of Example 1, except that 1,1-dimethylferrocene (decomposition temperature of 107°C and rate of weight decrease of 100wt%, Aldrich Co.) was used instead of ferrocene used in Example 1. According to the results of the performance evaluation, the maximum reflectivity R top was 66% at 780nm, the CNR was 63dB. and the reflectivity at 650nm was 34%. After recording an audio signal in the disc, the recorded audio signal could be reproduced using a CDP and DVDP. Also, the recording characteristics of the disc. evaluated by a CD-CATS, satisfied the standard of the CD. Example 6 An optical disc having a buffering layer with a thickness of 105nm was manufactured by the same method as that of Example 1, except that dicumilperoxide (decomposition temperature of 117°C and rate of weight decrease of 100wt%, Aldrich Co.) was used instead of AIBN used in Example 2. According to the result of the performance evaluation, the maximum reflectivity R top was 65 % at 780nm, the CNR was 62dB, and the reflectivity at 650nm was 35%. After recording an audio signal in the disc. the recorded audio signal could be reproduced using a CDP and DVDP. Also, the recording characteristics of the disc, evaluated by a CD-CATS, satisfied the standard of the CD. Comparative Examples 1 and 2 An optical disc was manufactured by the same method as that of Example 1, except that only 0.28g of a mixture of L-04 and S-04 was used to form a buffering layer instead of using the ferrocene (Comparative Example 1) and the Ni recording thin film was not used (Comparative Example 2). Optimal recording power at a recording rate of 2.4m/sec was measured based on β 4%, on discs obtained by the Comparative examples 1 and 2, and Example 1. As a result, the optical recording power was 10.5mW in the disc of Example 1, 12mW of the Comparative Example and 13mW of the Comparative Example 2. That is, it can be known that Example 1 requires less recording power than the Comparative Examples, so the recording sensitivity of the disc prepared by Example 2 is improved. Thus, the effects of the metal thin film and additive used in the present invention are proven. Also, after recording, BLER and reflectivity were measured at a power of 0.53kW/mw using a xenon lamp, a relative humidity of 50% and a temperature of 40°C in order to test the optical reliability of the disc. As a result, the BLER and reflectivity of the disc of Example 1 were 20 and 66%, respectively, without changes. However, in the case of the disc of Comparative Example 1, the BLER was changed from 50 to 1200 after 96 hours. Also, the BLER of the disc of Comparative Example 2, which was 10 initially, was seriously damaged to an extent such that BLER could not be measured after 96 hours. That is. the metal thin film of the present invention lower the BLER, improving the optical reliability. Comparative Example 3 An optical disc was manufactured by the same method as that of Example 1, except that a substrate having pregrooves with a depth of 80nm, a width of 100nm at the bottom thereof, and a width of 350nm at the middle thereof, and a coating solution obtained by dissolving 0.23g of dye mixture (L-04:S-04=1:1) and 0.26g ferrocene in 10ml diacetone alcohol (DAA), were used. The reflectivity of the optical disc before recording was 67% at 780nm. However. I 11R , which is a ratio representing the amplitude of the recording signal when an audio signal is recorded at 2X speeds by a recorder (Yamaha, Japan), was 0.55, which is less than the Standard of 0.60. Also, jitter was 50∼60ns, which is higher than the Standard of 35ns or less. Thus, the recorded audio signal could not be reproduced by a CD player. Comparative Example 4 An optical disc was manufactured by the same method as that of Example 1, except that a substrate having V-shaped pregrooves with a depth of 260nm, a width of 30nm at the bottom thereof, and a width of 300nm at the middle thereof was used. According to the results of the performance evaluation, the BLER was 1500, jitter was 50ns or more, and the maximum reflectivity R top was 65% at 780nm, so the reproduction was possible. However, the recording characteristics were poor. As described above, the optical recording medium of the present invention is recordable, and reproducible using a reproducing light having a wavelength of 630∼790nm, particularly, by a DVD player using a reproducing light having a short wavelength of 630∼660nm and a CD player using a reproducing light having a long wavelength of 770∼790nm. Also, due to a metal recording thin film formed on a substrate, the amplitude of a recording signal is increased and the optical reliability is also improved. Also, because of a thin buffering layer, the manufacturing costs can be reduced. Also, by adding a material whose weight is rapidly lost during decomposition to the buffering layer, the recording sensitivity and the optical reliability can be much improved.

Claims: An optical recording medium comprising a substrate (10) formed of a material transparent to a laser beam having pregrooves, and a metal recording thin film (20), a buffering layer (30) and a reflecting layer (40) which are stacked in sequence on the substrate (10), wherein the buffering layer (30) comprises one or more dyes having a refractive index (n) of 1.4 or more and an absorption coefficient (k) of 1.6 or less at a wavelength of 650nm, and a refractive index (n) of 1.8 or more and an absorption coefficient (k) of 0.001 or more at a wavelength of 780nm; the pregroove of the substrate (10) has a depth of 100∼300nm, a width of 100∼350nm at the bottom, and a width of 200∼550nm at the middle; and the buffering layer (30) further includes an additive which has a decomposition or sublimation temperature of 100∼200°C, and the weight of which rapidly decreases during decomposition. The optical recording medium of claim 1, wherein the decomposition temperature of the dye is 80∼250°C. The optical recording medium of claim 1 or 2, wherein the thickness of the buffering layer (30), measured at a region corresponding to the pregrooves of the substrate, ranges between 50∼200nm. The optical recording medium of any preceding claim, wherein the thickness of the buffering layer (30), measured at a region corresponding to the pregrooves of the substrate, ranges between 85∼105nm. The optical recording medium of any preceding claim, wherein the buffering layer (30) further includes an organic polymer. The optical recording medium of claim 5, wherein the content of the organic polymer is 30wt% or less based on the total weight of the buffering layer (30). The optical recording medium of claim 5 or 6, wherein the organic polymer is at least one selected from the group consisting of vinylalcohol resin, vinylacetate resin, acrylate resin, polyester resin, polyether resin, polystyrene resin, polyurethane resin, cellulose resin and fatty acid resin. The optical recording medium of any preceding claim, wherein the dye is at least one selected from the group consisting of anthraquinone, dioxadine, tripheno-dithiazine, phenanthrene, cyanine, phthalocyanine, merocyanine, pyrylium, xanthine, triphenylmethane, croconium, azo, indigoid, methine, azulene, squarium, sulfide and methal dithiolate. The optical recording medium of claim 8, wherein the additive has a rate of weight decrease of 20wt% based on the normal weight of the additive, when the weight decrease rate is measured at a temperature which is 50°C higher than the decomposition temperature of the additive. The optical recording medium of claim 8 or 9, wherein the additive emitting heat during decomposition is used when a dye, which absorbs heat during decomposition by taking a recording light or heat generated by the metal recording thin film onto which the recording light has irradiated, is used for the buffering layer (30). The optical recording medium of claim 10, wherein the additive emitting heat during decomposition is at least one selected from the group consisting of azo compound, peroxide compound and derivatives thereof. The optical recording medium of claim 11, wherein the azo compound is 2,2'-azobisisobutyronitrile (AIBN). The optical recording medium of claim 11, wherein the peroxide compound is dicumylperoxide. The optical recording medium of any preceding claim, wherein the additive absorbing heat during decomposition is used when a dye, which emits heat during decomposition by recording light or heat generated by the metal recording thin film onto which the recording light has irradiated, is used for the buffering layer (30). The optical recording medium of claim 14, wherein the additive absorbing heat during decomposition is at least one selected from the group consisting of metallocene, tetraethyllead complex and derivatives thereof, wherein the metallocene is expressed by a formula M(C 5 H 5 ) 2 , where M is titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), osmium (Os) or palladium (Pd). The optical recording medium of claim 15, wherein the metallocene is ferrocene and derivatives thereof. The optical recording medium of any preceding claim, wherein the content of the additive is 1∼20wt% based on the total weight of the buffering layer. The optical recording medium of any preceding claim, wherein the additive is at least one selected from the group consisting of metallocene expressed by a formula M(C 5 H 5 ) 2 , where M is titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), osmium (Os) or palladium (Pd), tetraethyllead complex, peroxide, azo compound and derivatives thereof. The optical recording medium of any preceding claim, wherein the pregroove of the substrate (10) has a depth of 230∼260nm, a width of 250∼260nm at the bottom, and a width of 410∼480nm at the middle. The optical recording medium of any preceding claim, wherein the pregroove of the substrate (10) has a "U" shape. The optical recording medium of any preceding claim, wherein the optical recording medium is compatibly reproducible at a wavelength of 630∼790nm. The optical recording medium of any preceding claim, wherein the optical recording medium has a reflectivity of 60% or more at a long wavelength of 770∼790nm, and a reflectivity of 30% or more at a short wavelength of 630∼660nm. The optical recording medium of any preceding claim, further comprising a protecting layer (50) on the reflecting layer (40).

Inventors: KIM SUNG-HOON, LEE SOO-HYUNG, MIN KYUNG-SUN, KIM, SUNG-HOON, LEE, SOO-HYUNG, MIN, KYUNG-SUN
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1998
Main classification: G11B7
IPC codes: B32B15, C09D129, B32B27, C09D125, C09D5, C09D133, G11B7, C09D1, B32B7, B32B23, C09D171, C09D175, C09D131