DE69622130T2 - Optical recording medium and optical recording method - Google Patents

Description

The present invention relates to an optical recording/playback medium, and more particularly to a recording/playback compact disc (CD) having a metal or non-metal recording layer and an optical recording method therefor.

Optical media is widely used as a high-density recording medium because the required recording area per unit recording amount is smaller than that of a conventional magnetic recording medium. Optical recording media are basically divided into three types according to their functions, i.e. ROM (Read Only Memory) which is suitable only for playing back information already recorded, WORM (Write Once Read Many) which is suitable only for recording once, and RW (Rewritable) which is suitable for erasing and re-recording after recording. The recorded information must be able to be played back in a player for a ROM type recording medium. Therefore, the "Red Book" standard for ROM type recording media must be observed, i.e. the recording medium must have a reflectivity of over 70% and a CNR (Carrier to Noise Ratio) of at least 47dB.

Optical recording media for recording reproduce information using a change in reflectivity due to physical deformation, a phase change or a change in magnetization of a recording layer before and after recording. To be used as recording media compatible with CDs (compact discs), the optical recording medium must have long stability and high recording sensitivity in addition to the high reflectivity and the CNR characteristics mentioned above. Various optical recording media have been proposed and partially improved by using different Materials have been implemented to enhance these features and simplify the production of the medium.

Figs. 1 to 5 of the accompanying schematic drawings are cross-sectional views illustrating the deposited structures of conventional optical recording media (discs).

According to Japanese Patent Laid-Open Publication No. 63-268142, as shown in Fig. 1, a recording medium has a structure including a substrate 1, a sensitizing layer 2 made of gelatin, casein or PVA disposed over the substrate 1, and a thin metal foil 3 made of Cr, Ni or Au having a thickness in the range of 50-500 Å disposed over the sensitizing layer 2. During optical recording in such a recording medium using a laser beam, the thin metal foil absorbs, and therefore the sensitizing layer 2 and the thin metal foil 3 are deformed to form recording dots. However, a recording medium having such a structure has disadvantages in maintaining long-term stability because the recording dots are exposed.

U.S. Patent No. 4,973,520 discloses a technology for achieving apparently very good recording characteristics of over 50 dB by forming a thin metal foil 2a of a three-layer structure including a first thin metal foil 1, a metal oxide and a second thin metal foil 2, the three-layer structure being deposited on a substrate 1a as shown in Fig. 2. However, since the dots on the thin metal foil 2a are formed by laser radiation, thereby exposing the recording dots, the recording stability also becomes poor.

As shown in Fig. 3, US Patent No. 4,983,440 attempts to compensate for these problems and discloses a technology for forming a thin two-layer metal foil 2b and 2b' as a recording layer 2a on a substrate 1b and for forming a protective layer 4 thereon to protect the recording layer 2a. However, the recording medium using this technology cannot be used as a medium compatible with conventional CD players because the reflectivity is below 20% due to the absence of a reflective layer. A high power light source is required if this non-compatible medium is to be used in an otherwise conventional CD player.

According to U.S. Patent No. 5,039,558, as shown in Fig. 4, the stability of the recording dots is improved by symmetrically adhering, by means of an adhesive layer 5, a pair of substrates 1c and 1c' on which thin metal foils 2c and 2c' and protective layers 4c and 4c' are applied. In this method, however, the thin metal foils 2c and 2c' cannot simultaneously function as a reflection foil and as a light-absorbing layer, so that the reflectivity does not reach 70% even in principle.

According to US Patent No. 5,328,813, a thin metal foil is provided as a recording layer on a substrate, and a solid metal oxide layer is formed thereon to improve the long-term stability of the recorded signals and the reflectivity up to 40% to 60%. However, it is a problem that the CNR still remains low.

In addition, U.S. Patent Nos. 4,990,388 and 5,155,723 disclose a technology for forming an organic dye layer 6 as a recording layer on a substrate 1d and forming a reflection film layer 7 and a protective layer 4d thereon, as shown in Fig. 5. When recording information on this medium, the dye layer 6 absorbs a recording laser beam and generates heat to deform the substrate 1d. Thus, the information can be recorded according to the change in reflectance before and after after recording in a deformed area, so that compatibility with conventional CD players can be achieved under the condition of more than 70% reflectivity and more than 47dB CNR after recording.

However, such discs have disadvantages including poor heat resistance and light fastness properties and the use of expensive organic inks. In addition, the ink must be applied to the substrate in a liquid state (dissolved in an organic solvent) by a spin coating process. Since the reflectivity varies significantly according to the thickness deviation of the coating layer, expensive equipment that can control the thickness of the coating layer within a deviation of ± 3% is required to reduce productivity due to the precise requirements for ink thickness.

U.S. Patent No. 4,499,178 describes an optical recording medium having a substrate made of plastic, glass, paper or foil. A thin metal recording foil overlies the substrate. A reflective metal layer overlies the thin metal recording foil and is separated from it by an organic or inorganic insulating layer. A signal is recorded on the medium by heating the thin metal recording foil with a laser beam to cause deformations of the recording foil without causing deformations of the reflective layer.

Preferred embodiments of the present invention contribute to providing an optical recording medium for recording/playback and an optical recording method that is compatible with conventional CD players and also enables lower manufacturing costs and high productivity.

It is also an object to provide an optical recording medium for recording/playback and an optical recording method having a high reflectivity and for recording signals, wherein the recording stability is improved.

Therefore, the present invention provides a method for optically recording a signal on an optical recording medium comprising a substrate, a thin metal recording foil formed on the substrate, a reflective layer disposed over the thin metal recording foil, and a deformable buffer layer located between the thin metal recording foil and the reflective layer, the method comprising the steps of: heating the thin metal recording foil between the substrate and the buffer layer with a laser beam to cause a deformation in at least one of the substrate, the thin metal recording foil and the buffer layer, characterized in that the substrate has a pregroove, that the reflective layer is deformable, and that heating the thin metal recording foil also causes a deformation of the reflective layer in an area of the pregroove, thereby forming points where the deformation takes place, these points detecting the signal represent.

The invention extends to an optical recording medium on which a signal is recorded by such a method.

For a better understanding of the invention, and to show how embodiments thereof may actually be carried into effect, reference will now be made, by way of example, to Figures 6 to 12 of the following diagrammatic drawings, in which:

Fig. 6 is a cross-sectional view illustrating the applied structure of an optical recording medium (disc) according to an example of the present invention;

Fig. 7 is a schematic cross-sectional view illustrating the structure of a dot forming a recorded area formed on the disc of Fig. 6;

Fig. 8 is a graph showing the reflection change depending on the thickness change of a buffer layer during recording relative to the thickness change of a metal foil of the disc of Fig. 6;

Fig. 9 is a graph illustrating the variation in reflectance with the change in thickness of a buffer layer in the disk of Fig. 6;

Fig. 10 is a graph illustrating the variation of write power and CNR according to an example of the present invention;

Fig. 11 is an AFM image of a substrate/thin metal layer on a recorded area; and

Fig. 12 is an AFM image of a buffer layer on a recorded area.

In the example of the optical recording medium of Fig. 6, a substrate 10 has a pregroove for guiding light during recording, a thin metal recording film 20 is formed on the substrate, and a buffer layer 30 and a reflective layer 40 are sequentially deposited on the substrate 10. A protective layer 50 is optionally deposited on the reflective layer 40 to protect the recording medium.

A concentrated recording laser beam heats the thin metal recording film 20 when optical recording is performed, and the heat is transferred to both the substrate 10 and the buffer layer 30. A region of the substrate 10 adjacent to a region of the heated thin metal film is deformed by thermal expansion, and the heated region of the thin metal recording film 20 swells up in the buffer layer 30 to form a dot. When the recorded information is reproduced, a recorded region (i.e., a dot) of the substrate 10 and a deformed region of the thin film 20 have a lower reflectance relative to a non-recorded region due to the following principles.

First, the reflectance is lower due to destructive interference caused by light reflected from an edge surface between the substrate 10 and the recording thin metal film 20 and by light reflected from an edge surface between the buffer layer 30 and the reflective layer 40. That is, as shown in Fig. 7, the thickness of the buffer layer 30 in a non-recorded area is first designated as "d₁" in order to make light "b₁" reflected from the reflective layer 40 and light "c" reflected from the recording thin metal film 20 in a pregroove of the substrate 10 constructively interfere with each other. In a recorded area, the substrate 10 and the recording thin metal film 20 swell upward by thermal expansion and cause the buffer layer 30 to deform. This reduces the thickness of the buffer layer 30 to a thickness "d₂" at which destructive interference occurs. In addition, swelling in the buffer layer 30 causes the reflective layer 40 to deform. Consequently, the reflectivity is lower due to the destructive interference caused by light "a" reflected from an edge surface between the substrate 10 and the thin metal foil 20 and by light "b₂" reflected from an edge surface between the buffer layer 30 and the reflective layer 40.

The above phenomenon results from the Fabry-Perot effect and, as shown in Fig. 8, changes the reflectivity of the recording medium relative to the thickness of the buffer layer. Therefore, the buffer layer must be made using materials that can be deformed in a range of thickness "d2" suitable for the destructive interference resulting from the deformation of the thin metal foil 20.

Second, the reflectance decreases due to scattering of incident light by slanted and rounded walls formed in the recorded area. That is, the substrate 10, the thin metal recording film 20 and the buffer layer 30 are deformed as shown in Fig. 7. Since there are slanted and rounded surfaces on both sides of the recorded area, a recording beam is scattered on the side walls of the deformed area of the thin metal film 20 so that the reflectance decreases relative to the non-recorded area where no scattering of light occurs. Consequently, the recorded area has a lower reflectance than a non-recorded area so that reproduction of recorded information is achieved due to the difference in reflectance between the recorded and non-recorded areas during playback.

Third, when the thin metal foil 20 absorbs the incident recording beam and the absorbing area is locally heated so that the repair increases greatly, the materials of the thin metal foil 20, the adjacent substrate 10 and the buffer layer 30 are locally thermally decomposed so that the overall reflectivity is reduced, thereby causing the reflected light "a" and "b₂" to decrease as shown in Fig. 7. An AFM image of the deformation of the substrate 10 and the thin metal foil 20 after removing the upper layers is shown in Fig. 11. Fig. 12 shows an AFM image of the deformation of the buffer layer 30 after removal of the reflective layer 40.

The disc described above enables a reflectivity of 70% or more in a non-recorded area by inserting a reflective layer 40 on a buffer layer 30 and adjusting the thickness of the buffer layer 30 so that the light "b" reflected by the reflective layer 40 and the light "c" reflected by the thin metal foil 20 constructively interfere with each other. In particular, the Fabry-Perot effect can be maximized by adjusting the reflectivity of the buffer layer 30 by including or not including additional dyes in the buffer layer 30.

In the above disc, the substrate 10 is transparent to a laser beam and can be made of polycarbonate, polymethyl methacrylate, epoxy, polyester or an amorphous polyolefin which has very good impact resistance and can expand when heated. The glass transition temperature for thermal deformation of the substrate 10 ranges from 80°C to 200°C and is preferably 100°C to 200°C. For guiding the laser beam during information recording, it is necessary to form a pregroove on the substrate 10 which has a depth of 30 to 450 nm and a width of 0.1 to 1.2 µm.

Since the thin metal foil 20 acts as a heat generating layer for absorbing a recording laser beam and generating heat and forms a partial mirror for forming a contrast before and after recording, the absorption value and the reflection must be constant. A suitable material has a thickness of 30 Å to 500 Å, a transmission of 5 to 95% and an absorption of 5 to 95%. In addition, a non-metal foil such as silicon and their compounds, such as silicon nitride, silicon germanium, silica and SiO.

One of the optical properties of a material for forming such a thin metal or non-metal film is a complex refractive index with a value (k) having an imaginary part greater than or equal to 0.01. When k is less than 0.01, absorption during recording is less, and a recorded area is deformed to a lesser extent, resulting in lower recording sensitivity. It was determined by computer simulation that materials for the thin metal foil or the non-metal foil having k values falling within a pentagon defined by vertices 7.15+3.93i, 7.15+5.851, 8.96+6.28i, 9.56+5.90i and 8.14+3.77i in the n,k plane would not provide effective recording sensitivity for a compact disk to be desirably recorded.

When the thickness of the thin metal foil 20 is larger than 500 Å, the recorded signals become smaller for the following reasons. First, the deformation of a recorded area becomes smaller due to the restricted expansion of the substrate 10 by the thin metal foil 20 during recording. Second, as shown in Fig. 9, the thicker the thin metal foil 20 is, the smaller the reduction in reflectance depending on the change in thickness of the protective layer during recording, thereby making the contrast between the recorded and non-recorded areas smaller.

When the thickness of the thin metal foil 20 is less than 30 Å, the heat generation due to absorption during recording becomes smaller, making it difficult to deform the substrate 10.

In addition, if the thermal conductivity of the thin metal film 20 is greater than or equal to 4W/cmºC, the heat cannot be concentrated on the thin metal film when it is heated during recording using the laser beam, and the heat is instead quickly transferred to the surroundings of the thin metal film 20, so that heating exceeding a certain temperature is difficult to achieve. Even if heating is possible, the size of the recorded areas may become so large that they reach an adjacent track. Therefore, the thermal conductivity of the thin metal film should be set to 4W/cmºC or less.

The linear expansion coefficient of the thin metal foil 20 is preferably 3 x 10-6 /°C. If it is smaller than this value, cracks may be generated in the thin metal foil 20 by the expansion of the deformed substrate 10 during recording, thereby reducing the probability of obtaining a uniform recording signal value.

As materials for the thin metal foil 20 suitable to satisfy the above conditions, at least one metal selected from the group consisting of Au, Al, Cr, Cu, Ni, Pt, Ag, Fe, Ti, Ta and alloys thereof can be used. The thin metal foil 20 made of such a material is deposited on the substrate 10 by a vacuum deposition method, an electron beam method or a sputtering method.

The buffer layer 30 for absorbing the deformation of the substrate 10 and the thin metal foil 20 can be easily deformed and is preferably made of an organic material having a suitable deformability. The glass transition temperature of the buffer layer 30 preferably ranges from 60°C to 180°C and should be lower than that of the substrate 10. If the glass transition temperature of the buffer layer 30 is lower than 60°C, the recording stability may be reduced. The buffer layer 30 made of an organic material is dissolved in an organic solvent to be then applied by a spin coating method. At this time, a material that dissolves the organic material well but does not damage the substrate must be used as the solvent. The thickness of the buffer layer 30 is set to have a reflectance of 70% or more as shown in Fig. 9. At this time, the range of 50 to 10,000 Å is suitable.

The organic material that can be used as the buffer layer 30 includes a vinyl alcohol resin, a vinyl acetate resin, an acrylate resin, a polyester resin, a polyether resin, a polystyrene resin, a urethane resin, a cellulose resin, a fatty acid, and a low molecular weight organic composition. Copolymers of the above materials can also be used as the buffer layer 30.

In order to facilitate thermal deformation of the substrate during recording, an organic dye may be added to the buffer layer 30. When the dye is added, the buffer layer 30 is also heated during recording, which facilitates deformation of the substrate. The amount of dye added should be less than 30 wt% of the buffer layer material. More than this amount of dye makes it difficult to obtain higher reflectivity. A dye that absorbs the incident light during recording is selected depending on a short (i.e., high-density optical discs) or long (i.e., low-density optical discs) wavelength of the recording laser beam. Preferably, dyes in the near infrared range that absorb in the wavelength range from 780 to 850 nm or dyes that absorb in the wavelength range from 610 to 700 nm are used. For example, cyanine, croconium, squarrylium, phthalocyanine or naphthalocyanine can be used.

The reflective layer 40 may be formed by a vacuum deposition method, an electron beam method, or a sputtering method. Metals such as Au, Al, Cu, Cr, Ni, Pt, Ag, Fe, Ti, Ta, and alloys thereof may be used as the material of the reflective layer 40, and the thickness thereof may range from 500 k to 1,500 Å.

The protective layer 50 for protecting the recording medium may be a transparent material having high impact resistance and cured by ultraviolet rays (UV). Curing epoxy or acrylate resin may be applied by a spin coating method and then cured by UV rays to form the protective layer 50.

Although the above example of the present invention has been described in terms of deformation of each layer (i.e., substrate, thin metal film, buffer layer, and reflective layer), it should be noted that deformation may alternatively occur in only one or two layers, depending on the power of the laser beam and/or the physical properties (e.g., elastic modulus, thermal conductivity, thermal expansion coefficient, glass transition temperature, etc.) of each layer. It should be noted that thermal deformation of the thin metal film itself is unusual due to its very high melting temperature. However, thermal deformation of the thin metal film may occur, depending on the power of the laser beam. For example, when the power of the laser beam is high, the thin metal recording film 20 may melt to form a rim around the center of the focused light by heating above the metal melting point.

In addition, the deformation may only take place in the substrate 10 if the material used for the buffer layer has more stable physical properties (for example, lower thermal expansion coefficient, higher glass transition temperature and/or higher elastic modulus) than the material used for the substrate 10. Note that the deformation in the thin metal foil 20 and in the buffer layer 30 may occur due to the heat transfer in the thin metal foil 20 during laser recording. However, the deformation in the thin metal foil 20 and in the buffer layer 30 may be so small that it is difficult to detect. In addition, such deformation may have little effect during recording.

If the material used for the substrate has stable physical properties compared to the material used for the buffer layer 30, then the deformation may occur mainly in the buffer layer 30. It is also possible that the deformation may occur mainly in both the substrate 10 and the buffer layer 30 if the materials for the substrate 10 and for the buffer layer 30 have similar physical properties.

Further numbered examples of embodiments of the present invention will now be described in further detail. It should be understood, however, that the present invention is in no way limited by such specific examples.

example 1

A thin Al foil of 10 nm was vacuum deposited on a 1.2 mm thick polycarbonate substrate having a pregroove of 170 nm depth, 0.5 µm width and 1.6 µm track pitch. A coating solution prepared by dissolving 0.9 g of cyanobiphenylepoxydamine in 10 mL of diacetone alcohol was spin-coated at a speed of 2,000 rpm to form a buffer layer thereon. At this time, the thickness of the buffer layer of the pregroove region was measured by scanning electron microscopy. (SEM) and the result was about 2,500 Å. After drying in a vacuum oven at 40ºC for 4 hours, Al of 1,000 Å was vacuum deposited to form a reflective layer. UV-curing epoxy acrylate resin was spin-coated thereon to be cured, thereby producing a disc.

The disc was evaluated by using an evaluator adjusting a laser beam to 780 nm. A CNR of 51 dB and a reflectance of 72% were obtained under 1.3 m/sec, 720 kHz and 8 mW recording conditions with 0.7 mW writing power. Then, at a recording speed of 4.8 m/sec with a writing power of 10 mW, a CNR of 42 dB was obtained. When the writing power of the above recording conditions is changed as shown in Fig. 10, a recorded signal has a CNR of 47 dB or higher, which can be reproduced at 4 mW or more. When audio was recorded in an RP-1000CD manufactured by Pioneer Corp., this disc was replayable in a compact disc player. An evaluation result of the recording characteristics using a CD-CATS was demonstrated to meet all points of the CD specification.

Examples 2-3

Unlike the first example, the thickness of the thin Al foil was changed to 7 nm (second example) and 12 nm (third example). However, the subsequent processes in manufacturing the disc were the same as in Example 1. In addition, the recording evaluation was carried out under the same evaluation conditions as those selected for the first example. The result showed that the disc manufactured with a 7 nm thin metal foil had a reflectance of 73% and a CNR of 48 dB, and the disc manufactured with a 12 nm thin metal foil had a reflectance of 69% and a CNR of 53 dB.

Examples 4-8

In contrast to the above examples, the metal was replaced by Au, Cu, Ag, Ni or Pt as indicated in the table below and the disc was manufactured by the same process as in the first example. Table 1

Examples 9-13

In contrast to the above examples, the material of the buffer layer was replaced with a PMMA with Mw of 25,000, a PVA with Mw of 35,000, or a polyethylene oxide with Mw of 32,000, all manufactured by Polyscience, or a fatty acid manufactured by Tokyo Kasei (Standard Kit FE M-1). The thickness of a buffer layer in a pregroove was set to 2,500 Å, and then the recording evaluation was performed in the same manner as in the first example. Table 2

Example 14

When forming the buffer layer, NK125 (Nippon Kankoh Shikisho Kenkyusho Co.) was added to the buffer layer in the range of 0.5 wt% of the buffer layer material. The subsequent processing steps were the same as those of the first example in producing the disc. When a disc was recorded at 1.3 m/sec and 8 mW, a CNR of 52 dB was obtained. When recorded at 4.8 m/sec, a CNR of 48 dB was obtained. In addition, the reflectivity was about 70%. The disc could be used as a CD-R.

In the above-described examples of the present invention, recording was enabled by adapting a buffer layer, and compatibility with a CD is possible. In particular, since expensive organic dyes are not used, the manufacturing cost can be reduced and the productivity can be greatly improved.

It is obvious to those skilled in the art that the foregoing exemplary embodiments of the present invention are merely illustrative in nature and do not limit the scope of the invention.

The steps described above can be used to form dots that can be optically recorded/read. The dots represent the areas that are deformed by the laser beam. The term "dot or dots that can be optically recorded/read" generally includes optically readable and detectable markings of all kinds. The deformed areas of the substrate and the thin metal layer and the buffer layer have different optical properties compared to those of the non-deformed areas.

Claims (16)

1. A method for optically recording a signal on an optical recording medium comprising a substrate (10), a thin metal recording foil (20) formed on the substrate (10), a reflective layer (40) disposed over the thin metal recording foil (20), and a deformable buffer layer (30) located between the thin metal recording foil (20) and the reflective layer (40), the method comprising the steps of: heating the thin metal recording foil (20) between the substrate (10) and the buffer layer (30) with a laser beam to cause deformation in at least one of the substrate (10), the thin metal recording foil (20), and the buffer layer (30), characterized in that the substrate has a pre-groove, that the reflective layer is deformable, and that the heating of the thin metal recording foil (20) also causes a deformation of the reflective layer (40) in an area of the pregroove, thereby forming points where the deformation takes place, the points representing the signal. 2. An optical recording medium on which a signal is recorded by a method according to claim 1. 3. An optical recording medium according to claim 2, further comprising a protective layer (50) disposed on the reflective layer (40). 4. An optical recording medium according to claim 2 or 3, wherein at least one of the pre-grooved substrate (10), the thin metal recording film (20) and the buffer layer (30) is arranged to deform. 5. An optical recording medium according to claim 4, wherein all of the substrate (10), the thin metal recording film (20) and the buffer layer (30) are arranged to deform in a region of the pregroove. 6. Optical recording medium according to claim 5, wherein the substrate (10), the thin metal recording film (20) and the buffer layer (30) are arranged to deform in a region of the pregroove so that the buffer layer (30) has a region (d2) of reduced thickness in this region. 7. Optical recording medium according to one of claims 2-6, wherein the buffer layer (30) contains an organic material. 8. Optical recording medium according to one of claims 2-7, wherein the reflective layer (40) is designed to deform in a region of the pregroove. 9. Optical recording medium according to one of claims 2-8, wherein the thin metal recording film (20) has a complex refractive index of which the imaginary part is greater than or equal to 0.01. 10. Optical recording medium according to one of claims 2-9, wherein the thin metal recording film (20) has a thickness in a range of 30 A to 500 A. 11. An optical recording medium according to any one of claims 2-10, wherein the thin metal recording film (20) contains a material selected from a group including Au, Al, Ag, Pt, Cu, Cr, Ni, Ti, Ta, Fe and alloys thereof. 12. An optical recording medium according to any one of claims 2-11, wherein the buffer layer (30) has a thickness in a range of 50 Å to 10,000 Å. 13. An optical recording medium according to any one of claims 2-12, wherein the buffer layer (30) has a glass transition temperature in a range of 60°C to 180°C. 14. Optical recording medium according to one of claims 2-13, wherein the buffer layer (30) contains an organic dye (preferably less than 30 weight percent). 15. An optical recording medium according to any one of claims 2-14, wherein the reflective layer (40) contains a material selected from a group consisting of Au, Al, Ag, Pt, Cu, Cr, Ni, Ti, Ta, Fe and alloys thereof. 16. An optical recording medium according to any one of claims 2-15, wherein the thin metal recording film (20) has a thermal conductivity of not more than 4W/cmºC.