Sasa, Noboru (Kanagawa, JP)
Hayashi, Yoshitaka (Kanagawa, JP)
Komoda, Hirotaka (Osaka, JP)
Watada, Atsuyuki (Shizouka, JP)
Tanaka, Kawori (Kanagawa, JP)
Fujii, Toshishige (Kanagawa, JP)
Kamezaki, Hisamitsu (Kanagawa, JP)

Plaque It!

11/159920

08/19/2008

06/23/2005

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Ricoh Company, Ltd. (Tokyo, JP)

428/64.4

430/270.12, 428/64.1

B32B3/02

430/270.14, 428/64.8

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JP2002334438	November, 2002			OPTICAL RECORDING MEDIUM
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JP2002367182	December, 2002			MULTILEVEL OPTICAL RECORDING MEDIUM

Mulvaney, Elizabeth

Cooper & Dunham, LLP

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Rule 1.53(b) continuation of U.S. Ser. No. 10/824,227, filed Apr. 14, 2004, now U.S. Pat. No. 6,933,032 the entire contents of which are herein incorporated by reference.

What is claimed is:

1. A write-once-read-many optical recording medium comprising: a first inorganic thin film; and an organic thin film, wherein the first inorganic thin film comprises at least “R” and “O”, wherein “R” represents at least one element selected from the group consisting of Y, Bi, In, Mo, V and lanthanum series elements; and “O” represents oxygen atom, the organic thin film comprises a colorant wherein the colorant is selected from the group consisting of polymethine colorant, naphthalocyanine colorant, phthalocyanine colorant, squarylium colorant, chroconium colorant, pyrylium colorant, naphthoquinone colorant, anthraquinone colorant, xanthene colorant, tripenylmethane colorant, azulene colorant, tetrahydrocholine colorant, phenanthrene colorant, triphenothiazine colorant, azo colorant, formazan colorant, and metal complex thereof, and the organic thin film is capable of suppressing at least one of deformation and breakage of the first inorganic thin film and receiving the change of state of the first inorganic thin film.

2. The write-once-read-many optical recording medium according to claim 1, wherein the organic thin film has a thickness of 10 nm to 10 μm.

3. The write-once-read-many optical recording medium according to claim 1, wherein the organic thin film has a thickness of 10 nm to 200 nm.

4. The write-once-read-many optical recording medium according to claim 1, wherein the change of state is at least one selected from the group consisting of fusing, change of composition, diffusion, change of crystalline state, oxidation and reduction.

5. The write-once-read-many optical recording medium according to claim 1, wherein the first inorganic thin film further comprises an element “M”, wherein the element “M” is at least one selected from the group consisting of Al, Cr, Mn, Sc, In, Ru, Rh, Co, Fe, Cu, Ni, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Pb, Mo, V and Nb.

6. The write-once-read-many optical recording medium according to claim 5, wherein the first inorganic thin film has a composition represented by R_xM_yO, wherein “x” and “y” are atomic ratios and satisfy the following condition: [x/(x+y)≧0.3]; and “M” represents the element “M.”

7. The write-once-read-many optical recording medium according to claim 1, wherein the first inorganic thin film comprises the element “R” as an oxide of “R” (RO) and as another form than the oxide.

8. The write-once-read-many optical recording medium according to claim 5, wherein the first inorganic thin film comprises the element “M” as an oxide of “M” (MO) and the element “R” as another form than the oxide.

9. The write-once-read-many optical recording medium according to claim 5, wherein the first inorganic thin film comprises the element “M” as an oxide of “M” (MO) and the element “R” as an oxide of “R” (RO).

10. The write-once-read-many optical recording medium according to claim 5, wherein the first inorganic thin film comprises the element “M” as an oxide of “M” (MO) and the element “R” as an oxide of “R”(RO) and as another form than the oxide.

11. The write-once-read-many optical recording medium according to claim 1, wherein the first inorganic thin film comprises a bismuth oxide.

12. The write-once-read-many optical recording medium according to claim 1, wherein the first inorganic thin film comprises elementary bismuth and a bismuth oxide.

13. The write-once-read-many optical recording medium according to claim 5, wherein the first inorganic thin film has a composition represented by Bi_a(4B)_bO_d, wherein “4B” represents at least one of Group 4B elements of the Periodic Table of Elements; and “a”, “b” and “d” are atomic percentages and satisfy the following conditions: 10≦a≦40, 3≦b≦20, 50≦d≦70.

14. The write-once-read-many optical recording medium according to claim 13, wherein the at least one of Group 4B elements is at least one of Si and Ge.

15. The write-once-read-many optical recording medium according to claim 5, wherein the first inorganic thin film has a composition represented by Bi_a(4B)_bX_cO_d, wherein “4B” represents at least one of Group 4B elements of the Periodic Table of Elements; “X” represents at least one element selected from the group consisting of Al, Cr, Mn, In, Co, Fe, Cu, Ni, Zn, Ti and Sn; and “a,” “b,” “c,” and “d” are atomic percentages and satisfy the following conditions: 10≦a≦40, 3≦b≦20, 3≦c≦20, 50≦d≦70.

16. The write-once-read-many optical recording medium according to claim 15, wherein the at least one of Group 4B elements is at least one of Si and Ge.

17. The write-once-read-many optical recording medium according to claim 1, wherein the organic thin film has a major absorption band at wavelengths longer than wavelengths at which information is at least one of recorded and reproduced.

18. The write-once-read-many optical recording medium according to claim 17, wherein the organic thin film has a complex refractive index with an imaginary part smaller than that of the first inorganic thin film at the wavelengths at which information is at least one of recorded and reproduced.

19. The write-once-read-many optical recording medium according to claim 17, wherein the organic thin film has an absorption band not belonging to the major absorption band in the vicinity of the wavelengths at which information is at least one of recorded and produced.

20. The write-once-read-many optical recording medium according to claim 1, further comprising a second inorganic thin film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to write-once-read-many (WORM) optical recording media. More specifically, it relates to write-once-read-many optical recording media on which information can be recorded at a high density even at blue-laser wavelengths and to processes for recording and/or reproducing information on the write-once-read-many optical recording media.

2. Description of the Related Art

1. Write-once-read-many Optical Recording Media Sensitive to Blue-laser Wavelengths or Shorter

With an increasing development of blue laser capable of recording of information at a very high density, write-once-read-many optical recording media sensitive to blue-laser wavelengths have been increasingly developed.

In conventional write-once-read-many optical recording media, laser beams are irradiated to a recording layer comprising an organic material to change the refractive index typically due to the decomposition and degeneration of the organic material, and thus recording pits are formed. The optical constant and decomposition behavior of the organic material used in the recording layer play an important role to form satisfactory recording pits.

For use in a recording layer of write-once-read-many optical recording media sensitive to blue-laser wavelengths, an organic material must have suitable optical properties and decomposition behavior with respect to light at blue-laser wavelengths. More specifically, the wavelengths at which recording and reproduction is performed (hereinafter briefly referred to as “recording-reproducing wavelengths”) are set at a tail on the longer-wavelength side of a major absorption band to increase the reflectance in unrecorded areas and to increase the change in refractive index caused by the decomposition of the organic material upon irradiation of laser to thereby yield a higher degree of modulation. This is because wavelengths at the tail on the longer-wavelength side of a major absorption band of such an organic material yield an appropriate absorption coefficient and a high refractive index. With reference to FIG. 1, a conventional write-once-read-many optical recording medium using an organic material in its recording layer has recording-reproducing wavelengths in the diagonally shaded area in FIG. 1.

However, no organic material having optical properties with respect to light at blue-laser wavelengths equivalent to those of conventional materials has not yet been found. To produce such an organic material having an absorption band in the vicinity of blue-laser wavelengths, the molecular skeleton must be downsized or the conjugate system must be shortened. However, this invites a decreased absorption coefficient and a decreased refractive index.

More specifically, there are many organic materials having an absorption band in the vicinity of blue-laser wavelengths, but they do not have a sufficiently high refractive index and fail to yield a high degree of modulation.

On conventional write-once-read-many optical recording media, information is recorded by the mechanism of deformation of a substrate as well as by change in refractive index due to decomposition and deformation of the organic material. For example, FIG. 3 illustrates a recorded area 101 on a substrate of a commercially available DVD-R medium observed by an atomic force microscope (AFM), showing that the substrate 105 deforms toward the reflective layer 103 , which deformation leads to modulation Examples of organic materials sensitive to blue-laser wavelengths can be found in Japanese Patent Application Laid-Open (JP-A) No. 2001-181524, No. 2001-158865, No. 2000-343824, No. 2000-343825, and No. 2000-335110.

However, these publications only teach the spectra of a solution of an organic material and a thin film prepared therefrom in their examples and fail to teach recording and/or reproducing information using the materials.

JP-A No. 11-221964, No. 11-334206, and No. 2000-43423 mention recording using an organic material in their examples but information is recorded at a wavelength of 488 nm. They only describe that satisfactory recording pits are-formed and fail to teach the recording conditions and recording densities.

JP-A No. 11-58955 mention recording using an organic material in the examples but information is recorded at a wavelength of 430 nm. It only describes that a satisfactory degree of modulation is obtained and fails to teach the recording conditions and recording densities.

JP-A No. 2001-39034, No. 2000-149320, No. 2000-113504, No. 2000-108513, No. 2000-222772, No. 2000-218940, No. 2000-222771, No. 2000-158818, No. 2000-280621, and No. 2000-280620 mention recording using an organic material at a wavelength of 430 nm and a numerical aperture NA of 0.65. However, the information is recorded at a low recording density in terms of a minimum pit of 0.4 μm, equivalent to that in DVD media.

JP-A No. 2001-146074 describes recording and/or reproducing at a wavelength of 405 to 408 nm but fails to teach a specific recording density. The recording herein is performed at a low density in which 14T-FEM signals are recorded.

The optical constants of the organic materials disclosed in the above publications at wavelengths around 405 nm, which is the center emitting wavelength of blue semiconductor laser now in practical use, are not equivalent to the required optical constant for recording layers of conventional write-once-read-many optical recording media. The publications fail to disclose examples in which information is recorded at a wavelength around 405 nm at a recording density higher than that in DVD media under specific conditions and fail to teach whether or not information can be recorded at a high density of 15 to 25 GB. In addition, most of the examples in the publications are performed using media of conventional configuration comprising a substrate, an organic material layer and a reflective layer, and colorants to be used therein must have optical properties and functions the same as conventional equivalents.

Such conventional write-once-read-many optical recording media can use only organic materials having a high refractive index and a relatively low absorption coefficient of about 0.05 to about 0.10 for ensuring a satisfactorily high degree of modulation and reflectance.

These conventional write-once-read-many optical recording media using organic materials have a major absorption band in the vicinity of the recording-reproducing wavelengths, thereby have a significantly varying optical constant depending on the wavelength, namely, show a large dependency on wavelength of the optical constant as shown in FIG. 2. They significantly change their recording properties such as recording sensitivity, degree of modulation, jitter and error rate as well as reflectance with varying recording-reproducing wavelengths due to individual difference of the laser or a varying environmental temperature.

The organic materials have insufficient absorptivity to the recording light, the thickness of the resulting organic layer cannot be reduced so much, and a substrate having deep grooves must be used. In this connection, a layer of an organic material is generally formed by spin coating, and such deep grooves are filled with the organic material to form a thick layer of the organic material. Such a substrate having deep grooves is difficult to prepare, and the resulting optical recording medium may have deteriorated quality.

In addition, such a large thickness of the organic material layer leads to a narrow recording power margin and other margins in recording-reproducing properties.

Examples of techniques on layer configurations and recording processes different from those of conventional CD media and DVD media can be found as follows.

JP-A No. 07-304258 discloses a technique for recording information on a medium comprising a substrate, a layer containing a saturable absorption colorant and a reflective layer in this order based on the change of the extinction coefficient (the “absorption coefficient” as used in the present invention) of the saturable absorption colorant.

JP-A No. 08-83439 discloses a technique for recording information on a medium comprising a substrate, a metal deposition layer, a light-absorptive layer and a protective sheet arranged in this order based on the discoloration or deformation of the metal deposition layer by action of heat generated from the light-absorptive layer.

JP-A No. 08-138245 discloses a technique for recording information on a medium comprising a substrate, a dielectric layer, a recording layer containing a photoabsorption material, and a reflective layer arranged in this order by changing the thickness of the recording layer to thereby change the thickness of the grooves.

JP-A No. 08-297838 discloses a technique for recording information on a medium comprising a substrate, a recording layer containing a photoabsorption material and a metal reflective layer arranged in this order by changing the thickness of the recording layer by a factor of 10 percent to 30 percent.

JP-A No. 09-198714 discloses a technique for recording information on a medium comprising a substrate, a recording layer containing an organic colorant, a metal reflective layer and a protective layer arranged in this order by increasing the groove width of the substrate broader than an unrecorded area by a factor of 20 percent to 40 percent.

Japanese Patent (JP-B) No. 2506374 discloses a technique for recording information on a medium comprising a substrate, an interlayer, and a metal thin film arranged in this order by deforming the metal thin layer to thereby form bubbles.

JP-B No. 2591939 discloses a technique for recording information on a medium comprising a substrate, a light-absorptive layer, an auxiliary recording layer and an optical reflective layer by deforming the auxiliary recording layer to a concave shape and deforming the optical reflective layer to a concave shape along the deformation of the auxiliary recording layer.

JP-B No. 2591940 discloses a technique for recording information on a medium comprising a substrate, a light-absorptive layer, a porous auxiliary recording layer and an optical reflective layer or comprising a substrate, a porous auxiliary recording layer, a light-absorptive layer and a reflective layer by deforming the auxiliary recording layer to a concave shape and deforming the reflective layer to a concave shape along the deformation of the auxiliary recording layer.

JP-B No. 2591941 discloses a technique for recording information on a medium comprising a substrate, a porous light-absorptive layer, and a reflective layer arranged in this order by deforming the light-absorptive layer to a concave shape and deforming the reflective layer to a concave shape along the deformation of the auxiliary recording layer.

JP-B No. 2982925 discloses a technique for recording information on a medium comprising a substrate, a recording layer containing an organic colorant and an auxiliary recording layer arranged in this order by allowing the auxiliary recording layer to mix with the organic colorant to thereby shift the absorption spectrum of the organic colorant to a shorter wavelength.

JP-A No. 09-265660 discloses a technique for recording information on a medium comprising a substrate, a multi-function layer having functions as a reflective layer and a recording layer, and a protective layer arranged in this order by forming a bump between the substrate and the multi-function layer. The publication specifies metals such as nickel, chromium and titanium and alloys of these metals as the material for the multi-function layer.

JP-A No. 10-134415 discloses a technique for recording information on a medium comprising a substrate, a metal thin layer, a deformable buffer layer, a reflective layer and a protective layer arranged in this order by deforming the substrate and the metal thin layer and reducing the thickness of the buffer layer in the deformed portion. The publication specifies metals such as nickel, chromium and titanium and alloys of these metals as the material for the metal thin layer and describes that a resin which is deformable and has an appropriate flowability is used in the buffer layer, which buffer layer may further comprise a colorant to accelerate the deformation.

JP-A No. 11-306591 discloses a technique for recording information on a medium comprising a substrate, a metal thin layer, a buffer layer and a reflective layer arranged in this order by deforming the substrate and the metal thin layer and changing the thickness and optical constant of the buffer layer in the deformed portion. The publication describes that a metal such as nickel, chromium or titanium or an alloy thereof is preferably used in the metal thin layer and that the buffer layer comprises a mixture of a colorant and an organic polymer, which colorant has a large absorption band in the vicinity of the recording-reproducing wavelengths.

JP-A No. 10-124926 discloses a technique for recording information on a medium comprising a substrate, a metal recording layer, a buffer layer and a reflective layer arranged in this order by deforming the substrate and the metal recording layer and changing the thickness and optical constant of the buffer layer in the deformed portion. The publication describes that a metal such as nickel, chromium or titanium or an alloy thereof is preferably used in the metal recording layer and that the buffer layer comprises a mixture of a colorant and a resin, which colorant has a great absorption band in the vicinity of the recording-reproducing wavelengths.

These conventional techniques do not intend to provide optical recording media sensitive to blue-laser wavelengths and do not teach layer configurations and recording processes usable at blue-laser wavelengths. In addition, according to the conventional techniques, the colorant in the recording layer must be capable of absorbing light and must have a major absorption band in the vicinity of the recording-reproducing wavelengths, thus the types of colorants to be used are severely restricted.

Most of the conventional techniques record information typically by mechanism of deformation. If information is recorded mainly by mechanism of deformation, the interference among recording marks increases and thereby margins in recording and/or reproducing properties decrease, even though a satisfactory low jitter and a high degree of modulation are obtained.

As a write-once-read-many optical recording medium according to “diffusion system”, for example, TDK Corporation has announced a medium having a configuration of a substrate, ZnS—SiO ₂ , Si, Cu, ZnS—SiO ₂ and Ag arranged in this order in CEATEC JAPAN 2003. The company has reported that the write-once-read-many optical recording medium has a degree of modulation of 65%, a jitter of 6% and a reflectance of 14%. An additional test made by the present inventors has revealed that when Si and Cu are arranged in adjacent layers, they gradually diffuse into another layer during storage and the recording medium shows deteriorated properties. This is a disadvantage of a medium in which components of two layers diffuse into and mix with each other. The medium requires two layers of a dielectric layer comprising ZnS—SiO ₂ to yield a satisfactorily high degree of modulation and requires many processes and high cost.

Thus, these conventional technologies are insufficient to provide write-once-read-many optical recording media sensitive to blue-laser wavelengths and do not teach layer configurations and recording processes usable at blue-laser wavelengths.

2. Multi-level Recordable Write-once-read-many Optical Recording Media

To increase the recording capacity, multi-level recording techniques have been developed. Recent home users generally treat large-capacity audio data and image-motion picture data, and the capacity of hard disks have increased. However, current CD or DVD optical recording media cannot provide sufficiently high recording capacities.

Under these circumstances, the ML (trademark; Multi Level) Technology has been proposed by Calimetrics, Inc. (CA) as a recording technique to increase conventional optical recording media. In short, the recording linear density is increased according to the ML Technology.

In the conventional CD or DVD optical recording media, the position or length of each recording mark edge varies corresponding to a target data message in recording, and the length of the recording mark is determined in reproduction (slice system). The current slice system will be illustrated in short below.

With reference to FIG. 4, a recording mark row (c) is initially formed on an optical recording medium using a recording waveform (b) corresponding to a target recording data (a).

Reproducing light is applied to the recording mark row (c) recorded on the optical recording medium to reproduce the information to thereby yield a reproducing signal waveform (d).

The reproducing signal waveform (d) is a dull waveform different from the recording waveform (b), a rectangular pulse, and is thereby formatted using an equalizer to yield an equalized waveform (e). More specifically, high-frequency components of the reproducing signal are amplified.

Next, the point of intersection of the equalized waveform (e) and the threshold is detected. A binary data (f) is then outputted as one (1) when the point of intersection is detected within the window and as zero (0) when it is not detected. The binary data (f) obtained by the detection of point of intersection is converted according to a non-return-to-zero (NRZ) procedure to thereby yield a decoded data (g).

In contrast, according to the multi-level recording, a mark having a reflectance at multiple levels is recorded in a fixed-length area “cell”, and the information is indicated by the multi-level reflectance. More specifically, one bit is indicated by the presence or absence of a recording mark in the conventional CD or DVD optical recording media. In contrast, recording marks are recorded at, for example, eight different levels of size and is read out as reflectance at eight different levels (FIG. 5). One recording mark indicates information corresponding to three bits, and the recording density can thereby be increased. Here, bidirectional arrow 107 indicates the size of each cell.

In the multi-level recording, the beam spot diameter of laser light in reproduction is generally larger than the cell length, and one recording mark indicates information corresponding to three bits. Thus, the recording linear density can be increased to thereby increase the recording capacity without narrowing the track pitch.

Examples of such multi-level recordable write-once-read-many optical recording media can be found in JP-A No. 2001-184647, No. 2002-25114, No. 2002-83445, No. 2002-334438, No. 2002-352428, No. 2002-352429 and No. 2002-367182. JP-A No. 2001-184647 discloses a concept of multi-level recording on an optical recording medium having a recording layer comprising an organic colorant and a concept of multi-level recording on the optical recording medium in a depth direction of the recording layer. However, this technique intends to provide a multi-level recordable write-once-read-many optical recording medium sensitive to red laser wavelengths, whose layer configuration and organic colorant used are the same as those of conventional CD or DVD write-once-read-many optical recording media.

Aforementioned JP-A No. 2002-25114 discloses a multi-level recordable optical recording medium including a substrate and a recording layer of an organic colorant, which substrate has a specific glass transition point, reflectance and thermal conductivity.

Aforementioned JP-A No. 2002-83445 discloses a multi-level recordable optical recording medium including a recording layer comprising an organic colorant, which organic colorant has specific thermal decomposition properties.

Aforementioned JP-A No. 2002-334438 and No. 2002-352428 each disclose a multi-level recordable optical recording medium having a recording layer comprising phthalocyanine or cyanine colorant, in which the relationships among the wavelength, numeral aperture NA and groove width are specified.

Aforementioned JP-A No. 2002-352429 discloses a multi-level recordable optical recording medium having a recording layer comprising an organic colorant, in which the relationship between the thickness of the recording layer on a groove and the groove depth is specified.

Aforementioned JP-A No. 2002-367182 discloses a multi-level recordable optical recording medium having a recording layer comprising an organic colorant, in which the reflectance in an unrecorded area is specified within a range of 40% to 80%.

To record information at a higher density, the cell length in the multi-level recording must be reduced to the same level as the minimum mark length in the conventional binary recording. Namely, the minimum mark in the multi-level recording is much shorter (smaller) than the minimum mark in the binary recording.

If multi-level recording can be performed at a sufficiently high density using a conventional recording material with a conventional layer configuration, this means that the minimum mark would be shortened even using the conventional recording material with a conventional layer configuration and means that the minimum mark length could be reduced in the binary recording and information could be recorded at a higher density. However, the recording density in the conventional binary recording technique cannot be actually increased unless a special recording-reproducing system is employed.

To provide multi-level recordable write-once-read-many optical recording media which are recordable at a higher density than conventional equivalents according to the binary recording, novel recording materials and layer configuration must be developed.

However, the aforementioned conventional technologies employ conventional recording materials and layer configurations in multi-level recording, although some of conditions such as the thickness of the recording layer and the material of the reflective layer are slightly modified. They cannot form shorter recording marks than conventional equivalents and cannot record and reproduce recording marks much smaller than conventional equivalents with a higher reliability. In short, they only achieve the reproduction of a somewhat smaller recording mark with good reliability by action of the recording and reproducing techniques, and simply apply the recording and reproducing techniques to write-once-read-many optical recording media.

In addition, the conventional techniques form recording marks typically by means of deformation (FIG. 3). The deformation presents no problem when the pitch between recording marks is sufficiently long, i.e., the recording linear density is low, or when the length of a cell in which a multi-level record is formed is not longer than the beam diameter of the reproducing light. However, the deformations interfere with each other and the interference becomes nonlinear when the recording linear density is high or when the length of a cell in which a multi-level record is formed is longer than the beam diameter of the reproducing light.

The phrase “the interference is linear” means that the deformation as a result of interference has a shape substantially indicated by the sum of the deformation in a cell and the deformation of an adjacent cell. FIGS. 6A, 6 B and 6 C are a plan view, a sectional view, and a sectional view as a sum, respectively of three recording marks which are formed in successive three cells mainly by means of deformation without interference.

FIGS. 7O, 7 A, 7 B, 7 C, 7 D and 7 E schematically illustrate a reproducing signal which varies depending on the difference of interference among the deformations in three cells. In this case, three recording marks are formed in successive three cells mainly by means of deformation and the total length of the three recorded cells is smaller than the diameter 109 of reproducing beam. If the interference in deformation is linear, the resulting deformation is as shown in FIG. 7B. However, if the interference in deformation is not linear, the resulting deformation is modified as shown in FIG. 7C or FIG. 7D.

The interfered deformation has a length smaller than the diameter 109 of reproducing beam, and the difference in the deformation is therefore not detected. Accordingly, a reproducing signal as shown in FIG. 7E can be substantially obtained even when the deformation varies as shown in FIGS. 7B, 7 C and 7 D.

Exact data can therefore be decoded by detecting the reflection levels at sampling times T ₁ , T ₂ and T ₃ shown in FIG. 7E.

FIGS. 8O, 8 A, 8 B, 8 C, 8 D, 8 E, 8 F and 8 G schematically illustrate the relationship between the interference in deformation and the reproducing signal when successive seven recording marks mainly based on deformation are formed in successive seven cells and the total length of the cells is larger than the diameter 109 of reproducing beam.

The interference in deformation in this case becomes more nonlinear than the case shown in FIGS. 7B, 7 C and 7 D, and the interfered deformation is as shown in FIGS. 8B, 8 C and 8 D when simply illustrated. The interfered deformations each have a length larger than the diameter 109 of reproducing beam, and the difference among the deformations can be clearly detected. Thus, reproducing signals shown in FIGS. 8E, 8 F and 8 G are obtained from the deformations in FIGS. 8B, 8 C and 8 D, respectively.

Accordingly, when reflection levels are detected at sampling times T ₁ through T ₇ shown in FIGS. 8E, 8 F and 8 G, different data corresponding to the different deformations are decoded, thus failing to decode the exact data.

As is described above, recording of data mainly based on deformation leads to different interference behaviors among recording marks depending on recording patters, and the resulting reproducing signals cannot be predicted. Thus, the data are not recorded and/or reproduced properly at a higher density.

3. Recording-reproducing Technique Using PRML System

As another possible solution to achieve high-density recording than the ML recording technique, the application of partial response and maximum likelihood (PRML) technique to optical recording media has been studied.

With an increasing recording linear density to achieve high-density recording, the reproducing signal has a dull waveform. In other words, with reference to FIG. 4, the reproducing signal waveform (d) is not a rectangular waveform as in the recording waveform (b). The high frequency components of the reproducing signal are amplified using an equalizer and the reproducing signal is converted to have an equalized waveform. When the reproducing signal has a dull waveform with an increasing density, a larger quantity of high frequency components must be amplified. In amplification of the high frequency components, signal degrading components are also amplified by the equalizer, thus inviting significantly decreased signal-to-noise ratio (SNR) of the reproducing signal. PRML is a reproducing signal processing system to prevent SNR of the reproducing signal from decreasing even in high density recording.

The PRML system will be briefly illustrated below.

FIG. 9 illustrates a recording data (a) as target information, a recording waveform (b), a recording mark row (c), a reproducing signal waveform (d) and equalized waveforms (e), (f) and (g).

More specifically, the equalized waveforms (e), (f) and (g) are obtained as a result of equalization of the reproducing waveform (d) by an equalizer depending on PR(1,1) characteristic, PR(1,2,1) characteristic and PR(1,2,2,1) characteristic, respectively. The PR(1,1) characteristic is a characteristic in which an impulse response appears at the rate of 1:1 at two successive identification points. The PR(1,2,1) characteristic is a characteristic in which an impulse response appears at the rate of 1:2:1 at three successive identification points. The PR(1,2,2,1) characteristic is a characteristic in which an impulse response appears at the rate of 1:2:2:1 at four successive identification points. The equalized waveforms (e), (f) and (g) in FIG. 9 show that an equalized waveform becomes duller with an increasing complexity of the PR characteristic.

In the PRML system, an increase in the signal degrading component in the equalizer can be suppressed by equalizing the reproduced waveform into a waveform of a PR characteristic which is closer to the characteristic of the reproduced waveform.

In the reproduction signal processing of PRML system, a Viterbi decoder which is a representative one of maximum likelihood decoders is generally used as a most-likelihood decoder in decoding of the equalized waveform signals. For example, if the reproduced waveform is equalized into a waveform of the PR(1,2,1) characteristic by the equalizer, the Viterbi decoder selects a series having the smallest error with respect to the sample series of the equalized waveform from all of the reproduced waveform series which satisfy the PR(1,2,1) characteristic and estimates and outputs recording data (binary data, decoded data) used as a source for generating the selected reproduced waveform series.

Thus, the PRML system realizes high-density recording even using a conventional optical system. However, even the PRML system cannot record and reproduce information with high reliability when the interference among recording marks (intersymbol interference) becomes large and becomes nonlinear, namely, when a predictable interference among recording marks occurs. In other word, the PRML system can be applied onto to such a case in which a predictable interference among recording marks occurs. If an interference among recording marks different from the predicted one occurs, the advantages of the PRML system are not obtained.

The deformation of recording marks must be prevented to suppress the interference among recording marks at a predictable level.

By providing write-once-read-many optical recording media on which information can be recorded by multi-level recording at a wavelength of blue-laser wavelengths or shorter, recording marks with higher quality than those obtained by the conventional binary recording technique can be formed. Information can be recorded on the resulting write-once-read-many optical recording media by the conventional binary recording technique at a wavelength of blue-laser wavelengths or shorter and also by multi-level recording at a higher density by the application of the PRML system. Requirements to achieve write-once-read-many optical recording media sensitive to blue-laser wavelengths or shorter wavelengths can be considered as requirements to provide write-once-read-many optical recording media that allow multi-level recording at a wavelength of blue-laser wavelengths or shorter.

The requirements to write-once-read-many optical recording media that allow multi-level recording at a wavelength of blue-laser wavelengths or shorter are the following requirements (1), (2) and (3):

• (1) smaller recording marks;
• (2) less interference among recording marks; and
• (3) higher stability of recording marks.

In most of the conventional write-once-read-many optical recording media, information is recorded mainly based on deformation as described above.

In binary recording, the minimum mark has a sufficient size with respect to the diameter of reproducing beam (approximately half the diameter of reproducing beam), the amplitude derived from the minimum mark is sufficiently large, and the deformation in the minimum mark is large.

In contrast, in multi-level recording, the minimum mark has an insufficient size with respect to the diameter of reproducing beam, and the amplitude derived from the minimum mark in multi-level recording is approximately one-half to one-tenths or less the amplitude derived from the minimum mark in binary recording, and the deformation in the minimum mark is very small.

Conventional CD or DVD write-once-read-many optical recording media have a layer of an organic colorant having optical absorptivity arranged directly adjacent to a substrate. Thus, the substrate deforms to a large extent. The degree of modulation is primarily affected by the deformation of substrate and is secondarily affected by the decomposition of the organic colorant. A deformation of the substrate within an elastic deformation region may be relieved, for example, by extraneous heat. A deformation of the substrate exceeding the elastic deformation region is limited, but its shape may significantly vary with the heat applied upon the formation of an adjacent recording mark or with the deformation of the adjacent recording mark.

FIGS. 10 and 11 illustrate these phenomena.

FIG. 10 shows recording marks in a write-once-read-many optical recording medium having a conventional structure of a substrate, a colorant layer, an Ag reflective layer and a protective layer arranged in this order.

FIG. 10 illustrates a waveform A of a reproducing signal; an atomic force micrographic (AFM) image B of the surface of the substrate after removing the protective layer, Ag reflective layer and colorant layer; and a deformation C of the cross section of the substrate as determined based on the AFM image B. FIG. 10 shows that the recorded area is much largely deformed with a concave shape at a center part of the recording mark. The interference in deformation (interference within one recording mark) is nonlinear, as illustrated in FIGS. 7C, 7 D, 8 C and 8 D.

FIG. 11 illustrates recording marks obtained by recording the information as in FIG. 10 on the conventional write-once-read-many optical recording medium and applying a weak direct-current light of about one-fifths of the recording power to the medium.

FIG. 11 illustrates a waveform A of a reproducing signal; an atomic force micrographic (AFM) image B of the surface of the substrate after removing the protective layer, Ag reflective layer and colorant layer; and a deformation C of the cross section of the substrate as determined based on the AFM image B. FIG. 11 shows that the deformation of substrate changes and thus the waveform of the reproducing signal changes upon irradiation of the weak direct-current light. This is probably because the application of the weak direct-current light relieves the strain in the deformed portion of the substrate.

The fact that the shape of deformed portion of the substrate varies upon irradiation of such a weak direct-current light shows that the colorant layer on the recording mark should have a sufficient optical absorptivity and that the conventional write-once-read-many optical recording medium generates the degree of modulation mainly based on deformation.

Recording mainly based on deformation invites the following problems:

• (1) the interference in deformation within one recording mark increases, and the waveform of the reproducing signal varies depending on the deformation, i.e., depending on the recording mark length;
• (2) the interference in deformation among recording marks increases, and the waveform of the reproducing signal varies depending on the deformation, i.e., depending on the recording pattern such as the types of recording marks between anterior and posterior tracks or between adjacent tracks; and
• (3) the deformation is relieved in reproduction, in recording onto an adjacent track, in leaving at high temperatures or in leaving for a long period of time, and the waveform of the reproducing signal varies.

These problems invite the following disadvantages:

• (a) deteriorated jitter, error rate and other properties;
• (b) narrowed recording power margins in jitter, error rate and other properties;
• (c) unreasonable asymmetry largely shifted from zero under recording conditions to yield the optimum jitter or minimum error rate;
• (d) unstable formation of small recording marks; and
• (e) unpredictable interference among recording marks.

These disadvantages and problems also occur in conventional binary recording but are significant in write-once-read-many optical recording media for recording at a higher density, i.e., write-once-read-many optical recording media corresponding to the multi-level recording and/or PRML system.

In addition, the conventional write-once-read-many optical recording media each having a recording layer comprising an organic material have the following disadvantages (i), (ii), (iii) and (iv):

• (i) very narrow or small degree of freedom in selection of the organic material;
• (ii) very large dependency on wavelength;
• (iii) deep grooves of the substrate for satisfactory recording-reproducing properties; and
• (iv) no recording in “lands” between grooves.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to solve these problems and to provide a write-once-read-many optical recording medium that can yield a high degree of modulation by recording marks with small deformation and to provide a process for recording and/or reproducing information for the medium.

Specifically, the present invention provides a write-once-read-many optical recording medium containing a first inorganic thin film; and at least one of a second inorganic thin film and an organic thin film, wherein the first inorganic thin film comprises at least “R” and “O,” wherein “R” represents at least one selected from the group consisting of Y, Bi, In, Mo, V and lanthanum series elements; and “O” represents oxygen atom, and the second inorganic thin film and the organic thin film are capable of suppressing at least one of deformation and breakage of the first inorganic thin film and receiving the change of state of the first inorganic thin film. Information can be easily recorded on the write-once-read-many optical recording medium of the present invention at high density by binary recording or multi-level recording even at blue-laser wavelengths of 500 nm or less, typically at wavelengths around 405 nm.

The write-once-read-many optical recording medium may contain at least the first inorganic thin film and the organic thin film in a first aspect, may contain at least the first inorganic thin film and the second inorganic thin film in a second aspect and may contain at least the first inorganic thin film, the second inorganic thin film and the organic thin film in a third aspect.

The present invention further provides a process for recording and/or reproducing information on the write-once-read-many optical recording medium, the process including forming a recorded area by the photoabsorption function of at least one of the first inorganic thin film and the organic thin film at wavelengths at which recording and reproduction is performed. The process of the present invention can easily record and reproduce information on the write-once-read-many optical recording medium at a high density by binary recording or multi-level recording even at blue-laser wavelengths of 500 nm or less, typically at wavelengths around 405 nm.

Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between the recording-reproducing wavelengths and the major absorption band of an organic material in a recording layer of a conventional write-once-read-many optical recording medium.

FIG. 2 is a diagram showing a large dependency on wavelength of the optical constant of the conventional write-once-read-many optical recording medium.

FIG. 3 is an atomic force micrograph of the deformation of the substrate of the conventional write-once-read-many optical recording medium.

FIG. 4 is an illustration of data decoding according to a conventional level slice system and shows a recording data (a) as a target information, a recording waveform (b) corresponding to the recording data (a), a recording mark row (c) formed in an optical recording medium, a reproducing signal waveform (d) of the recording mark row (c), an equalized waveform (e) as a result of equalization of the reproducing signal waveform (d) by an equalizer, a binary data (f) obtained by detecting the point of intersection of the equalized waveform (e) and the threshold, and a decoded data (g) obtained by subjecting the binary data (f) to NRZ conversion.

FIG. 5 is a schematic diagram of recording marks in multi-level recording.

FIGS. 6A, 6 B and 6 C are a plan view, a sectional view of deformation without interference, and a sectional view of added deformation, respectively; of three recording marks formed in three successive cells mainly based on deformation and show linear interference in deformation.

FIGS. 7O, 7 A, 7 B, 7 C, 7 D and 7 E are diagrams showing different reproducing signals due to different interference in deformation of three recording marks formed in three successive cells mainly based on deformation when the total length of the recorded cells is smaller than the diameter of reproducing beam, in which FIG. 70 illustrates the diameter of reproducing beam, FIG. 7A is a plan view of the recording marks, FIG. 7B illustrates an added deformation, FIGS. 7C and 7D illustrate a nonlinearly interfered deformation, respectively, and FIG. 7E illustrates a reproducing signal obtained in the cases of FIGS. 7B, 7 C and 7 D.

FIGS. 8O, 8 A, 8 B, 8 C, 8 D, 8 E, 8 F and 8 G are diagrams showing different reproducing signals due to different interference in deformation of seven recording marks formed mainly based on deformation in seven successive cells when the total length of the recorded cells is larger than the diameter of reproducing beam, in which FIG. 80 illustrates the diameter of reproducing beam, FIG. 8A is a plan view of the recording marks, FIG. 8B illustrates an added deformation, FIGS. 8C and 8D illustrate a nonlinearly interfered deformation, respectively, and FIGS. 8E, 8 F and 8 G illustrate a reproducing signal obtained in the cases of FIGS. 8B, 8 C and 8 D, respectively.

FIG. 9 is a diagram showing data decoding according to the PRML system and shows a recording data (a) as a target information, a recording waveform (b) corresponding to the recording data (a), a recording mark row (c) formed in an optical recording medium, a reproducing signal waveform (d) of the recording mark row (c), an equalized waveform (e) as a result of equalization of the reproducing signal waveform (d) based on the PR(1,1) characteristic by an equalizer, an equalized waveform (f) as a result of equalization of the reproducing signal waveform (d) based on the PR(1,2,1) characteristic by the equalizer, and an equalized waveform (g) as a result of equalization of the reproducing signal waveform (d) based on the PR(1,2,2,1) characteristic by the equalizer.

FIG. 10 is a diagram showing the relationship between the deformation of a substrate and the reproducing signal of a conventional write-once-read-many optical recording medium.

FIG. 11 is a diagram showing the relationship between the deformation of a substrate and the reproducing signal of a conventional write-once-read-many optical recording medium when a weak DC light is applied to the medium after recording.

FIG. 12 is a diagram showing the relationship between the major absorption band and the recording-reproducing wavelengths of a write-once-read-many optical recording medium of the present invention.

FIG. 13 is a diagram illustrating the “major absorption band” as used in the present invention.

FIG. 14 is a diagram showing binary recording on a write-once-read-many optical recording medium according to Example 1-1.

FIG. 15 is an AFM image of deformation of the substrate-of a write-once-read-many optical recording medium according to Comparative Example 1-1.

FIG. 16 is a diagram showing eight-level recording on a write-once-read-many optical recording medium according to Example 1-8.

FIG. 17 is a diagram showing eight-level recording on a write-once-read-many optical recording medium according to Comparative Example 1-2.

FIGS. 18A and 18B are an AFM image and a cross sectional view along the lines L-L of FIG. 18A, respectively, of deformed substrate of a write-once-read-many optical recording medium according to Comparative Example 1-2.

FIG. 19 is a diagram showing the relationship between the deformation height on the surface of substrate and the amount of drops of a write-once-read-many optical recording medium according to Comparative Example 1-2.

FIG. 20 is a diagram showing eight-level recording on a write-once-read-many optical recording medium according to Example 1-9.

FIG. 21 is a diagram of scanning electron microscopic observation of the BiFeO surface of the write-once-read-many optical recording medium according to Example 1-9.

FIG. 22 is a diagram showing absorption factors Q of write-once-read-many optical recording media according to Example 1-1 and Comparative Example 1-1 and of a commercially available CD-R medium.

FIG. 23 is a transmission electron micrograph of a recorded area of a write-once-read-many optical recording medium according to Example 1-1 as a specimen cut by focused ion beam (FIB).

FIG. 24 is a diagram showing binary recording on a write-once-read-many optical recording medium according to Example 2-1-2.

FIG. 25 is a diagram showing binary recording on a write-once-read-many optical recording medium according to Example 2-1-1.

FIG. 26 shows jitters of write-once-read-many optical recording media according to Example 2-1-1 and Example 2-1-2, respectively.

FIG. 27 is a diagram of reproducing signal levels in the space and recording marks of the write-once-read-many optical recording media according to Example 2-1-1 and Example 2-1-2, respectively.

FIG. 28 is an AFM image of a deformed substrate of a write-once-read-many optical recording medium according to Comparative Example 2-1.

FIG. 29 is a diagram showing the relationship between the recording power and the jitter (σ/Tw) of a write-once-read-many optical recording medium according to Example 2-20 in one track recording at a varying minimum mark length (2T).

FIG. 30 is a diagram showing the relationship between the recording power and the jitter (σ/Tw) of the write-once-read-many optical recording medium according to Example 2-20 in continuous recording at a varying minimum mark length (2T).

FIG. 31 is a diagram showing the suitability for high-density recording of write-once-read-many optical recording media according to Example 2-1-1 and Comparative Example 2-1.

FIG. 32 is a diagram showing the suitability for high-density recording of write-once-read-many optical recording media according to Example 2-11 and Comparative Example 2-1.

FIG. 33 is a diagram showing the relationship among the recording power and the jitter (σ/Tw) and the degree of modulation in recording at a recording density of 2T=0.231 (μm) on a write-once-read-many optical recording medium according to Example 2-23.

FIG. 34 is a diagram showing the relationship among the recording power and the jitter (σ/Tw) and the degree of modulation in recording at a recording density of 2T=0.222 (μm) on a write-once-read-many optical recording medium according to Example 2-23.

FIG. 35 is a diagram showing the relationship among the recording power and the jitter (σ/Tw) and the degree of modulation in recording at a recording density of 2T=0.214 (μm) on a write-once-read-many optical recording medium according to Example 2-23.

FIG. 36 is a diagram showing the relationship among the recording power and the jitter (σ/Tw) and the degree of modulation in recording at a recording density of 2T=0.205 (μm) on a write-once-read-many optical recording medium according to Example 2-23.

FIG. 37 is a diagram showing eight-level recording on a write-once-read-many optical recording medium according to Example 2-1-1.

FIG. 38 is a diagram showing eight-level recording on a write-once-read-many optical recording medium according to Comparative Example 2-3.

FIGS. 39A and 39B are an AFM image of the surface and a cross sectional view taken along the lines L-L of FIG. 39A, respectively, of a substrate of a write-once-read-many optical recording medium according to-Comparative Example 2-3.

FIG. 40 is a diagram showing the relationship between a deformation height and the drops in degree of modulation of a write-once-read-many optical recording medium according to Comparative Example 2-3.

FIG. 41 is a diagram showing eight-level recording on a write-once-read-many optical recording medium according to Example 2-30.

FIG. 42 is a scanning electron micrograph of deformation of a BiFeO surface of the write-once-read-many optical recording medium according to Example 2-30.

FIG. 43 is a scanning electron micrograph of deformation of a BiFeO surface of the write-once-read-many optical recording medium according to Example 2-31.

FIG. 44 is a diagram showing a reproducing signal obtained from a write-once-read-many optical recording medium according to Example 2-31.

FIG. 45 is a scanning electron micrograph of deformation of a BiFeO surface of the write-once-read-many optical recording medium according to Comparative Example 2-4 at a cell length of 0.32 μm.

FIG. 46 is a diagram showing a reproducing signal obtained from a write-once-read-many optical recording medium according to Comparative Example 2-4 at a cell length of 0.32 μm.

FIG. 47 is a scanning electron micrograph of deformation of a BiFeO surface of the write-once-read-many optical recording medium according to Comparative Example 2-4 at a cell length of 0.24 μm.

FIG. 48 is a diagram showing a reproducing signal obtained from a write-once-read-many optical recording medium according to Comparative Example 2-4 at a cell length of 0.24 μm.

FIG. 49 is a scanning electron micrograph of deformation of a BiO surface of a write-once-read-many optical recording medium according to Example 2-32.

FIG. 50 is a diagram showing a reproducing signal obtained from a write-once-read-many optical recording medium according to Example 2-32.

FIG. 51 is a diagram showing absorption factors Q of write-once-read-many optical recording media according to Example 2-1-1 and Comparative Example 2-1 and of a commercially available CD-R medium.

FIG. 52 is a diagram showing absorption factors Q of write-once-read-many optical recording media according to Example 2-23 and Comparative Example 2-1 and of a commercially available CD-R medium.

FIG. 53 is a transmission electron micrograph of a recorded area of a write-once-read-many optical recording medium according to Example 2-1-1 as a specimen cut in a radius direction by focused ion beam (FIB).

FIG. 54 is a transmission electron micrograph of a recorded area of a write-once-read-many optical recording medium according to Example 2-1-2 as a specimen cut in a radius direction by focused ion beam (FIB).

FIG. 55 is a transmission electron micrograph of a recorded area of a write-once-read-many optical recording medium according to Example 2-31 as a specimen cut in a guide groove direction by focused ion beam (FIB).

FIG. 56 is a transmission electron micrograph of an unrecorded area of a write-once-read-many optical recording medium according to Example 2-42 as a specimen cut in a radius direction by focused ion beam (FIB).

FIG. 57 is a transmission electron micrograph of a recorded area of a write-once-read-many optical recording medium according to Example 2-42 as a specimen cut in a radius direction by focused ion beam (FIB).

FIG. 58 is a diagram showing the relationship between x/(x+y) and the jitter (σ/Tw) in conventional binary recording on a write-once-read-many optical recording medium according to Example 2-14.

FIG. 59 is a diagram showing the relationships among x/(x+y), the degree of modulation and the reflectance in conventional binary recording on a write-once-read-many optical recording medium according to Example 2-14.

FIG. 60 is a diagram showing the relationship between the recording power and the jitter in conventional binary recording on a write-once-read-many optical recording medium according to Example 2-19.

FIG. 61 is a diagram showing the relationship between the recording power and the jitter in conventional binary recording on a write-once-read-many optical recording medium according to Example 2-28.

FIG. 62 is a diagram showing an eye pattern (eye diagram) in conventional binary recording on a write-once-read-many optical recording medium according to Example 2-28.

FIG. 63 is a diagram showing the sigma to dynamic range (SDR) at different cell lengths in multi-level recording on a write-once-read-many optical recording medium according to Example 2-33.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Write-once-read-many Optical Recording Media

The write-once-read-many optical recording media according to the present invention each comprise a first inorganic thin film, and at least one of a second inorganic thin film and an organic thin film and may further comprise a reflective layer, a cover layer and other layers according to necessity.

The write-once-read-many optical recording medium according to a first embodiment comprises at least the first inorganic thin film such as an RO film, wherein “R” is an element as mentioned below and “O” is oxygen atom, and the organic thin film and may further comprise a reflective layer, a cover layer and other layers-according to necessity.

The write-once-read-many optical recording medium according to a second embodiment comprises at least the first inorganic thin film and the second inorganic thin film and may further comprise a reflective layer, a cover layer and other layers according to necessity.

The write-once-read-many optical recording medium according to a third embodiment comprises at least the first inorganic thin film, the second inorganic thin film and the organic thin film and may further comprise a reflective layer, a cover layer and other layers according to necessity.

In a write-once-read-many optical recording medium comprising, for example, a substrate, the first inorganic thin film such as an RO film, the organic thin film and the reflective layer arranged in this order, the thickness of the first inorganic thin film and the organic thin film must be optimized. If not, the first inorganic thin film largely deforms and may break in some cases. For example, a write-once-read-many optical recording medium having the above configuration can yield satisfactory recording-reproducing properties in the vicinity of the optimum recording power, but its first inorganic thin film largely deforms, may break in some cases and the medium shows decreased margins of the jitter and error rate at a recording power higher than the optimum recording power. More specifically, the medium shows a rapidly and discontinuously increasing degree of modulation with an increasing recording power.

The deformation increasingly affects the properties of the medium not only with an increasing recording power but also with an increasing recording mark length. Therefore, the asymmetry may be deteriorated to a negative region under such recording conditions as to yield the optimum jitter and error rate.

This is because, in one aspect, recording on the first inorganic thin film in the medium having this configuration is performed mainly by a recording mechanism accompanied with a relatively large change such as deformation and fusing. In other words, this configuration may often invite a relatively large change such as deformation and fusing of the first inorganic thin film in recording. In addition, the substrate is arranged in direct contact with the first inorganic thin film, and a large quantity of heat propagates into the substrate. Thus, the substrate expands and deforms to thereby further deform the first inorganic thin film, which may invite breakage of the first inorganic thin film.

Accordingly, the medium of the present invention may preferably further comprise the second inorganic thin film in addition to the first inorganic thin film. The second inorganic thin film works to suppress at least one of the deformation and breakage of the first inorganic thin film and to receive the change of state of the first inorganic thin film, such as fusing, change in composition, diffusion, change in crystalline state, oxidation and/or reduction.

The second inorganic thin film may preferably be arranged adjacent to the first inorganic thin film to effectively exhibit the aforementioned functions. However, it can also exhibit its functions even when another layer is interposed between the first inorganic thin film and the second inorganic thin film.

To yield a high degree of modulation using recording marks with less deformation, the following factors (1) to (5) are important:

• (1) to prevent a light-absorptive layer from change of state such as fusing, change in composition including decomposition and degradation, diffusion, change in crystalline state, oxidation and/or reduction to thereby prevent the layer from largely deforming;
• (2) to form a layer in the vicinity of the light-absorptive layer for preventing deformation and/or breakage of the light-absorptive layer to thereby prevent the light-absorptive layer from largely deforming;
• (3) to prevent a light-absorptive layer from change of state such as fusing, change in composition including decomposition and degradation, diffusion, change in crystalline state, oxidation and/or reduction to thereby prevent the layer from conducting a large quantity of heat to an adjacent layer that easily deforms, such as the substrate, in other words to allow the light-absorptive layer to consume heat generated in the light-absorptive layer to thereby reduce the deformation of the adjacent layer such as the substrate;
• (4) to have a layer that induces a large change in optical constant to thereby yield a sufficient degree of modulation even with a reduced deformation; and
• (5) to perform recording in such a manner as to make fuzzy or unclear interface with an adjacent layer to thereby yield a sufficient degree of modulation even with a reduced deformation.

In consideration of these factors, the combination use of the first inorganic thin film and the second inorganic thin film can much reduce adverse effects of the deformation of recording marks and prevent increased adverse effects of such deformation with an increasing recording power. Thus, the problems in conventional technologies can be effectively solved. The second inorganic thin film herein works to suppress at least one of the deformation and breakage of the first inorganic thin film and to receive the change of state of the first inorganic thin film, such as fusing, change in composition, diffusion, change in crystalline state, oxidation and/or reduction.

In conventional write-once-read-many optical recording media, the absorption coefficient at the recording-reproducing wavelengths is reduced by at least one of decomposition and degradation of the organic material, and the resulting large change in refractive index is used to modulate the amplitude. An organic material layer used herein works as a heat generation layer due to its optical absorptivity and as a recording layer based on change in refractive index (a real part of complex refractive index) caused by the decomposition and/or degradation.

In contrast, according to one embodiment of the write-once-read-many optical recording media of the present invention, a function as a main heat generation layer is separated from such a conventional organic thin film, the first inorganic thin film having photoabsorptivity is formed in addition to the organic thin film, and the second inorganic thin film is further formed.

According to the present invention, recording marks are formed based on at least one of the following mechanisms (1) to (11):

(1) deforming at least one of the first inorganic thin film and the second inorganic thin film;

(2) changing the complex refractive index of at least one of the first inorganic thin film and the second inorganic thin film;

(3) changing the composition of at least one of the first inorganic thin film and the second inorganic thin film;

(4) fusing the first inorganic thin film;

(5) diffusing constitutional elements of the first inorganic thin film into at least one of the second inorganic thin film and the organic thin film;

(6) changing at least one of the crystalline state and crystalline structure of the first inorganic thin film;

(7) oxidizing and/or reducing a constitutional element of the first inorganic thin film;

(8) changing the composition distribution of the first inorganic thin film;

(9) changing the volume of the organic thin film;

(10) changing the complex refractive index of the organic thin film; and

(11) forming a cavity in the organic thin film.

Preferred recording mechanisms to form recording marks are those relating to the change of state of at least one of the first inorganic thin film and the second inorganic thin film, i.e., the recording mechanisms (1) to (8), of which recording mechanisms (2) to (8) are more preferred. According to these mechanisms, the “change of state” such as change in composition, fusing, change in crystalline state, oxidation and/or reduction, diffusion of constitutional elements into an adjacent layer can be utilized. Thus, the first inorganic thin film can have a significantly varied complex refractive index and can have an unclear interface with the adjacent layer to thereby reduce the influence of multiple reflection. Accordingly, a high degree of modulation can be yielded even with a small deformation.

Namely, information can be recorded not mainly based on deformation but mainly based on any of the above recording mechanisms.

1. Functions of First Inorganic Thin Film

According to the present invention, the first inorganic thin film performs a major photoabsorption function.

The first inorganic thin film exhibits normal dispersion, does not have a large absorption band within a range of certain wavelengths and its complex refractive index less depends on wavelength, in contrast to organic materials. Accordingly, the use of the first inorganic thin film can suppress large variation of recording properties such as recording sensitivity, degree of modulation, jitter and error rate and of reflectance even with varying recording-reproducing wavelengths derived from the individual difference of laser or varying environmental temperature.

In conventional write-once-read-many optical recording media, the organic thin film performs both functions as a recording layer and a light-absorptive layer. The constitutional organic material must thereby have a high refractive index n and a relatively low absorption coefficient k at the recording-reproducing wavelengths. Thus the organic thin film must have a relatively large thickness to reach a temperature at which the organic material decomposes. In a phase-change optical recording medium, the substrate must have very deep grooves.

In the optical recording media of the present invention, however, there is no need for the organic thin film to perform a major photoabsorption function and recording function. The organic thin film herein can have a smaller thickness than conventional equivalents.

The organic thin film having such a smaller thickness realizes the use of a substrate having shallow grooves and thus having satisfactory transfer ability (moldability). This type of substrate can be more easily prepared at lower cost than conventional equivalents. In addition, the resulting optical recording media have significantly improved signal quality.

Reproduction according to any of the aforementioned recording mechanisms is impervious to the shape of grooves in the substrate and has a large allowance (margin) with respect to variation of the shape of substrate. This type of substrate can be more easily prepared at lower cost than conventional equivalents.

The organic thin film having such a smaller thickness realizes broader recording power margins.

The first inorganic thin film performs a photoabsorption function as well-as a recording function.

More specifically, the photoabsorption function of the first inorganic thin film causes any of the following changes of state of the first inorganic thin film itself:

(1) deformation (however, the degree of deformation is less than conventional equivalents);

(2) change in complex refractive index;

(3) change in composition;

(4) fusing;

(5) diffusion of constitutional elements into an adjacent layer;

(6) change in crystalline state and/or crystalline structure;

(7) oxidation and/or reduction of constitutional elements; and

(8) change in compositional distribution.

For example, to perform a recording function and a photoabsorption function with respect to a recording-reproducing wavelength of 500 nm or less, the first inorganic thin film preferably comprises an element capable of absorbing light at a recording-reproducing wavelength of 500 nm or less as element R and/or M.

For yielding a larger change in complex refractive index, change in composition, change in crystalline state, fusing or diffusion of constitutional elements into an adjacent layer, the first inorganic thin film preferably comprises, as Element R, an element having a relatively low melting point as an elementary substance or as an oxide.

Thus, Element R of the first inorganic thin film should be at least one selected from Y, Bi, In, Mo, V and lanthanum series elements. Among them, bismuth oxide BiO, wherein “R” is Bi, and a mixture of bismuth and bismuth oxide are preferred for multi-level recording. The first inorganic thin film preferably further comprises at least one element M selected from Al, Cr, Mn, Sc, In, Ru, Rh, Co, Fe, Cu, Ni, Zn, Li, Si, Ge, Zr, Ti, Hf, Sn, Pb, Mo, V and Nb.

Advantages of the combination use of R and O, or R, M and O in the first inorganic thin film are as follows.

(1) The presence of an oxide can increase the hardness of the first inorganic thin film to thereby suppress the deformation of the first inorganic thin film itself and/or of an adjacent layer such as the substrate.

(2) The presence of the oxide can increase the storage stability.

(3) The use of an element capable of highly absorbing light at wavelengths of 500 nm or less, such as Bi, can improve the recording sensitivity.

(4) The use of an element having a low melting point or being easily diffusible, such as Bi, can form recording marks that yield a high degree of modulation even with relatively small deformation.

(5) A satisfactory inorganic thin film can be prepared by a vapor phase epitaxy such as sputtering.

When the first inorganic thin film is represented by R _x M _y O, wherein “x” and- ^“y ” are atomic ratios, the ratio of “x” to the total of “x” and “y” [x/(x+y)] is preferably 0.3 or more. Thus, the deformation of the first inorganic thin film itself and/or the deformation of an adjacent layer such as the substrate can be reduced to thereby reduce interference among recording marks.

Element R is preferably Bi for further better recording-reproducing properties.

The first inorganic thin film is preferably represented by Bi _a (4B) _b O _d or Bi _a (4B) _b X _c O _d , wherein “4B” is at least one of Group 4B elements of the Periodic Table of Elements for better recording-reproducing properties and for better storage stability. Examples of Group 4B elements are C, Si, Ge, Sn and Pb, of which Si and Ge are typically preferred. “X” is at least one element selected from Al, Cr, Mn, In, Co, Fe, Cu, Ni, Zn, Ti and Sn.

When the first inorganic thin film is represented by Bi _a (4B) _b X _c O _d , the element “X” works to cause a larger change in complex refractive index or in composition, to cause fusing or to yield a further diffusion of constitutional elements into an adjacent layer.

The first inorganic thin film may comprise the oxide RO alone but may further comprise Element R in another form than an oxide (this form of element other than an oxide is hereinafter referred to as “elementary”), in addition to RO (oxide of Element R) (hereinafter referred to as “R+RO”).

When the first inorganic thin film further comprises Element M in addition to R and O, these elements may be contained as at least one of (1) a ternary compound of R−M−O, (2) a mixture of the elementary R and an oxide of Element M (R+MO), (3) a mixture comprising an oxide of Element R and an oxide of Element M (RO+MO), (4) a mixture of Element R, an oxide of Element R, and an oxide of Element M (R+RO+MO), and a combination of configurations (1) to (4). In other words, the “first inorganic thin film” for use in the present invention may comprise any of compounds, elements, and mixtures as mentioned above.

For example, oxidation of Element R not in the state of oxide upon recording can significantly change the complex refractive index of the first inorganic thin film. By using this recording mechanism of oxidation, information can be recorded not based on deformation and can be recorded with less intersymbol interference.

However, if first inorganic thin film contains a large amount of Element R and/or Element M not in the state of oxide, the medium may have decreased storage stability. Accordingly, the content of elementary R and/or elementary M is preferably lower than the content of the oxide of Element R and/or the oxide of Element M. The ratio of the former to the latter is preferably set according to the balance of properties such as recording sensitivity, jitter and storage stability.

Information can be recorded based on reduction when Elements R and M are in the state of oxide, and the same advantages as in oxidation can be obtained.

According to the present invention, Element R and/or an oxide thereof performs a major photoabsorption function and a recording function, and above specified elements as Element R can exhibit specific advantages.

When the first inorganic thin film has a composition represented by R _x M _y O, wherein “x” and “y” are atomic ratios, the ratio [x/(x+y)] is preferably 0.3 or more for better recording-reproducing properties. However, a first inorganic thin film having a ratio [x/(x+y)] less than 0.3 can be used for fine control of recording-reproducing properties and storage stability. The ratio [x/(x+y)] is not limited to the range of 0.3 or more in the present invention.

The first inorganic thin film preferably has a thickness of 20 to 500 angstroms (2-50 nm).

2. Functions of Second Inorganic Thin Film

The second inorganic thin film functions to suppress the deformation and/or breakage of the first inorganic thin film and to receive change of state of the first inorganic thin film, such as fusing, change in composition, diffusion, change in crystalline state, oxidation and/or reduction.

The first inorganic thin film is capable of absorbing light in a relatively large quantity at the recording-reproducing wavelengths and generally has a relatively small thickness for better reflectance, although the thickness of the first inorganic thin film may be set according to its absorption coefficient. A first inorganic thin film having a small thickness arranged directly in contact with the substrate may largely deform or break due-to expansion of the substrate, even though the film has a high hardness.

Accordingly, the second inorganic thin film is used to suppress the deformation and/or breakage of the first inorganic thin film in the present invention. The second inorganic thin film may be arranged between the substrate and the first inorganic thin film to effectively suppress the deformation and/or breakage of the first inorganic thin film. However, the advantages of the second inorganic thin film can also be obtained even if the medium comprises the substrate, the first inorganic thin film and the second inorganic thin film arranged in this order. This is probably because the first inorganic thin film has a relatively small thickness, and the second inorganic thin film can easily exhibit its advantages through such a thin first inorganic thin film.

The second inorganic thin film performs a recording function in addition to the function of suppressing the deformation and/or breakage of the first inorganic thin film. As is described above, information is recorded on the write-once-read-many optical recording media of the present invention, for example, by:

(1) deforming at least one of the first inorganic thin film and the second inorganic thin film;

(2) changing the complex refractive index of at least one of the first inorganic thin film and the second inorganic thin film;

(3) changing the composition of at least one of the first inorganic thin film and the second inorganic thin film;

(4) fusing the first inorganic thin film;

(5) diffusing constitutional elements of the first inorganic thin film into at least one of the second inorganic thin film and the organic thin film;

(6) changing at least one of the crystalline state and crystalline structure of the first inorganic thin film;

(7) oxidizing or reducing a constitutional element of the first inorganic thin film; and/or

(8) changing the composition distribution of the first inorganic thin film.

When the second inorganic thin film is arranged adjacent to the first inorganic thin film, it functions to receive the change of state of the first inorganic thin film and to have an unclear interface with the first inorganic thin film.

Such an “unclear interface” means that the interface between the two thin films becomes a different state from that before recording. For example, the unclear interface means that the complex refractive index becomes gradient in the vicinity of the interface between the first inorganic thin film and the second inorganic thin film typically by mixing or diffusion of components of the first inorganic thin film and the second inorganic thin film at the interface. In this case, the complex refractive index varies discontinuously at the interface in an unrecorded area but varies gradually in the vicinity of the interface in a recorded area.

Thus, the second inorganic thin film can receive the change of state of the first inorganic thin film such as fusing, change in composition, diffusion, change in crystalline state, oxidation and/or reduction and make the interface between the first inorganic thin film and the second inorganic thin film unclear. The influence of, for example, multiple reflection can be suppressed to thereby yield a high degree of modulation.

The second inorganic thin film has another essential function to control its thermal conductivity. Thus, fine recording marks with less variation can be efficiently formed.

The second inorganic thin film further functions to control the reflectance, tracking signals and recording sensitivity by appropriately selecting the material and thickness thereof.

Materials that do not undergo decomposition, sublimation or formation of cavities due to heat generated from the first inorganic thin film are preferably used in the second inorganic thin film. Examples of such materials are Al ₂ O ₃ , MgO, BeO, ZrO ₂ , UO ₂ , ThO ₂ and other simple oxides; SiO ₂ , 2MgO—SiO ₂ , MgO—SiO ₂ , CaO—SiO ₃ , ZrO ₂ —SiO ₂ , 3Al ₂ O ₃ -2SiO ₂ , 2MgO-2Al ₂ O ₃ -5SiO ₂ , Li ₂ O-Al ₂ O ₃ -4SiO ₂ and other silicate-containing oxides; Al ₂ TiO ₅ , MgAl ₂ O ₄ , Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ , BaTiO ₃ , LiNbO ₃ , PZT [Pb(Zr,Ti)O ₃ ], PLZT [(Pb,La)(Zr,Ti)O ₃ ], ferrite and other double oxides; Si ₃ N ₄ , Si _6-z Al _Z O _Z N _8-Z , AlN, BN, TiN and other nitride-based nonoxides; SiC, B ₄ C, TiC, WC and other carbide-based nonoxides; LaB ₆ , TiB ₂ , ZrB ₂ and other boride-based nonoxides; CdS, MoS ₂ and other sulfide-based nonoxides; MoSi ₂ and other silicide-based nonoxides; amorphous carbon, graphite, diamond and other carbon-based nonoxides. The second inorganic thin film may comprise an organic substance.

For example, the second inorganic thin film preferably mainly comprises SiO ₂ , ZnS or ZnS—SiO ₂ for better optical transparency to the recording-reproducing light or for better productivity. It may also preferably mainly comprise ZrO ₂ for sufficient thermal insulation. It may also preferably comprise an oxide selected from ZnS, ZrO ₂ , Y ₂ O ₃ and SiO ₂ or an oxide comprising ZrO ₂ , TiO ₂ , SiO ₂ and “X,” wherein “X” is at least one selected from Y ₂ O ₃ , CeO, Al ₂ O ₃ , MgO, CaO, NbO and oxide of rare earth metals.

The thermal conductivity and hardness of the second inorganic thin film play an important role to effectively receive the change of state of the first inorganic thin film, such as fusing, change in composition, diffusion, change in crystalline state, oxidation and/or reduction. To have a suitable thermal conductivity and hardness, the second inorganic thin film preferably mainly comprises ZnS. If it comprises ZnS—SiO ₂ , the ratio of ZnS is preferably increased. When the second inorganic thin film mainly comprising ZnS is arranged adjacent to a reflective layer mainly comprising Ag, the medium may further comprise a sulfuration-resistant layer for preventing sulfuration of Ag.

In general, the second inorganic thin film is preferably transparent to light at the recording-reproducing wavelengths for higher reflectance. However, it can have some function for absorbing light at the recording-reproducing wavelength for controlling recording sensitivity.

The second inorganic thin film preferably has a thickness of 20 to 2000 angstroms (2-200 nm) and generally preferably has a thickness larger than that of the first inorganic thin film.

3. Functions of Organic Thin Film

Functions of the organic thin film are roughly classified as (a) a heat insulating function typically in a configuration in which the organic thin film is sandwiched between the reflective layer and the first inorganic thin film; (b) a function of yielding a high degree of modulation; (c) a function of compensating the reproducing signal waveform; (d) a function of controlling, for example, the reflectance and tracking signals; and (e) a function of controlling the recording sensitivity.

The heat insulating function (a) is as follows. When the write-once-read-many optical recording media have a reflective layer adjacent to the first inorganic thin film, energy absorbed by the first inorganic thin film may not be efficiently converted to heat and information may not be recorded at a suitable recording power. In this case, by arranging an organic thin film between the first inorganic thin film and the reflective layer, the organic thin film even in a very small thickness can serve to insulate the heat.

Such organic thin films are often prepared by spin coating. In this case, the resulting organic thin film has a larger thickness in grooves than in lands, and the grooves work to sufficiently insulate heat and the lands work to dissipate heat. Thus, crosstalk can be controlled. The use of the organic thin film as a heat insulating layer in groove recording can improve the recording-reproducing properties.

The organic thin film performs the function (b) of yielding a high degree of modulation based on the following mechanisms:

(1) the organic thin film changes its volume as a result of recording;

(2) it changes its complex refractive index as a result of recording;

(3) it forms cavities as a result of recording;

(4) it receives the change of state of the first inorganic thin film as a result of recording; and

(5) it receives the deformation of the reflective layer.

The phrase “change of state of the first inorganic thin film” as used herein means and includes, for example, deformation, change in complex refractive index, change in composition, fusing, diffusion or mixing of constitutional elements into an adjacent layer, change in crystalline state and/or crystalline structure, oxidation and/or reduction, and change in composition distribution.

The function (c) of compensating the reproducing signal waveform is a function of converting the reproducing signal waveform into a desired waveform typically so as to yield a high-to-low single recording polarity. This function is achieved by arranging the organic thin film adjacent to the first inorganic thin film. If the organic thin film is not arranged, the reproducing signal waveform may become nonuniform and may not allow the recording polarity to be high-to-low single polarity.

The organic thin film can control its complex refractive index and thickness in very wide ranges and thereby performs the function (d) of controlling the reflectance and tracking signals.

According to the present invention, the first inorganic thin film performs a major photoabsorption function. However, the organic thin film can work as a secondary light-absorptive layer by controlling its complex refractive index, particularly the imaginary part of the complex refractive index and thus performs the function (e) of controlling the recording sensitivity.

The organic thin film preferably has its major absorption band at wavelengths longer than the recording-reproducing wavelength (FIG. 12, wherein the recording-reproducing wavelength is diagonally shaded). This configuration broadens the scope of selection of the organic material and reduces change in complex refractive index at around the recording-reproducing wavelength even though the write-once-read-many optical recording medium uses an organic thin film.

When the organic thin film functions as a secondary light-absorptive layer, the organic thin film preferably has an imaginary part of complex refractive index smaller than that of the first inorganic thin film at the recording-reproducing wavelength. An excessively large imaginary part of complex refractive index of the organic thin film at the recording-reproducing wavelength may increase wavelength dependency.

The organic thin film functioning as a light-absorptive layer more preferably has an absorption band not belonging to the major absorption band at the vicinity of the recording-reproducing wavelength in addition to the above configuration.

The term “major absorption band” as used herein refers to an absorption band having the maximum absorption at visible ray wavelengths (FIG. 13) and generally means an absorption band based on HOMO-LUMO transition. The term “absorption band not belonging to the major absorption band at around the recording-reproducing wavelength” means and refers to an absorption band based on a transition other than HOMO-LUMO transition (FIG. 13). In FIG. 13, vertically oriented ellipse 111 designates the major absorption band of an absorption spetrum and horizontally oriented ellipse 113 the absorption band not belonging to the major absorption band.

Thus, even when the organic thin film performs a secondary photoabsorption function, the wavelength dependency can be reduced by allowing the organic thin film to have an absorption band not belonging to the major absorption band at around the recording-reproducing wavelength.

In the above description, the exemplified organic thin film comprises one organic material having a major absorption band and an absorption band not belonging to the major absorption band. However, the organic thin film for use in the present invention may comprise a mixture of two or more organic materials so as to have an absorption spectrum as shown in FIG. 13. The resulting write-once-read-many optical recording medium can also have a significantly reduced wavelength dependency as compared with conventional equivalents.

According to the present invention, information is recorded by a mechanism not mainly based on deformation. However, the present invention intends not to exclude deformation but to reduce deformation to a level at which the interference among recording marks can be predicted. Accordingly, the write-once-read-many optical recording media of the present invention can utilize deformation of, for example the first inorganic thin film, the second inorganic thin film and/or the reflective layer.

The organic thin film can control the tendency of deformation of an adjacent layer by controlling its thickness. Namely, the recording sensitivity can also be controlled by adjusting the thickness of the organic thin film when deformation is utilized in recording.

Thus, the organic thin film can control the recording sensitivity by changing its complex refractive index and/or thickness.

In the present invention, the first inorganic thin film performs a major recording function and a major photoabsorption function, and the organic thin film does not have to have such functions. Accordingly, there is no need of utilizing the change in real part of complex refractive index of the organic thin film at the recording-reproducing wavelength, and there is no need for the organic thin film of having a photoabsorption function at the recording-reproducing wavelength. Thus, conventional strict requirements in optical constant of the organic material are not required in the present invention. In this connection, the real part of complex refractive index may change as a result of recording. The present invention can employ organic materials having a large absorption band at red laser wavelengths but no large absorption band at blue laser wavelengths, such as colorants for CD-R or DVD-R media, even in recording and reproduction at blue-laser wavelengths.

Conventional equivalents must control wavelengths and thereby require colorants having a complicated substituent or being hardly synthetically prepared in a recording layer. In contrast, the organic thin film for use in the present invention does not require such a complicated control of wavelengths and can employ low-cost organic materials.

In addition, the organic thin film can employ colorants and other organic materials having a large absorption band at wavelengths far from the recording-reproducing wavelength, and the write-once-read-many optical recording media can significantly solve the conventional problems such as large variation in recording properties such as recording sensitivity, degree of modulation, jitter and error rate and reflectance with varying recording-reproducing wavelength caused by individual difference of laser or by change in environmental temperature. In this connection, the refractive index exhibits anomalous dispersion and varies largely with a varying wavelength at wavelengths in the vicinity of such a large absorption band. In contrast, the refractive index exhibits normal dispersion and varies gradually with a varying wavelength at wavelengths far from the large absorption band.

In some embodiments of the present invention, the organic thin film has its major absorption band at wavelengths longer than the recording-reproducing wavelength. However, the relationship between the major absorption band of the organic thin film and the recording-reproducing wavelength is not specifically limited thereto and can be arbitrarily set.

However, the organic thin film preferably has its major absorption band at wavelengths far from the recording-reproducing wavelength for higher reflectance, since the first inorganic thin film plays a major role as a light-absorptive layer. In this case, the organic thin film may have its major absorption band at wavelengths either longer than or shorter than the recording-reproducing wavelength.

Thus, the present invention can be applied to wide ranges of recording-reproducing wavelength including red laser wavelengths, and blue laser wavelengths or shorter and can yield a target optical recording medium corresponding to the recording-reproducing wavelength used by selecting materials satisfying the above requirements from among conventional organic materials such as colorants as mentioned below.

Colorants are preferably used as the organic material in the organic thin film.

The organic thin film preferably has its major absorption band at wavelengths far from the recording-reproducing wavelength for higher reflectance. For example, when the recording-reproducing is performed at red laser wavelengths, the organic material can have its major absorption band either longer than or shorter than the recording-reproducing wavelength. In contrast, when the recording-reproducing wavelength are equal to or shorter than blue laser wavelengths, the organic material should preferably have its major absorption band at wavelengths longer than the recording-reproducing wavelength. If not, the organic material must have a smaller molecular skeleton and a shorter conjugated system. This configuration may invite decreased decomposing ability or may invite insufficient formation of the organic thin film due to decreased solubility or increased crystallinity.

For satisfactory thermal decomposition properties and for forming a satisfactory thin film, therefore, an organic material having its major absorption band at wavelengths longer than the recording-reproducing wavelength is preferably used when the recording-reproducing wavelength is equal to or shorter than blue-laser wavelengths.

Examples of colorants satisfying the above requirements are polymethine colorants, naphthalocyanine colorants, phthalocyanrne colorants, squarylium colorants, chroconium colorants, pyrylium colorants, naphthoquinone colorants, anthraquinone (indanthrene) colorants, xanthene colorants, triphenylmethane colorants, azulene colorants, tetrahydrocholine colorants, phenanthrene colorants, triphenothiazine colorants, azo colorants, formazan colorants, and metal complexes of these compounds.

A layer of such a colorant can be prepared according to a conventional procedure such as vapor deposition, sputtering, chemical vapor deposition (CVD) and coating using a solvent. For example, the layer can be prepared by coating in which the colorant such as a dye is dissolved in an organic solvent and the solution is applied according to a conventional coating procedure such as spraying, roller coating, dipping or spin coating.

Examples of the organic solvent are methanol, ethanol, isopropanol and other alcohols; acetone, methyl ethyl ketone, cyclohexanone and other ketones; N,N-dimethylacetamide, N,N-dimethylformamide and other amides; dimethyl sulfoxide and other sulfoxides; tetrahydrofuran, dioxane, diethyl ether, ethylene glycol monomethyl ether and other ethers; methyl acetate, ethyl acetate and other esters; chloroform, methylene chloride, dichloroethane, carbon tetrachloride, trichloroethane and other halogenated aliphatic hydrocarbons; benzene, xylenes, monochlorobenzene, dichlorobenzene and other aromatic hydrocarbons; hexane, pen