JPH04321946A - Optical recording medium - Google Patents

Abstract

PURPOSE:To enhance the reflectance of an unrecorded part by successively laminating a high-refractive-index layer and a low-reflective-index layer on the uppermost low-refractive-index layer as a recording layer and buffer- amplifying the respective reflected waves at each interface. CONSTITUTION:A high-refractive-index layer and a low-refractive-index layer are successively laminated on a substrate more than once. The attenuation coefficient of the high-refractive-index layer is preferably controlled to <=0.1, and the org. dyestuff such as cyanine dye and phthalocyanine dye and Si, ZnS, etc., as the inorg. material are used. The attenuation coefficient of the low- refractive-index layer is controlled almost to zero, and the inorg. matter such as SiO2 and the org. matter such as 'Teflon (R)' are used. A high-refractive- index layer is laminated on the uppermost low-refractive-index layer, and at least one among the high-refractive-index layers is used as the recording layer. This recording layer is irradiated with a laser beam to break the multiplex structure, and the layer is recorded while lowering the reflectance. The amplified reflected waves are buffer-amplified at each interface in this recording medium, and a high reflectance of >=75% is obtained at the unrecorded part as a whole.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium, and more particularly to an optical disc that can be played back by a read-only optical disc (CD) playback device.

0002

BACKGROUND OF THE INVENTION As for optical information recording media, read-only discs such as compact discs (CDs) have become widely used, and research into write-once optical discs and rewritable optical discs, which are expected to have great demand in the future, has followed. It is being actively carried out. Among these, write-once optical discs that use organic dyes such as cyanine dyes, phthalocyanine dyes, and naphthalocyanine dyes as recording materials can be formed by spin coating, compared to optical disks that use inorganic materials such as tellurium as recording materials. For this reason, it is attracting new attention as it can be expected to reduce costs. In recent years, read-only optical discs, i.e. write-once optical discs (CD-WO), which can be played back by a compact disc (CD) playback device and can be recorded once, have been proposed, and the compact disc standard has been adopted. Research and development of compliant write-once optical discs is being actively conducted. In particular, various disk structures are being studied in order to increase the reflectance of the unrecorded portion of the disk to 70% or more with respect to the reproduction laser beam.

[0003] JP-A-2-87339, JP-A-Hei 2-
No. 87340 and Japanese Unexamined Patent Publication No. 2-79235 disclose that in order to increase the reflectance, a recording layer having a specific optical constant and a metal reflective layer such as gold, silver, copper, or aluminum are provided on a substrate in a predetermined manner. A structure in which layers are laminated to a certain thickness has been proposed. According to this structure, it is said that the reflectance for laser light can be increased to 70% or more. Several proposals have been made regarding the structure of a disc to increase the reflectance in the unrecorded area, but as described in these publications, a structure in which a recording layer and a reflective layer are sequentially laminated on a substrate is proposed. These are common.

[0004] In the structure described in the above-mentioned publication, the reflective layer for increasing the reflectance includes a layer having a low refractive index and a large extinction coefficient.
It is necessary to use gold, silver, copper, aluminum, and platinum. These reflective films made of gold or the like are expensive, have weak adhesion at the interface with the dye, and have been pointed out to have a peeling problem. Further, although a reflective layer using inexpensive aluminum has been announced, corrosion resistance of aluminum is considered to be a problem.

[0005]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an optical recording medium having a novel structure for increasing the reflectance of unrecorded areas.

[0006]

[Means for Solving the Problems] The optical recording medium of the present invention has a high refractive index layer and a low refractive index layer laminated one or more times in sequence on a substrate, and a high refractive index layer is stacked on the topmost low refractive index layer. The refractive index layers are laminated, at least one of the high refractive index layers is used as a recording layer, and the amplitudes of reflected waves at each interface are adjusted so as to interfere and amplify each other.

The above-mentioned substrate may be made of thermoplastic resin such as vinyl chloride, acrylic resin, polyolefin resin, polycarbonate resin, or polyvinyl acetal resin, or thermoplastic resin such as epoxy resin, unsaturated polyester resin, or vinyl ester resin. Some are made of materials such as hardened resin and glass, and are preferably plate-shaped. The substrate preferably has an extinction coefficient of 0 or approximately 0 at the laser emission wavelength.

[0008] The high refractive index layer preferably has a higher refractive index than the substrate at the laser transmission wavelength and has a small extinction coefficient since multiple interference is used to increase the reflectance. When the extinction coefficient is small, light can be transmitted to other layers without absorbing much light. The high refractive index layer is
The refractive index is 1 or more larger than the substrate at the laser emission wavelength,
Preferably, the extinction coefficient is 0.1 or less. However, the outermost high refractive index layer may have a small extinction coefficient. The high refractive index layer is made of an organic dye, an inorganic substance, or the like. Examples of organic dyes include cyanine dyes, phthalocyanine dyes, naphthalocyanine dyes, merocyanine dyes, polymethine dyes, anthraquinone dyes, and porphyrin dyes. Among these dyes, those having a resonance absorption band peak on the wavelength side lower than the laser beam emission wavelength tend to exhibit a high refractive index and a low extinction coefficient at this wavelength. These dyes are dissolved in an organic solvent and then formed into a high refractive index layer by spin coating or, if possible, by vacuum evaporation. Inorganic substances include Si, ZnS, CdS, ZnSe, and Zn.
A high refractive index layer is formed by forming a film using a vacuum evaporation method, a sputtering method, or the like. At least one of the high refractive index layers is a recording layer. If the recording layer has an extinction coefficient greater than 0 at the recording laser wavelength, light is absorbed by this recording layer and recording is performed under the influence of deformation and the like. When this recording layer is unrecorded, it is preferable that the extinction coefficient is as small as possible in order to achieve high reflectance, and the recording layer is in contact with the substrate so that the recording layer absorbs as much light as possible. It is preferable that the arrangement be such that sufficient recording can be performed with a weak recording laser beam power.

[0009] The low refractive index layer preferably has a lower refractive index than the high refractive index layer at the laser transmission wavelength and has a small extinction coefficient since multiple interference is used to increase the reflectance. It is preferable that the low refractive index layer has a refractive index smaller than that of the high refractive index layer by one or more at the laser transmission wavelength and an extinction coefficient of 0 or almost 0. As the material for the low refractive index layer, inorganic materials such as SiO2, MgF2, CeF3, and LaF3, and organic materials such as Teflon, polystyrene, nitrocellulose, polyvinylpyrrolidone, hydroxyethylcellulose, polyvinyl alcohol, and silicone resin can be used. The inorganic material is formed into a film by a vacuum evaporation method or a sputtering method, and the organic polymer is formed into a film by a spin coating method or a sputtering method after being dissolved in an organic solvent to form a low refractive index layer.

The optical recording medium of the present invention may have a protective layer laminated as the outermost layer. Various synthetic resins such as ultraviolet curing resins and thermosetting resins can be used for the protective layer. Also,
As the protective layer, a film of silicon dioxide or the like may be formed by sputtering, vacuum evaporation, or the like.

It is preferable that the refractive index of the substrate and each layer is around 1.5 for the substrate, the low refractive index layer and the protective layer laminated as necessary, and 2.7 or more for the high refractive index layer. In this case, the reflectance of the unrecorded area can be sufficiently increased (for example, the reflectance can be increased to 70% or more).

In the optical recording medium of the present invention, a high refractive index layer and a low refractive index layer are sequentially laminated on a substrate, and a high refractive index layer is laminated on the topmost low refractive index layer. . In this way, the high refractive index layer is present in contact with the substrate, and
Since the high refractive index layer is the outermost layer, the number of amplitude reflected waves contributing to the reflectance increases, making it easier to obtain a high reflectance. The fact that the three layers of the high refractive index layer, the low refractive index layer, and the high refractive index layer are sequentially laminated on the substrate is because a high reflectance of, for example, 70% or more can be obtained with the least number of laminated layers. preferable.

As described above, the film thickness of each layer is adjusted so that the amplitude reflected waves based on each interface interfere with each other and are amplified, but it is preferable to determine it using the matrix method as follows. .

In the following, a case will be explained in which a first high refractive index layer, a low refractive index layer, a second high refractive index layer, and a protective layer are sequentially laminated on a substrate. The wavelength of the reproduction laser beam is indicated by λ. Also, the characteristic matrix of the layer is M,
The optical constant at wavelength λ is N, the refractive index at wavelength λ is n,
Denote the extinction coefficient k at the wavelength λ, the film thickness d, and the phase distance γ, which are the substrate, the first high refractive index layer, the low refractive index layer, the second high refractive index layer, and the protective layer. that it is of layers,
They are indicated by subscripts 0, H1, L, H2 and P, respectively. Further, in the following, i represents an imaginary number of 1. Note that N=n+ik.

Characteristic matrices MH1, ML and MH of the first high refractive index layer, low refractive index layer and second high refractive index layer
2 is given by matrices shown by Equations 1, 2, and 3 below.

[Math 1]

[Math 2]

[Math 3]

γH1, γL and γH2 appearing in the above matrix are expressed by the following equations 4, 5 and 6.

[Equation 4] γH1=2πdH1nH1/λ-idH1kH
1/λ

[Formula 5] γL = 2πdL nL /λ-idL
kL/λ

[Equation 6] γH2=2πdH2nH2/λ-
idH2kH2/λ

The property matrix of the layered structure is the product of the property matrices of each layer. If the characteristic matrix of the laminated structure of the first high refractive index layer, low refractive index layer, and second high refractive index layer described above is MMUL,
This is expressed by the following number 7.

[Equation 7] However, in Equation 1, M11, M12, M21, and M22 represent components of the matrix. For example, M11 is the following number 8
It is expressed as

[Formula 8] M11=cosγH1・cosγL・cosγ
H2-(NL/NH1)・sinγH1・sinγL・
cosγH2-(NH2/NL)・cosγH1・si
nγL・sinγH2-(NH2/NH1)・sinγ
H1・cosγL・sinγH2

The reflectance (energy reflectance) R of the optical recording medium having the above structure is:
In the case of normal incidence, it is calculated by the following equation 9.

[Math. 9]

In the above equation, the optical constant of each layer, the reproduction wavelength, and the thickness of each layer are included in the equation. The reproduction wavelength can be specified, and by specifying this and the material of each layer, the optical constants can be determined. Therefore, in this case,
In Equation 9 above, the unknowns are the three film thicknesses dH1, dL, and dH2. Therefore, two of these are approximately λ/4
n (where λ is the above-mentioned reproduction wavelength, n is the refractive index of each layer at this reproduction wavelength), and the remaining film thickness is set to the above-mentioned R.
is determined so that it is maximized. λ/4n corresponds to the shortest phase distance for aligning the phases of the amplitude reflected waves. However, since at least the recording layer has a non-zero extinction coefficient, the shortest phase distance for aligning the phases of the amplitude reflected waves slightly deviates from λ/4n. Therefore, two of the above three film thicknesses are set to approximately λ
/4n and the other one is determined so that the above-mentioned R is maximized, so that the thickness of each layer of the optical recording medium can be easily designed so that the reflectance of the unrecorded part is 70% or more. Can be done. Furthermore, even when there are many layers, the film thickness can be designed in the same way. When there is no protective layer, the above Np
can be replaced with the optical constant of air.

Recording on the optical recording medium in the present invention is carried out by irradiating a recording laser beam to cause a change in the recording layer, thereby changing the reflectance of that portion. For example, the recording layer is irradiated with a recording laser beam, the irradiated part absorbs the light, the absorbed light is converted into heat, the irradiated part is melted, vaporized, or decomposed and deformed, and then the recording layer Recording can be achieved by deforming the interface between adjacent layers to break the multiple interference structure and lower the reflectance of the irradiated area. Those that involve changes in the recording layer are write-once types. Reproduction of recorded information can be performed by irradiating the same laser beam with a power lower than that used for recording and detecting changes in reflectance between recorded and unrecorded areas.

[0021]

[Operation] In the optical recording medium of the present invention, a large number of thin films having different complex refractive indexes are laminated, and the transmitted light is reflected at the interface of each layer, and the amplitude reflected waves based on each interface are amplified by interfering with each other. The amplitudes of the reflected waves are aligned in phase, and a high overall reflectance can be obtained without using a metal reflective layer.

[0022]

[Example] Next, an example of the present invention will be shown.

Example 1 1,1'-dibutyl-3,3,3',3 was formed as a high refractive index layer on a glass substrate (refractive index at 780 nm: 1.5, extinction coefficient 0). '-tetramethyl-4,5,4'
, 5'-dibenzoindodicarbocyanine perchlorate (refractive index at 780 nm is 2.7, extinction coefficient is 0.0
5) was dissolved in 5 cc of 1,1,1,3,3,3-hexafluoropropanol, and a film with a thickness of 70 nm was formed by spin coating. In addition, a low refractive index layer of SiO2 (refractive index at 780 nm is 1.5,
(extinction coefficient is 0) by sputtering method.
A film was formed to a nm size. Furthermore, as a high refractive index layer, Si (780
A film having a refractive index of 3.1 and an extinction coefficient of 0.1) was similarly formed into a film with a thickness of 62 nm by the sputtering method. Figure 1 shows the reflection spectrum of this multilayer film. The reflectance at the recording/reproducing wavelength of 780 nm was 75%.

Example 2 Polycarbonate substrate (outer diameter: 120 mm, inner diameter: 15
mm, track pitch: 1.6 μm, manufactured by Idemitsu Petrochemical Co., Ltd., refractive index at 780 nm is 1.5, extinction coefficient is 0
1,1'-dibutyl-3,3,3',3'-tetramethyl-4,5,4',
5'-dibenzoindicabocyanine perchlorate (refractive index at 780 nm is 2.7, extinction coefficient is 0.05
) was dissolved in 5 cc of 1,1,1,3,3,3-hexafluoropropanol, and a film with a thickness of 70 nm was formed by spin coating. On top of this, a low refractive index layer of SiO2 (refractive index at 780 nm: 1.5, extinction coefficient: 0) was formed to a thickness of 130 nm by sputtering. Further, as a high refractive index layer, Si (refractive index at 780 nm: 3.1, extinction coefficient: 0.1) was similarly formed into a 62 nm film by sputtering. The reflectance of this disk at 780 nm was 75%. An optical recording evaluation device (
Using OMS-2000 (product name of Nakamichi Co., Ltd.), a semiconductor laser with a wavelength of 780 nm was used to increase the linear velocity of 1 from the substrate side.
．． A signal was recorded at 25 m/sec and a duty ratio of 50/50. The recording signal is the same single frequency as the CD EFM signal, 11T (197KHz), 7T (309KHz), 3
T (718 KHz) was used. Recorded signal at 780n
When the data was reproduced using a 0.5 mW semiconductor laser, the reflectance decreased in the recording part and the highest C/N was approximately 50 dB. In addition, EFM was applied to the optical disc obtained above.
Record the signal and play it on a commercially available compact disc player (
DP-8000, manufactured by Kenwood, reproduction light wavelength 78
0 nm). At this time, as determined from the eye pattern of the reproduced amplitude signal, I11/
ITOP was 0.65 and I3/ITOP was 0.35.

Example 3 A high refractive index layer of 1,1'-dibutyl-3,3,3', 3'-tetramethyl-4,5,4
',5'-dibenzoindodicarbocyanine perchlorate (refractive index at 780 nm is 2.7, extinction coefficient is 0.
05) was dissolved in 5 cc of 1,1,1,3,3,3-hexafluoropropanol, and a film with a thickness of 70 nm was formed by spin coating. On top of this, as a low refractive index layer, SiO2 (refractive index at 780 nm is 1.5, extinction coefficient is 0) is deposited at 130 nm by sputtering.
A film was formed on m. Further, as a high refractive index layer, bis(tributylsiloxy)silicon naphthalocyanine (refractive index at 780 nm: 4.0, extinction coefficient: 0.2) was similarly formed into a 50 nm film by vacuum evaporation. Figure 2 shows the reflection spectrum of this multilayer film. Recording/reproducing wavelength of 780nm
The reflectance was 81%.

Example 4 Polycarbonate substrate (outer diameter: 120 mm, inner diameter: 15
mm, track pitch: 1.6 μm, manufactured by Idemitsu Petrochemical Co., Ltd., refractive index at 780 nm is 1.5, extinction coefficient is 0
1,1'-dibutyl-3,3,3',3'-tetramethyl-4,5,4',
5'-dibenzoindicabocyanine perchlorate (refractive index at 780 nm is 2.7, extinction coefficient is 0.05
) was dissolved in 5 cc of 1,1,1,3,3,3-hexafluoropropanol, and a film with a thickness of 70 nm was formed by spin coating. On top of this, a low refractive index layer of SiO2 (refractive index at 780 nm: 1.5, extinction coefficient: 0) was formed to a thickness of 130 nm by sputtering. Further, as a high refractive index layer, bis(tributylsiloxy)silicon naphthalocyanine (refractive index at 780 nm: 4.0, extinction coefficient: 0.2) was similarly formed into a 50 nm film by vacuum evaporation. When the optical recording characteristics of the optical disk obtained as described above were evaluated in the same manner as in Example 2, a C/N of around 45 dB was obtained by recording a single signal of 3 to 11 T at 4 mW. FIG. 3 shows the relationship between recording power and C/N for each frequency signal. In FIG. 3, graph 1 shows the characteristics at 3T, graph 2 shows the characteristics at 7T, and graph 3 shows the characteristics at 11T.

[0027]

Effects of the Invention The optical recording medium according to claim 1 has a high reflectance in the unrecorded portion at the laser wavelength for recording and reproduction.

[Brief explanation of drawings]

FIG. 1 is a reflection spectrum of the optical disc obtained in Example 1.

FIG. 2 is a reflection spectrum of the optical disc obtained in Example 3.

FIG. 3 is a graph showing the recording characteristics of the optical disc obtained in Example 4.

[Explanation of symbols]

1... Graph showing recording characteristics at frequency signal 3T, 2... Graph showing recording characteristics at frequency signal 7T, 3... Graph showing recording characteristics at frequency signal 11T.

Claims (1)

[Claims] Claim 1: A high refractive index layer and a low refractive index layer are sequentially laminated one or more times on a substrate, and a high refractive index layer is laminated on top of the uppermost low refractive index layer. An optical recording medium in which at least one of the layers is a recording layer, and the amplitudes of reflected waves at each interface are adjusted so as to interfere and amplify each other.