JPH0877605A - Optical recording medium - Google Patents

Abstract

PURPOSE: To form a transmissivity varying layer by a coating method such as spin coating method to improve the productivity of a recording medium and to effectively reduce a size of optical spot in order to further improve the durability of reproduction in comparison with a medium of the prior art. CONSTITUTION: A mask layer obtained by coating of a material including photochromic compound as a transmissivity varying substance is provided on a transparent substrate and a reflection layer or photorecording layer is provided to obtain an optical recording medium. As a material of such mask, a mixture where asymmetric diallylethene compound is dispersed into the polymer liquid crystal or a compound where asymmetric diallylethene compound is combined with polymer liquid crystal by the covalent bond method may be used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an information recording medium such as an optical disk, and more particularly to an information recording medium suitable for efficiently recording and reproducing information at a recording density below the optical resolution. .

[0002]

2. Description of the Related Art Conventionally, in a technique for reproducing a minute signal below a reproduction limit in an optical disk device or the like, for example, a technical digest of optical data storage (Technical Digest of
Optical Data Storage, 199
1, Vol No. 5) Topical Meeting 19
91, volume 5, pp112-115, pp116-
119 is shown. This is characterized by using a plurality of layers of magneto-optical recording film and providing a layer for masking the surrounding information by a transmittance changing substance alone, in addition to the information recording layer. Since the information (bits) recorded in the information recording layer appears in the raised portion, even when the recording density becomes high, the interference with the adjacent bits is suppressed and the optical resolution is improved. Become.

[0003]

However, when the transmittance changing substance is formed as a film by itself as in the prior art as described above, there are the following inconveniences.

When the transmittance changing substance is formed by a method such as vapor deposition with a vacuum film, it is difficult to increase the film thickness. Therefore, it is not possible to use a substance with a small change in transmittance that cannot obtain desired characteristics unless the film thickness is increased, and the range of material selection is reduced.

When a conventional photochromic compound is used as a mask layer, for example, in a conventional diarylethene compound, a colored isomer is excited by light irradiated during reproduction, and a photochromic reaction (from the colored isomer to a non-existent state) occurs from this excited state. There is a problem that the photochromic reaction to the colored body) progresses, the amount of transmitted light to be detected changes, and the reproduction durability is poor (the reproduction causes deterioration of the mask layer).

The present invention is intended to solve these problems, and the transmittance changing layer can be formed by a coating method,
It is an object of the present invention to provide an optical recording medium with high productivity, which is made of a material that can effectively obtain good transmittance change characteristics.

[0007]

According to the present invention, a mask layer formed by applying a material containing a photochromic compound as a transmittance changing substance is provided on a transparent substrate, and a reflective layer or an optical recording layer is further provided. It is a characteristic optical recording medium. The present invention also provides an optical recording medium in which the mask layer material is a mixture of an asymmetric diarylethene compound dispersed in a polymer liquid crystal, and the mask layer material is an asymmetric diarylethene compound in a polymer liquid crystal. The present invention provides an optical recording medium which is a compound bound by a covalent bond.

According to the present invention, when the compound containing the transmittance changing substance is formed as the mask layer, the recording or reproducing light beam is transmitted through the substrate by the action of the transmittance changing substance of the mask layer. The spot diameter is reduced.

Further, if the temperature does not rise, the exchange rate to the ring-closed compound does not increase, that is, the transmittance does not change, that is, by using the asymmetric diarylethene compound as a photochromic compound having a thermal threshold value, It is intended to prevent deterioration of the mask layer due to reproduction.

Specific examples of the optical recording medium according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a CD-ROM and a partial-RO.
Optical recording medium to which the present invention is applied, such as M (P-ROM)
In the same figure, the main surface of the substrate 11 is
The unevenness 12 corresponding to the groove is formed. The main of this board 11
The mask layer 13 and Si as an enhancement film are formed on the surface.₃ N
_Four Film 14, magneto-optical recording film 15, Si as a heat insulating film₃ N _Four 
A film 16, an Al film 17 as a reflective film, and an ultraviolet ray as a protective film
The curable resin protective layers 18 are laminated in that order.
It Among them, the enhancement film 14 outputs a signal due to the interference effect.
In order to increase the force, the heat insulating film 16 reduces heat conduction.
It is provided to improve the sensitivity.

The substrate 11 is made of PC, PM
Amorphous polymers such as MA and polyolefin are used.

Examples of the material of the mask layer made of the transmittance changing substance include photochromic compounds such as spiropyran, spirooxazine, azobenzene, fulgide, diarylethene, triarylmethane and indigo.

Particularly, a diarylethene derivative which is a photon-mode chromic compound that does not cause thermal isomerization is preferably used. This substance has an absorption maximum of light in the vicinity of a wavelength of 600 nm, and easily changes to a colorless body by irradiation with light of 600 nm, and the light transmittance remarkably increases. Further, it has a transmittance change characteristic that it returns to its original state immediately after the light is blocked.

Also, a polymer liquid crystal having methyl methacrylate, a copolymer of methyl acrylate and cyclohexyl acrylate, or cyclohexyl methacrylate as a liquid crystal component, and a mesogen molecule exhibiting liquid crystallinity as a pendant in a side chain through a predetermined alkyl spacer. A mixture prepared by dispersing and mixing is also suitably used as the mask layer.
The mixture has a response rate of 30 when the absorption rate changes due to light absorption.
It is preferable because it is as fast as nsec.

Further, a compound in which a photochromic compound is covalently bonded to a polymer liquid crystal is also used. That is, a copolymer of a liquid crystal monomer having an addition polymerization property and a photochromic monomer, a liquid crystal compound having an unsaturated double bond in a reactive polymer, and a copolymer obtained by subjecting a photochromic compound having a double bond to an addition reaction Polymers may be mentioned.

Regarding the polymer liquid crystal used, for example, cyanobiphenyl or cyanophenylbenzoate having a positive dielectric anisotropy, or methoxybiphenyl, methoxyphenylbenzoate, or methoxyphenyl (4-diphenylaniline having a negative dielectric anisotropy is used. It is possible to use those having a (phenylbenzoate) structure and a main chain structure of polyacrylate type, polymethacrylic type, polyether type, polyester type, polysiloxane type and the like.

Examples of the photochromic compound dispersed in the polymer liquid crystal include the above-mentioned spiropyran, spirooxazine, azobenzene, fulgide, diarylethene, triarylmethane, and indigo.

Particularly preferred are diarylethene derivative compounds which are photon-mode photochromic compounds which do not cause thermal isomerization. The above compounds have a small time dependency of the change in the absorptivity of 50 nsec or less and are suitable as a material for the mask layer.

The change in the absorptance shows the maximum absorption at a wavelength of 510 nm and an absorption edge at 650 nm when irradiated with 365 nm of UV light. The rise time is 50 nsec or less.

As described above, when the conventional diarylethene compound is used as the mask layer material, there is a problem in the reproduction durability of the recording medium.

Therefore, by connecting an asymmetric molecule to the substituent of the diarylethene compound and imparting asymmetry to the diarylethene compound, the rate of exchange of the compound with a ring-closed compound increases when the temperature rises. The above problem could be solved by providing

As the derivative of the asymmetric molecule in the diarylethene compound, those having different aryl groups at the 1 and 2 positions are preferably used.

For example, an asymmetric diarylmaleimide derivative having two thiophene groups on one of the aryl groups is synthesized, and this is copolymerized with styrene, methylmethacrylate, cyclododecylmethacrylate, etc. to synthesize a copolymer for use. be able to. Examples of the thiophene group bonded to one of the aryl groups include a 2-cyanothiophene group and a 2-methoxybenzothiophene group.

These asymmetric type diarylethene compounds have an exchange rate to a ring-closed compound of about 50% at 150 ° C. That is, the photochromic reaction (photochromic reaction from a colored isomer to a non-colored body) has a thermal threshold value. A mixture in which the asymmetric diarylethene compound is dispersed and blended in the above-mentioned polymer liquid crystal and a compound in which the diarylethene compound is bonded to the polymer liquid crystal by a covalent bond are also used.

As the diarylethene compound, it is also possible to use a diarylfluorocyclopentene derivative, which is mentioned as being excellent in the thermal stability of the colored body and the durability against repeated coloring and decoloring. The following compounds are used as the diarylfluorocyclopentene derivative.

[0027]

Embedded image That is, a copolymer of a liquid crystal monomer having addition polymerizability and the diarylethene compound can be mentioned. The polymer liquid crystal used is methyl methacrylate, a copolymer of methyl acrylate and cyclohexyl acrylate, or cyclohexyl methacrylate, and as a liquid crystal component, a mesogen molecule showing liquid crystallinity is pendant to a side chain through a predetermined alkyl group spacer. And a polymer liquid crystal having Further, for example, it has a cyanobiphenyl or cyanobiphenylbenzoate having a positive dielectric anisotropy, or a methoxybiphenyl, methoxyphenylbenzoate, or methoxyphenyl (4-phenylbenzoate) structure having a negative dielectric anisotropy, and has a main chain A structure having a polyacrylate type, a polymethacrylic type, a polyether type, a polyester type, a polysiloxane type, or the like can be used.

The mask layer in the optical recording medium of the present invention can be formed by a coating method such as a spin coating method or a bar coating method. The thickness of the mask layer can be increased, but it is preferably in the range of 0.2 μm to 20 μm, and particularly preferably in the range of 0.5 μm to 10 μm.

Next, the operation of the present invention as described above will be explained with reference to FIG. 1. The light beam for irradiating the pits 12 to read out information is the substrate 11 as shown by an arrow AA.
It is incident on the optical disc from the side. By the way, in the present invention,
A mask layer 13 made of a light transmittance changing substance is formed on the substrate 11. Therefore, when the light beam travels to the mask layer 13, the spot diameter is reduced by the transmittance changing substance.

The light beam whose spot diameter has been reduced enters the pit 12, is reflected by the reflection film 14, passes through the substrate 11, and is output from the optical disc as indicated by an arrow BB. This reflected light beam is incident on a photoelectric detection means (not shown), converted into an electric signal here, and then processed by a known method.

As described above, according to the present invention, since the transmittance changing layer can be formed by the coating method, the productivity is excellent, and the film thickness of the transmittance changing layer can be increased, so that the transmittance changing efficiency is high. It is possible to obtain a favorable change in transmittance even when a substance having a small value is used.

[0032]

EXAMPLES The present invention will be described in more detail with reference to the following examples. (Embodiment 1) FIG. 1 is a sectional view of a recording medium of this embodiment.

Main surface of polycarbonate disc substrate 11
Concavities and convexities 12 corresponding to pits are formed on the upper surface of the substrate.
A mask layer 13 and an enhancement film 14 are formed on the main surface of 11.
(Si ₃ N_Four Film 70 nm), recording film 15 (Tb₂₀Fe₇₃
Co₇ Film 20 nm), heat insulating film 16 (Si₃ N_Four Membrane 20n
m), the reflective film 17 (Al film 50 nm), the protective film 18 (S
i₃ N_Four Films 50 nm) are laminated in that order.
It Among them, the enhancement film 14 outputs a signal due to the interference effect.
In order to increase the force, the heat insulating film 16 reduces heat conduction.
It is provided to improve the sensitivity.

As the material of the mask layer 13, a photochromic compound such as 5-methoxyindole group, 2-
On one of the aryl groups having a cyanothiophene group, 2-
Using a copolymer obtained by copolymerizing an asymmetric diarylmaleimide derivative having a cyanothiophene group and a 2-methoxybenzothiophene group and cyclodecyl methacrylate,
this

[0035]

Embedded image Polymer of monomer represented by (polymer liquid crystal: molecular weight 6
10,000%) was applied by a spin coating method using a mixture in which 4% by weight was dispersed, and after drying, a mask layer 13 having a thickness of 0.3 μm was formed. This is sample 1. (Embodiment 2) FIG. 2 is a sectional view of a recording medium of this embodiment.

Concavities and convexities 12 corresponding to the guide grooves are formed on the main surface of the substrate 11. The recording film 15 is formed on this main surface.
Is formed, and the reflective film 17 and the protective film 18 are respectively formed on the recording film 15. For the recording film 15, for example, a material such as an amorphous metal magnetic film capable of recording information by a magneto-optical effect or the like is used. A mask layer 13 is formed on the substrate 11. Here, as the material of the mask layer 13, the asymmetric diarylmaleimide used in Example 1 and the monomer of the formula (II) were used as TH
A mask layer 13 having a thickness of 0.4 μm was formed by spin coating using a compound obtained by copolymerizing FBN as a solvent with AIBN [azoisobisbutyronitrile] as an initiator. This is sample 2. (Example 3) Instead of using the polymer of the monomer of the formula (II) used in Example 1 as the polymer liquid crystal, a polymer liquid crystal having cyanophenylbenzoate in the side chain and a main chain of polyacrylate was used. An optical recording medium was prepared in the same manner as in Example 1 except that it was used. The thickness of the mask layer is
It was 0.5 μm. This is sample 3. (Example 4) Instead of using the copolymer of the monomer of the formula (II) used in Example 1 as the polymer liquid crystal, a polymer liquid crystal having methoxyphenylbenzoate in the side chain and a main chain of polymethacrylate. An optical recording medium was prepared in the same manner as in Example 1 except that was used. The thickness of the mask layer was 0.4 μm. This is sample 4. Example 5 The procedure of Example 2 was repeated except that a monomer in which methoxyphenyl (4-phenylbenzoate) was covalently bonded to methacrylate was used instead of the monomer of the formula (II) used in Example 2. A recording medium was created. The thickness of the mask layer was 0.5 μm. This is sample 5. (Example 6) Instead of using the polymer of the monomer of the formula (II) used in Example 1 as the polymer liquid crystal, a polymer liquid crystal having cyanobiphenyl in the side chain and having polymethacrylate as the main chain is used. An optical recording medium was prepared in the same manner as in Example 1 except for the above. The thickness of the mask layer is 0.4 μm
And This is sample 6. (Example 7) A mask layer was formed in the same manner as in Example 1 except that a diarylfluorocyclopentene compound was used as the photochromic compound as the material of the mask layer 13 and the compound was dispersed in the polymer liquid crystal of Example 1. This is referred to as Sample 7. Here, the following compounds were used as the diarylfluorocyclopentene compounds.

[0037]

[Chemical 3] Table 1 shows the photoresponsiveness of the materials used for forming the mask layer in Examples 1 to 7 described above.

[0038]

[Table 1] (Comparative Example 1) An optical recording medium was prepared in the same manner as in Example 1 except that the mask layer 13 was not provided. This is sample 8.

Next, a recording / reproducing experiment was conducted on each sample medium using a recording / reproducing evaluation machine (Nakamichi OMS-2000). Rotate each sample medium at 2400 rpm
Then, an RF signal of 12.5 MHz was written at a position with a radius of 36 mm so that the recording mark length would be 0.4 μm. The linear velocity of the medium at this time is 10 m / s.
At this time, the recording power is 7 mW and the recording bias magnetic field is 20
0 Oe, NA of the objective lens of the optical head is 0.55,
The laser wavelength is 780 nm.

Next, the laser power during reproduction was changed to find the condition that the reproduction C / N ratio (dB) was maximized.
The results are shown in Table-2.

[0041]

[Table 2] * Numerical values indicate reproduction C / N ratio (dB).

From the results shown in Table 2, the medium of Comparative Example 1 (Sample 8) has a reproducing laser power of 1.0 to 1.25 mW.
Results in the maximum reproduction C / N ratio. This is because at 0.75 mW or less, noise of the element that receives the reproduction beam cannot be ignored, and at 1.25 mW or more, the temperature of the recording film rises during reproduction and the magneto-optical effect decreases. Is.

On the other hand, in Example samples 1 to 7,
The maximum reproduction C / N ratio of 40- is about 1.5-1.75 mW.
It showed 43 dB. This value was 5 to 8 dB higher than that of the comparative example having no mask layer.

The reproduction power of the maximum reproduction C / N ratio is 0.5
The reason why the mask layer is larger than mW is considered that the mask layer absorbs or reflects about 5 to 10% of the reproduction light.

The temperature of the central portion of the reproducing beam becomes slightly higher than that of the peripheral portion, the transmittance of the mask layer in the central portion is increased to give a super-resolution effect, and the temperature between the central portion and the peripheral portion is high. It is considered that there is some reflection or refraction effect of light at the boundary portion where the refractive index is changed due to the difference, and as a result, the effect of narrowing the reproduction beam is increased. Comparative Example 2 Instead of the diarylethene compound used in Example 1, a symmetric diarylethene compound (1,2-bis- (2-methyl-3-benzothienyl)) was used.
-3,34,4,5,5-hexafluorocyclopentene

[0046]

[Chemical 4] An optical recording medium was prepared in the same manner as in Example 1 except that was used. The thickness of the mask layer was 0.4 μm. This is sample 9.

This medium had a maximum transmittance change of 10% with an incident light (780 nm) and a response time of 50 nsec.

Next, the results of comparison of the reproduction durability between Samples 1 to 7 (Example) and Sample 9 (Comparative Example 2) are shown in the table.
3 shows. The recording condition is that the rotation speed of each sample medium is 24
Rotate at 00 rpm and set R of 12.5 MHz so that the recording mark length becomes 0.4 μm at the position of radius 36 mm.
I wrote the F signal. The linear velocity of the medium at this time is 10 m /
s, the recording laser power at this time is 7 mW, the recording bias magnetic field is 200 Oe, and the NA of the objective lens of the optical head is
Is 0.55 and the laser wavelength is 780 nm. Next, the change of the reproduction C / N ratio (dB) was measured when the reproduction laser power was set to 2 mW and the number of reproduction was increased.

The recording / reproducing experiment was carried out using a recording / reproducing evaluation machine (Nakamichi OMS-2000).

[0050]

[Table 3] * Numerical values indicate reproduction C / N ratio (dB).

As is clear from Table 3, in the samples 1 to 7 (Examples 1 to 7), the reproduction C / N ratio (dB) showed almost no change even after 5000 times of reproduction. On the other hand, in Sample 9 (Comparative Example 2),
A decrease in the reproduction C / N ratio was observed after about 500 times.

Even in the mask layer, the reproduction light is 5 to 10%.
Since it absorbs to some extent, when a conventional diarylethene compound having no thermal threshold value is used for the mask layer, it is considered that reproduction deterioration due to change in transmittance occurs. On the other hand, when the mask layer of the present invention having a thermal threshold value is used, reproduction deterioration does not occur.

[0053]

As described above, according to the optical recording medium of the present invention, the transmittance changing layer can be formed by a coating method such as spin coating, and productivity can be improved. Further, the light spot can be effectively reduced. Furthermore, there is an effect that the reproduction durability is improved as compared with the conventional medium.

[Brief description of drawings]

FIG. 1 is a cross-sectional view illustrating an optical recording medium having a mask layer of the present invention.

FIG. 2 is a cross-sectional view illustrating an optical recording medium showing one embodiment of the present invention.

[Explanation of symbols]

 11 substrate 12 pits (unevenness) 13 mask 14 enhance film 15 recording film 16 heat insulating film 17 reflective film 18 protective film AA information reading beam BB reflected beam output from optical disc

Claims (3)

[Claims] 1. An optical recording medium comprising a transparent substrate, a mask layer formed by coating a material containing a photochromic compound as a transmittance changing substance, and a reflection layer or an optical recording layer. 2. The optical recording medium according to claim 1, wherein the material is a mixture in which an asymmetric diarylethene compound is dispersed in a polymer liquid crystal. 3. The optical recording medium according to claim 1, wherein the material is a compound in which an asymmetric diarylethene compound is covalently bonded to a polymer liquid crystal.