WO2006070900A1

WO2006070900A1 - Optical recording medium, optical recording method and optical reproducing method

Info

Publication number: WO2006070900A1
Application number: PCT/JP2005/024187
Authority: WO
Inventors: Hiroshi Kubo
Original assignee: Fujifilm Corporation
Priority date: 2004-12-27
Filing date: 2005-12-22
Publication date: 2006-07-06
Also published as: TW200703307A

Abstract

The optical recording medium comprises a substrate, an in-groove and an on-groove, and further, a recording layer and a cover layer formed in this order on at least one surface of the substrate, wherein the recording layer is formed plane-wise and has an almost uniform thickness and the in-groove is an optical recording section. The optical recording method comprises applying the converged light to record information in an optical recording section. The optical reproducing method comprises applying the converged light and detecting the reflected light to reproduce the information stored in the optical recording section.

Description

DESCRIPTION

OPTICAL RECORDING MEDIUM, OPTICAL RECORDING METHOD AND OPTICAL REPRODUCING METHOD

TECHNICAL FIELD

The invention relates to an optical recording medium, an optical recording method and an optical reproducing method for computer use, business use and public use.

BACKGROUND ART

The rapid spread of networks such as the Internet, and high- vision TVs has progressed in recent years. Also, the commencement of broadcasting of HDTV (High Definition Television) is near at hand. In such circumstances, a large capacity optical recording medium enabling the recording of image information simply at low cost is demanded. DVD-Rs (Digital Versatile Disks) satisfactorily play role of a large capacity optical recording medium. However, demands for the development of a high-capacity and high-density optical recording medium are steadily increasing and it is therefore also necessary to develop an optical recording medium that can meet these demands. This is the reason why development is under way of an optical recording medium which enables high-density recording by using light having a wavelength shorter than that used for DVD-Rs and has a larger capacity.

For example, a record reproducing method is disclosed in which in an optical recording medium having a recording layer containing an organic dye, information is recorded and reproduced by applying laser light having a wavelength of 530 nm or less from the recording layer side to the reflecting layer side (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818 and 2000-228028).

In these methods, an optical recording medium provided with a recording layer containing a porphyrin compound, an azo type dye, a metal azo type dye, a quinophthalone type dye, a trimethinecyanine dye, a dicyanoviiiylphenyl skeleton dye, a cumarin compound, a naphthalocyanine compound or the like is irradiated with blue (wavelength: 430 nm, 488 nm) or bluish green (wavelength: 515 nm) laser light to thereby record or reproduce information.

Also, an optical recording medium is proposed which can record and reproduce information using two laser lights having wavelength regions different from each other, from the viewpoint of compatibility with a CD-R system currently used.

For example, an optical recording medium is proposed which uses a mixture of a dye used in CD-Rs and a dye used in DVD-Rs whereby information can be recorded and reproduced by both an infrared laser light having a wavelength close to 780 nm and a visible laser light having a wavelength close to 650 nm (see, for example, JP-ANos. 2000-141900, 2000-158816, 2000-185471, 2000-289342 and 2000-309165).

However, the studies made by the inventors of the present invention clarified that the optical recording media described in the above publications fail to obtain the sensitivity required for practical use or do not reach satisfactory levels in other characteristics such as reflectance and the degree of modulation when recording information by irradiating the medium with laser light having a wavelength shorter than 600 nm or less and particularly 450 nm or less, and there is a necessity for further improvement in such media. Particularly, it was confirmed that the optical recording media described in the above publications exhibited deteriorated recording characteristics when they were irradiated with laser light having a wavelength of 450 nm or less.

In optical recording media that have been developed so far, light is generally incident on the substrate side. However, a short-wavelength laser and a pickup using an object lens having a high numerical aperture (NA) have been put to use to attain high densification.

An example of such media is a DVR-blue ("ODS 2001 Technical Digest" pp.139-141) comprising a combination of a bluish violet laser having a wavelength of about 405 nm and an object lens having an NA of 0.85. In an optical recording medium using such a short wavelength laser and a high NA optical system, the influence of coma aberration caused by waipage of the disk is increased to so great an extent that it cannot be ignored.

In order to eliminate the influence of coma aberration, laser light for recording and reproduction is incident through a cover layer having a thickness of about 0.1 mm which is much thinner than that of a conventional substrate (thickness: for example, CD: 1.2 mm, DVD: 0.6 mm) to thereby improve defective recording or reproduction.

As the structure of the above optical recording medium, a medium structure constituted of cover layer / (transparent adhesive layer) / dielectric layer / recording layer / dielectric layer / reflecting layer / substrate in this order from the laser light incident side is adopted (there is a case where the above cover layer and the above transparent adhesive layer are integrated using an ultraviolet ray-curable resin by spin coating) in the case of a rewritable phase-change type optical recording medium.

Here, in the case of a DVD-blue, the total thickness of the cover layer and the transparent adhesive layer is about 0.1 mm and the thickness of the substrate is about 1.1 mm.

On the other hand, a dye type additional storage type optical recording medium generally has the following medium structure in order from the laser light incident side of: substrate / recording layer / reflecting layer / protective layer in the case of CD-Rs and substrate / recording layer / reflecting layer / (protective layer) / adhesive layer / (protective layer) / (reflecting layer) / substrate in the case of DVD-Rs. In the case of DVD-Rs, there is a case where the above layers in parenthesis are omitted.

When it is intended to apply this structure to an optical recording medium having the above cover layer, the following structure is considered: cover layer / (transparent adhesive layer) / transparent block layer / recording layer / reflecting layer / substrate.

Here, the transparent block layer is a transparent thin film layer formed by sputtering to prevent the occurrence of the phenomenon whereby when a transparent adhesive layer made of, for example, an ultraviolet ray-curable resin is applied to a dye layer which is a recording layer, a dye contained in the dye layer is eluted whereby the dye layer is damaged. As the transparent block layer, a dielectric layer made of, for example, ZnS is used.

However, the formation of the dielectric layer incurs film forming costs and offers the possibility of the dye layer being damaged. There is also the problem that the formation of a new interface offers the possibility of recording abilities being impaired.

As other means, there is a method in which the transparent cover layer is applied using a pressure sensitive double coated tape (including a case where no support is present). However, this method has the problem that it is costly and the optical characteristics are impaired by the tackifier layer.

Also, an air sandwich type optical recording medium (generally, a write-once type having an outside diameter of 130 mm) is proposed and used, the optical recording medium being produced by applying two substrates to each other via a spacer or projections disposed at the outside and inside peripheries such that each recording layer formed on the disk substrate faces inside. This medium is a type in which a dye type or metal type additional storage recording layer is decomposed or perforated by focused laser light to record information. Because, basically, the recording layer is an open system and therefore, the decomposition of the dye and the perforation are easily attained, the medium has the advantage that a desired degree of modulation is easily obtained. On the contrary, there are problems concerning preservability due to, for example, oxidation or corrosion of the recording layer.

To sum up the foregoing, when the recording medium is made to have a structure in which a cover layer is formed, the layer is formed on every disk by spin coating or sputtering, giving rise to problems such as reduced productivity, damage to the dye layer, reduction in optical properties, low degree of modulation and an increase in difference between inside and outside signal levels. Also, the above air sandwich type poses the problem that it has low reliability.

Also, optical information recording mediums which allow an image to be formed by laser light in JP-ANo. 2000-113516, 2001-283464 and 2000-173096.

DISCLOSUREOFTHEINVENTION

PROBLEMTOBESOLVEDBYTHEINVENTION

In view of the above situation, it is an object of the invention to provide an optical recording medium which is reduced in difference between inside and outside signal levels and is superior in productivity while retaining high degree of modulation, high density and high reliability, an optical recording method and an optical reproduction method.

MEANS FOR SOLVING PROBLEM The above problem is solved by the invention shown below. The invention relates to:

< 1 > An optical recording medium comprising a substrate, an in-groove and an on-groove, and further, a recording layer and a cover layer formed in this order on at least one surface of the substrate, wherein the recording layer is formed plane-wise and has an almost uniform thickness, and the in-groove is an optical recording section.

<C2^> The optical recording medium of the above <C 1 !>, wherein the recording layer contains a dye as the recording material and the dye is selected from a cyanine dye, an oxonol dye, an azo dye, a phthalocyanine dye, a benzotriazole dye, and a triazine dye.

<C3^> The optical recording medium of the above <C 1^>, wherein the following relationship is established between the average thickness Ta of the recording layer, and the maximum thickness Tmax and minimum thickness Tmin of the recording layer within the disk.

Ta < Tmax < 1.05Ta

0.95Ta < Tmin < Ta

<4> The optical recording medium of the above <1>, wherein the material of the substrate is a material selected from a polycarbonate and an amorphous polyolefin.

<C5^> The optical recording medium of the above O^>, wherein the depth of the in-groove (groove depth) is 20 to 150 nm.

<C6^> The optical recording medium of the above <Q!>, wherein the material of the cover layer is a material selected from a polycarbonate, a cellulose triacetate and an amorphous polyolefin.

<C7^> The optical recording medium of the above <C 1 ^>, wherein the recording layer has a layer thickness ranging from 20 to 500 nm. <8> The optical recording medium of the above < 1 >, the recording medium further comprising an adhesive layer between the recording layer and the substrate.

<3!> The optical recording medium of the above <C l^>, wherein the cover layer has a thickness of 30 to 130 μm and the substrate has a thickness of 0.3 to 1.2 mm.

<Cl0^> The optical recording medium of the above <O!>, wherein the in-groove, the on-groove and the recording layer form a clearance among them, the clearance having any one of a vacuum atmosphere, an atmosphere under reduced pressure, a nitrogen atmosphere under reduced pressure and an inert gas atmosphere under reduced pressure.

< 11 > The optical recording medium of the above < 1 > , the recording medium further comprising a reflecting layer formed between the substrate and the recording layer, wherein a clearance is formed between the reflecting layer and the recording layer.

<C12^> The optical recording medium of the above <C 11 ^>, wherein the clearance formed between the reflecting layer and the recording layer has any one of a vacuum atmosphere, an atmosphere under reduced pressure, a nitrogen atmosphere under reduced pressure and an inert gas atmosphere under reduced pressure.

< 13 > The optical recording medium of the above < 11 > , wherein the material of the reflecting layer is selected from Cr, Ni, Pt, Cu, Ag, Au, Al and stainless steel.

<Cl4]> The optical recording medium of the above <C ll^>, wherein the reflecting layer has a layer thickness of 10 to 300 nm.

<C15> An optical recording method comprising: applying laser light having a wavelength of 450 nm or less from the cover layer side of the optical recording medium according to the above <Cl>; converging the light through the object lens having a numerical aperture of 0.7 or more; and applying the converged light to store information in an optical recording section.

<16> An optical reproducing method comprising: applying laser light having a wavelength of 450 nm or less from the cover layer side of the optical recording medium according to the above <U1U>; converging the light through the object lens having a numerical aperture of 0.7 or more; applying the converged light; and detecting the reflected light to reproduce the information stored in an optical recording section.

EFFECT OF THE INVENTION

The invention can solve the above prior art problem and provide a high density, highly reliable and inexpensive optical recording medium and an optical reproduction method.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a partial sectional view showing an example of the layer structure of an optical recording medium according to the invention.

Fig. 2 is a partial sectional view showing another example of the layer structure of an optical recording medium according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION <C Optical recording medium of the invention^

Fig. 1 shows a partial sectional view showing an example of the layer structure of an optical recording medium according to the invention.

As shown in Fig. 1, the recording medium has a structure in which an on-groove Ia and an in-groove Ib are formed on one surface of a substrate I₃ and a recording layer 3 and a cover layer 4 are formed in this order on the side on which the on-groove Ia and the in-groove Ib are formed.

Here, the on-groove 1 a means portions forming convex parts as viewed from the direction in which a laser light 12 is applied to the cover layer 4 through an object lens 11. On the other hand, the in-groove Ib means portions forming concave parts as viewed in the same direction as above.

The thickness of the recording layer 3 is almost uniform. This makes it possible to reduce a difference between inside and outside signal levels and a difference in signal levels within the circumference, which makes possible to widen a margin for signal errors and to increase reliability.

Also, by making the thickness almost uniform, the productivity can be improved.

Here, the above term "almost uniform" means that when the average thickness is Ta, the thickness (Tmax) in the maximum thickness part in one disk falls in the range satisfying the following equation: "Ta < Tmax < 1.05Ta (preferably Ta < Tmax < 1.03Ta)", and the thickness (Tmin) in the minimum thickness part in one disk falls in the range satisfying the following equation: "0.95Ta < Tmin < Ta (preferably 0.97Ta < Tmin < Ta)". When the thickness falls in the above ranges, a difference between the inside and outside signal levels can be decreased.

Also, the optical recording medium of the invention may have a structure in which a reflecting layer 2 is formed between the substrate 1 and the recording layer 3 as shown in Fig. 2. The performance such as tracking can be improved by forming the reflecting layer 2.

The layers or members represented by the same symbols in Figs. 1 and 2 respectively mean materials having the same function and therefore, their explanations will be omitted.

In the optical recording medium of the invention which has the above structure, the in-groove Ib is an optical recording portion. Specifically, the optical recording medium has a structure in which information is recorded in the in-groove Ib.

Also, a clearance Ic (in the case of Fig. 2, a clearance formed by the reflecting 2 and the recording 3) formed by the in-groove Ib₅ the on-groove Ia and the recording layer 3 has any one of a vacuum atmosphere, an atmosphere under reduced pressure, a nitrogen atmosphere under reduced pressure and an inert gas atmosphere under reduced pressure. If the clearance has any one of the above atmospheres, decomposed gas which is derived from a dye and generated when recording information is easily released and therefore, the medium can be highly sensitized. Such an effect can be expected also in other recording systems associated with a change in form such as a hole forming type.

In order to make the clearance Ic have any one of the above atmospheres, a method may be adopted in which the clearance Ic is put in any one of the above atmospheres when the substrate 1 on which the desired reflecting layer 2 is formed is applied to the cover film corresponding to the cover layer 4 on which the recording layer 3 is formed by using an adhesive or the like. The atmosphere of the clearance Ic can be confirmed by mass-spectroscopic analysis in a vacuum circumstance.

The optical recording medium of the invention can be manufactured by applying a substrate provided with an in-groove and an on-groove formed on at least one surface thereof and with an optional reflecting layer thereon to a cover film to be a cover layer on which a recording layer is formed by using an adhesive or the like such that the surface on which the in-groove and the on-groove are foπned and the surface on which the recording layer is formed respectively face inside. The method of producing an optical recording medium will be explained by explaining the substrate and each layer of the optical recording medium of the invention.

(Substrate on which an optional reflecting layer is formed)

As the substrate, a proper material selected arbitrarily from various materials used as substrate materials for conventional optical recording mediums may be used.

Specific examples of the substrate material include glass; polycarbonates, acryl resins such as polymethylmethacrylate; vinyl chloride resins such as polyvinyl chloride and vinyl chloride copolymers; epoxy resins; amorphous polyolefms; polyesters; and metals such as aluminum. These materials may be combined as desired.

Among the above materials, polycarbonate and amorphous polyolefin are preferable and polycarbonate is particularly preferable from the viewpoint of moisture resistance, dimensional stability and low costs. Also, the thickness of the substrate is preferably 0.3 to 1.2 mm in consideration of retention of strength, prevention of waipage of a disk and handling characteristics.

Grooves (hereinafter, the term "groove" means an "on-groove" and an "in-groove", as the case may be) that are tracking guide channels or express information such as address signals are formed on the substrate as mentioned above. It is preferable to use a substrate on which grooves having a narrower track pitch than those of CDs or DVD-Rs are formed to achieve higher recording density. The track pitch of the grooves is designed to be preferably in a range from 200 to 400 nm and more preferably in a range from 250 to 350 nm. Also, the depth (groove depth) is designed to be preferably in a range from 20 to 150 nm and more preferably in a range from 25 to 80 nm.

A light reflective material having high reflectance to laser light is used in the reflecting layer that is arbitrarily formed on the surface of the substrate on which surface the groove is formed. The reflectance of this light reflective material is preferably 70% or more.

Examples of the light reflective material having high reflectance may include metals or semimetals such as Mg, Se, Y, Ti₅ Zr, Hf, V, Nb, Ta, Cr, Mo₃ W, Mn, Re, Fe, Co₅ Ni, Ru, Rh, Pd, Ir, Pt, Cu₅ Ag, Au, Zn, Cd, Al, Ga, In, Si, Ge, Te, Pb, Po, Sn and Bi and stainless steel. These light reflective materials may be used either singly or in combinations of two or more or as alloys. Among these materials, Cr, Ni, Pt, Cu, Ag, Au, Al and stainless steel are preferable. Au₅ Ag₅ Al or alloys of these metals are particularly preferable and Au, Ag or alloys of these metals are most preferable.

The reflecting layer may be formed on the substrate by applying the above light reflecting material by vapor deposition, sputtering or ion plating. The layer thickness of the reflecting layer is usually in a range of 10 to 300 nm and preferably in a range of 50 to 120 nm.

The substrate on which the arbitrary reflecting layer is formed is manufactured as mentioned above.

(Cover film)

Though any material may be used as the cover film which is to be the cover layer insofar as it is a transparent material, the material used for the cover film is preferably a material having a moisture absorbance of 5% or less at 23⁰C under 50% RH. It is more preferable to use polycarbonate, cellulose triacetate or amorphous polyolefin.

Here, the term "transparent" means that a material is so transparent that it transmits recording light and reproducing light (transmittance: 85% or more).

The thickness of the cover layer is preferably in a range of 30 to 130 μm and more preferably in a range of 50 to HOμm from the viewpoint of resistance to adhesion of dusts and to damages and a reduction in coma aberration.

The recording layer permits the recording of information by using at least laser light having a wavelength of 450 nrn or less and contains a dye as a recording material. Examples of the dye to be contained in the recording layer include at least one of cyanine dyes, oxonol dyes, azo dyes, phthalocyanine dyes, benzotriazole dyes and triazine dyes. Among these dyes, phthalocyanine dyes are preferable.

The dyes described in JP-ANos. 4-74690, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513 and 2000-158818 are preferably used as the recording material.

The recording layer is formed in the following manner: recording materials such as dyes are dissolved in a proper solvent to prepare a coating solution, then the coating solution is applied to the above cover film to form a coating film and, as required, the coating film is dried.

As the method of applying the above coating solution, it is preferable to take measures such as web coating on the cover film and continuous vapor deposition. These web coating and continuous vapor deposition make possible to form a recording layer having an almost uniform thickness efficiently. Also, the recording layer can be formed continuously by these measures and therefore, these measures are superior in mass-productivity and can reduce the production of defectives whereby productivity can be improved as compared with a method of forming a recording layer every medium by spin coating or dip coating.

The concentration of the recording material in the coating solution is generally in a range of 0.01 to 15% by mass, preferably in a range of 0.1 to 10% by mass, more preferably in a range of 0.5 to 5% by mass and most preferably in a range of 0.5 to 3% by mass.

Examples of the solvent used in the coating solution may include esters such as butyl acetate, ethyl lactate and cellosolve acetate; ketones such as methyl ethyl ketone, cyclohexanone and methyl isobutyl ketone; chlorinated hydrocarbons such as dichloromethane, 1,2-dichloroethane and chloroform; amides such as dimethylformamide; hydrocarbons such as methylcyclohexane; ethers such as tetrahydrofuran, ethyl ether and dioxane; alcohols such as ethanol, n-propanol, isopropanol, n-butanol and diacetone alcohol; fluorine type solvents such as 2,2,3, 3-tetrafluoropropanol; and glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether and propylene glycol monomethyl ether.

The above solvents may be used either singly or in combinations of two or more taking the solubility of the recording material into account. Various additives such as an antioxidant, ultraviolet absorber, plasticizer and lubricant may be further added to the coating solution according to the purpose.

In the case of using a binder, examples of the binder may include natural organic high-molecular materials such as gelatin, cellulose derivatives, dextrin, rosin and rubber; and synthetic organic high-molecular materials, for example, hydrocarbon type resins such as polyethylene, polypropylene, polystyrene and polyisobutylene, vinyl type resins such as polyvinyl chloride, polyvinylidene chloride and polyvinyl chloride/polyvinyl acetate copolymers, acryl resins such as polymethylacrylate and polymethylmethacrylate and initial condensates of thermosetting resins such as polyvinyl alcohol, polyethylene chloride, epoxy resins, butyral resins, rubber derivatives and phenol-formaldehyde resins. When the binder is used as the material of the recording layer, the amount of the binder to be used is generally in a range of 0.01 to 50 equivalents (mass ratio) and preferably in a range of 0.1 to 5 equivalents (mass ratio) to the recording material. The concentration of the recording material in the coating solution prepared in this manner is generally in a range of 0.01 to 10% by mass and preferably in a range of 0.1 to 5% by mass.

The layer thickness of the recording layer is generally in a range of 20 to 500 nm, preferably in a range of 30 to 300 nm and more preferably in a range of 25 to 80 nm.

It is preferable to compound various fading preventives to improve the light fastness of the recording layer.

As the fading preventive, a singlet oxygen quencher is usually used. As the singlet oxygen quencher, those described in already known publications such as patent specifications may be utilized.

Specific examples of the fading preventive may include fading preventives as described in JP-ANos. 58-175693, 59-81194, 60-18387, 60-19586, 60-19587, 60-35054, 60-36190, 60-36191, 60-44554, 60-44555₃ 60-44389, 60-44390, 60-54892, 60-47069, 63-209995 and 4-25492, Japanese Patent Publication (JP-B) Nos. 1-38680 and 6-26028, the specification of DT Patent No. 350399 and J. Japan Chemical Society, the October issue, (1992), pp. 1141.

The amount of the fading preventive such as a singlet quencher to be used is generally in a range of 0.1 to 50% by mass, preferably of 0.5 to 45% by mass, more preferably of 3 to 40% by mass and particularly preferably of 5 to 25% by mass based on the amount of the dye.

A cover film on which the recording layer is formed is manufactured in the manner as above.

It is preferable to apply the substrate on which the aforementioned optional reflecting layer is formed to the cover film on which the recording layer is formed in such a manner as will be explained below.

Although no particular limitation is imposed on the method of applying the above cover film to the substrate, the following methods (1) to .(3) may be adopted.

(I) A first method is one in which, for example, the innermost peripheral part and the outermost peripheral part are mechanically bound without providing an adhesive layer as shown in Figs. 1 and 2.

(2) A second method is one in which an adhesive (including a tackifier in this specification) is applied to the recording layer of the cover layer side to form an adhesive layer and this adhesive layer is stuck (applied) to the substrate side on which the optional reflecting layer is formed or the both are applied through a tackifier sheet used in place of the adhesive layer.

(3) A third method is one in which a dielectric layer (also called a transparent block layer or a barrier layer) is formed on the recording layer of the cover layer side (or on the adhesive layer in the case where the adhesive layer is formed) in the second method to apply the both.

As the dielectric layer which is a transparent block layer, materials such as an oxide, nitride, carbide or sulfide made of any one or more of Zn, Si, Ti, Te, Sm, Mo and Ge may be used wherein these materials may be hybridized like ZnS-SiO₂. The material of the transparent block layer is not particularly limited to a dielectric material but any material may be used as the material of the transparent block layer insofar as it has a transmittance of 90% or more for laser wavelength. The thickness of the transparent block layer is preferably 1 to 100 nm and more preferably 1 to 10 nm.

The application method will be hereinafter explained in detail.

First, an adhesive is dissolved in a proper solvent to prepare a coating solution. The prepared coating solution is applied to the recording layer of the cover film formed by, for example, web coating. As the coating method, a method such as curtain coating or spray coating is preferably applied.

Thereafter, the surface of the substrate on which surface the groove is formed or the surface of the substrate on which surface of the reflecting layer is formed is applied to the coated surface. Then, the cover film is irradiated with light from above or heated to cure the adhesive, thereby making the substrate adhere to the cover film firmly. The application of light for curing may be earned out by applying light from the substrate side when, particularly, the reflecting layer is not present. Also, it is preferable to apply the cover film to the substrate after the coated cover film is cut into almost the same size as the substrate.

As the circumstance under which the substrate is applied to the cover film, various circumstances including an atmospheric pressure circumstance may be adopted. However, any of a vacuum atmosphere, atmosphere under reduced pressure, nitrogen atmosphere under reduced pressure or inert gas atmosphere under reduced pressure may be preferably adopted.

Here, the reduced pressure means pressure less than the atmospheric pressure and is preferably about 0.3 to 10^'5 Torr (39.9 Pa to 1.33 X 10^'3 Pa).

The optical recording medium of the invention is manufactured in the above manner.

The production method like this makes it possible to carry out the applying operation continuously, and therefore the productivity can be improved. Also, it is possible to apply the cover film such that it is stuck firmly to the substrate, whereby the material cost can be reduced to attain cost reduction.

The above production methods are examples and various known processes and various modifications made within the scope which can be practiced by a person skilled in the art may be introduced.

As the material constituting the adhesive, a pressure sensitive adhesive (tackifier) is preferably used though various materials such as photocurable resins may be used. As the tackifier, a tackifier made of an acrylate type resin may be used. An isocyanate hardener is preferably added to the tackifier.

Also, when a photocurable resin is used as the adhesive, it preferably has a small hardening shrinkage factor to prevent warpage of the disk. As this photocurable resin, an ultraviolet ray-curable resin is preferable. Specific examples of the ultraviolet ray-curable resin may include ultraviolet ray-curable resins (ultraviolet ray-curable adhesives) such as "SD-640", "SD-347" and "SD-318" manufactured by Dainippon Ink and Chemicals, Incoiporate. Also, the thickness of the adhesive layer is preferably in a range of 1 to 1,000 nm, more preferably in a range of 2 to 100 nm and particularly preferably in a range of 3 to 50 nm to exhibit elasticity.

The application temperature is preferably in a range of 23 to 50⁰C, more preferably in a range of 24 to 40⁰C and still more preferably in a range of 25 to 37°C.

The coating film is preferably irradiated with ultraviolet rays by using a pulse type illuminator (preferably an ultraviolet illuminator) to prevent warpage of a disk. The pulse interval is preferably lower than the order of msec and more preferably lower than the order of μ sec. The amount of light of one pulse to be applied is preferably 3 kW/cm or less and more preferably 2 kW/cm or less though no particular limitation is imposed on it.

The number of pulses is preferably 20 or less and more preferably 10 or less though no particular limitation is imposed on it.

Next, the following explanations are furnished as to a method of optically recording information in the optical recording medium of the invention and a method of optically reproducing stored information.

(Optical recording method)

The optical recording method of the invention comprises applying laser light having a wavelength of 450 nm or less from the cover layer side of the foregoing optical recording medium of the invention, converging the light through the object lens having a numerical aperture of 0.7 or more and applying the converged light to record information in an optical recording section. Specifically, information is recorded in the following manner.

First, the optical recording medium of the invention is irradiated with laser light for recording from the cover layer side while, for example rotating the recording medium at a constant linear speed. By irradiation with laser light, the recording layer absorbs the light, which raises the recording layer temperature locally, causing physical or chemical changes in the local part of the recording layer, thereby changing the optical characteristics of the local part of the recording layer to record information. The information is recorded by tracking the in-groove.

Any laser light source may be used as the laser light for recording insofar as it oscillates and emits laser light having a wavelength of 450 nm or less. Examples of the laser light source may include a bluish violet semiconductor laser having an oscillation wavelength range from 390 to 415 nm and a bluish violet SHG laser having a center oscillation wavelength of 425 nm.

Also, the NA of the object lens used for the pickup is designed to be 0.7 or more and preferably 0.85 or more to obtain high recording density.

(Optical reproducing method)

In this method, laser light having a wavelength of 450 nm or less is applied from the cover layer side of the optical recording medium of the invention to converge the light through the object lens having a numerical aperture of 0.7 or more, and the converged light is applied to detect the reflected light to reproduce the information stored in the optical recording section.

Specifically, the optical recording medium is irradiated with laser light from the cover layer side while rotating the recording medium at a constant linear speed in the same manner as in the aforementioned method of recording information, to detect the reflecting light, whereby the information can be reproduced. EXAMPLES

The invention will be explained hereinbelow in detail by way of examples, which, however, are not intended to limit the invention.

(Example 1)

Ag was deposited by sputtering on the surface of a 1.1-mm-thick and 120-mm-diameter injection-molded polycarbonate resin (trade name: PANLITE AD5503, polycarbonate manufactured by Teijin Limited.) on which surface grooves constituted of a spiral in-groove and on-groove (depth of the in-groove: 40 nm, width of the in-groove: 120 nm, track pitch: 320 nm) were formed, to form a reflecting layer 100 nm in thickness.

12 g of a dye A (trade name: ORAZOL GN, manufactured by Ciba Specialty Chemicals Inc., maximum absorption: 340 nm, 640 nm) which was a phthalocyanine type organic compound was mixed in 1 liter of 2,2,3, 3-tetrafluoropropanol and the mixture was subjected to ultrasonic treatment for two hours to dissolve, thereby preparing a coating solution for forming a recording layer.

This coating solution was applied using a web to a polycarbonate sheet (trade name: PURE-ACE, manufactured by Teijin Limited, thickness: 0.07 mm) as a cover film and dried to manufacture a cover film on which a recording layer is formed. The drying was carried out during web-conveying in an atmosphere of30^oC and 45% RH.

The manufactured cover film was punched by a disk puncher provided with a punching knife at each of the center hole part and outside part (hereinafter, the punched cover film referred to as "cooky"). The cover layer was applied to the substrate such that the recording layer of the cooky faces the reflecting layer of the substrate under an atmosphere of a vacuum of 3 X 10^"3 Torr by means of vacuum contact to manufacture an optical recording medium.

When the both were applied to each other, an ultraviolet ray-curable resin (trade name: SD-318, manufactured by Dainippon Ink and Chemicals, Incorporate.) was applied in advance to the inside peripheral part and outside peripheral part of the cooky and irradiated with ultraviolet rays. The inside of the system when the both were applied was a nitrogen atmosphere.

When the produced optical recording medium was cut, it was confirmed that a clearance part (corresponds to 1 c in Fig. 2) was formed between the reflecting layer and the recording layer and also that the recording layer was formed plane-wise. Also, when the recording medium was analyzed by mass spectroscopy in a vacuum system, nitrogen gas that was the same as the atmosphere when the both were applied and was released from the clearance part was detected.

(Comparative Example 1)

A reflecting layer was formed in the same manner as in Example 1.

Then, 12 g of a phthalocyanine type dye (trade name: ORAZOL BLUE GN, manufactured by Ciba Specialty Chemicals Inc.), was mixed in 1 liter of 2,2,3 ,3-tetrafluoropropanol and the mixture was subjected to ultrasonic treatment for 2 hours to dissolve, thereby preparing a coating solution. This prepared coating solution was applied to the above reflecting layer by a spin coating method at a rotation changed between 300 and 4000 rpm in the condition of 23⁰C and 50% RH. Thereafter, the substrate was stored at 23⁰C under 50% RH for one hour to form a recording layer.

SiO₂ was deposited on the recording layer by a RF sputtering method to form a transparent block layer 10 nm in thickness.

An ultraviolet ray-curable adhesive (trade name: SD-318, manufactured by Dainippon Ink and Chemicals, Incorporate.) was applied to a transparent block layer by a spin coating method at a rotation of 100 to 300 rpm. The adhesive layer was overlapped with a polycarbonate sheet (trade name: PURE-ACE₅ manufactured by Teijin Limited, thickness: 0.07 mm) as a cover film and then, the rotation was changed between 300 and 4,000 rpm to spread the ultraviolet ray-curable resin on the entire surface. Then, the ultraviolet ray-curable resin layer was irradiated with ultraviolet rays from an ultraviolet ray radiation lamp from the cover film side to cure the resin, thereby manufacturing an optical recording medium.

The optical recording mediums manufactured in Example 1 and Comparative Example 1 were subjected to a confocal microscope type thickness meter to measure the average thickness of the recording layer, maximum thickness and minimum thickness. The results are shown in Table 1 below.

[Table 1]

It is confirmed from Table 1 that with regard to the optical recording medium of Example I₅ the maximum thickness and minimum thickness of the recording layer respectively fall in a range of "average thickness ± 3% of the average thickness" and therefore, a uniform layer is formed.

With regard to the optical recording mediums manufactured in Example 1 and Comparative example 1, 3T signals were recorded using DDU-1000 (manufactured by Pulsetech) which oscillated and emitted 405 nm laser light and a reproducing test was made using 405 nm laser light to measure the internal-external difference of signal levels and the degree of modulation, and as a result, the optical recording medium of Example 1 was reduced in a difference between inside and outside signal levels and a difference in signal levels within the circumference in respect to recording sensitivity, reflectance, degree of modulation, jitter and asymmetry, to show that the optical recording medium of Example 1 was superior to the optical recording medium of Comparative Example 1 in recording and reproducing characteristics.

INDUSTRIAL APPLIC ABIITY

The optical recording medium of the invention can be applied to an optical recording method and optical reproducing method using laser light.

Claims

1. Ail optical recording medium comprising a substrate, and an in-groove and an on-groove, and further, a recording layer and a cover layer formed in this order on at least one surface of the substrate, wherein the recording layer is formed plane-wise and has an almost uniform thickness, and the in-groove is an optical recording section.

2. The optical recording medium of Claim 1, wherein the recording layer contains a dye as a recording material and the dye is selected from a cyanine dye, an oxonol dye, an azo dye, a phthalocyanine dye, a benzotriazole dye, and a triazine dye.

3. The optical recording medium of Claim 1, wherein the following relationship is established between the average thickness Ta of the recording layer, and the maximum thickness Tmax and minimum thickness Tmin of the recording layer within the disk.

Ta < Tmax < 1.05Ta 0.95Ta < Tmin < Ta

4. The optical recording medium of Claim 1, wherein the material of the substrate is a material selected from a polycarbonate and an amorphous polyolefin.

5. The optical recording medium of Claim 1, wherein the depth of the in-groove (groove depth) is 20 to 150 nm.

6. The optical recording medium of Claim 1, wherein the material of the cover layer is a material selected from a polycarbonate, a cellulose triacetate and an amorphous polyolefin.

7. The optical recording medium of Claim 1, wherein the recording layer has a layer thickness ranging from 20 to 500 nm.

8. The optical recording medium of Claim 1, further comprising an adhesive layer between the recording layer and the substrate.

9. The optical recording medium of Claim 1, wherein the cover layer has a thickness of 30 to 130 μm and the substrate has a thickness of 0.3 to 1.2 mm.

10. The optical recording medium of Claim I₅ wherein a clearance is formed between the in-groove, the on-groove and the recording layer form a clearance among them, the clearance having any one of a vacuum atmosphere, an atmosphere under reduced pressure, a nitrogen atmosphere under reduced pressure and an inert gas atmosphere under reduced pressure.

11. The optical recording medium of Claim I₅ the recording medium further comprising a reflecting layer formed between the substrate and the recording layer, wherein a clearance is formed between the reflecting layer and the recording layer.

12. The optical recording medium of Claim 11, wherein the clearance formed between the reflecting layer and the recording layer has any one of a vacuum atmosphere, an atmosphere under reduced pressure, a nitrogen atmosphere under reduced pressure and an inert gas atmosphere under reduced pressure.

13. The optical recording medium of Claim 11, wherein the material of the reflecting layer is selected from Cr, Ni, Pt, Cu, Ag, Au, Al and stainless steel.

14. The optical recording medium of Claim 11, wherein the reflecting layer has a layer thickness of 10 to 300 nm.

15. An optical recording method comprising: applying laser light having a wavelength of 450 nm or less from the cover layer side of the optical recording medium as claimed in Claim 1; converging the light through an object lens having a numerical aperture of 0.7 or more; and applying the converged light to record information in the optical recording section.

16. An optical reproducing method comprising: applying laser light having a wavelength of 450 nm or less from the cover layer side of the optical recording medium as claimed in Claim 1; converging the light through an object lens having a numerical aperture of 0.7 or more; applying the converged light; and detecting reflected light to reproduce information stored in the optical recording section.