JP2006260699A - Information recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an information recording medium having high reliability of recording data and excellent durability to repetitive recording of data in the information recording medium using a Bi-Ge-Te based phase transition material as a recording layer. <P>SOLUTION: The information recording medium capable of rewriting information plural times by irradiation with a laser beam on a condition of 76.0 nsec≤(λ/NA)/V≤115.0 nsec (λ=390 to 420 nm) when the wavelength of the laser beam, the numerical aperture of an objective lens for condensing the laser beam and a recording linear speed are defined as λ(nm), NA and V(m/sec), respectively, is characterized in that compositions of Bi, Ge and Te in the recording layer are in a compositional range enclosed by compositional points B4, B2, C3, D5, D2 and C4 in a triangular composition diagram of Bi, Ge and Te. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an information recording medium on which information is recorded by irradiation with an energy beam, and more particularly to a phase change optical disc compatible with a blue laser such as HD DVD.

  In recent years, the market for read-only optical disks such as DVD-ROM and DVD-Video has been expanded. Following this, the market for rewritable DVDs such as DVD-RAM, DVD-RW, DVD + RW, etc., is rapidly expanding as a computer backup medium and a video recording medium replacing VTR. With the expansion of this market, in recent years, there has been an increasing demand for improvement in transfer rate and access speed and increase in capacity for recordable DVDs.

  In recordable erasable DVDs such as DVD-RAM and DVD-RW, a phase change recording method using a phase change material for a recording layer on which information is recorded is employed. In the phase change recording method, information of “0” and “1” is basically recorded corresponding to the crystalline state and the amorphous state of the phase change material, respectively. In addition, since the refractive index of the phase change material is different between the crystalline state and the amorphous state, each layer constituting the recording type DVD is configured so that the difference in reflectance between the portion changed into the crystal and the portion changed into the amorphous is maximized. The refractive index, film thickness, etc. are designed. The crystallized portion and the amorphous portion are irradiated with laser light, and the difference in the amount of reflected light from each portion of the optical disk is detected to detect information “0” and “1” recorded in the recording layer.

  In addition, in order to make the predetermined position amorphous (usually, this operation is called “recording”), a relatively high power laser beam is irradiated and the temperature of the recording layer becomes equal to or higher than the melting point of the recording layer material. To heat. On the other hand, in order to crystallize a predetermined position (usually, this operation is called “erasing”), a crystal having a temperature of the recording layer equal to or lower than the melting point of the recording layer material is irradiated with a relatively low power laser beam. Heat to a temperature near the conversion temperature. In this way, by adjusting the power of the laser light applied to the predetermined portion of the recording layer, the state of the predetermined portion can be reversibly changed between the amorphous state and the crystalline state.

  As a method for improving the transfer rate in the recordable DVD as described above, a method of increasing the number of rotations of the medium and performing recording / erasing in a short time is general. However, in this case, the recording / erasing characteristic when information is overwritten on the medium becomes a problem. This problem will be described in detail below.

  Consider a case where the predetermined position of the medium is changed from amorphous to crystalline. Increasing the rotation speed of the medium shortens the time for the laser beam to pass through a predetermined position of the medium, and at the same time shortens the time for which the predetermined position is held at the crystallization temperature. If the time for which the temperature is maintained at the crystallization temperature is too short, the crystal cannot be sufficiently grown, so that amorphous remains. This remaining amorphous state is reflected in the reproduction signal, and the reproduction signal quality deteriorates.

  As a method for solving the above problem, a method of adding Sn to a Ge—Sb—Te phase change recording material generally used for a recording layer of a recordable DVD is conventionally known. Other than that, for example, in Japanese Patent Application Laid-Open No. 2001-322357, by using a material in which a metal such as Ag, Al, Cr, or Mn is added to a Ge—Sn—Sb—Te-based material as a recording layer material. It is disclosed that an information recording medium capable of high-density recording, excellent in repeated rewriting performance, and little deterioration in crystallization sensitivity with time can be obtained. JP-A-2-14289 also discloses a Ge—Sb—Sn—Te recording layer material.

  Conventionally, a practical composition range has been proposed using a Bi—Ge—Te phase change material as a recording layer material (see, for example, Patent Document 1). Furthermore, a practical composition range of a Bi-Ge-Te phase change material that can cope with 2 × speed and 5 × speed with a DVD-RAM has been proposed (see, for example, Patent Document 2).

  JP-A-62-273439 and JP-A-1-220236 disclose Bi-Ge-Se-Te phase change recording materials, and JP-A-1-287836 discloses Bi. A practical range of the -Ge-Sb-Te phase change recording material is defined.

  Furthermore, PCOS2001 reports a Ge—Sn—Sb—Te-based material as a recording material that can cope with the double to quadruple speed of DVD-RAM. In ISO / ODS2002, an information recording medium capable of supporting 2 × and 5 × speeds of DVD-RAM has been reported, and this 5 × speed medium can be increased to 5 × speed by adding a nucleation layer to an 8-layer structure. Is possible.

  In addition, as a technology for increasing the capacity of a recordable DVD, the laser spot diameter is reduced by shortening the wavelength of the laser beam to 405 nm and increasing the objective lens NA to 0.85, so that information with higher density can be obtained. Is well known (Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 756-761, Part 1, No. 2B, Feb. 2000). This method is used as a main technology of the so-called Blu-ray Disc, and the influence on the disc tilt is reduced by adopting a substrate having a thickness of 0.1 mm thinner than that of a conventional DVD. The 0.1 mm thick substrate plays important roles such as mechanical protection of the recording layer and electrochemical protection (corrosion prevention).

  The basic structure of a rewritable optical disk such as a conventional DVD-RAM, DVD-RW, etc. has a first dielectric layer, a phase change recording layer, a second dielectric layer, and a 0.6 mm thick polycarbonate (PC) substrate. It has a four-layer structure in which reflective layers are sequentially laminated, and is manufactured by bonding a substrate having a thickness of 0.6 mm from the reflective layer side. However, in the above-described Blu-ray Disc, when the laminated structure similar to that of the conventional optical disc is used, it is difficult to maintain the rigidity of the substrate because the substrate is as thin as 0.1 mm. Therefore, in Blu-ray Disc, a reflective layer, a second dielectric layer, a phase change recording layer, and a first dielectric layer are sequentially stacked on a thick substrate, for example, a 1.1 mm thick PC substrate (conventional rewritable type). Finally, a substrate having a thickness of 0.1 mm is formed as a cover layer (protective layer) from the first dielectric layer side. The cover layer of the Blu-ray Disc is formed by attaching a 0.1 mm thick sheet on the first dielectric layer with an ultraviolet curable resin adhesive, and spinning the ultraviolet curable resin on the first dielectric layer. A method of forming a cover layer by applying uniformly by a coating method and curing an ultraviolet curable resin by ultraviolet irradiation has been proposed.

  As a recording material for Blu-ray Disc, for example, an Ag-In-Sb-Te recording material disclosed in Japanese Patent No. 2941848 can be used. This patent also discloses in detail the composition of a recording material in which a fifth element and a sixth element are added to an Ag—In—Sb—Te recording material.

  As another method for increasing the capacity of a recordable DVD, an optical disk is prepared by laminating layers in the same order as in the past on a 0.6 mm thick substrate, the wavelength of the laser beam is 405 nm, and the objective lens NA is set to 0.0. A method of recording information as 65 is also proposed. This method has a larger laser spot diameter and a lower recording density because the objective lens NA is smaller than a method using a cover layer having a thickness of 0.1 mm such as the aforementioned Blu-ray Disc. However, there is an advantage that the rigidity of the substrate can be easily maintained and the recording layer can be easily multi-layered. There is also an advantage that the influence of dust and scratches on the medium can be reduced.

  In the above-described techniques such as DVD-RAM, DVD-RW, DVD + RW, and Blu-ray Disc, a so-called wobble track that causes the recording track to meander is employed. Address information, a synchronization signal, and the like are recorded in the wobble. The recording signal is reproduced as a sum signal, and the wobble signal is reproduced as a difference signal, thereby improving the format efficiency. Further, since it is possible to obtain a synchronization signal from a wobble signal, it is known that this is an extremely effective means for improving the reliability of address information and recorded information.

Japanese Patent Laid-Open No. 62-209741 JP 2004-155177 A

  As described above, in order to increase the capacity of an information recording medium, it is essential to use a blue laser (wavelength 390 to 420 nm), and various recording layer materials have been proposed. An object of the present invention is an information recording medium compatible with a blue laser using a Bi-Ge-Te phase change material as a recording layer, which has high reliability of recorded data and excellent durability against repeated recording of data. It is to provide a recording medium.

According to the aspect of the present invention, when the wavelength of the laser beam is λ [nm], the numerical aperture of the objective lens for condensing the laser beam is NA, and the recording linear velocity is V [m / sec], Information that can be rewritten multiple times by irradiating the laser beam under the condition of 76.0 [nsec] ≦ (λ / NA) /V≦115.0 [nsec] and λ = 390 to 420 [nm]. A recording medium comprising a substrate and a recording layer provided on the substrate and formed of a phase change material containing Bi, Ge, and Te, wherein the composition of Bi, Ge, and Te in the recording layer is There is provided an information recording medium characterized by being in a composition range surrounded by the following points on a triangular composition diagram of Bi, Ge, and Te.
B4 (Bi _2.5 , Ge _47.0 , Te _50.5 )
_{B2 (Bi 1.5, Ge 48.0,} Te 50.5)
_{C3 (Bi 3.5, Ge 48.0,} Te 48.5)
D5 (Bi _6.0 , Ge _49.0 , Te _45.0 )
D2 (Bi _6.0 , Ge _45.5 , Te _48.5 )
C4 (Bi _4.0 , Ge _47.0 , Te _49.0 )

  The present inventors have proposed a practical composition range of a Bi—Ge—Te phase change material that can cope with 2 × speed and 5 × speed in Patent Document 2. Thereafter, the present inventors made extensive studies, and as a result, an HD DVD having a recording layer formed of a Bi—Ge—Te phase change material having a composition range defined in Patent Document 2 was produced. It has been found that the following problems occur when recording / reproduction is performed under the standard speed (single speed) conditions (laser wavelength 405 nm, objective lens numerical aperture NA 0.65, recording linear velocity 5.6 m / sec).

(Problem 1)
The reproduction signal quality of recorded information is greatly deteriorated. In particular, the reproduction signal amplitude is greatly reduced. This cause is presumed to be due to recrystallization of the recording mark according to the analysis of the experimental data by the present inventors. Recrystallization is a cooling process immediately after the recording layer material is heated to a melting point or higher (amorphized) by irradiating a predetermined position on the recording layer with a laser beam. Crystallization occurs from the outer edge of the molten region, and the size of the recording mark Is a phenomenon (shrink). When the recording mark shrinks, the size of the recording mark becomes small, so that the reproduction signal amplitude decreases. In addition, when the recording mark shrinks, the crystal size of the recrystallized portion is different from that of the normally crystallized portion, so that reflectance dispersion resulting from this occurs and noise is generated. Therefore, when the shrinkage of the recording mark due to recrystallization is too large, reproduction signal deterioration occurs due to the above-described causes. Recording mark shrinkage can be solved by reducing the crystallization speed of the recording layer material.

(Problem 2)
If a higher power laser beam is irradiated to record a wide recording mark and increase the playback signal amplitude for a recording layer with high recrystallization, the recording mark recorded on the adjacent track is erased. (Cross erase), the signal quality of adjacent tracks is significantly degraded.

(Problem 3)
When a higher-power laser beam is irradiated to record a wide recording mark and increase the reproduction signal amplitude, damage to the recording layer due to many rewrites increases, and the number of rewrites decreases.

  When the above-described problem 1 (record mark shrinking) and problem 2 (cross-erasing) occur, the track pitch cannot be narrowed for high density, so that the effect of reducing the beam diameter by the blue laser can be fully utilized. It becomes impossible to do.

  The present invention has been made to solve the above-mentioned problems. The information recording medium of the present invention solves all of the above-mentioned problems 1 to 3 even when recording / reproduction is performed under the standard speed (1 × speed) conditions of HD DVD. An object of the present invention is to provide an information recording medium that can be solved.

In an information recording medium using a Bi-Ge-Te phase change material as a recording layer, the present inventors conducted a verification experiment to determine the composition of Bi, Ge, and Te in the recording layer as a triangle of Bi, Ge, and Te. By making the composition range surrounded by the following composition points on the composition diagram, for example, all of the above problems 1 to 3 can be solved even when recording / reproduction is performed under the standard speed (1 × speed) conditions of HD DVD. I found that I can do it.
B4 (Bi _2.5 , Ge _47.0 , Te _50.5 )
_{B2 (Bi 1.5, Ge 48.0,} Te 50.5)
_{C3 (Bi 3.5, Ge 48.0,} Te 48.5)
D5 (Bi _6.0 , Ge _49.0 , Te _45.0 )
D2 (Bi _6.0 , Ge _45.5 , Te _48.5 )
C4 (Bi _4.0 , Ge _47.0 , Te _49.0 )

  More specifically, when the wavelength of the laser beam is λ (nm), the numerical aperture of the objective lens for condensing the laser beam is NA, and the recording linear velocity is V (m / sec), the information recording medium Under recording / reproducing conditions such that the parameter (λ / NA) / V representing the time required for the laser beam spot to pass through a certain point above is in the range of 76.0 to 115.0 nsec (λ = 390 to 420 nm). By setting the composition of Bi, Ge and Te in the recording layer to the composition range surrounded by the above composition points, all the above problems 1 to 3 can be solved, and the reliability of the recorded data is high. The present inventors have found through verification experiments that an information recording medium excellent in durability against repeated recording of data can be provided.

As the principle of suppressing recrystallization, the following hypothesis is described in Patent Document 2 described above. In the information recording medium of the present invention, it is considered that recrystallization is suppressed by the same principle. Bi-Ge-Te phase change materials include compounds of GeTe, Bi ₂ Te ₃ , Bi ₂ Ge ₃ Te ₆ , Bi ₂ GeTe ₄ and Bi ₄ GeTe _{7 to} the extent that has been clarified so far. To do. When a part of the melted region is recrystallized immediately after being melted by irradiating a laser beam to record information (amorphize) in a predetermined part of the recording layer, it depends on the composition of the recording layer. It is considered that the compound, Bi, Ge and Te mentioned above are recrystallized from the outer edge of the melting region in order from the material having the highest melting point. When these substances are arranged in descending order of melting point, they are as follows.
Ge: about 937 ° C
GeTe: about 725 ° C
Bi ₂ Ge ₃ Te ₆ : about 650 ° C.
Bi ₂ Te ₃ : about 590 ° C
Bi ₂ GeTe ₄ : about 584 ° C
Bi ₄ GeTe ₇ : about 564 ° C.
Te: about 450 ° C.
Bi: about 271 ° C

Since the melting point of Ge is the highest as described above, a phase change material in which Ge is excessively added to the composition material on the line connecting GeTe and Bi ₂ Te ₃ on the triangular composition diagram with Bi, Ge and Te as vertices. In the information recording medium used as the recording layer, it is considered that Ge is easily segregated at the outer edge of the melted region. When Ge is excessively present at the outer edge portion of the melting region, the crystallization speed of the outer edge portion of the melting region is reduced, and recrystallization from the outer edge portion can be suppressed. Therefore, it is possible to suppress the occurrence of recrystallization “bands” caused by rewriting many times.

  If recrystallization from the outer edge of the melting region can be suppressed as described above, it is not necessary to increase the laser power to improve the reproduction signal amplitude, and it is not necessary to melt a wide region, and the recording mark recorded on the adjacent track The problem of erasing (problem 2: cross erase) can be solved.

  Furthermore, if the recrystallization from the outer edge of the melted region can be suppressed as described above, the recording power irradiated per recording can be lowered, so that damage to the recording layer due to multiple rewrites (Problem 3) Can be suppressed. That is, the rewriting durability can be improved. Thus, the information recording medium of the present invention can solve all the above problems 1 to 3.

  In the information recording medium of the present invention, it is preferable that a thermal diffusion layer is further provided on the side of the recording layer opposite to the laser beam incident side. The thermal conductivity of the thermal diffusion layer is preferably 100 [W / m · K] or more. In the information recording medium of the present invention, by providing the thermal diffusion layer on the side opposite to the laser beam incident side of the recording layer, the reliability for multiple rewriting is improved. In particular, by providing a thermal diffusion layer having a thermal conductivity of 100 W / m · K or more, the reliability with respect to multiple rewrites is dramatically improved.

  In the information recording medium of the present invention, a protective layer is further provided between the recording layer and the thermal diffusion layer, and the refractive index n and the extinction coefficient among the complex optical constants in the wavelength range of 390 nm to 420 nm of the protective layer. It is preferable that a relationship of k ≦ 0.1 and 13 ≦ n × d ≦ 35 is established between k and the thickness of the protective layer d (nm). Through the verification experiment, the present inventors can reduce cross erase and provide contrast (crystal part) by providing the protective layer having the complex optical constant as described above between the recording layer and the thermal diffusion layer. It was found that the difference in reflectance between the amorphous part and the amorphous part can be increased. Details thereof will be described below.

  In an information recording medium provided with at least a protective layer, a phase change recording layer, an intermediate layer, and a thermal diffusion layer from the laser beam incident side, the thermal diffusion layer in the groove portion is adjacent to the recording layer in the land portion due to the step of the groove. May be configured to exist. When land / groove recording is adopted for such an information recording medium, the heat applied to the land recording layer easily spreads toward the heat diffusion layer of the adjacent groove, and the heat leaking from the land into the groove increases. . As a result, the cross erase of the groove may be increased. As an improvement measure, it is necessary to make the intermediate layer thick so that the heat diffusion layer in the groove portion does not exist next to the recording layer in the land portion. However, if the intermediate layer is made thicker, the distance between the recording layer and the heat diffusion layer becomes longer, so that heat applied to the recording layer during recording cannot be quickly released, and damage to the recording layer due to heat increases. That is, the reliability when rewritten many times decreases.

  In order to solve the above-mentioned problems, the present inventors have conducted extensive research. As a result, the thermal conductivity of the thermal diffusion layer is set to 100 W / m · K or more, and the complex optics in the wavelength range of 390 nm to 420 nm of the protective layer is used. Of the constants, assuming that the refractive index is n, the extinction coefficient is k, and the film thickness is d (nm), if the conditions are k ≦ 0.1 and 13 ≦ n × d ≦ 36, Even if the heat diffusion layer in the groove portion exists next to the recording layer in the land portion, the heat generated in the land portion can be quickly released in the film thickness direction and prevented from spreading to the adjacent groove portion. I found.

  In the information recording medium of the present invention, the recording layer preferably has a thickness of 5 to 12 [nm]. As a result of verification experiments, the inventors have found that the reliability of the information recording medium for multiple rewrites is improved by setting the thickness of the recording layer to 12 nm or less. This is considered to be because by reducing the film thickness of the recording layer, it was possible to suppress phenomena such as flow of the recording layer material, composition variation, and segregation that occurred during many rewrites.

  The information recording medium of the present invention preferably further comprises an interface layer provided in contact with at least one surface of the recording layer. The inventors of the present invention have found that the reliability for multiple rewriting can be improved even if an interface layer is provided in contact with at least one surface of the recording layer.

  In the information recording medium of the present invention, the information recording medium has a disc shape, and the substrate is formed with concentric or spiral grooves, and a recording track is formed between the grooves and the grooves. It is preferable to use as. In this case, the track pitch TP of the recording track is preferably in the range of 0.5 × (λ / NA) to 0.6 × (λ / NA), and in particular, the wavelength λ of the laser beam is It is preferable that λ = 400 to 410 [nm], the numerical aperture NA of the objective lens is NA = 0.6 to 0.65, and the track pitch TP is 0.34 [μm] or less. .

  According to the inventor's verification experiment, it was found that good recording / reproducing characteristics can be obtained even in such a structure. Specifically, the inventors of the present invention have conducted the information recording according to the present invention in which the track pitch TP of the recording track is in the range of 0.5 × (λ / NA) to 0.6 × (λ / NA) through a verification experiment. It has been found that good recording / reproducing characteristics can be obtained with respect to a medium. In particular, under the conditions where the wavelength λ of the laser beam is λ = 400 nm to 410 nm, the numerical aperture NA of the objective lens is NA = 0.6 to 0.65, and the track pitch TP is 0.34 μm or less. It has been found that better recording / reproducing characteristics can be obtained.

  In the information recording medium of the present invention, the information recording medium has a disc shape, and the substrate has a concentric or spiral groove formed thereon, and at least one of the groove and the groove is recorded. It is preferably used as a track, and at least one of the grooves and the grooves meander. In this case, the track pitch TP of the recording track is preferably in the range of 0.3 × (λ / NA) to 0.4 × (λ / NA), and in particular, the wavelength λ of the laser beam is It is preferable that λ = 400 to 410 [nm], the numerical aperture NA of the objective lens is NA = 0.6 to 0.65, and the track pitch TP is 0.4 [μm] or less. . According to the inventor's verification experiment, it was found that good recording / reproducing characteristics can be obtained even in such a structure.

  Further, the composition of the Bi-Ge-Te phase change material as in the information recording medium of the present invention is within the above range on the triangular composition diagram of Bi, Ge, and Te (B4, B2, C3, D5, D2, C4 ) In the blue region having a wavelength of 405 nm as compared with the conventional Ge—Sb—Sn—Te, Ge—Sb—Te, and Ag—In—Sb—Te phase change recording materials. Since the difference (Δn, Δk) between the optical constants of the amorphous portion and the amorphous portion becomes large, the difference in reflectance (contrast) between the recorded portion and the unrecorded portion can be increased, and the reproduction signal amplitude can be increased.

  Further, in the Bi—Ge—Te phase change material used in the information recording medium of the present invention, Si, Sn, Pb, etc., which are homologous elements, may be used instead of Ge, and an appropriate amount of the element may be used. Si, Sn, Pb or the like may be added. By adding an appropriate amount of Si, Sn, Pb or the like, the linear velocity range that can be handled can be easily adjusted. That is, the composition of the recording layer material is Bi-Ge-Te in a range surrounded by the above points (B4, B2, C3, D5, D2, C4) on the triangular composition diagram having Bi, Ge, and Te as vertices. A material having a composition in which a system phase change material is used as a base material and a part of Ge is substituted with at least one element of Si, Sn, and Pb may be used as the recording layer.

  Further, in the Bi—Ge—Te recording layer material used for the information recording medium of the present invention, Sb which is a homologous element may be used instead of Bi, and an appropriate amount of Sb is added. Also good. By adding an appropriate amount of Sb, the applicable linear velocity range can be easily adjusted. That is, the range of the composition of the Bi-Ge-Te recording layer material surrounded by the above points (B4, B2, C3, D5, D2, C4) on the triangular composition diagram with Bi, Ge, and Te as vertices. A recording layer material having a composition in which a Bi-Ge-Te phase change material is used as a base material and a part of Bi is substituted with Sb may be used.

  Further, in the Bi—Ge—Te recording layer material used for the information recording medium of the present invention, the above points (B4, B2, C3, D5, D2) on the triangular composition diagram having Bi, Ge, and Te as apexes. , C4) as a base material, a part of Ge is substituted by at least one element of Si, Sn and Pb, and a part of Bi is substituted. A recording layer material having a composition substituted by Sb may be used.

  Furthermore, B may be added to the Bi—Ge—Te recording layer material used for the information recording medium of the present invention. If B is added, recrystallization is further suppressed, and an information recording medium exhibiting excellent performance can be obtained.

In the information recording medium of the present invention, a nucleation layer containing Bi ₂ Te ₃ , SnTe, PbTe or the like may be provided adjacent to the recording layer. In this case, the effect of suppressing recrystallization is further improved.

  In the information recording medium of the present invention, if the composition of the recording layer maintains the above-described composition range (the range surrounded by B4, B2, C3, D5, D2, and C4), even if impurities are present. Even if it is mixed, the effect of the present invention is not lost as long as the atomic% of impurities is within 1%.

  In this specification, the information recording medium of the present invention may be expressed as a phase change optical disk or simply an optical disk. However, as the information recording medium of the present invention, heat is generated by irradiation of an energy beam. Therefore, the present invention can be applied to any information recording medium in which the atomic arrangement changes and information is recorded. Therefore, the information recording medium of the present invention can be applied to an information recording medium such as an optical card regardless of its shape.

  In the present specification, the energy beam described above may be expressed as a laser beam, or simply a laser beam or light. However, as described above, the energy capable of generating heat on the information recording medium of the present invention. Since an effect can be obtained with a beam, an energy beam such as an electron beam may be used.

  The information recording medium of the present invention is premised on a configuration in which the substrate is disposed on the light incident side of the recording layer. However, the substrate is disposed on the opposite side of the recording layer from the light incident side. Alternatively, a protective material such as a protective sheet thinner than the substrate may be disposed.

According to the information recording medium of the present invention, a Bi—Ge—Te phase change material is used as the recording layer, and the composition of Bi, Ge, and Te in the recording layer is as follows on the triangular composition diagram of Bi, Ge, and Te. Therefore, the wavelength of the laser beam is λ (nm), the numerical aperture of the objective lens for condensing the laser beam is set to NA and the recording linear velocity. When V (m / sec) is set, information is recorded / reproduced under the condition of 76.0 nsec ≦ (λ / NA) /V≦115.0 nsec (where λ = 390 nm to 420 nm) (for example, Even in the case of recording / reproducing at normal speed (1 × speed) on an HD DVD, all of the above problems 1 to 3 (record mark shrink, cross erase and heat damage problems) can be solved, and the recorded data of An information recording medium with high reliability and excellent durability against repeated data recording can be provided.
B4 (Bi _2.5 , Ge _47.0 , Te _50.5 )
_{B2 (Bi 1.5, Ge 48.0,} Te 50.5)
_{C3 (Bi 3.5, Ge 48.0,} Te 48.5)
D5 (Bi _6.0 , Ge _49.0 , Te _45.0 )
D2 (Bi _6.0 , Ge _45.5 , Te _48.5 )
C4 (Bi _4.0 , Ge _47.0 , Te _49.0 )

  Examples of the information recording medium of the present invention will be described below, but the present invention is not limited to this.

[Information recording medium and manufacturing method thereof]
The information recording medium manufactured in Example 1 is a phase change type optical disk, and a schematic cross-sectional view thereof is shown in FIG. As shown in FIG. 1, an optical disc 10 manufactured in this example has a first protective layer 2, a first interface layer 3, a recording layer 4, a second interface layer 5, a second protective layer 6, a thermal diffusion layer on a substrate 1. The layer 7 and the ultraviolet curable resin layer 8 are sequentially laminated. Next, a method for producing the optical disc 10 of this example will be described. The optical disk 10 manufactured in this example employs a so-called land / groove recording method in which information is recorded on both the groove and the land.

  First, a polycarbonate substrate having a diameter of 120 mm and a thickness of 0.6 mm was used as the substrate 1. The substrate 1 was manufactured by injection molding, and a groove having a track pitch of 0.34 μm was formed in the information recording area of the radius of 23.8 mm to 58.6 mm of the substrate 1. In this example, since the land / groove recording method is adopted, the track pitch in this example is the distance from the center of a predetermined land track to the center of the groove track adjacent to the land track. In this example, the track was wobbled at a cycle of 93 channel bits.

Next, (ZnS) ₈₀ (SiO ₂ ) ₂₀ was formed as a first protective layer 2 on the substrate 1 to a thickness of 60 nm by sputtering. Next, Ge ₈₀ Cr ₂₀ -N was formed as a first interface layer 3 on the first protective layer 2 to a thickness of 7 nm by sputtering.

Next, a Bi-Ge-Te phase change film having a thickness of 12 nm was formed as the recording layer 4 on the first interface layer 3 by sputtering. In this example, the recording layer 4 having various compositions on the line connecting Ge ₅₀ Te ₅₀ and Bi ₂ Te ₃ on the triangular composition diagram with Bi, Ge, and Te as vertices was formed (B-series optical disc). At this time, Ge ₅₀ Te ₅₀ and Bi ₆ Ge _43.5 Te _50.5 targets were used as the sputtering target, and they were formed by simultaneous sputtering. Further, the sputtering power applied to each target was adjusted so that the composition of the recording layer 4 was a desired composition.

Specifically, the composition of the recording layer 4 is Bi _1.5 Ge _48.0 Te _50.5 (sample number B2), Bi _2.0 Ge _47.5 Te _50.5 (sample number B3), and Bi. _The optical disk 10 to be _2.5 Ge _47.0 Te _50.5 (sample number B4) was produced. In this example, for comparison, the composition of the recording layer 4 is Bi _1.0 Ge _49.0 Te _50.0 (sample number B1), Bi _3.0 Ge _46.0 Te _51.0 (sample number). B5) and Bi _4.0 Ge _45.0 Te _51.0 (sample number B6) were also produced.

On the recording layer 4 formed by the above method, Ge ₈₀ Cr ₂₀ -N was formed as a second interface layer 5 with a thickness of 2 nm by sputtering. Next, (ZnS) ₈₀ (SiO ₂ ) ₂₀ was formed as a second protective layer 6 (also referred to as an intermediate layer) with a thickness of 10 nm on the second interface layer 5 by sputtering. Furthermore, Ag—Ca—Cu was formed as a thermal diffusion layer 7 on the second protective layer 6 by sputtering so as to have a thickness of 150 nm.

  Next, a UV resin is applied as an ultraviolet curable resin layer 8 on the heat diffusion layer 7, and a polycarbonate transparent substrate (not shown) having a thickness of 0.6 mm is further placed thereon. The transparent substrate was laminated on the ultraviolet curable resin layer 8 by irradiating UV through the transparent substrate and curing the UV resin. The optical disc 10 shown in FIG. 1 was obtained by the above manufacturing method.

  Using an initialization apparatus (not shown), various optical disks 10 obtained by the above production method are irradiated with elliptical laser beams having a wavelength of 810 nm, a beam spot major axis of 96 μm, and a minor axis of 1 μm. Then, initialization was performed.

  In this example, as shown in FIG. 1, an optical disc was manufactured with a film configuration similar to that of a conventional product such as a DVD-RAM. However, each layer was laminated on the substrate 1 in the reverse order of FIG. The same effects as those of the present invention can be obtained even with this optical disc.

[Information recording / playback device]
FIG. 2 shows a schematic configuration diagram of an information recording / reproducing apparatus for recording and reproducing information with respect to various optical disks produced in this example. As shown in FIG. 2, the information recording / reproducing apparatus 20 used in this example mainly includes a motor 21 for rotating the optical disk 10 manufactured in this example, and an optical head 22 for irradiating the optical disk 10 with laser light. And an L / G servo circuit 23 for tracking control, a reproduction signal processing system 24, and a recording signal processing system 27. As shown in FIG. 2, the reproduction signal processing system 24 includes a preamplifier circuit 25 that adjusts the gain of the reproduction signal, and a 2-10 demodulator 26 that reproduces information based on the reproduction signal. As shown in FIG. 2, the recording signal processing system 27 includes a 2-10 modulator 30 that modulates an input signal by a predetermined modulation method, a recording waveform generation circuit 29 that generates a recording signal waveform, and laser light emission. And a laser drive circuit 28 to be controlled.

The optical head 22 used in this example includes a semiconductor laser having a wavelength of 405 nm and an objective lens having a numerical aperture NA of 0.65 (not shown). Generally, when a laser beam having a wavelength λ is collected by an objective lens having a numerical aperture NA, the spot diameter of the laser beam is approximately 0.9 × λ / NA. Is about 0.6 μm. However, in this example, the polarization of the laser light is circularly polarized. In this example, since the track pitch TP is 0.34 μm, between the track pitch TP, the wavelength λ, and the numerical aperture NA,
TP = 0.55 × (λ / NA)
The relationship is established.

  In addition, since the optical disk manufactured in this example is a land / groove recording type optical disk, the information recording / reproducing apparatus 20 shown in FIG. 2 also supports the land / groove recording system. In the information recording / reproducing apparatus 20 of this example, the tracking for the land and the groove can be arbitrarily selected by the L / G servo circuit 23 in FIG.

  The operation of the information recording / reproducing apparatus 20 will be described below with reference to FIG. As a motor control method for recording / reproducing, a ZCLV method is adopted in which the number of revolutions of the disk is changed for each zone for recording / reproducing. In this example, when recording information, the mark edge method is used, and information is recorded on the optical disc 10 by the ETM, RLL (1, 10) modulation method. In this modulation method, information is recorded with a mark length of 2T to 11T. Here, T represents a clock cycle at the time of recording information, and in this example, T = 15.4 ns. That is, in this example, the shortest 2T mark length is about 0.17 μm, and the longest 11T mark length is about 0.935 μm. In this example, the recording linear velocity 5.64 m / sec was set to 1 × speed.

  First, a signal necessary for information recording is input to the 2-10 modulator 30 from the outside of the recording apparatus. Next, the signal input to the 2-10 modulator 30 is modulated by the above-described modulation method, and a 2T to 11T digital signal is output. Next, the 2T to 11T digital signals output from the 2-10 modulator 30 are input to the recording waveform generation circuit 29.

  The recording waveform generation circuit 29 generates a multi-pulse recording waveform necessary for laser light irradiation during information recording based on the input 2T to 11T digital signal. In this example, the high power level region of the multi-pulse recording waveform is composed of a high power pulse having a width of about T / 2 and a low power pulse having a width of about T / 2 formed between the high power pulses. Formed with a series of pulse trains. In addition, the region between the series of pulse trains of the multi-pulse recording waveform was composed of intermediate power level pulses. At this time, the pulse intensity at a high power level for forming (amorphizing) the recording mark on the recording layer and the pulse intensity at the intermediate power level for crystallizing the recording mark are optimal for each optical disc to be recorded and reproduced. Adjusted to value.

  Further, in the recording waveform generation circuit 29, the digital signal waveforms of 2T to 11T are made to correspond to “0” and “1” alternately in time series, and in the case of “0”, a laser pulse of an intermediate power level is generated. In the case of “1”, a series of pulse trains composed of the above-described high power pulse and low power pulse was irradiated. At this time, the portion on the optical disk 10 irradiated with the intermediate power level laser pulse becomes a crystal, and the portion irradiated with the series of pulse trains composed of the high power pulse and the low power pulse described above becomes amorphous (mark portion). Change. Further, when forming the series of pulse trains composed of the high power pulse and the low power pulse described above, the recording waveform generation circuit 29 determines the start pulse width and the end pulse of the multi-pulse waveform according to the space length before and after the mark portion. It has a multi-pulse waveform table corresponding to the method of changing the pulse width of the tail (adaptive recording waveform control), so that the multi-pulse recording waveform that can eliminate the influence of thermal interference between marks as much as possible It has occurred.

  Next, the multi-pulse recording waveform generated by the above-described recording waveform generating circuit 29 is transferred to the laser driving circuit 28, and the laser driving circuit 28 stores the multi-pulse recording waveform in the optical head 22 based on the input multi-pulse recording waveform. Controls the emission of the semiconductor laser. The laser light emitted from the semiconductor laser is narrowed down onto the recording layer of the optical disk 10 by the objective lens in the optical head 22, and information is recorded by irradiating the laser light at a timing corresponding to the multi-pulse recording waveform. It was.

  Next, the reproducing operation of the information recorded as described above will be described. First, a laser beam is irradiated onto the recording mark of the optical disk 10 from the optical head 22, and the reflected light from the recording mark and a portion other than the recording mark (unrecorded portion) is detected by the optical head 22 to obtain a reproduction signal. The amplitude of the reproduction signal is amplified with a predetermined gain by the preamplifier circuit 25 and transferred to the 2-10 demodulator 26. The 2-10 demodulator 26 demodulates information based on the input reproduction signal and outputs reproduction data. With the above operation, the reproduction of the recorded mark is completed.

  In the measurement of an error rate described later, a random pattern signal including 2T to 11T was recorded and reproduced.

[Evaluation of recording layer material]
In order to evaluate the effects of recording / erasing performance, signal quality, and cross erase on the various optical disks produced in this example, the optical disk was mounted on the information recording / reproducing apparatus described above, and an error occurred in 1 × speed recording of HD DVD. The rate (error rate after rewriting the random pattern 10 times) was measured.

  Specifically, (1) error rate after rewriting a random pattern 10 times in one track (here, error rate 1), (2) random pattern in order from the inner circumference to the outer circumference of five consecutive tracks After the recording, the error rate measured on the center track of 5 tracks (here, the error rate is 2), and the signals recorded on both adjacent tracks from the state of (3) and (2) are continuously irradiated. Then, after erasing (crystallization), three types of error rates were measured: an error rate measured at the center track (here, error rate 3). By looking at the ratio between the error rate 1 and the error rate 3, it is possible to evaluate the deterioration of the recording / reproduction characteristics due to the cross erase.

  In this example, the error rate after 1000 rewrites in 1 × speed recording was measured in order to test the rewrite life. Furthermore, in this example, in order to evaluate the influence of recrystallization in the recording mark, at a recording linear velocity equivalent to 1 × speed of HD DVD and a recording linear velocity equivalent to 2 × speed of HD DVD (11.28 m / sec). A single frequency signal of 8T was recorded, and the amplitude ratio of the reproduced signal of information recorded at each recording linear velocity (amplitude at 1 × speed recording / amplitude at 2 × speed recording) was measured. At this time, in order to eliminate the influence due to the error of the laser power setting, recording was performed with the optimum power set to 1.7 times the recording start power.

  The result of the measurement is shown in FIG. In this example, since the land / groove recording method is adopted, the average value of the evaluation result in the land and the evaluation result in the groove is shown here. The 1 × speed recording error rate in FIG. 3 is the error rate 2 of the above-mentioned three types of error rates, and the 1 × speed recording error rate ratio (cross erase performance) is the above-mentioned three types of error rates. It is the ratio of error rate 1 and error rate 3 (error rate 3 / error rate 1).

The target values for each evaluation item in FIG. 3 were as follows.
(1) 1 × speed recording bit error rate: 5 × 10 ⁻⁵ or less (2) 1 × speed recording bit error rate ratio: 5 or less (3) Bit error rate after 1000 rewrites: 1 × 10 ⁻⁴ or less (4) Amplitude ratio: 0.85 or more

In FIG. 3, the evaluation results are represented by ◎, ○, and ×, and these evaluation criteria are as follows.
(1) 1 × speed recording bit error rate ◎: 1.0 × 10 ⁻⁵ or less, ○: 5.0 × 10 ⁻⁵ or less, ×: larger than 5.0 × 10 ⁻⁵ (2) 1 × speed recording bit error Rate ratio ◎: 3 or less, ○: 5 or less, ×: Greater than 5 (3) Bit error rate after rewriting 1000 times 1 × speed recording ◎: 1.0 × 10 ⁻⁵ or less, ○: 5.0 × 10 ⁻⁵ Hereinafter, x: larger than 5.0 × 10 ⁻⁵ (4) Amplitude ratio ◎: 0.9 or more, ○: 0.85 or more, x: smaller than 0.85 (5) Overall evaluation ◎: All the above evaluation items When ◎: ○: There is no x in the above evaluation items and there is at least one ×: There is at least one x item in the above evaluation items

As is clear from FIG. 3, in the sample B1 (Bi _1.0 Ge _49.0 Te _50.0 ), the bit error rate and amplitude ratio items after 1000 rewrites of the 1 × speed recording did not reach the targets. For this reason, comprehensive evaluation was x.

In sample B2 (Bi _1.5 Ge _48.0 Te _50.5 ), as shown in FIG. 3, the target values were achieved in all items, and the item of 1 × speed recording error rate ratio was ◎ evaluation, Items other than were rated as ○. Therefore, in the sample B2, the overall evaluation was “good”.

In the sample B3 (Bi _2.0 Ge _47.5 Te _50.5 ), as shown in FIG. 3, the target values are achieved in all items, and the items of the 1 × speed recording bit error rate and the 1 × speed recording error rate ratio are achieved. Was evaluated as ◎, and other items were evaluated as ○. Therefore, in the sample B3, the overall evaluation was “good”.

In sample B4 (Bi _2.5 Ge _47.0 Te _50.5 ), as shown in FIG. 3, all items were evaluated as “good”, and the overall evaluation was “good”.

In sample B5 (Bi _3.0 Ge _46.0 Te _51.0 ), as shown in FIG. 3, the item of the amplitude ratio did not reach the target, and the overall evaluation was x. In sample B6 (Bi _4.0 Ge _45.0 Te _51.0 ), the items of the 1 × speed recording bit error rate ratio and the amplitude ratio did not reach the targets, and the overall evaluation was x.

From the above measurement results, B series optical discs (optical discs having a recording layer having a composition on a line connecting Ge ₅₀ Te ₅₀ and Bi ₂ Te ₃ on a triangular composition diagram with Bi, Ge and Te as vertices) When the Bi-Ge-Te phase change material having a Ge amount of 47 to 48 at% (composition of samples B2 to B4) was used, it was found that the target value was achieved in all of the above evaluation items.

In Example 2, the composition of the recording layer is such that Ge is added more than the composition on the line connecting Ge ₅₀ Te ₅₀ and Bi ₂ Te ₃ on the triangular composition diagram with Bi, Ge and Te as vertices. A recording layer was formed. In Example 2, an optical disk was produced in the same manner as in Example 1 except that the composition of the recording layer was changed. Also in this example, optical disks (C-series optical disks) having various recording layers having different compositions were produced.

In this example, Ge ₅₆ Te ₄₄ and Bi ₆ Ge _43.5 Te _50.5 targets were used as the sputtering target, and the recording layer was formed by co-sputtering. At this time, the sputtering power applied to each target was adjusted so that the composition of the recording layer was a desired composition. Specifically, the optical disc has a recording layer composition of Bi _3.5 Ge _48.0 Te _48.5 (sample number C3) and Bi _4.0 Ge _47.0 Te _49.0 (sample number C4). Was made. In this example, for comparison, the composition of the recording layer is Bi _2.0 Ge _51.5 Te _46.5 (sample number C1), Bi _3.0 Ge _49.0 Te _48.0 (sample number C2). ), Bi _4.5 Ge _46.0 Te _49.5 (sample number C5) and Bi _5.0 Ge _45.0 Te _50.0 (sample number C6).

  The same evaluation as in Example 1 was performed on the C-series optical disk manufactured in this example. The results are shown in FIG. In addition, the target value of each evaluation item and the evaluation criteria of ◎, ○, and × in FIG. 4 are the same as those in the first embodiment.

As is clear from FIG. 4, in the sample C1 (Bi _2.0 Ge _51.5 Te _46.5 ), the items of the 1 × speed recording error rate and the bit error rate after 1000 rewrites did not reach the target, and the overall evaluation was X. In sample C2 (Bi _3.0 Ge _49.0 Te _48.0 ), the bit error rate item after 1000 rewrites did not reach the target, and the overall evaluation was x.

In sample C3 (Bi _3.5 Ge _48.0 Te _48.5 ), as shown in FIG. 4, the target value was achieved for all evaluation items, and the bit error rate item after 1000 rewrites was evaluated as ○. The other items were evaluated as ◎. Therefore, in the sample C3, the overall evaluation was “good”.

In sample C4 (Bi _4.0 Ge _47.0 Te _49.0 ), as shown in FIG. 4, the target value was achieved in all evaluation items, the amplitude ratio item was ○ evaluation, and the other items Was an evaluation. Therefore, in the sample C4, the overall evaluation was “good”.

Sample _{C5 (Bi 4.5 Ge 46.0 Te 49.5} ) and sample _{C6 (Bi 5.0 Ge 45.0 Te 50.0} ), as is clear from FIG. 4, both the target items of the amplitude ratio It was not achieved and the overall evaluation was x.

  From the above measurement results, in the case of a C-series optical disc, when the Bi—Ge—Te phase change material having a Ge amount of 47 to 48 at% (composition of samples C3 and C4) was used as the recording layer, the above evaluation items were obtained. It was found that the target value was achieved with all of the above.

  In Example 3, the recording layer was formed so that the composition of the recording layer was such that Ge was added in excess on the triangular composition diagram having Bi, Ge, and Te as vertices on the composition line of the recording layer of the C-series optical disc. Formed. In Example 3, an optical disk was manufactured in the same manner as in Example 1 except that the composition of the recording layer was changed. Also in this example, optical disks (D series optical disks) having various recording layers with different compositions were produced.

In this example, Bi _6.0 Ge _54.0 Te _40.0 and Bi _6.0 Ge _43.5 Te _50.5 targets were used as the sputtering target, and the recording layer was formed by co-sputtering. At this time, the sputtering power applied to each target was adjusted so that the composition of the recording layer was a desired composition. Specifically, the composition of the recording layer is Bi _6.0 Ge _45.5 Te _48.5 (sample number D2), Bi _6.0 Ge _46.5 Te _47.5 (sample number D3), Bi _6. Optical discs having ₀ Ge _48.0 Te _46.0 (sample number D4) and Bi _6.0 Ge _49.0 Te _45.0 (sample number D5) were prepared. In this example, for comparison, the composition of the recording layer is Bi _6.0 Ge _44.0 Te _50.0 (sample number D1) and Bi _6.0 Ge _50.0 Te _44.0 (sample number D6). The optical disc to be) was also produced.

  The same evaluation as in Example 1 was performed on the D-series optical disk manufactured in this example. The results are shown in FIG. In addition, the target value of each evaluation item and the evaluation criteria of ◎, ○, and × in FIG.

As is clear from FIG. 5, in the sample D1 (Bi _6.0 Ge _44.0 Te _50.0 ), the items of the 1 × speed recording error rate and the bit error rate after 1000 rewrites did not reach the target, and the overall evaluation was X.

In sample D2 (Bi _6.0 Ge _45.5 Te _48.5 ), as shown in FIG. 5, the target values were achieved in all items. The items of 1 × speed recording error rate ratio and amplitude ratio were evaluated as “◎”, and the items of 1 × speed recording error rate and bit error rate after 1000 rewrites were evaluated as “◯”. Therefore, in the sample D2, the overall evaluation was “good”.

In sample D3 (Bi _6.0 Ge _46.5 Te _47.5 ), as shown in FIG. 5, the target value was achieved in all items, and the item of 1 × speed recording error rate was evaluated as “Good”, otherwise The item was ◎ evaluation. Therefore, in the sample D3, the overall evaluation was “good”.

In sample D4 (Bi _6.0 Ge _48.0 Te _46.0 ), as shown in FIG. 5, the target values were achieved in all items. The items of 1 × speed recording error rate ratio and amplitude ratio were evaluated as “◎”, and the items of 1 × speed recording error rate and bit error rate after 1000 rewrites were evaluated as “◯”. Therefore, in the sample D4, the overall evaluation was “good”.

In sample D5 (Bi _6.0 Ge _49.0 Te _45.0 ), as shown in FIG. 5, the target values were achieved for all items, all items were evaluated as ◯, and overall evaluation was also ◯. It was.

In sample D6 (Bi _6.0 Ge _50.0 Te _44.0 ), as is clear from FIG. 5, the items of the 1 × speed recording error rate and the bit error rate after 1000 rewrites did not reach the target, and were evaluated comprehensively. Was x.

  From the above measurement results, in a D-series optical disc, when a Bi—Ge—Te phase change material having a Ge amount of 45.5 to 48 at% (composition of samples D2 to D5) was used as the recording layer, It was found that the target values were achieved for all the evaluation items.

[Comparative Example 1]
In Comparative Example 1, the composition of the recording layer is such that Te is added in excess of the composition on the line connecting Ge ₅₀ Te ₅₀ and Bi ₂ Te ₃ on the triangular composition diagram with Bi, Ge and Te as vertices. A recording layer was formed. In Comparative Example 1, an optical disk was produced in the same manner as in Example 1 except that the composition of the recording layer was changed. Also in this example, optical disks (A series optical disks) having various recording layers having different compositions were produced.

In this example, Ge ₄₉ Te ₅₁ and Bi _6.0 Ge _43.5 Te _50.5 targets were used as the sputtering target, and the recording layer was formed by co-sputtering. At this time, the sputtering power applied to each target was adjusted so that the composition of the recording layer was a desired composition. Specifically, the composition of the recording layer is Bi _1.0 Ge _48.0 Te _51.0 (sample number A1), Bi _2.0 Ge _47.0 Te _51.0 (sample number A2), Bi3 _{. 0} Ge _46.0 Te _51.0 (sample number A3), Bi _4.0 Ge _44.5 Te _51.5 (sample number A4) and Bi _5.0 Ge _43.5 Te _51.5 (sample number A5) An optical disc was prepared.

  The same evaluation as in Example 1 was performed on the A-series optical disk manufactured in this example. The results are shown in FIG. The target value of each evaluation item and the evaluation criteria of ◎, ○, and × in FIG. 6 are the same as in Example 1.

  As is clear from FIG. 6, in all the samples A1 to A5, the items of the bit error rate and the amplitude ratio after 1000 rewrites did not reach the target, and the overall evaluation was x. That is, it was found that the A-series optical disc is not practical as an information recording medium for HD DVD 1 × speed.

[Comparative Example 2]
In Comparative Example 2, the recording layer was recorded such that the composition of the recording layer was such that Ge was added in excess on the triangular composition diagram with Bi, Ge, and Te as vertices as compared to the composition line of the recording layer of the D-series optical disc. A layer was formed. In Comparative Example 2, an optical disk was produced in the same manner as in Example 1 except that the composition of the recording layer was changed. Also in this example, optical disks having various recording layers with different compositions (E-series optical disks) were produced.

In this example, Bi _7.0 Ge _53.0 Te _40.0 and Bi _6.0 Ge _43.5 Te _50.5 targets were used as sputtering targets, and a recording layer was formed by co-sputtering. At this time, the sputtering power applied to each target was adjusted so that the composition of the recording layer was a desired composition. Specifically, the composition of the recording layer is Bi _6.5 Ge _45.0 Te _48.5 (sample number E1), Bi _7.0 Ge _47.0 Te _46.0 (sample number E2) and Bi _{7. An} optical disc to be ₅ Ge _49.5 Te _43.0 (sample number E3) was produced.

  The same evaluation as in Example 1 was performed on the E-series optical disc manufactured in this example. The results are shown in FIG. In addition, the target value of each evaluation item and the evaluation criteria of ◎, ○, and × in FIG.

  As is clear from FIG. 7, in the sample E1, the item of the 1 × speed recording error rate ratio did not reach the target, and the overall evaluation was x. In Samples E2 and E3, the items of the 1 × speed recording error rate and the 1 × speed recording error rate ratio did not reach the target, and the overall evaluation was “x”. That is, it has been found that E-series optical discs are not practical as information recording media for HD DVD 1 × speed.

[Optimum recording layer composition range]
From the evaluation results of Examples 1 to 3, Comparative Example 1 and Comparative Example 2 described above, the practical composition range of the recording layer as an information recording medium for HD DVD 1 × speed is the composition range surrounded by the following composition points: I found out that FIG. 8 shows the composition range more specifically. The range surrounded by the thick line in FIG. 8 (including on the line) is the optimum composition range.
B4 (Bi _2.5 , Ge _47.0 , Te _50.5 )
_{B2 (Bi 1.5, Ge 48.0,} Te 50.5)
_{C3 (Bi 3.5, Ge 48.0,} Te 48.5)
D5 (Bi _6.0 , Ge _49.0 , Te _45.0 )
D2 (Bi _6.0 , Ge _45.5 , Te _48.5 )
C4 (Bi _4.0 , Ge _47.0 , Te _49.0 )

  In addition, 100 optical disks each having recording layers of composition points B4, B2, C2, D5, D2, and C4 shown on the triangular composition diagram of FIG. The number of optical disks to be found (pass rate) was examined. As a result, it was found that, for all the optical disks having the composition points B4, B2, C2, D5, D2 and C4, the overall evaluation was “good” for 90 or more of 100 optical disks, and the productivity was excellent.

  In Example 4, the bit error rate was measured by changing the recording linear velocity for the optical disc having the recording layer having the above composition range, and the optimum recording linear velocity range in the optical disc of the present invention was examined. In this example, the recording linear velocity was changed in the range of 4.0 m / sec to 8.4 m / sec. Since the wavelength λ of the laser light is 405 nm and the numerical aperture NA of the objective lens is 0.65, the parameter (λ / NA) / V representing the time for the laser light spot to pass through a certain point on the optical disk. Is changed in the range of 74.2 ≦ (λ / NA) /V≦155.8.

In this example, the above measurement was performed on an optical disc having a recording layer composition of Bi _3.5 Ge _48.0 Te _48.5 (C4 composition shown in FIG. 8). The results are shown in FIG. In FIG. 9, the horizontal axis represents the parameter (λ / NA) / V, and the vertical axis represents the bit error rate. Here, the target level of the bit error rate is set to 5.0 × 10 ⁻⁵ .

As is apparent from FIG. 9, the bit error rate is within the range of the parameter (λ / NA) / V of 76.0 to 124.6, that is, the recording linear velocity of 5.0 m / sec to 8.2 m / sec. It was 5.0 × 10 ⁻⁵ or less and was at the target level. However, when the recording linear velocity was 8.4 m / sec ((λ / NA) /V=74.2), the bit error rate was 5.3 × 10 ⁻⁵ and the target was not achieved. When the recording linear velocity is 4 ((λ / NA) /V=155.8) and 4.4 m / sec ((λ / NA) /V=141.6), the bit error rate is 8. It became ^0x10-5 and ^5.2x10-5 , and the target was not achieved.

The above measurement was similarly performed on an optical disc having a recording layer having the composition points B4, B2, C2, D5 and D2 in FIG. As a result, in all optical disks, the bit error rate is 5.0 × in the recording linear velocity range of 5.0 m / sec to 8.2 m / sec (76.0 ≦ (λ / NA) /V≦124.6). ^10-5 or less, and the target was not achieved in other linear speed ranges.

  In Example 5, 100 optical discs each having a different material (refractive index) and film thickness of the second protective layer provided between the recording layer and the thermal diffusion layer were produced, respectively, and the 1 × speed recording bit error rate ( Error rate 2) Pass rate in the evaluation items of the bit error rate ratio of 1 × speed recording (error rate 3 / error rate 1) and the bit error rate after 1000 rewrites (○ There are 100 optical discs to be evaluated) Was measured.

Specifically, the material for forming the second protective layer is formed of (ZnS) ₈₀ (SiO ₂ ) ₂₀ (refractive index n = 2.3) or SiO ₂ (refractive index n = 1.5), and (ZnS) ₈₀ (SiO ₂ ) ₂₀ was produced in a range of 5 to 16 nm, and for SiO ₂ , 100 various optical disks were produced each having a thickness in the range of 8 to 25 nm. An optical disk was manufactured in the same manner as in Example 1 except that the material (refractive index) and film thickness of the second protective layer were changed. The material of the second protective layer is not limited to the above material, and any material can be used as long as it is a transparent film having an extinction coefficient k ≦ 0.1.

Further, in this example, the above measurement was performed on an optical disc having a recording layer composition of Bi _3.5 Ge _48.0 Te _48.5 (composition of C4 shown in FIG. 8). The results are shown in FIG. FIG. 10 shows not only the pass rate of each evaluation item but also the complex optical constant (refractive index n and extinction coefficient k) and optical path length n × d (d: film thickness) at a wavelength of 405 nm.

First, the evaluation result when (ZnS) ₈₀ (SiO ₂ ) ₂₀ (refractive index n = 2.3) is used as the material of the second protective layer will be described. In addition, in the notation of the pass rate in FIG. 10, for example, when 90 optical discs out of 100 were evaluated as “good”, the pass rate was displayed as 90/100.

  When the film thickness is d = 5 nm (optical path length n × d = 11.5 nm), the pass rate of the 1 × speed recording bit error rate is 75/100 as shown in FIG. The pass rate was 50/100, and a large number of optical disks with large cross erase were confirmed.

  When the film thickness d = 6 nm to 16 nm (optical path length n × d = 13.8 to 34.5 nm), the pass rate of all evaluation items is 90/100 or more as shown in FIG. It was confirmed that the information recording medium for double speed was practical.

  When the film thickness d = 16 nm (optical path length n × d = 36.8 nm), as shown in FIG. 10, the pass rate of the 1 × speed recording bit error rate is 75/100, and the bit error rate after 1000 rewrites is The acceptance rate was 50/100, and a large number of optical discs whose signal quality deteriorated after 1000 rewrites were confirmed.

Next, the evaluation result when SiO ₂ (refractive index n = 1.5) is used as the material of the second protective layer will be described.

  When the film thickness is d = 8 nm (optical path length n × d = 12.0 nm), the pass rate of the 1 × speed recording bit error rate is 75/100 as shown in FIG. The pass rate was 45/100, and a large number of optical disks with large cross erase were confirmed.

  When the film thickness d = 9 nm to 24 nm (optical path length n × d = 13.5 to 36.0 nm), as shown in FIG. 10, the pass rate is 90/100 or more for all evaluation items, and the HD DVD1 It was confirmed that the information recording medium for double speed was practical.

  When the film thickness is d = 25 nm (optical path length n × d = 37.5 nm), as shown in FIG. 10, the pass rate of the 1 × speed recording bit error rate is 75/100, and the bit error rate after 1000 rewrites. The pass rate was 40/100, and many optical discs whose signal quality deteriorated after 1000 rewrites were confirmed.

  From the results of FIG. 10, by appropriately selecting the formation material and film thickness of the second protective layer so that the extinction coefficient k is k ≦ 0.1 and the optical path length n × d is about 13 to 35, A pass rate of 90/100 or more was obtained for all the items evaluated in this example, and it was confirmed that the information recording medium for HD DVD 1 × speed was practical.

[Optimum configuration]
The optimum composition and optimum film thickness of each layer constituting the information recording medium of the present invention will be described below.

(First protective layer)
The substance present on the light incident side of the first protective layer is a plastic substrate such as polycarbonate or an organic substance such as an ultraviolet curable resin. Moreover, these refractive indexes are about 1.4-1.6. In order to effectively cause reflection between the organic material and the first protective layer, the refractive index of the first protective layer is desirably 2.0 or more. The refractive index of the first protective layer is optically greater than or equal to the refractive index of the substance existing on the light incident side (corresponding to the substrate in this embodiment), and the first protective layer is within a range where no light is absorbed. The higher the refractive index, the better. Specifically, the first protective layer has a refractive index n of 2.0 to 3.0 and is formed of a material that does not absorb light, and in particular, a metal oxide, carbide, nitride, sulfide, It is desirable to contain selenide.

The thermal conductivity of the first protective layer is desirably at least 2 W / m · K. In particular, a ZnS—SiO ₂ -based compound has a low thermal conductivity, and is optimal as the first protective layer. Furthermore, the material with the addition of SnO ₂ or ZnS to SnO _2, CdS, SnS, GeS, the added material a sulfide such as PbS or SnO ₂ in the _Cr 2 _O 3, _Mo 3 _O transition metal oxides, such as _4, Not only has a low thermal conductivity, but is also more thermally stable than a ZnS—SiO ₂ -based material. Therefore, the thickness of the first interface layer provided between the first protective layer and the recording layer is 2 nm or less. Even in this case, since the melt into the recording layer does not occur, particularly excellent characteristics as the first protective layer are exhibited.

  When the wavelength of the laser beam is about 405 nm, the optimum film thickness of the first protective layer is 50 nm to 90 nm in order to effectively use the optical interference between the substrate and the recording layer.

(First interface layer)
Since the melting point of the phase change material used for the recording layer of the information recording medium of the present invention is as high as 650 ° C. or higher, a thermally extremely stable first interface layer may be provided between the first protective layer and the recording layer. desirable. Specifically, as a material for forming the first interface layer, refractory oxides such as Cr ₂ O ₃ , Ge ₃ N ₄ , SiC, refractory nitrides, and refractory carbides are desirable. It is stable and does not deteriorate due to film peeling even after long-term storage.

  In addition, it is more desirable that the first interface layer contains a material such as Bi, Sn, or Pb that promotes crystallization of the recording layer, since an effect of suppressing recrystallization of the recording layer can be obtained. In particular, Te, oxides of Bi, Sn, Pb, Te oxides of Bi, Sn, Pb, mixtures of oxides and germanium nitride, Te oxides of Bi, Sn, Pb, oxides and transition metal oxides A mixture with a transition metal nitride is desirable. Since transition metals easily change their valence, even if elements such as Bi, Sn, Pb, and Te are liberated, the transition metal changes its valence, and the transition metal and Bi, Sn, Pb, Te, etc. This is because a bond occurs between the two to produce a thermally stable compound. In particular, Cr, Mo, and W are excellent materials because they have a high melting point and can easily change the valence, so that a thermally stable compound is easily generated with the metal.

  It is desirable that the Bi, Sn, and Pb Te compounds and oxides in the first interface layer be as much as possible in order to promote crystallization of the recording layer. However, compared with the second interface layer, the first interface layer is likely to be heated to a high temperature due to laser beam irradiation, and problems such as the interface layer material being melted into the recording film occur. Therefore, Te compounds of at least Bi, Sn, and Pb are generated. It is necessary to keep the oxide content to 70% or less.

  The effect is exhibited if the thickness of the first interface layer is 0.5 nm or more. However, when the film thickness of the first interface layer is less than 2 nm, the material for forming the first protective layer may pass through the first interface layer and dissolve in the recording layer, which may deteriorate the reproduction signal quality after many rewrites. Therefore, the film thickness of the first interface layer is desirably 2 nm or more. Further, when the thickness of the first interface layer is greater than 10 nm, there is an optical adverse effect, and there are adverse effects such as a decrease in reflectivity and a decrease in signal amplitude. Therefore, the film thickness of the first interface layer is desirably 2 nm to 10 nm.

(Recording layer)
As described above, if the composition of the Bi-Ge-Te phase change recording layer material is within the range surrounded by the following composition points B4, B2, C3, D4, D2 and C4, for example, HD Even when information is recorded / reproduced at a standard speed (1 × speed) on a DVD, the above-mentioned problems 1 to 3 (record mark shrink, cross erase and heat damage problems) can be solved, and the recorded data It is possible to provide an information recording medium with high reliability and excellent durability against repeated data recording.
B4 (Bi _2.5 , Ge _47.0 , Te _50.5 )
_{B2 (Bi 1.5, Ge 48.0,} Te 50.5)
_{C3 (Bi 3.5, Ge 48.0,} Te 48.5)
D5 (Bi _6.0 , Ge _49.0 , Te _45.0 )
D2 (Bi _6.0 , Ge _45.5 , Te _48.5 )
C4 (Bi _4.0 , Ge _47.0 , Te _49.0 )

  Further, within the composition range of the recording layer, Si, Sn, Pb, or the like, which is a family element, may be used instead of Ge. In addition, by adding an appropriate amount of Si, Sn, Pb or the like, the applicable linear velocity range can be easily adjusted. For example, when Si is added and a part of Ge is replaced with Si, SiTe having a melting point higher than that of Ge or GeTe and having a low crystallization speed is generated. Is suppressed. Further, when GeTe is replaced by SnTe or PbTe, the nucleation speed is improved, so that the lack of erasure during high-speed recording can be compensated.

  Furthermore, when B is added to the Bi—Ge—Te phase change material used for the recording layer of the information recording medium of the present invention, recrystallization is further suppressed, so that an information recording medium exhibiting superior performance can be obtained. can get. This is presumably because B not only has the effect of suppressing recrystallization, like Ge, but also because B atoms are very small and can be segregated quickly.

  As long as the recording layer material used in the information recording medium of the present invention maintains the above-mentioned composition range relationship, even if impurities are mixed, the atomic percentage of impurities is within 1%. The effect of the present invention is not lost.

  In the structure of the information recording medium of the present invention, the recording layer preferably has a thickness of 5 nm to 12 nm. In particular, when the recording layer is formed with a film thickness of 8 nm to 12 nm, it is possible to suppress the deterioration of the reproduction signal due to the flow of the recording film at the time of many rewrites, and further to optimize the modulation degree optically.

(Second interface layer)
Since the melting point of the phase change material used for the recording layer of the information recording medium of the present invention is as high as 650 ° C. or higher, a thermally extremely stable second interface layer is provided between the second protective layer and the recording layer. It is desirable. Specifically, the second interface layer is preferably a high-melting point oxide such as Cr ₂ O ₃ , Ge ₃ N ₄ , or SiC, a high-melting point nitride, or a high-melting point carbide. These materials are thermally stable and do not deteriorate due to film peeling even after long-term storage.

  In addition, it is more desirable that the second interface layer contains a material such as Bi, Sn, or Pb that promotes crystallization of the recording layer, because an effect of suppressing recrystallization of the recording layer can be obtained. In particular, Te, oxides of Bi, Sn, Pb, Te oxides of Bi, Sn, Pb, mixtures of oxides and germanium nitride, Te oxides of Bi, Sn, Pb, oxides and transition metal oxides A mixture with a transition metal nitride is desirable. Since transition metals easily change their valence, even if elements such as Bi, Sn, Pb, and Te are liberated, the transition metal changes its valence, and the transition metal and Bi, Sn, Pb, Te, etc. This is because a bond occurs between the two to produce a thermally stable compound. In particular, Cr, Mo, and W are excellent materials because they have a high melting point and can easily change their valence, so that a thermally stable compound is easily generated with the metal.

  The Bi, Sn, and Pb Te compounds and oxides in the second interface layer are desirably as much as possible in order to promote crystallization of the recording layer. However, the second interface layer is likely to be heated to a high temperature by laser beam irradiation, and problems such as the second interface layer material being melted into the recording layer occur. It is necessary to keep it below 70%.

  The above effects are exhibited when the thickness of the second interface layer is 0.5 nm or more. However, if the film thickness is thinner than 1 nm, the material for forming the second protective layer may pass through the second interface layer and dissolve in the recording layer, which may deteriorate the reproduction signal quality after many rewrites. Therefore, the film thickness of the second interface layer is preferably 1 nm or more. Further, when the thickness of the second interface layer is greater than 5 nm, there is an optical adverse effect, and there are adverse effects such as a decrease in reflectivity and a decrease in signal amplitude. Therefore, the film thickness of the second interface layer is desirably 1 nm to 5 nm.

(Second protective layer)
The second protective layer is a material that does not absorb light, and particularly preferably contains a metal oxide, carbide, nitride, sulfide, or selenide. The thermal conductivity of the second protective layer is desirably at least 2 W / m · K. In particular, a ZnS—SiO ₂ -based compound is optimal as the second protective layer because of its low thermal conductivity. Furthermore, the addition of SnO _2, or ZnS to SnO _2, CdS, SnS, GeS, material obtained by adding a sulfide, such as PbS, or SnO ₂ in the _{Cr 2 O 3, Mo 3 O} 4 such as a transition metal oxide material Is not only low in thermal conductivity but also more thermally stable than ZnS—SiO ₂ -based material, so that even when the thickness of the second interface layer is 1 nm or less, the second to the recording layer is Since the material for forming the interface layer does not melt, it exhibits particularly excellent characteristics as the second protective layer.

(Thermal diffusion layer)
As a material for forming the thermal diffusion layer, a metal or alloy having high reflectivity and high thermal conductivity is desirable, and a material having a total content of Al, Cu, Ag, Au, Pt, and Pd of 90 atomic% or more is desirable. In addition, as a material for forming the thermal diffusion layer, a material having a high melting point and high hardness such as Cr, Mo, and W, and an alloy of these materials are also desirable. When these materials are used, recording at the time of many rewrites is possible. Deterioration due to flow of the layer material can be prevented.

  Specifically, in particular, when the thermal diffusion layer is formed of a material containing Al at 95 atomic% or more, it is inexpensive and has not only the effects of high CNR, high recording sensitivity, and excellent resistance to multiple rewrites, but also cross erase. The effect that the reduction effect becomes extremely large is obtained. In particular, when the thermal diffusion layer is formed of a material containing 95 atomic% or more of Al, an information recording medium that is inexpensive and excellent in corrosion resistance can be realized. As additive elements for Al, Co, Ti, Cr, Ni, Mg, Si, V, Ca, Fe, Zn, Zr, Nb, Mo, Rh, Sn, Sb, Te, Ta, W, Ir, Pb, B and C is excellent in terms of corrosion resistance, but when Co, Cr, Ti, Ni, or Fe is used as an additive element, there is a great effect in improving corrosion resistance.

  The thickness of the thermal diffusion layer is desirably 40 nm to 200 nm. When the thickness of the thermal diffusion layer is less than 40 nm, the heat generated in the recording layer is difficult to diffuse. Therefore, the recording layer is likely to be deteriorated especially when rewritten about 100,000 times, and cross erase occurs. It may be easier. Further, when the thickness of the thermal diffusion layer is less than 40 nm, light is transmitted, so that it is difficult to use the thermal diffusion layer and the reproduction signal amplitude may be reduced. On the other hand, when the thickness of the thermal diffusion layer is greater than 200 nm, productivity deteriorates, and internal warping of the thermal diffusion layer causes warpage of the substrate, making it impossible to accurately record and reproduce information. There is a case. Moreover, if the film thickness of a thermal diffusion layer is 40 nm-90 nm, it is excellent in terms of corrosion resistance and productivity, and is more desirable.

  The thermal conductivity of the thermal diffusion layer used in the information recording medium of the present invention is preferably 100 W / m · K or more. By setting the thermal conductivity to such a value, a cross erase reduction effect can be exhibited.

  As described above, in the information recording medium of the present invention, the wavelength of the laser beam is λ (nm), the numerical aperture of the objective lens for condensing the laser beam is NA, and the recording linear velocity is V (m / sec). , Even if information is recorded / reproduced under the condition of 76.0 nsec ≦ (λ / NA) /V≦115.0 nsec (where λ = 390 nm to 420 nm), the recording mark shrinks, cross erases, and heat It is an information recording medium that can solve all the problems of damage, has high reliability of recorded data, and is excellent in durability against repeated recording of data. Therefore, the information recording medium of the present invention is suitable as an information recording medium for HD DVD 1 × speed, for example.

FIG. 1 is a schematic cross-sectional view of a phase change recording optical disk manufactured in Example 1. FIG. FIG. 2 is a schematic optical drawing of the recording / reproducing apparatus used for the evaluation of the optical disk produced in Example 1. FIG. 3 shows the evaluation results of the optical disk produced in Example 1. FIG. 4 shows the evaluation results of the optical disk produced in Example 2. FIG. 5 shows the evaluation results of the optical disc produced in Example 3. FIG. 6 shows the evaluation results of the optical disc produced in Comparative Example 1. FIG. 7 shows the evaluation results of the optical disk produced in Comparative Example 2. FIG. 8 is a triangular composition diagram of the Bi—Ge—Te phase change material, and shows an optimum composition range of the Bi—Ge—Te phase change material used for the recording layer of the present invention. FIG. 9 is a diagram showing the relationship between the recording linear velocity and the bit error rate in the information recording medium of the present invention. FIG. 10 is a diagram showing the relationship between the configuration of the second protective layer film and the bit error rate in the information recording medium of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 1st protective layer 3 1st interface layer 4 Recording layer 5 2nd interface layer 6 2nd protective layer 7 Thermal diffusion layer 8 UV curable resin layer

Claims (12)

When the wavelength of the laser beam is λ [nm], the numerical aperture of the objective lens for condensing the laser beam is NA, and the recording linear velocity is V [m / sec], 76.0 [nsec] ≦ ( An information recording medium in which information can be rewritten a plurality of times by irradiating the laser beam under the condition of λ / NA) /V≦115.0 [nsec] and λ = 390 to 420 [nm],
A substrate,
A recording layer provided on the substrate and formed of a phase change material containing Bi, Ge, and Te;
An information recording medium characterized in that the composition of Bi, Ge and Te in the recording layer is in a composition range surrounded by the following points on the triangular composition diagram of Bi, Ge and Te.
B4 (Bi _2.5 , Ge _47.0 , Te _50.5 )
_{B2 (Bi 1.5, Ge 48.0,} Te 50.5)
_{C3 (Bi 3.5, Ge 48.0,} Te 48.5)
D5 (Bi _6.0 , Ge _49.0 , Te _45.0 )
D2 (Bi _6.0 , Ge _45.5 , Te _48.5 )
C4 (Bi _4.0 , Ge _47.0 , Te _49.0 ) The information recording medium according to claim 1, further comprising a thermal diffusion layer on a side opposite to the laser beam incident side of the recording layer. The information recording medium according to claim 2, wherein the thermal diffusion layer has a thermal conductivity of 100 [W / m · K] or more. Further, a protective layer is provided between the recording layer and the thermal diffusion layer, and the refractive index is n and the extinction coefficient is k among the complex optical constants in the wavelength range of 390 to 420 [nm] of the protective layer, 4. The information recording medium according to claim 2, wherein the relationship of k ≦ 0.1 and 13 ≦ n × d ≦ 35 is established when the thickness of the protective layer is d [nm]. . The information recording medium according to any one of claims 1 to 4, wherein the recording layer has a thickness of 5 to 12 [nm]. The information recording medium according to claim 1, further comprising an interface layer provided in contact with at least one surface of the recording layer. The information recording medium has a disc shape, and concentric or spiral grooves are formed on the substrate, and both of the grooves and the grooves are used as recording tracks. Item 7. The information recording medium according to any one of Items 1 to 6. 8. The information recording medium according to claim 7, wherein a track pitch TP of the recording track is in a range of 0.5 * ([lambda] / NA) to 0.6 * ([lambda] / NA). The wavelength λ of the laser beam is λ = 400 to 410 [nm], the numerical aperture NA of the objective lens is NA = 0.6 to 0.65, and the track pitch TP is 0.34 [μm. The information recording medium according to claim 8, wherein: The information recording medium has a disk-like shape, and concentric or spiral grooves are formed on the substrate, and at least one of the grooves and the grooves is used as a recording track. The information recording medium according to claim 1, wherein at least one of the grooves meanders. The information recording medium according to claim 10, wherein a track pitch TP of the recording track is in a range of 0.3 × (λ / NA) to 0.4 × (λ / NA). The wavelength λ of the laser beam is λ = 400 to 410 [nm], the numerical aperture NA of the objective lens is NA = 0.6 to 0.65, and the track pitch TP is 0.4 [μm. The information recording medium according to claim 11, wherein:

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