JP4998224B2 - Optical information recording medium - Google Patents

Description

The present invention relates to an optical information recording medium for recording or reproducing information by laser beam irradiation, in particular, it relates to an optical information recording medium comprising a phase change material as the super resolution mask layer.

  An optical information recording medium (commonly called “optical disk”) that reproduces information by laser light irradiation is a read-only optical disk that can only reproduce information, and a write-once optical disk that can record information only once and cannot be rewritten. The optical disc is classified into a rewritable optical disc such as a magneto-optical disc and a phase change optical disc that can rewrite information.

The capacity of the optical disk basically depends on the beam diameter of the laser beam used for recording / reproducing information. The smaller the beam diameter, the higher density information can be reproduced without error. Here, even if the light emitted from the laser is focused through the objective lens, it is not focused as one point due to the influence of diffraction, but is formed as a beam having a finite width.
This is generally called the diffraction limit. When the wavelength of the laser light is λ and the numerical aperture of the objective lens is NA (Numerical Aperture), λ / (4NA) is the limit of the reproduction resolution.

  Therefore, for example, when λ = 405 [nm] and NA = 0.65, a pit having a length of 156 [nm] or less cannot be read accurately. Therefore, for example, in order to accurately read a pit having a length of 156 [nm] or less, it is necessary to make the wavelength of the laser light shorter than 405 [nm] or make the NA of the objective lens larger than 0.65.

  However, the current laser technology has a limit in providing a short wavelength laser, and there is a limit in that it is expensive to manufacture an objective lens having a large numerical aperture. Furthermore, as the numerical aperture of the objective lens increases, the distance between the pickup and the disk decreases. For this reason, if recording and reproduction of high-density information is performed to the limit, there is a risk of collision between the optical head and the optical disk. When this collision occurs, the surface of the disk is often damaged, and sometimes data may be lost.

As a technique for exceeding the diffraction limit of an optical disk, a technique related to medium super-resolution has been known (Patent Document 1). In this medium super-resolution, a mask layer (also referred to as a micro-aperture forming layer or a super-resolution layer) whose optical characteristics (reflectance) change nonlinearly with temperature is used.
This utilizes the fact that the beam diameter of the laser beam is effectively reduced because the reflectivity of the mask layer changes in a region where the temperature is higher than a certain temperature during laser irradiation. As a result, a minute recording mark can be reproduced, and a high density can be realized.

  The required characteristics of the mask layer are (1) the temperature change of the reflectance has a threshold value (the reflectance changes stepwise around the threshold temperature), and (2) around the threshold value. There are two types of change in reflectance.

As an example of a material having these two characteristics, there is a chalcegonide phase change material such as GeSbTe or AgInSbTe used for a recording layer of a phase change optical disk. The phase change material has a clear threshold temperature at which the reflectivity changes, that is, the melting point, and its optical characteristics are greatly different between the solid phase (crystalline state) and the molten state, and is therefore suitable for use as a mask layer ( Patent Documents 1 and 2).
JP 2004-030891 JP-A-2005-238221 JP-A-10-275360

  In a super-resolution optical disk that uses a phase change material as a mask layer, reproduction is performed by using a change in optical characteristics between when the phase change material is in a crystalline state and when it is in a molten state. In some areas, the phase change material is always melted.

  In phase change optical discs (hereinafter, phase change optical discs represent ordinary “rewritable optical discs”), recording is performed by melting an area irradiated with a recording pulse. Resolution reproduction is equivalent to recording with DC light. Therefore, the super-resolution optical disk is subjected to a higher heat load than the phase change optical disk, and higher repeatability is required than the phase change optical disk to ensure a predetermined number of reproductions.

  In this case, the technique disclosed in the above-mentioned Patent Document 3 (Japanese Patent Laid-Open No. 10-275360), that is, the technique of adding an interface layer such as a nitride in contact with the recording layer is a repetitive rewrite of the phase change optical disk. It is effective for improving the number of times.

However, the upper limit of the number of rewrites is still limited to about 10 ³ to 10 ⁴ times in each of the related technologies described above, and the number of repetitive recordings with DC light of about 10 ⁵ to 10 ⁶ times, that is, phase change. It has been very difficult to realize the number of reproductions of about 10 ⁵ to 10 ⁶ times in a super-resolution optical disk using materials.

(Object of invention)
An object of the present invention is to provide a high-speed and high-capacity super-resolution optical disk with high reliability that can improve the above-mentioned disadvantages of the related art and in particular can ensure a sufficient number of reproducible times.

In order to achieve the above object, an optical information recording medium according to the present invention is an optical information recording medium for recording or reproducing information by laser beam irradiation, and comprises a mixture of Ga oxide and Cr oxide. And the composition of the protective layer or interface layer is represented by (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x , where x is 0.5 ≦ x ≦ It is characterized by 0.8. In the present invention, for the recording layer formed of a super-resolution layer represented by (GeTe) _1-y (Bi ₂ Te ₃ ) _y , the value of y is 0.3 ≦ y ≦ 1, The super-resolution layer is in contact with a dielectric layer (protective layer) made of a mixture of Ga oxide and Cr oxide.

  Since the present invention is configured as described above, it is an optical information recording medium using a phase change material, and the number of repeated reproductions can be greatly improved, which is particularly effective for a high-speed crystallization material. Therefore, it is possible to provide an unprecedented excellent optical information recording medium capable of realizing a high-speed and large-capacity optical information recording medium.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
Here, the basic configuration content of this embodiment will be described first, and then the overall content will be described.
FIG. 1 shows a basic laminated structure of an optical information recording medium in this embodiment. As shown in FIG. 1, the optical information recording medium in this embodiment includes a recording layer 11 and protective layers 1 and 2 on both sides of the recording layer 11 , and records information on the recording layer 11 by laser light irradiation. Or it is the structure which performs reproduction | regeneration. Among these, the recording layer 11 described above is composed of a phase change layer that changes phase by laser light irradiation, and the protective layer is composed of a mixture of Ga oxide and Cr oxide.

Here, for the protective layers 1 and 2, the composition of the oxide mixture constituting the protective layers 1 and ₂ is (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x in the present embodiment. Furthermore, x in this formula was set to a value in the range of 0.5 ≦ x ≦ 0.8.

In the present embodiment, the phase change layer having the composition of the recording layer 11 described above is a super-resolution layer whose optical characteristics are changed by laser light irradiation. Further, for this super-resolution layer, the composition is composed of a substance represented by (GeTe) _1-y (Bi ₂ Te ₃ ) _y , and the value of the symbol y in the above formula is set to 0. It is characterized in that the value is in the range of 3 ≦ y ≦ 1.

Further, the protective layers 1 and 2 are made of another oxide different from (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x, and each of the protective layers 1 and 2 and the recording layer are recorded. In this case, interface layers 11A and 11B are provided between the layer 11 and the interface layer is formed of a mixture of oxides represented by (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x. The value of the symbol x in the formula was set to a value in the range of 0.5 ≦ x ≦ 0.8. In this case, the thickness of each of the interface layers 11A and 11B is set to be thinner than the thickness of the protective layers 1 and 2 as will be described later.

Here, the phase change layer having the composition of the recording layer 11 described above may be formed of a phase change material containing a large amount of elements such as Sb. Specifically, for example, the phase change layer having the composition of the recording layer 11 may be formed of Ge ₁ Sb ₂ Te ₄ , InSb, or GaSb.

This will be described more specifically below.
First, the configuration of FIG. 1 described above represents an example of a ROM medium. A configuration in which a protective layer 1, a super-resolution mask layer (super-resolution layer) 11, which is a recording layer, a protective layer 2, and a reflective layer 12 are sequentially laminated on a substrate 10 on which uneven pits (not shown) are formed. It has become. As the substrate 10, plastic such as polycarbonate (PC) or glass is used.

As the super-resolution mask layer 11, a general phase change material can be used, but a material having a high crystallization speed is particularly preferable in order to realize a high speed. In the case of a material having a slow crystallization rate, when it is cooled after melting and returned to the solid phase, it does not become a crystalline state but becomes an amorphous state.
Here, both the amorphous state and the molten state are states in which the atomic arrangement is disordered, and the optical characteristics are similar. As a result, the reflectivity becomes substantially the same, so that the melted region does not function as an effective mask layer, and high density cannot be realized.

  On the other hand, if the material has a sufficiently high crystallization rate, it will be in a crystalline state when cooled and then returned to the solid phase after melting. While the molten state is a disordered atomic arrangement, the crystalline state is an ordered atomic arrangement, and the optical constants are different between the two states. As a result, the reflectivity differs greatly, so that the molten region functions as an effective mask layer. The influence of the crystallization speed on the super-resolution effect is schematically shown in FIG.

  Whether it becomes crystalline or amorphous when cooled and returned to the solid phase after melting depends on the critical cooling rate Rc of the substance and the actual cooling rate R (temperature decrease per unit time before and after the melting point) If R (actual cooling rate) is smaller than Rc (critical cooling rate), recrystallization occurs after melting. Since the actual cooling rate R increases as the linear velocity during regeneration increases, it is necessary to use a material having a large Rc (critical cooling rate), that is, a material with a high crystallization rate, in order to realize high-speed regeneration.

In a phase change optical disk, a material with a high crystallization speed is used to realize high-speed recording, but in a region where a recording mark is formed, that is, a region where a high power laser is irradiated and returned to a solid phase after melting, The recording layer is in an amorphous state.
On the other hand, in a super-resolution optical disk using a phase change material, the phase change layer needs to be in a crystalline state rather than an amorphous state after melting.

  Therefore, in order to realize super-resolution capable of high-speed reproduction, a material having a higher crystallization speed than the recording layer used in the phase change optical disk is required. In a phase change optical disk, increasing the crystallization speed of the recording layer may degrade the storage stability of the amorphous state, but it is necessary to stably form the amorphous state when applied to super-resolution. Therefore, a material having a crystallization speed as fast as possible can be used.

A (GeTe)-(Bi ₂ Te ₃ ) pseudo binary composition is known as a recording layer of a phase change optical disk having a high crystallization speed, and the crystallization speed increases as the (Bi ₂ Te ₃ composition ratio) increases. _(GeTe) in the composition represented by the _{1-y Bi 2 Te 3)} y, if y ≧ 0.3, a crystalline state immediately after the film formation, the linear velocity 30 [m / s] as high linear velocity It was found that it was difficult to form an amorphous state even when melted underneath, and it was suitable for high-speed super-resolution reproduction.

However, the present inventor has found that increasing y, that is, increasing the Bi composition in the film causes a problem that the number of reproducible times decreases. This is because Bi tends to cause mutual diffusion with the ZnS—SiO ₂ protective layer material used for the optical disc.
Therefore, as a result of studying a material that hardly causes interdiffusion with Bi and has high durability against a thermal load, it was tried that a dielectric layer made of a mixture of Ga oxide and Cr oxide is suitable. The inventor found out as a result of mistakes. In this embodiment, this dielectric layer is used as the protective layers 1 and 2.

The dielectric layers (protective layers 1 and 2) may be formed by sputtering a mixed target of Ga ₂ O ₃ and Cr ₂ O ₃ using a rare gas or a rare gas and oxygen.
Here, the dielectric layers made of a mixture of Ga oxide and Cr oxide are in contact with the recording layer (super-resolution layer 11) and have interface layers 11A and 11B having a thickness of about 5 nm (see FIG. 4). ), Or as protective layers 1 and 2 having a thickness of 10 nm or more as shown in FIG.

On the PC substrate 10, (Ga ₂ O ₃ ) — (Cr ₂ O ₃ ) as a dielectric layer (protective layer) 1 has a thickness of 80 [nm], and as a super-resolution layer (recording layer) 11 (GeTe) _{0 .5 (Bi} 2 _{Te 3) 0.5} thickness 15 [nm], the dielectric layer (protective layer) as a _{2 (Ga 2 O 3) -} (Cr 2 O 3) a thickness 15 [nm], the reflection As the layer 12, Ag Pd Cu was sequentially laminated by sputtering at a thickness of 100 nm.
The _Ga 2 _{O 3 -Cr} 2 _O 3 is a dielectric layer (protective _{layer) 1, (Ga 2 O 3} ) 50 [mol%] - a target made of _{(Cr 2} O 3) 50 [mol%], A film was formed by sputtering in an Ar gas atmosphere. As for this Ga ₂ O ₃ —Cr ₂ O ₃ , when the film composition actually formed was analyzed, it was confirmed that it was the same as the target composition within the range of the analysis error (detection accuracy).

  As the PC substrate, a substrate on which (1-7) modulated random pattern pits having a depth of 40 [nm] and a shortest pit length of 100 [nm] were formed in advance was used. Using an optical head with a wavelength of 405 [nm] and an objective lens NA = 0.65, reproduction characteristics were evaluated at a linear velocity of 6.6 [m / s]. FIG. 3 shows the relationship between error rate and reproduction power.

Since the diffraction limit when this optical head is used is 156 [nm], data cannot be correctly reproduced with a reproduction power of 0.5 to 2.5 [mW] where super-resolution does not occur. In the reproduction power of 3 [mW] or more, the GeBiTe layer melts and super-resolution occurs, so that data can be reproduced with a low error rate. The error rate remained low even after repeated reproduction 10 ⁵ times at 3 [mW].

(Comparative Example 1)
A disk (Disk) having the configuration shown in FIG. 4 (Table 1) was prepared using the same substrate as in Example 1 described above, and the error rate after reproduction was measured 10 ³ times and 10 ⁵ times. The reproduction power was set to a power that gives the lowest error rate in the initial reproduction for each disk. The configuration in this case is shown in FIG.

Here, in FIG. 4 (Table 1), GaO—CrO means “(Ga ₂ O ₃ ) 50 [mol%] − (Cr ₂ O ₃ ) 50 [mol%]”.
Further, “(ZnS) 80 [mol%]-(SiO ₂ ) 20 [mol%]” was used as ZnS—SiO ₂ .

  Further, FIG. 5 is an explanatory diagram in the case where interface layers 11A and 11B are provided between the super-resolution layer 11 and the protective layers 1 and 2. FIG. In all the discs A, B, C, and D, the thickness of the super-resolution layer is 15 [nm], and when the interface layers 11A and 11B are provided, the thickness of each of the interface layers 11A and 11B is 5 [Nm], and the thickness of the reflective layer was 100 [nm]. The reproduction characteristics of each disk (Disk) obtained in this case are shown in FIG. 6 (Table 2).

In the disk (Disk) A using ZnS-SiO ₂ as a dielectric layer without an interface layer as the protective layers 1 and 2, the number of reproducible times is extremely low, and the error rate cannot be measured after 10 ³ times reproduction. It was.
The disk (Disk) B in which GeN, which is generally used in a rewritable phase change optical disk, is applied to the interface layers 11A and 11B can realize 10 ³ times of reproduction, but measures the error rate after 10 ⁵ times of reproduction. I couldn't.

On the other hand, discs C and D using Ga ₂ O ₃ —Cr ₂ O ₃ as interface layers 11A and 11B or protective layers (dielectric layers) 1 and 2 have low errors even after 10 ⁵ times of reproduction. It was confirmed that the rate could be realized.

On a 2-inch glass substrate, ZnS—SiO ₂ has a thickness of 50 nm as the protective layer (dielectric layer) 1 and Ga ₂ O ₃ —Cr ₂ O ₃ layer has a thickness of 5 nm as the interface layer 11A. The Bi ₂ Te ₃ layer is 15 nm thick as the super-resolution layer 11, the Ga ₂ O ₃ layer is 5 nm thick as the interface layer 11 B, and the protective layer (dielectric layer) 2 is ZnS-SiO 2. _A sample was formed by sequentially laminating ₂ with a thickness of 50 [nm] by sputtering. This sample was heated and cooled in the range from room temperature to 650 ° C., and the temperature dependence of reflectance and transmittance was measured.

FIG. 7 (Table 3) shows the value of x in the composition (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x and the change of the sample when it returns to room temperature after heating.

Bi ₂ Te _{3 had} a melting point of about 610 ° C., and in sample 2 or sample 3, a change in optical properties accompanying melting was observed at around 610 ° C. In the temperature range from 600 ° C. to 650 ° C., there was no particular change in the optical characteristics, and a reversible change in the optical characteristics (the same change during the temperature increase and during the temperature decrease) was shown during the cooling process.

On the other hand, in sample 1, the sample became transparent at a temperature lower than 600 ° C. during the temperature increase, and a reversible change in optical characteristics could not be observed. This change is considered to be caused as a result of ZnS—SiO ₂ of the dielectric layer diffusing into BiTe of the super-resolution layer 11.

In Sample 4, although a change in optical properties was observed at around 610 ° C., many voids (defects) that were thought to be caused by agglomeration that occurred during melting occurred. This is presumably because Cr ₂ O ₃ has high thermal durability but poor wettability with a phase change layer having a high Bi content, and the melted phase change layer becomes granular.

From these results (FIG. 7), mutual diffusion between ZnS-SiO ₂ constituting the protective layers (dielectric layers) 1 and ₂ and Bi ₂ Te ₃ layer constituting the super-resolution layer 11 is prevented. And 0.5 <= x <= 0.8 was specified as a suitable composition range which can ensure repetition durability.

In the second embodiment, as described above, the mutual diffusion prevention between ZnS-SiO ₂ and Bi has been described. However, the protective layer (or the interface layer) according to the present invention is not limited to Bi, and is mutually compatible with ZnS-SiO _2. A phase change material (for example, Ge ₁ Sb ₂ Te ₄ , InSb, GaSb, etc.) containing a large amount of elements such as Sb that easily causes diffusion also functions effectively as an anti-diffusion.

In the embodiment or comparative example described above, only the read-only optical information recording medium to which the super-resolution layer is applied has been described. However, each of these materials is configured to be applied to a recordable optical disk. May be.
In this case, for example, the Ga ₂ O ₃ —Cr ₂ O ₃ layer, the phase change super-resolution layer, the Ga ₂ O ₃ —Cr ₂ O ₃ layer, the recording layer, the dielectric layer, and the reflective layer are sequentially arranged from the laser incident surface. A laminated structure is formed, and Co ₃ O ₄ or the like can be used as the recording layer.

  As described above, by providing the dielectric layer made of the oxide of Ga and the oxide of Cr adjacent to the mask layer made of the phase change material as the protective layer or the interface layer, a sufficient number of times of reproduction can be achieved. Thus, it is possible to realize a high-resolution and high-capacity super-resolution optical disc that can remarkably improve reliability.

As described above, the description has centered on prevention of mutual diffusion of ZnS—SiO ₂ and Bi. However, this idea is common to the entire disk device mainly composed of the recording layer. The usefulness of the present invention is high in that the effective use of the change material is activated and leads to the development of a better material.

1 is a schematic configuration diagram showing an embodiment of an optical information recording medium according to the present invention. FIG. 2A is a diagram showing a mechanism for forming a mask in super-resolution using a phase change material in the recording layer of the optical information recording medium disclosed in FIG. 1, and FIG. FIG. 2B is an explanatory diagram showing a case where the crystallization speed is low. It is an example of the reproduction characteristic evaluation in 1st Example, and is a diagram which shows the relationship between reproduction power and an error rate. 6 is a table (Table 1) showing the configuration contents of comparative disks A, B, C, and D created in Comparative Example 1. FIG. It is explanatory drawing which shows the laminated structure of the optical information recording medium at the time of incorporating an interface layer in the Example of FIG. FIG. 5 is a chart (Table 2) showing measurement results of disks A, B, C, and D in FIG. 4. 10 is a table (Table 3) showing measurement results of temperature dependency of the optical information recording medium performed in Example 2. FIG.

Explanation of symbols

1, 2 Protective layer (dielectric layer)
10 Substrate 11 Super-resolution layer (super-resolution mask layer) which is a recording layer
11A, 11B Interface layer 12 Reflective layer

Claims (6)

An optical information recording medium having a structure that includes a recording layer and protective layers on both sides of the recording layer, and records or reproduces information on the recording layer by laser light irradiation,
The recording layer is composed of a phase change layer that changes phase by laser light irradiation, the phase change layer is a super-resolution layer whose optical characteristics change by laser light irradiation, and the protective layer is an oxide of Ga and an oxide of Cr And the composition is composed of
The composition of the mixture of oxides constituting the protective layer is (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x, and the value x is in the range of 0.5 ≦ x ≦ 0.8. An optical information recording medium characterized by that. The optical information recording medium according to claim 1 ,
The super-resolution layer is made of a material whose composition is represented by (GeTe) _1-y (Bi ₂ Te ₃ ) _y , and the value of the symbol y is in a range of 0.3 ≦ y ≦ 1. An optical information recording medium characterized by having a certain value. In the optical information recording medium according to claim 1 or 2 ,
Each protective layer is made of another oxide different from (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x, and an interface layer is formed between each protective layer and the recording layer, respectively. And this interface layer is formed of a mixture of oxides represented by (Ga ₂ O ₃ ) _1-x (Cr ₂ O ₃ ) _x , and the value of the symbol x in this case is 0.5 ≦ x ≦ 0 An optical information recording medium having a value in the range of .8. In the optical information recording medium according to claim 3 ,
An optical information recording medium characterized in that the thickness of each interface layer is set to be thinner than the thickness of each protective layer. The optical information recording medium according to claim 1,
An optical information recording medium, wherein the phase change layer having the composition of the recording layer is composed of a phase change material containing a large amount of elements such as Sb. The optical information recording medium according to claim 1,
An optical information recording medium, wherein the phase change layer having the composition of the recording layer is formed by using Ge1Sb ₂ Te ₄ , InSb, or GaSb as a composition.

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