JP3691501B2 - Optical recording medium - Google Patents

Description

  The present invention relates to a phase change optical recording medium capable of high-speed recording.

  With the increase in recording speed of DVDs, the composition ratio margin of the recording layer composition has been very limited. Many of patent documents employ a system in which Ga, Ge, In or the like is added based on Sb—Te (see Patent Documents 1 to 16 and Non-Patent Document 1). This is because the chalcogen elements (S, Se, Te) have the characteristic of “combining with many elements to form various amorphous states”, which is essential for phase change materials, especially Te. This is one of the reasons that has attracted attention as a constituent element. However, according to the study by the present inventors, when Te is added in the optimum Ga—Sb, Ge—Sb, and In—Sb composition region targeted for high-speed recording equivalent to DVD 8 × speed, Te becomes a crystal of the recording layer material. It has been found that good high-speed recording characteristics cannot be obtained because the conversion speed is lowered. The recording layer of the present invention contains at least one element selected from Au, Ag, and Cu as essential constituent elements, and these are crystalline particles inside the recording layer. There are no specific features.

Japanese Patent Application Laid-Open No. 60-179954 Japanese Patent Publication No. 03-052651 Japanese Patent Publication No. 04-001933 JP 05-286249 A Japanese Patent Application Laid-Open No. 07-0665414 Japanese Patent Laid-Open No. 07-120867 JP 08-212604 A JP 2000-190636 A JP 2000-339750 A Japanese Patent Application Laid-Open No. 2001-067722 JP 2002-264514 A JP 2002-283726 A JP 2002-331758 A JP 2003-006859 A Japanese Patent No. 2941848 Japanese Patent No. 3214210 “Phase-Change optical data storage in GaSb”, Applied Optics, Vol. 26, no. 22115, November, 1987

An object of the present invention is to provide a phase change optical recording medium suitable for high-speed recording equivalent to DVD 8 × speed, easy to initialize a recording material, and having a uniform reflectance distribution after initialization.
As one of optical recording media in which information can be rewritten by irradiation with a semiconductor laser beam, a so-called phase change optical recording medium using a phase transition between crystal and amorphous or between crystal and crystal is known. This phase-change optical recording medium is capable of single-beam repetitive recording and has a simpler optical system on the drive side, and is widely used worldwide as a recording medium related to computers and audiovisuals. . Optical recording media such as CD-Rs and CD-RWs have been used for high-speed recording with the spread, and phase change optical recording media are also increasing in capacity and density of recording media from the perspective of high-density image recording. Therefore, high speed recording is essential.
Such a phase change optical recording medium generally has a substrate and a recording layer, and protective layers having heat resistance and translucency are provided on both sides of the recording layer. A metal reflection layer made of metal or alloy is laminated on the protective layer opposite to the light beam incident direction. Recording and erasing can be performed only by changing the power of the laser beam. In general, the recording system uses a crystalline state as an unrecorded / erased state and an amorphous state as a recording mark (amorphous mark).

  The recording principle of the phase change optical recording medium is as follows. To switch the crystalline state / amorphous state of the recording layer, a focused laser beam pulsed at three output levels is used. In this case, the highest output level is used to melt the recording layer, the intermediate output level is used to heat the recording layer just below the melting point and higher than the crystallization temperature, and the lowest level is used to heat the recording layer. Or it is used for control of cooling. The recording layer melted by the laser pulse with the highest output level becomes amorphous or microcrystalline by the subsequent rapid cooling, resulting in a decrease in reflectivity and a recording mark. Further, all laser pulses with an intermediate output become crystalline and can be erased. As described above, by changing the writing laser pulse between the output levels, a crystalline region and an amorphous region can be alternately formed in the recording layer, and information is stored.

In order to realize high-speed recording, a phase change material having a high crystallization speed is required as a recording layer material. As such a phase change material, Ge-Te, Ge-Te-Se, In-Sb, Ga-Sb, Ge-Sb-Te, Ag-In-Sb-Te, etc. have been fast so far. In addition, it has been attracting attention because of its high erasure ratio during high-speed recording.
However, in order to realize high-speed recording, it is not sufficient to simply increase the crystallization speed of the recording layer material, and there is a problem of “ensuring easy initialization” as another big problem. For example, a Ga—Sb phase change material known as one of high-speed recording materials has been reported to have an extremely high crystallization speed (Non-patent Document 1), but the crystallization temperature is as high as 350 ° C. Initialization is difficult, and even if initialization is performed by irradiating with a high initialization power, the reflectance after the initialization varies, which greatly affects the recording characteristics.

In the process of developing an optical recording medium targeted for high-speed recording equivalent to DVD 8 × speed, the present inventors have proposed a Ga—Sb system that the present inventors consider to be particularly suitable for 8 × speed recording as a method for solving the above problems. Focusing on the phase change material, at least one element selected from Au, Ag, and Cu is added thereto, and crystalline particles are generated in the recording layer in the process of initial crystallization. The present invention has been completed by remarkably improving and finding that the reflectance distribution after initialization becomes uniform.
That is, the above-mentioned problems are solved by the following inventions 1) to 6) (referred to as the present inventions 1 to 6).
1) It has at least a first protective layer, a phase change recording layer, a second protective layer, and a metal reflective layer on a transparent substrate, and the phase change recording layer is made of an alloy represented by the following composition formula: Optical recording medium.
(X ₁ ) αSbβ (X ₂ ) γ
(Where X ₁ is at least one element selected from Ga, Ge, In, X ₂ is at least one element selected from Au, Ag, Cu, α, β, γ are atomic%, 2 ≦ α ≦ 20, 55 ≦ β ≦ 95, 0 <γ ≦ 10, α + β + γ = 100)
2) The alloy contains 1 to 40 atomic% of at least one additive element selected from Al, Zn, Mg, Tl, Pb, Sn, Bi, Cd, Hg, Se, C, N, Mn, and Dy. The optical recording medium according to claim 1.
3) The optical recording medium according to 1) or 2), wherein the protective layer comprises a mixture of ZnS and SiO ₂ .
4) The optical recording medium according to any one of 1) to 3), wherein the metal reflective layer is made of Ag or an alloy containing Ag as a main component.
5) The optical recording medium according to 4), wherein a third protective layer mainly comprising Si or SiC is provided between the second protective layer and the metal reflective layer.
6) The optical recording medium according to any one of 1) to 5), wherein the optimum recording linear velocity is in a range of 28.0 ± 3 m / s.

Hereinafter, the present invention will be described in detail.
In the present invention, an alloy having the composition formula of the present invention 1 is used as a phase change material that is suitable for high-speed recording equivalent to DVD 8 × speed, is easy to initialize, and has a uniform reflectance distribution after initialization. Sb, which is a main constituent element, is an excellent phase change material capable of realizing high-speed recording. The crystallization speed can be adjusted by changing the Sb ratio, and the crystallization speed can be increased by increasing the ratio. However, with Sb alone, it is difficult to realize a recording layer material having a high crystallization speed equivalent to DVD 8 × speed and excellent storage stability. Therefore, a method for increasing the crystallization speed without impairing overwrite characteristics and storage reliability. As a method for adding at least one element selected from Ga and In, and for improving storage reliability, Ga, In, and especially Ge are added.

Ga can increase the crystallization speed with a small addition amount, and also has an effect of increasing the crystallization temperature of the phase change material, which is effective in the stability (storage reliability) of the mark.
Although Ge does not have an effect of increasing the crystallization speed, it is an important element similar to Ga because it does not raise the crystallization temperature as much as Ga and can improve storage reliability with a small amount of addition.
In has the same effect as Ga and has the advantage of not raising the crystallization temperature as much as Ga. Therefore, when considering the problem of initialization, it is effective to use it as an element supplementing Ga.
Such an X ₁ -Sb-based phase change material (X ₁ is at least one element selected from Ga, Ge, and In) is a recording layer having a high crystallization speed and excellent storage stability by adjusting the composition ratio. Material can be designed. However, as described above, such a material generally has a drawback of a high crystallization temperature, which newly causes a problem of initialization failure.

As a result, the present inventors have previously added “crystal nuclei” in the recording layer by adding at least one element selected from Au, Ag and Cu to the recording layer material to generate crystalline particles. It was found that the initialization problem of the high-speed crystallization material can be solved.
It is considered that the metal crystalline particles are generated through the following process of [Formula 1], where X ₂ ⁰ is the metal atom present in the recording layer.
[Formula 1] nX ₂ ⁰ + heating → (X ₂ ⁰ ) _n (crystalline particles)
Further, when metal atoms are present as ions in the recording layer, the following process of [Formula 2] is also conceivable.
[Formula 2] X ₂ ⁺ + Sb ²⁺ + hν → X ₂ ⁰ + Sb ⁴⁺ (reduction reaction with Sb)
The reduction reaction by Sb is usually a photosensitive reaction, and it is said that the reaction proceeds by ultraviolet irradiation or the like. In this case, for example, metal atoms are generated by the influence of ultraviolet rays used when curing an ultraviolet curable resin. It is also possible.
Metal atoms X ₂ ⁰ present in the recording layer in any case, are converted by heat obtained by laser irradiation at the time of initialization into fine metal crystal particles as shown in [Equation 1], such " It seems that the particles that act as “crystal nuclei” are uniformly formed inside the recording layer, thereby facilitating initialization and enabling uniform initialization of the reflectance distribution.

Furthermore, since Au, Ag, and Cu are additive elements effective for storage reliability, it is possible to design a phase change material that not only solves initialization failure but also has excellent storage reliability.
Therefore, the present invention 1 pays attention to the high-speed crystallization characteristic of the X ₁ -Sb compound (X ₁ is at least one element selected from Ga, Ge, In) as the recording layer material, and uses the characteristic. On the other hand, the problem of poor initialization caused by a high crystallization temperature, which is a defect of the above compound, is produced by adding at least one element selected from Au, Ag, and Cu to generate crystalline particles in the recording layer material. An optical information recording medium satisfying both the characteristics of high-speed recording and storage reliability according to the first aspect of the present invention, the initialization of the recording material being easy, and the reflectance distribution after the initialization is uniform Can be realized.

  In order to design a phase change recording material suitable for high-speed recording equivalent to DVD 8 × speed, the additive amount γ of Au, Ag, and Cu in the recording layer material is set to 10 atomic% or less. As described above, Au, Ag, and Cu are elements that are excellent in storage reliability and effective in improving the initialization failure of the high-speed recording material, but on the other hand, the crystallization limit speed of the recording layer material is lowered. It also has the property of preventing high-speed recording. Therefore, if the addition amount of Au, Ag, and Cu exceeds 10 atomic%, it becomes difficult to design a phase change recording material suitable for high-speed recording equivalent to DVD 8 × speed, so the upper limit of the addition amount is 10 atomic%. There is a need. However, if the amount is too small, the effect of adding Au, Ag, and Cu becomes unclear, so the lower limit is preferably 1 atomic%.

Regarding the composition ratio of Sb, Ga, Ge, and In, α in the formula needs to be 2 atomic% or more. For example, when α becomes _{X 1} from Ga and / or In is less than 2 atomic%, or if β is less than 55 atomic%, the crystallization rate is lowered, the recording linear velocity of 28 m / s, which corresponds to DVD8 speed Overwriting below becomes difficult. Further, when α is less than 2 atomic%, the storage reliability is also lowered. Ga and / or In can increase the crystallization speed with a small addition amount, and in particular, Ga has an effect of increasing the crystallization temperature of the phase change material. is there. However, when α in the formula is larger than 20 atomic%, initialization becomes extremely difficult. In particular, when the amount of Ga added is large, the crystallization temperature becomes excessively high, and the crystalline state has a uniform and high reflectance at the time of initialization. It becomes difficult to get. On the other hand, In has the same effect as Ga and has the advantage of not raising the crystallization temperature as much as Ga. Therefore, when considering the problem of initialization, it is effective to use it as an element supplementing Ga. However, excessive addition of In repeatedly degrades the recording characteristics and causes a decrease in reflectivity.

When X ₁ is Ge alone, Ge does not raise the crystallization temperature as much as Ga, and storage reliability can be drastically improved by addition of a small amount. Therefore, a recording material having particularly excellent storage reliability can be realized. Ge is particularly effective for stabilizing an amorphous recording layer having a high crystallization speed. The effect appears when the addition amount is 2 atomic% or more, and the effect increases as the addition amount increases. On the other hand, Ge has an effect of decreasing the crystallization speed, and excessive addition also causes adverse effects such as an increase in jitter due to overwriting. Therefore, the upper limit is preferably 20 atomic% or less. Even if the addition amount of Ge is 20 atomic%, if β exceeds 95 atomic%, the increase in crystallization speed becomes abrupt and mark formation becomes difficult, and the storage reliability decreases, so β is 95 atomic%. The following is preferable.
Furthermore, if X ₁ is composed of Ga and / or In and Ge, Ga and / or by reducing the amount of In added correspondingly Ge, good storage reliability by making the 2 atomic percent or more α Obtainable.

As a phase change material suitable for high-speed recording equivalent to DVD 8 × speed as in the present invention 2 and easy to initialize and having a uniform reflectance distribution after initialization, an alloy having a composition formula defined in the present invention 1 is used. Further, an alloy in which at least one element selected from Al, Zn, Mg, Tl, Pb, Sn, Bi, Cd, Hg, Se, C, N, Mn, and Dy is added as an additional element constitutes the present invention. It is even more suitable as a recording material. Since each of these elements has a unique effect of improving recording characteristics and storage reliability, the characteristics of the X ₁ -Sb-X ₂ alloy can be further improved by adding an appropriate amount.
The effect of the additive element will be described. In addition to Ga and In, Tl, Pb, Sn, Bi, Al, Zn, Mg, Cd, and Hg also have an effect of increasing the crystallization limit speed. Among them, Sn, which has an atomic number closest to Sb and is considered to have a high affinity with Sb, is preferable, and increases the crystallization limit speed and improves the overwrite characteristics. However, if the addition amount is too large, it causes deterioration of reproduction light and initial jitter, so the composition range needs to be 40 atomic% or less for any element.

Regarding storage reliability, Al, C, N, and Se are also effective in addition to Ge. Further, Al and Se contribute to high-speed crystallization. Se also contributes to an improvement in recording sensitivity.
Furthermore, it has been found through research by the present inventors that Mn and Dy have the same effect as In, and in particular, Mn is an additive element with excellent storage reliability that does not require much addition of Ge. . The optimum addition amount of Mn is 1 to 15 atomic%, and if it is less than 1 atomic%, the effect does not appear, and if it exceeds 15 atomic%, the reflectivity in the unrecorded state (crystalline state) becomes too low.
The film thickness of the recording layer is 6 to 20 nm. When the thickness is less than 6 nm, the recording characteristics are remarkably deteriorated by repeated recording. When the thickness is more than 20 nm, the recording layer easily moves due to repeated recording, and the increase in jitter becomes severe. Furthermore, in order to reduce the difference in absorption between crystal and amorphous as much as possible to improve the erasing characteristics, it is preferable that the thickness of the recording layer is thin, so the preferred thickness is 8 to 16 nm.

The optimum thickness of the first protective layer is selected from the thermal and optical conditions, but is usually 40 to 200 nm, preferably 40 to 100 nm.
The film thickness of the second protective layer is directly related to the cooling of the recording layer, so that it is 2 nm or more in order to obtain good erasing characteristics and repeated recording durability. If it is thinner than this range, defects such as cracks are caused, and the recording durability is repeatedly lowered, and the recording sensitivity is deteriorated, which is not preferable. However, if the thickness exceeds 20 nm, the cooling rate of the recording layer becomes slow, so that mark formation becomes difficult and the mark area becomes small. A preferable film thickness range is 4 to 15 nm.
In the present invention 3, a mixture of ZnS and SiO ₂ is used for the protective layer. A mixture of ZnS and SiO ₂ is excellent in heat resistance, low thermal conductivity, and chemical stability required for the protective layer, has a small residual stress in the film, and has a recording sensitivity, an erasing ratio, etc. even by repeated recording / erasing. It is suitable as a protective layer constituting the present invention in that the characteristic deterioration is difficult to occur and the adhesiveness to the recording layer is excellent.

When pure Ag or an Ag alloy is used for the metal reflection layer as in the present invention 4, when a protective layer containing sulfur such as a mixture of ZnS and SiO ₂ is used, sulfur corrodes Ag due to a sulfurization reaction and becomes a defect. A malfunction will occur. Therefore, a third protective layer is preferably provided to prevent such a reaction. An appropriate material is selected for the third protective layer from the viewpoints of the following (1) to (5). A preferable specific example includes a material mainly composed of Si or SiC.
(1) It has a barrier ability to prevent the sulfurization reaction of Ag.
(2) It is optically transparent to laser light.
(3) Low thermal conductivity due to formation of amorphous marks.
(4) Good adhesion to the protective layer and the metal reflective layer.
(5) It is easy to form.
The thickness of the third protective layer is required to be at least 2 nm in order to prevent the sulfurization reaction of Ag.
The thickness of the reflective layer is usually 100 to 300 nm.

  According to the present invention, it is possible to provide a phase change optical recording medium suitable for high-speed recording equivalent to DVD 8 × speed, easy to initialize a recording material, and having a uniform reflectance distribution after initialization.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further more concretely, this invention is not restrict | limited at all by these Examples, the initialization apparatus used, etc. For example, a surface recording type rewritable optical recording medium in which the order of film formation is reversed, or an optical recording medium that is the same or different via a resin protective layer instead of the bonding substrate 8 as seen in a DVD system. The present invention can also be applied to a recording medium or the like in which two sheets are bonded together.
FIG. 1 is a schematic cross-sectional view of a phase change optical disk (hereinafter referred to as an optical disk) manufactured as an example and a comparative example. The first protection is provided on the upper surface of a transparent substrate 1 provided with a laser light guide groove. A layer 2, a phase change recording layer 3 that reversibly changes between crystalline and amorphous, a second protective layer 4, a third protective layer 5, a metal reflective layer 6, and a resin protective layer 7, and finally the same as the substrate It has a layer structure in which a substrate for bonding 8 is attached.

[Examples 1 to 5]
A first protective layer 2, a phase change recording layer 3, a second protective layer 4, a third protective layer 5, and a metal reflective layer 6 are formed in this order on the substrate 1 by a sputtering method, and a spin coating method is formed thereon. Then, the resin protective layer 7 was applied, and the bonding substrate 8 was further bonded to produce an optical disk. Next, 'PCR DISK INITIALIZER' manufactured by Hitachi Computer Equipment Co., Ltd. is used as an initialization device, the optical disk is rotated at a constant linear velocity, and a laser beam with a power density of 10 to 20 mW / μm ² is sent at a constant rate in the radial direction. It was initialized by irradiating while moving by the amount.
As the substrate 1, a substrate with a guide groove made of polycarbonate having a diameter of 12 cm and a thickness of 0.6 mm and having a track pitch of 0.74 μm was used. The first protective layer 2, the thickness 65nm of ZnS (80 mol%) - was _SiO 2 (20 mol%). The phase change recording layer 3 has a thickness of 16 nm, Ga ₉ Sb ₈₆ Ag ₅ (Example 1), Ge ₁₆ Sb ₇₉ Ag ₅ (Example 2), In ₁₃ Sb ₈₂ Ag ₅ (Example 3), Ga ₉ Ge ₃ Sb ₈₅ Ag ₃ (Example 4) and Ga ₈ In ₄ Sb ₈₃ Ag ₅ (Example 5). The second protective layer 4 was made of ZnS (80 mol%)-SiO ₂ (20 mol%) having a thickness of 14 nm. The third protective layer 5 was made of SiC having a thickness of 4 nm. The metal reflective layer 6 was made of Ag having a thickness of 140 nm. As the bonding substrate 8, a polycarbonate substrate having a diameter of 12 cm and a thickness of 0.6 mm, which is the same as that of the substrate 1, was used.

[Comparative Example 1]
An optical disc was manufactured by the same layer configuration and manufacturing method as in Example 1 except that the recording layer was changed to Ga ₁₀ Sb ₉₀ . Compared to Example 1, this comparative example has a composition in which Ag is not contained in the recording layer material.

[Comparative Example 2]
An optical disc was manufactured by the same layer configuration and manufacturing method as in Example 1 except that the recording layer was changed to Ga ₉ Sb ₈₁ Ag ₅ Te ₅ . This comparative example is obtained by adding 5 atomic% of Te to the recording layer material of Example 1.

For each optical disc after initialization, the reflectance distribution and the recording characteristics were evaluated.
The reflectance distribution was evaluated by measuring the signal width of the Rf signal obtained from the optical disc. The recording characteristics were evaluated using an optical disk evaluation apparatus (DDU-1000 manufactured by Pulstec Corp.) having a pickup with a wavelength of 660 nm and NA of 0.65, a recording linear velocity of 28 m / s (equivalent to 8 times the speed of DVD), and a linear density of 0. The evaluation was performed by evaluating the C / N ratio when the 3T single pattern was overwritten 10 times by the EFM + modulation method under the condition of 267 μm / bit. The evaluation criteria are as follows.
Evaluation of reflectance distribution: The reflectance distribution after initialization of DVD + RW (currently shown as Comparative Example 3 in Table 2) that is currently commercially available for 2.4 × speed recording was compared as an approximate reference.
For Example 1 and Comparative Example 1, the state of the recording layer after initialization was observed with a transmission electron microscope (TEM), and the difference in crystal state was also evaluated (see FIG. 2).
Evaluation of recording characteristics: When realizing a rewritable optical disk system, the C / N ratio is required to be at least 45 dB, and if it is 50 dB or more, a more stable system can be realized. The case where the C / N ratio was less than 45 dB was indicated as “×”, the case where it was 45 dB or more and less than 50 dB was indicated as “◯”, and the case where it was 50 dB or more was indicated as “◎”.
The results are summarized in Tables 1 to 3. In each of Examples 1 to 5, the reflectance distribution after initialization was small, the CN ratio was high as 45 dB or more, and the recording characteristics were good. Further, as shown in FIG. 2, the TEM observation result regarding the reflectance distribution after the initialization shows an image in which small crystal grain sizes are uniformly distributed in Example 1, whereas it is small in Comparative Example 1. An image in which the crystal grain size and the large crystal grain size are mixed unevenly was obtained, and it was confirmed that this was the cause of the reflectance distribution.
In addition, the Rf signal after initialization in Comparative Example 1 is very wide as compared with Examples 1 to 5, and therefore, the reflectance after initialization is uneven and it is difficult to perform sufficient initialization. I found out.
In Comparative Examples 1 and 2, the recording characteristics were evaluated in the same manner as in Example 1. However, in Comparative Example 1, good characteristics could not be obtained due to the large variation in reflectance. Compared to Example 1, the crystallization speed of the recording material was lowered, and good results could not be obtained with the recording characteristics at a recording linear velocity of 28 m / s.

1 is a schematic cross-sectional view of a phase change optical disc according to an embodiment. The figure which shows the result of having observed the state of the recording layer after initialization with the transmission electron microscope.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Plastic substrate 2 1st protective layer 3 Phase change recording layer 4 2nd protective layer 5 3rd protective layer 6 Metal reflective layer 7 Resin protective layer 8 Substrate for bonding

Claims (6)

An optical recording comprising at least a first protective layer, a phase change recording layer, a second protective layer, and a metal reflective layer on a transparent substrate, wherein the phase change recording layer is made of an alloy represented by the following composition formula Medium.
(X ₁ ) αSbβ (X ₂ ) γ
(Where X ₁ is at least one element selected from Ga, Ge and In, X ₂ is at least one element selected from Au, Ag and Cu, α, β and γ are atomic%, 2 ≦ α ≦ 20, 55 ≦ β ≦ 95, 0 <γ ≦ 10, α + β + γ = 100)   The alloy includes 1 to 40 atomic% of at least one additive element selected from Al, Zn, Mg, Tl, Pb, Sn, Bi, Cd, Hg, Se, C, N, Mn, and Dy. The optical recording medium according to claim 1. The optical recording medium according to claim 1, wherein the protective layer is made of a mixture of ZnS and SiO ₂ .   The optical recording medium according to claim 1, wherein the metal reflective layer is made of Ag or an alloy containing Ag as a main component.   5. The optical recording medium according to claim 4, wherein a third protective layer comprising Si or SiC as a main component is provided between the second protective layer and the metal reflective layer. The optical recording medium according to claim 1, wherein the optimum recording linear velocity is in a range of 28.0 ± 3 m / s.