JPH04223191A - Phase change type optical recording medium - Google Patents

Abstract

PURPOSE:To achieve improvement of repetitive durability of rewriting operation by a method wherein an optical recording material layer is made to be a thinned film layer of a specific thickness or under and besides, this thinned film layer is formed of a material having such ratio of the number of atoms as Ge2Sb2Te5 admixed with an additive. CONSTITUTION:In a phase change type optical recording medium having Ge, Se, and Te as main constitutive elements of an optical recording material layer, the optical recording material layer is formed by a thinned film layer of under 20 nm film thickness. Further, the thinned film layer is preferably formed of a material produced by adding an additive of maximum 5 atom % to a material having such ratio of the number of atoms as Ge2Sb2Te5. Furthermore, as the additive, Cu, Au, Ag, Co, or Ni may be preferably used excepting Ge-Sb-Te series materials. Thereby, flow of the materials is restrained, and sudden increase of an error rate due to repetition of rewriting operation can be adjusted.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to an optical recording medium that utilizes a reversible optical change in the crystalline state of an optical recording material layer by irradiation with laser light, in which Ge, Sb, and Te are added to the optical recording material layer. The present invention relates to a phase change type optical recording medium in which the main constituent element is a phase change type optical recording medium.

0002

BACKGROUND OF THE INVENTION In recent years, there has been an increasing demand for higher density and larger capacity information recording, and research is being actively conducted both domestically and internationally. In particular, optical recording media have a higher recording density than conventional magnetic recording media, and since information can be recorded and reproduced without contact between the recording and reproducing heads and the recording medium, there is less damage to the recording medium. Additionally, due to its long lifespan, it is seen as a promising high-density, large-capacity recording method for recording and reproducing vast amounts of information. There are three types of optical recording media depending on the purpose: read-only type, write-once type, and rewritable type.
It can be roughly divided into types. A read-only type is a read-only recording medium from which information can only be read, while a write-once type can record and reproduce information as needed.
It is an optical recording medium in which recorded information cannot be erased. On the other hand, rewritable media is an optical recording medium that can record and reproduce information, as well as erase and rewrite recorded information, and has the highest expectations for use as data files for computers. It is something. Regarding rewritable optical recording media, two recording methods, a magneto-optical method and a phase change method, are being developed, and both methods are still in the process of research and development in terms of recording materials, writing mechanisms, etc. Among the above methods, the phase change method generally focuses a laser beam or the like whose pulse output and pulse width are controlled on the recording surface of the medium and heats it. The optical recording material on the surface of the optical recording medium, such as the laser beam, undergoes a phase change due to heating by the laser beam, causing a transition from a crystalline state to an amorphous state or a phase transition. Optical recording media record and erase information based on the difference in reflectance between phase-changed states, for example, a crystalline state and an amorphous state.

The normal structure of this phase change type optical recording medium is to form a protective layer made of ceramic such as ZnS on the surface of a substrate such as polycarbonate having many tracking grooves, and record data on the protective layer. It has a structure in which a material layer is provided, a protective layer made of ceramic or the like is again formed on the recording material layer, and a surface protective layer of an organic material is sequentially deposited. Furthermore, a reflective/cooling layer made of Al or the like may be provided between the protective layer and the organic surface protective layer. The purpose of this reflective/cooling layer is to reflect laser light, etc. incident from the substrate to increase the utilization efficiency of the laser light, etc., and to cool the recording material layer from its molten state when it changes from a crystalline state to an amorphous state. The purpose is to increase speed. When the recording material layer is in a molten state, the ceramic protective layer acts as a heat insulating layer. In addition, for the above-mentioned optical recording medium,
Laser light and the like are incident on the surface of the medium opposite to the side having the recording material layer.

In such a phase-change optical recording medium, the optical recording material is initially in a crystalline state, and the crystalline recording material is irradiated with a laser beam or the like and melted, then rapidly cooled to form an amorphous spot. The state in which this is formed is the information recording state. In the erasing operation, a spot in the amorphous state of the optical recording material is irradiated with a laser beam or the like to anneal it and return it to a crystalline state. In this way, a phase change type optical recording medium is a recording medium that records information based on the difference in reflectance between a crystalline state and an amorphous state.

[0005] As an optical recording material for the above-mentioned phase change type optical recording medium, G having a crystal composition of Ge2 Sb2 Te5 is used.
A material whose main constituent elements are e, Sb, and Te is used.

[0006]

The biggest problem with such phase-change optical recording media is their durability against repeated rewriting operations. Practically speaking, it is required to repeat the rewrite operation 106 times, that is, to repeat the operation of recording and reproducing information 106 times, and further erasing the recorded information. However, conventional optical recording media have a problem in that if the number of rewrites exceeds 105, defects will be formed on the medium and rewriting errors will increase.

The cause of this problem is that the optical recording material melted during the phase change from a crystalline state to an amorphous state, or conversely from an amorphous state to a crystalline state, during a rewrite operation is affected by the centrifugal force caused by the rotation of the medium. Alternatively, it is thought that the protective film flows due to internal stress or the like, and voids are generated in a part of the material.

One way to deal with this problem is to increase the viscosity coefficient by changing the material components. An optical recording material whose main constituent elements are Ge, Sb, and Te has a minimum viscosity coefficient when the atomic ratio is Ge2 Sb2 Te5. Therefore, as the ratio of Ge, Sb, and Te in the material differs from the atomic ratio of Ge2 Sb2 Te5, the viscosity coefficient increases and the flow of the material decreases. Also,
Even when components other than Ge, Sb, or Te are added to this material, the viscosity coefficient increases and the flow of the material decreases.

However, the atomic ratio of the material is Ge2
As Sb2 deviates from Te5, the crystallization speed slows down, the crystallization ratio of recrystallization during erasing in a rewriting operation, that is, the erasing ratio during overwriting decreases, and rewriting operation errors increase. In this way, additives are added to the material,
Merely increasing the viscosity coefficient will not solve this problem.

[0010] Therefore, an object of the present invention is to reduce the amount of light melted during rewriting without changing the composition of the material, or while suppressing the change in the composition of the material within a range where the effects of the delay in crystallization speed as described above are not observed. The object of the present invention is to suppress the flow of a recording material layer and provide an optical recording medium that has high durability against repeated rewriting operations.

[0011]

[Means for Solving the Problems] As a new method for suppressing the flow of the molten optical recording material layer, which is the above-mentioned problem, the measures taken in the present invention are to reduce the optical recording material layer to a thin layer of less than 20 nm. It is something to do.

Furthermore, the thinned layer has an atomic ratio of Ge2.
It is preferable to use a material containing Sb2Te5 and a maximum of 5 atomic percent of additives. In addition to Ge-Sb-Te-based materials, additives include Cu.
, Au, Ag, Co or Ni.

[0013]

[Operation] According to the above means, by making the optical recording material layer a thin layer, the coefficient of frictional resistance in the flow of the molten optical recording material during rewriting increases, and as a result, the flow of the material is suppressed. . In addition, in an optical recording material layer with a thickness of less than 20 nm and with suppressed flow, the occurrence of voids due to repeated rewriting operations is reduced, and the rapid error rate in optical recording media with this optical recording material layer is reduced. increase is suppressed.

[0014] Furthermore, this thinned layer has an atomic ratio of Ge2.
In the case of forming the Sb2Te5 material with additives added, the viscosity coefficient of the material layer increases, thereby enhancing the effect of the thinning layer and further suppressing the flow. If the amount of the above-mentioned additive is at most 5 atomic%, there will be no effect on the error rate due to the recrystallization rate during rewriting due to the additive, and the optical recording material will be able to achieve a practical rewriting operation. An optical recording medium with repeated durability is formed.

[0015]

[Example] (Example 1) Ge-Sb-T in the present invention
The thin film of e-based optical recording material is formed by RF magnetron sputtering. The optical recording medium substrate is in the shape of a polycarbonate disc, and a protective film of Zn is coated on the surface.
S film, Ge2 Sb2 Te5 film which is an optical recording material,
Sputtered films are formed in this order: a ZnS film as a protective film on the opposite side, and then an Al film as a reflective/cooling film. The thickness of each film is 100nm, Xnm, 170nm, and 100nm, respectively, and the thickness X of the Ge2 Sb2 Te5 film, which is an optical recording material film, is 15nm, 19nm, and 25nm, and the influence on the durability of repeated rewriting operations is investigated. . The number of repetitions of the rewriting operation and the error rate in each case are shown in the attached FIG. 1.

[0016] The diameter of the optical recording disk is 86 mm,
After initializing this disk, overwriting is repeatedly performed using a semiconductor laser having a wavelength of 830 nm while rotating the disk at 3600 rpm. The recording capacity of the optical recording disk of this embodiment is approximately 6 x 105 bytes, and the writing conditions are a frequency of 5.8 MHz, a pulse width of 60 ns, a peak power of 12 mW, and a bias power of 6 mW.

The CN ratio in the rewrite operation is 52 dB, and the erasure ratio is 29 dB, and there is no change due to repetition. As shown in the attached Figure 1, Ge2 Sb2 T which is this example
In the optical recording material layer containing only e5, its film thickness is 15 nm.
In the case of m, the error rate is 10-5 or less even after 106 repeated rewriting operations, and has the durability necessary for practical use.

(Example 2) The recording medium in this example is an optical recording disk similar to that in Example 1, and the optical recording material is a material having an atomic ratio of Ge2 Sb2 Te5,
This is a material to which 5 atomic% of Sb is added. The other conditions are the same as in Example 1, so explanations will be omitted. The influence on the durability of repeated rewriting operations will be investigated in three cases in which the thickness X of the optical recording material film is 15 nm, 19 nm, and 25 nm. The number of repetitions of the rewriting operation and the error rate in this embodiment are summarized in the attached FIG. 2.

The CN ratio in the rewriting operation is 53 dB, and the erasure ratio is 26 dB, and there is no change with repetition, but when the amount of Sb added is further increased, the erasure ratio decreases,
The error rate of rewriting operations due to unerased data increases. As shown in the attached Figure 2, Ge2 Sb2 which is this example
In the optical recording material layer in which 5 atomic% Sb is added to Te5, the overall error rate is lower than in Example 1, and when the film thickness is 19 nm, there are no errors even after 106 repeated rewriting operations. The rate is below 10-5.

Similar results are obtained when Ge or Te is added instead of Sb. From the above, Ge-Sb-
In the Te-based optical recording material layer, Ge2 Sb2 Te5
The upper limit of the amount of Ge, Sb, or Te that can be added to the optical recording material layer is 5 atomic %, and if the thickness of this optical recording material layer is less than 20 nm, it has the durability necessary for practical use.

(Example 3) The recording medium in this example is an optical recording disk similar to that in Example 1, and the optical recording material is a material with an atomic ratio of Ge2 Sb2 Te5.
This is a material to which 5 atomic% of Cu is added. The other conditions are the same as in Example 1, so explanations will be omitted. The influence on the durability of repeated rewriting operations will be investigated in three cases in which the thickness X of the optical recording material film is 15 nm, 19 nm, and 25 nm. The number of repetitions of the rewriting operation and the error rate in this embodiment are summarized in the attached FIG. 3.

The CN ratio and erase ratio in the rewriting operation are almost the same as in Example 2, and the amount of addition is also the same as in Example 2.
If it exceeds 5 atomic percent, the erase ratio will decrease and the error rate of rewriting operations due to unerased data will increase. As shown in the attached Figure 3, Ge2 Sb2 Te which is this example
Similarly to Example 2, in the optical recording material layer to which 5 atomic% of Cu was added, the overall error rate decreased compared to Example 1, and when the film thickness was 19 nm, the error rate was 106 times. Even after repeated rewriting operations, the error rate is 10-5 or less.

Similar results are obtained when Au, Ag, Co or Ni is added instead of Cu. From the above,
Ge2S in the Ge-Sb-Te-based optical recording material layer
The upper limit of the amount of additives added to b2 Te5 is 5 atomic %, and if the thickness of this optical recording material layer is less than 20 nm, it has the durability necessary for practical use.

As described above, the results of the experiments shown in FIGS. 1 to 3 show that when the thickness of the optical recording material layer is less than 20 nm, the error rate sharply increases with respect to the 106 rewriting operations required at the practical stage. This shows that it is an optical recording medium with no increase in . Further, the upper limit of the amount of various additives added to the optical recording material layer is 5 atomic %. Furthermore, G
1 for an optical recording material layer consisting only of e2 Sb2 Te5
If the upper limit of the film thickness is 20 nm for an optical recording material layer with a thickness of 5 nm or less and a doping amount of 5 atomic%, the optical recording material has an extremely low error rate of 10-5 or less, which is sufficient for practical use, for 106 rewriting operations. It becomes a recording medium.

The reason why the above results were obtained is that first, the optical recording material layer was made into a thin layer. By making the film thinner, the coefficient of frictional resistance in the flowing state of the molten material increases and the flow can be suppressed, and the flow rate and the thickness of the thinner layer can be explained as follows. The flow of a thin film such as the optical recording material layer of an optical recording medium is
Since the layer is an extremely thin film and its flow rate is relatively slow, it is only necessary to consider the region of laminar flow, and furthermore, the flow is formed on the surface of the protective layer sandwiching the optical recording material layer. The flow in the velocity boundary layer is expressed by the following equation.

[0026] u=(dp/dx)・(de2/4μ)
・・・・・・(1)
u
:Flow velocity (cm/s)
(d
p/dx): pressure gradient
de
:Effective diameter
μ
: Viscosity coefficient of material de=4・(flow path cross-sectional area)/wetted edge length
=4・a・ρ/2(a+ρ)=2ρ (a>>ρ)

・・・・・・
(2)
a
: Recording mark width

(~103 nm)

ρ: Optical recording material layer thickness (1)
In the equation, the pressure gradient (dp/dx) corresponds to the driving force of the flow, such as the centrifugal force due to the rotation of the optical recording medium or the internal stress of the protective film. From equations (1) and (2), it can be seen that the flow rate u of the optical recording material is proportional to the square of the effective diameter de of the channel, that is, the square of the optical recording material layer thickness ρ.

As described above, by reducing the thickness of the optical recording material layer, the flow of the recording material layer is suppressed. The conventional film thickness of this optical recording material layer is about 25 nm or more,
In this region, the error rate increases rapidly after approximately 105 rewriting operations, and the method has not reached the stage of practical use. However, in the thin layer of less than 20 nm implemented in the present invention, a rapid increase in error rate is suppressed even after 106 repetitions required for practical use.

Furthermore, as shown by the above equation (1), the flow rate is inversely proportional to the viscosity coefficient μ, and it can be seen that the flow of the optical recording material layer can also be suppressed by controlling the viscosity coefficient. As described above, by adding additives to the material layer and increasing its viscosity coefficient, the flow of the optical recording material layer is suppressed, but on the other hand, as the material differs from the atomic ratio of Ge2 Sb2 Te5, the optical recording material The speed of crystallization during erasing of the rewriting operation slows down, the erasing ratio decreases,
There is a problem in that the error rate of the rewriting operation of the recording medium increases, and an appropriate ratio of the additive has not been sought. In this example, Ge2 Sb2
Additives to the optical recording material layer containing Te5 as the main component include Ge, Sb, or Te, as well as Cu, A
As a result of experiments with the addition of components such as u, Ag, Co, or Ni, the upper limit of the amount added is that the flow of the material layer is suppressed due to an increase in the viscosity coefficient, and the error rate does not increase due to a delay in the crystallization rate. 5 atomic%.

[0029]

As explained above, the present invention makes the optical recording material layer in an optical recording medium thinner to a thickness of less than 20 nm, and further comprises a material with an atomic ratio of Ge2 Sb2 Te5 as the main component and a maximum of 5 atomic %. Since the thin layer is formed using a material to which the following additives have been added, the following effects can be achieved.

In optical recording media using conventional Ge-Sb-Te optical recording materials, the error rate due to repeated rewriting operations is lower than the number of repetitions (10
6), so it cannot be said that it has reached the stage of practical application.

Furthermore, even if additives are added to the optical recording material layer in order to suppress the flow of the material layer, which is the cause of an increase in the error rate, this will cause a delay in the recrystallization speed, and this method alone will not solve the above problem. cannot be resolved.

However, in the present invention, the flow of the material is suppressed by forming the optical recording material layer with a thin layer of less than 20 nm, thereby suppressing the rapid increase in error rate due to repeated rewriting operations. Is possible.

Furthermore, in addition to the above-mentioned thinning, the atomic ratio Ge2 Sb2 Te was added to a maximum of 5 atomic %.
By forming the optical recording material layer using the material No. 5, there is no delay in crystallization speed, which was a problem in the past.
Since the effect of flow suppression can be enhanced, 10
Even after repeating the rewriting operation six times, it is possible to obtain an optical recording medium with a sufficiently low error rate.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the error rate of a rewriting operation versus the number of repetitions in a first embodiment of the present invention.

FIG. 2 is a graph showing the error rate of a rewrite operation versus the number of repetitions in a second embodiment of the present invention.

FIG. 3 is a graph showing the error rate of a rewriting operation versus the number of repetitions in a third embodiment of the present invention.

Claims (4)

[Claims] 1. A phase-change optical recording medium containing Ge, Sb, and Te as main constituent elements of an optical recording material layer, characterized in that the optical recording material layer is a thin layer with a thickness of less than 20 nm. Phase change optical recording medium. 2. In claim 1, the optical recording material layer comprises Ge2 Sb2 Te5 and 5 atomic %
A phase change optical recording medium characterized by comprising the following additives. 3. The phase change optical recording medium according to claim 2, wherein the additive is at least one component selected from the group consisting of Ge, Sb, and Te. 4. The phase change optical recording medium according to claim 2, wherein the additive is at least one component selected from the group consisting of Cu, Au, Ag, Co, and Ni.