JPH05116455A - Phase-change optical recording medium - Google Patents

Abstract

PURPOSE:To obtain a phase-change optical recording medium having little possibility of an error in reproducing information and a large power margin by a method wherein in an optical recording medium provided with a recording layer made of a phase-change material, an erasing ratio in the recording layer is increased. CONSTITUTION:A recording layer contains a crystalline core material 11, which serves as a crystalline nucleus when a phase-change material 12 changes in phase from an amorphous state to a crystalline state and does not melt when the phase-change material 12 changes either to an amorphous phase or to a crystalline phase. In this manner, the phase-change material is crystallized with a uniform crystal diameter while an erasing ratio is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording technique, and more particularly to a phase change optical recording medium in which a recording layer of a phase change material whose optical properties change according to a phase change between amorphous and crystalline is provided on a substrate.

[0002]

2. Description of the Related Art FIG. 3 shows a conventional phase change optical recording medium.
A light incident side protective layer 2, a recording layer 3 of a phase change material, a light transmitting side protective layer 4, a reflective layer 5, and a surface protective layer 6 are provided on a substrate 1 to form a phase change optical recording medium.

In optical recording using such a phase-change optical recording medium, a high-power laser beam with a diameter of about 1 micron is irradiated onto the recording layer, and the irradiated portion is melted and rapidly cooled to change to an amorphous state. Then, the information is recorded by irradiating the intermediate layer laser beam on the recording layer and maintaining the irradiated portion at a crystallization temperature or higher for a predetermined time to change the state to a crystalline state to erase the information. Information is reproduced by irradiating the recording layer with low-power laser light and detecting the difference in optical properties between the crystalline state and the amorphous state as the difference in the amount of reflected light. Then, by modulating the intensity of the laser beam with the high output and the intermediate output in accordance with the information, new information can be overwritten on the previous information.

[0004]

However, in the conventional phase change optical recording material, since the crystal grain size is different in the same recording layer due to the difference in heating and cooling process when the crystal phase is formed, the signal before the overwriting is performed. Remains somewhat after overwriting (Proceedings of the 51st JSAP Academic Lecture, 1199, 1)
990). As a result, the rate at which the previous signal disappears (hereinafter referred to as the “erase rate”) is as small as about 25 to 30 dB, so that information reproduction errors increase and the power margin of the intermediate output (erase level) of the laser light decreases. was there.

As an attempt to solve this problem, there is a method of increasing the cooling rate after irradiating the recording layer with a laser beam to prevent the crystal grain size from increasing, but it was not sufficient.

In view of the above, the present invention prevents the crystal grain size from increasing in the same recording layer by preventing the crystal grain size from increasing due to the difference in heating / cooling process, thereby increasing the erasing rate and reducing the information reproduction error. An object is to provide a phase change optical recording medium having a large power margin.

[0007]

The present invention provides a phase change optical recording medium in which a recording layer of a phase change material whose optical properties change with the phase change to either the amorphous or the crystalline is provided on a substrate. In the recording layer, a crystal nucleus material which becomes a nucleus of the crystal when the phase change material undergoes a phase change from amorphous to crystal and which does not melt when the phase change material undergoes a phase change to either amorphous or crystal is formed in the recording layer. The problem is solved by dispersing with a uniform distribution.

The phase change material used in the present invention includes:
Ge-Sb-Te, In-Sb-Te, In-Ge-S, which are phase change optical recording materials used for conventional optical recording media.
b-Te or the like may be used, or a material such as GeTe, which is considered to have a low crystallization rate, because it is not necessary to generate crystallization nuclei in the phase change material itself.

On the other hand, the following materials can be used as the crystal nucleus material. Each of the crystalline phases of the phase change material has a NaCl type structure with a lattice constant of about 6 Å, and its melting point is about 600 ° C. Therefore,
As a property of the above-mentioned crystal nucleus material which does not melt during recording and erasing but becomes a crystal nucleus during crystallization, it has a NaCl type crystal structure with a lattice constant of about 6 Å and a melting point of 1000.
It is desirable that the material be at a temperature of about ° C or higher.

CeSe, CeTe, DyTe, ErTe, Eu are available as crystal nucleus materials having such properties.
S, EuSe, HoTe, LaS, LaSe, NdS
e, PrSe, PuTe, SmSe, SrS, TbT
e, ThSe, TmTe, UTe, YTe, YbSe,
CeP, DySb, ErSb, HoSb, LaP, Nd
P, PrP, ThP, ScSb, TbSb, TmSb,
USb, YbSb, CeAs, GeAs, LaAs, N
dAs, PrAs, PuAs, SmAs, TbAs, T
hAs, PbSe, etc. can be used. Of these, P
Compounds other than bSe include elements of Group 3A of the periodic table (rare earth elements and actinide elements).

In the present invention, it is important to disperse the crystal nucleus material in the recording layer in a substantially uniform distribution. The form of the dispersion is as shown in FIG. The phase change material 12 may have a form in which it is not solid-dissolved in the phase change material 12 and is dispersed uniformly, or the crystal nucleus material 13 is formed in an island shape or a columnar shape on the adjacent protective layer. The phase change material 14 may be in contact therewith. Moreover, the dispersion density of the crystal nucleus material 11 is determined by the phase change material 12
Has a sufficiently high density that coarse grains do not grow when crystallization occurs.

As shown in FIG. 1, in order to mix the crystal nucleus material in the phase change material without forming a solid solution, in addition to the method of simultaneously sputtering the target of the crystal nucleus material and the target of the phase change material, There is a method of producing by the simultaneous vapor deposition method using the vapor deposition source of and the vapor deposition source of the phase change material.

Further, as shown in FIG. 2, when the crystal nucleus material is formed in an island shape or a column shape on the adjacent protective layer surface, after forming the protective layer, the crystal nucleus material layer is formed by a sputtering method or a vapor deposition method. There is a method in which the phase change material is formed to be extremely thin, and the phase change material is further stacked thereon by a sputtering method, a vapor deposition method, or the like. When forming a layer by sputtering or vapor deposition,
It is known that when it is extremely thin, it has an island shape or a column shape, and this method forms the crystal nucleus material into an island shape or a column shape. As another method, after forming a layer of a crystal nucleus material having a constant thickness by a sputtering method or the like, a part of the crystal nucleus material is removed by a method such as reverse sputtering, and an island-shaped or columnar crystal nucleus material is removed. Alternatively, the phase change material may be stacked thereon by a sputtering method or a vapor deposition method. On the contrary, after forming the phase change material, the crystal nucleus material may be formed into an island shape or a column shape by these methods.

[0014]

In the present invention, the crystal nucleus material is dispersed in the recording layer containing the phase change material in a substantially uniform distribution. And
This crystal nucleus material always serves as a nucleus of the crystal when the phase change material undergoes a phase change from amorphous to crystal, so that the crystal grains formed are mutually limited in size by adjacent crystal grains. To be done. As a result, the crystal grain size becomes substantially constant according to the density of dispersion of the crystal nucleus material. Moreover, since the crystal nucleus material does not melt when the phase change material undergoes phase change to either the amorphous state or the crystal state, the dispersion density of the crystal nucleus material does not change even if the superheat cooling process is different. Therefore, even if the superheat cooling process is different, the crystal grain size of the crystal phase formed becomes substantially constant, and the erasing rate can be improved.

Further, since the crystal nucleus material does not melt even when the phase change material undergoes a phase change to either the amorphous state or the crystal state, the recording and erasing are performed without changing the density or dispersion state of the crystal nucleus material. Can be repeated, and a stable improvement in the erase rate is realized.

[0016]

EXAMPLES Examples of the present invention will be specifically described below.

(1) Example 1 In the first example, the recording layer was made of Ge ₂ Sb ₂ Te ₅ as a phase change material, DyTe as a crystal nucleus material, and a Ge ₂ Sb ₂ Te ₅ target and a DyTe target. It was produced by the original simultaneous sputtering method. An optical recording medium using this recording layer was manufactured as follows. That is, on a 1.2 mm thick polycarbonate substrate, a 160 nm thick (ZnS) ₈₀
(SiO ₂ ) ₂₀ light incident side protective layer, 25 nm thick recording layer, 30 nm thick SiO ₂ light transmissive side protective layer, 100 nm thick Al-Ti light transmissive side reflection layer The layer and the layer are sequentially formed by a sputtering method. Next, thickness 1.2
An optical recording medium is obtained by forming a surface protective layer by adhering a polycarbonate protective plate having a thickness of mm with an ultraviolet curable resin.

This optical recording medium was rotated at a rotational speed of 1800 rpm, and a semiconductor laser beam having a wavelength of 830 nm was focused on a recording layer by an objective lens having a numerical aperture of 0.5 for evaluation. Recording was performed by intensity-modulating the laser light with two values of peak power and bias power. Frequency 3.7MH
The signal of z was recorded and the signal of the frequency of 1.39 MHz was overwritten, and the reduction amount of the signal level of 3.7 MHz at this time was defined as the erasing rate.

It was found that the optical recording medium of this example provided an erasing rate of 35 dB or more, and that the erasing rate was improved over the comparative example. The same value was obtained for the erasing rate after recording the signal of 3.7 MHz 10 ⁵ times, and the repeatability was also good.

When the recording layer of this medium was observed with a transmission electron microscope, the crystal grain size was more uniform than that of the medium of the comparative example.

Here, the produced recording layer is Ge ₂ Sb ₂ T.
Phase-separated into e ₅ and DyTe, Ge ₂ Sb ₂ Te ₅ undergoes a crystal-amorphous phase change, and DyTe becomes a crystal nucleus at the time of crystallization and disperses in a form schematically shown in FIG. Ge
₂ Sb ₂ Te ₅ is N with a lattice constant of 6.0 Å.
It has an aCl type crystal phase and its melting point is about 600 ° C.
Is. On the other hand, DyTe has a NaCl structure with a lattice constant of 6.09 angstrom and has a melting point of 1500 ° C. or higher. Therefore, DyTe does not melt at the time of recording and erasing, and it becomes a crystal nucleus when Ge ₂ Sb ₂ Te ₅ is crystallized.

When the recording layer is irradiated with laser light and the recording layer at the irradiated portion is kept at the crystallization temperature or higher for a predetermined time, the formed crystal grains are always crystallized with the crystal nucleus material 11 as the nucleus. At this time, since the crystal nucleus material 11 is uniformly dispersed at a sufficiently high density, the crystal grain size to be formed is limited by the crystal grains to be formed adjacent to each other, and is almost constant according to the density of the crystal nucleus material 11. It is considered that the erasing rate is high. Further, since the crystal nucleus material 11 does not melt, the crystal grain size that is formed is always constant and does not differ even if the irradiation of laser light is repeated, and it is considered that the repeatability is good.

(2) Example 2 In an optical recording medium having the same structure as in Example 1 except for the recording layer, a DyTe layer having a thickness of 5 nm was formed as a recording layer by a sputtering method and Ge ₂ having a thickness of 20 nm was formed. Sb ₂ T
The e ₅ layer was formed by the sputtering method. Where DyT
Since the e layer is as thin as 5 nm, it is dispersed in an island shape or a column shape as shown in FIG. 2, and this DyTe serves as the crystal nucleus material 13, while Ge ₂ Sb ₂ Te ₅ serves as the phase change material 14.

When this optical recording medium was evaluated in the same manner as in Example 1, an erasing rate of 35 dB or more, which was higher than that in Comparative Example, was obtained. Further, the repeatability was also good as in Example 1. When the recording layer of this medium was observed with a transmission electron microscope, the crystal grain size was more uniform than that of the medium of the comparative example.

(3) Comparative Example As a comparative example, the recording layer was formed of Ge ₂ Sb ₂ Te having a thickness of 25 nm.
₅ and with other than the in the optical recording medium of the same structure as in Example 1,
It was manufactured in the same manner as in Example 1. When the erase rate was measured by the same method as in Example 1, the erase rate of this example was about 3
It was as small as 0 dB. In addition, when the recording layer of this medium was observed with a transmission electron microscope, there were ring-shaped coarse crystal grains around the amorphous marks, and some large crystals were found on the amorphous marks where they seemed to be erased. There was a large variation in crystal grain size due to the presence of grains.

[0026]

According to the phase-change optical recording medium of the present invention, the crystal nucleus material is dispersed in the recording layer in a substantially uniform distribution, and this crystal nucleus material causes the phase change material to change phase from amorphous to crystalline. In doing so, the phase change material in the recording layer is crystallized with a uniform crystal grain size by becoming a nucleus of the crystal, and the phase change material undergoes a phase change to either the amorphous state or the crystal state to the other side. In this case, the recording and erasing can be repeated without changing the number of crystal nuclei and the mixed state without melting and a stable high erasing rate can be obtained, there are few data reproduction errors, and the power margin can be increased. You can

Further, since the number of crystal nuclei does not change and the recording material does not easily move, the number of overwriting can be increased. Further, since the crystallization speed is high without the generation of crystal nuclei, it is possible to transfer data at high speed.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a form of dispersion of a recording layer according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a form of dispersion of recording layers according to a second embodiment of the present invention.

FIG. 3 is a sectional view of a conventional phase change optical recording medium.

[Explanation of symbols]

1 ... Substrate, 2 ... Light incident side protective layer, 3 ... Recording layer, 4 ... Light transmitting side protective layer, 5 ... Reflective layer, 6 ...
.Surface protective layer, 11, 13 ... Crystal nucleus material, 12, 1
4 ... Phase change material

Claims (1)

[Claims] 1. A phase-change optical recording medium comprising a recording layer of a phase-change material, the optical properties of which change with the phase change to either the amorphous state or the crystalline state, provided on a substrate. A crystal nucleus material that becomes a nucleus of the crystal when the phase changes to a crystal and does not melt when the phase change material changes to an amorphous phase or a crystal is dispersed in the recording layer in a substantially uniform distribution. A phase change optical recording medium characterized by the above.