JPH0832482B2 - Optical information recording medium - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium in which an information signal can be recorded and reproduced at high speed and high density and rewritable by using means such as a laser beam.

2. Description of the Related Art A technique for irradiating a thin film such as a metal or a dye with a laser beam to cause a local change to record / reproduce information at high density is known, and a type capable of additional recording is a so-called write once ( It has been commercialized as a WRITE-ONCE) type optical disc device. On the other hand, the rewritable type is still in the research stage, but it has been proposed so far to use a thin film of a material containing chalcogen such as Te or Se or its compound (chalcogenide) as a main component in the recording layer. There is. Reversible phase changes between the amorphous and crystalline phases can occur relatively easily in these materials, especially in systems containing Te as the main component, and the optical constants change significantly between them. Since then, various compositions have been studied.

Te has absorption in the infrared region of light, melting point 400 ° C
It has a low viscosity, and because its structure is basically composed of two-coordinate chain-like atomic bonds, it has a high viscosity and tends to form an amorphous phase when cooled from the liquid phase. It has desirable characteristics as a rewritable recording material. However, Te alone has a low crystallization temperature (Tx) and cannot obtain a stable amorphous phase at room temperature. Therefore, it has been attempted to add various additive elements to Te to obtain a stable amorphous phase or to control the crystallization speed.

For example, Ge ₁₅ Te ₈₁ Sb ₂ S ₂ (Japanese Patent Publication No. 47-26897), Te
₉₂ Ge ₂ As ₅ (Applied Physics Letters (APPL
IED PHYSICS LETTERS), 18 (1971) P254), Te ₈₇ Ge ₈ Sn ₅
(Ibid. 46 (1985) P734).

All of these stabilize the amorphous phase of Te by adding Ge. In addition, an attempt to increase the crystallization rate was made by TeGe
There are Au-based alloys (Japanese Patent Application No. 60-61137), TeGeSnAu-based alloys (Japanese Patent Application No. 60-112420), etc., and by adding Au, the chain structure of Te is disrupted and the diffusion rate of atoms is increased. Has been successful.

Problems to be Solved by the Invention An object of the present invention is to perform recording and erasing at a much higher speed than in conventional systems.

Conventionally, in a recording-erasing method using a phase change between amorphous and crystalline, generally, a change from a crystalline phase to an amorphous phase is a recording direction, and conversely, a change from an amorphous phase to a crystalline phase is an erasing direction. The reason is that crystallization is a relatively time-consuming process such as annealing of the amorphous phase or slow cooling from the liquid phase, whereas amorphization is a fast process of rapid cooling from the liquid phase. . In other words, the recording time can be increased because the irradiation time of the laser beam can be shortened.

However, when a new signal is recorded while erasing a recorded information signal, that is, simultaneous erasure, the crystallization speed also needs to be sufficiently high. That is, the time required for crystallization (erasing) must be made as short as the time required for amorphization (recording). Until now, as a means for realizing this so-called simultaneous recording, two light spots for recording, reproduction and erasure have been used, and the length of the light spot for erasure has been recorded relative to the length of the light spot for recording and reproduction. A method has been adopted in which the irradiation time of the erasing light spot is lengthened by making it longer. However, this method requires a technique for accurately arranging two light spots on the same track, and there is a problem in that the optical system is complicated in terms of device design. Moreover, if the rewriting speed is further increased from the speed of several hundred kilobytes / sec at the highest as before, and recording is performed at several megabytes / sec like a magnetic disk, for example, the relative speed to the optical spot recording medium is several tens. The actual irradiation time will be tens of nanoseconds, which is extremely short. In such a case, the method of supplementing with the optical system as described above cannot be used, and a material having a truly high crystallization rate is required.

Means to solve the problem The ternary thin film of Te, Ge, Sb is adopted as the recording phase, and its composition range is especially defined as a stoichiometric compound composition or an equivalent single-phase composition. Choose to be.

In the three elements of action Ge-Sb-Te, there exists a ternary stoichiometric compound composition such as Ge ₂ Sb ₂ Te ₅ , GeSb ₂ Te ₄ and GeSb ₄ Te ₇ between GeTe and Sb ₂ Te ₃ . In a stable stoichiometric compound composition, the difference in free energy between a liquid state (also an amorphous state) and a crystalline state is large, and a driving force for crystallization can be increased. In addition, since the crystal phase is a stable single phase, there is no problem that the characteristics do not change even if recording and erasing are repeated without changing the phase. Further, this ternary system exhibits excellent characteristics as a rewritable optical recording medium, such as a relatively low melting point, an amorphous phase is easily formed, and a sufficiently stable amorphous phase is obtained due to a high crystallization temperature. You can

Example As shown in FIGS. 2a to 2c, the optical information recording medium of the present invention is made of PMMA, resin such as polycarbonate, aluminum,
Si on a substrate 1 with a smooth surface such as metal such as copper or glass
The recording layer 2 is formed by sandwiching a dielectric 3 such as O ₂ , Al ₂ O ₃ or ZnS. Although the dielectric layer is not always necessary for the present invention, it is effective for reducing the thermal damage to the resin substrate due to the repeated irradiation of the laser beam or the deformation and evaporation of the recording layer itself. It is also possible to attach the light reflecting layer 4 on the dielectric layer on the side opposite to the side where the laser beam is incident for the purpose of increasing the absorption efficiency of the laser beam, and further attach the protective plate 5 thereon.

The invention is characterized by the composition of the recording layer. The recording layer is composed of ternary elements Te, Sb and Ge, and the basic composition is
As shown in FIG. 1b, it is located on the line connecting the two stoichiometric compound compositions GeTe and the two composition points representing Sb ₂ Te _{3, and} can be expressed by the following composition formula.

xGeTe · (1-x) Sb ₂ Te ₃ (0 <x <1) Hereinafter, the basic concept of forming the recording layer in the recording medium of the present invention, specific constituent elements, and the reason for determining the concentration thereof will be described. Describe.

As described above, in a Te-based recording medium, when Te is contained in an excessive amount more than capable of forming a compound with other additives, this causes the erasing speed to be limited. Therefore, a method of increasing the concentration of the additive and fixing Te as a compound having a stoichiometric composition can be considered. However, Te alloys containing only one element, such as CdTe, SnTe, PbTe, Sb ₂ Te ₃ , GeTe, A
uTe _{2 and the} like have the following reasons: (1) If the melting point is too high and the laser beam irradiation time is short (recording pulse), it cannot be melted easily. That is, the recording sensitivity is low.

(2) The crystallization temperature is too low to form a stable amorphous phase. That is, it lacks reliability.

Therefore, it is unsuitable as a recording layer for either or both reasons. Among them, GeTe has a stable amorphous phase and a relatively low melting point of 725 ° C, but the output of the current practical laser diode is at most high.
Considering that it is about 30-40 mW, there are drawbacks that it is not easy to amorphize and the moisture resistance is low. Therefore, the present inventors have investigated a ternary compound containing Te and GeSbTe
It was found that the ternary compound (1) was excellent as a recording thin film in the following points. This system has several properties 1-4 that are specific and important to this system as described below.

First, there are multiple stoichiometric compounds between GeTe and Sb ₂ Te ₃ , namely Ge ₂ Sb ₂ Te ₅ , GeSb ₂ Te ₄ and GeSb ₄ Te.
_{As in 7} , there is a ternary compound phase between the binary compound phases at both ends. Figure 3 shows the GeTe-Sb ₂ Te ₃ pseudo-binary phase diagram by N.Kh.Abrikosov et al. As described above, in the stoichiometric compound phase, the free energy in the crystalline state is low and the difference in energy level from the amorphous phase is large, so that a high crystallization rate can be obtained.

Second, the compound phases have crystal structures very similar to each other, and exhibit very close values in crystallization temperature, melting point, and the like. This has the following effect: even if the composition of the recording film does not exactly match one of the stoichiometric phases, it can be considered as a mixture of the three phases as a whole, and over a wide composition range. The advantage is that the characteristics do not change. Especially GeTe-Sb ₂ Te ₃
On the line of (3), the characteristics which are completely the same as the above three stoichiometric compound compositions were obtained.

The third point is that the crystallization temperature is sufficiently high, and thus a stable amorphous phase can be obtained. The measurement example of the crystallization temperature of the Ge-Sb-T ternary thin film containing the composition on the line of GeTe-Sb ₂ Te ₃ in FIGS.
The result of examining the composition dependence is shown as a contour map. This shows that the GeTe-Sb ₂ Te ₃ pseudo-binary system can secure a crystallization temperature sufficiently higher than room temperature over a wide range.

The fourth point was found during the crystallization process. In other words, in the GeTe-Sb ₂ Te ₃ pseudo-binary system, the stable crystal phase at room temperature is hexagonal, but as the initial phase of crystallization, a single plane consisting of Ge, Sb, and Te ternary It was found that a cubic cubic metastable phase appears. The atomic structure in the liquid state or the amorphous state is much more isotropic than the atomic structure in the crystalline state. Therefore, it is important that the resulting crystal form is as isotropic as possible by shortening the diffusion distance of atoms during crystallization and crystallization. It is considered to be advantageous in shortening the time. It is known that the face-centered cubic lattice is one of the most isotropic crystal systems.
The metastable phase is likely to occur when heating and cooling are performed relatively quickly, such as laser irradiation.

From the above reason, it is understood that the GeSbTe thin film is suitable as a rewritable optical information recording medium.

Next, the manufacturing method of the present invention will be described. The recording medium of the present invention can be formed by a method such as vacuum deposition and sputtering. For sputtering, it is also possible to use an alloy target indexed from the desired composition,
A composite mosaic target having an area corresponding to each composition can also be used. In the case of the vacuum evaporation method, it is convenient to control a composition by preparing a plurality of sources and employing a co-evaporation method. In this case, for example, three sets of electron guns and their power supplies,
It is desirable that a film thickness sensor (for example, a quartz oscillator) be prepared and the deposition rate from each source can be controlled completely independently. Vacuum degree during vapor deposition is 10-4 Torr to 10-7 Torr
Is enough.

 Hereinafter, the present invention will be described in more detail with reference to specific examples.

Example 1 Test pieces of Ge-Sb-Te ternary recording media having various compositions were prepared by the above vacuum deposition method, and their characteristics were examined. The characteristics were evaluated from three points: 1. phase transformation temperature Tx 2. minimum laser irradiation power required for amorphization p 3. laser irradiation time d required for starting crystallization. For the Tx measurement, Pyrex glass having a thickness of 0.3 mm and a diameter of 8 mm was used as a substrate for Tx measurement, and the recording layer had a thickness of about 100 nm. Tx is defined as the temperature at which the optical transmittance of the sample piece in the as-depo state begins to change when gradually heated. The temperature is raised at a speed of 1 ° C / s, and the change in optical transmittance during that time is changed to He-Ne.
An inflection point was detected by monitoring with a laser. Thereby, the thermal stability of the amorphous phase can be evaluated. For the measurement of the amorphization sensitivity p and the crystallization speed d, a PMMA plate having a length of 12 mm, a width of 10 mm and a thickness of 1.2 mm was used as a substrate, and 100 nm ZnS, 100 nm GeSbTe ternary recording film, and 200 nm Z
nS was sequentially vapor-deposited, and the same PMMA plate as the substrate was laminated on it using an ultraviolet curing resin. p is a value obtained by measuring the irradiation power required to start amorphization by irradiating a laser beam having a certain pulse width on the surface of the recording film in a crystalline state. In this case, each sample was pre-blackened (crystallized) by performing laser irradiation with a power of 4 mW for a length of 10 μs in advance, and then fixing the irradiation pulse width to 50 ns to obtain only the laser output. Irradiation power was measured by changing and irradiating, and the amorphization was confirmed as a change in optical reflectance. Thereby, the recording sensitivity is evaluated. In addition, d is a lens system that allows light from a laser diode to have a diameter of 1 μm on the recording film in the as-depo state.
It is the irradiation time required to start crystallization when irradiated as a spot of about m. In this case, the irradiation power was changed in the range of 2 to 25 mW and the irradiation time was changed in the range of 10 to 1000 ns to obtain the fastest crystallization condition. As a result, the erasing speed can be evaluated.

FIG. 4a shows an example of measurement of Tx in the GeTe—Sb ₂ Te ₃ system. It can be seen that the transmittance sharply decreased at a certain temperature and crystallization occurred.

b corresponds to each composition point on the ternary composition diagram of Ge-Sb-Te
It is a hot line diagram which plotted Tx and connected the point of the same temperature.
It has been found that a transition temperature well above room temperature can be obtained over a wide composition range. c is the result of examining the relationship between the composition ratio x of GeTe and Tx with respect to the composition on the line connecting the composition points of GeTe and Sb ₂ Te ₃ .

The phase transformation temperature of the xGeTe * (1-x) Sb ₂ Te ₃ (0 <x <1) system is 100 ° C. or higher in both cases of x> 0, which is sufficiently high with respect to room temperature and the amorphous phase is stable. I understand that Further, in the region of x> 0.1, it was found that Tx was 140 ° C. or higher and an extremely stable amorphous state was obtained.

FIG. 5 shows the results of measuring the laser irradiation time required to start crystallization for the same system. In the figure, each curve plots the crystallization start pulse width d of the ternary composition of Ge-Sb-Te when the laser power is set to 8 mW, corresponding to each composition point, and connecting the points at the same temperature at a constant velocity. It is a diagram. From this, Ge
It can be seen that with the composition on the Te-Sb ₂ Te ₃ line, crystallization starts with an extremely short irradiation time of about 30 ns to 100 ns, and the irradiation time required for crystallization increases as it deviates from the line. However, when the deviation from the line composition is not so large, crystallization can be started by laser irradiation for about 200 ns. It was confirmed that the laser irradiation time to start crystallization was shortened up to about 14 mW by further increasing the laser power.

Next, the amorphization sensitivity was examined. On the contrary, it can be said that the fact that the crystallization speed is high has a possibility that the amorphous is hardly formed. The melting point of the Ge-Sb-Te ternary film near the above line was found to be relatively close between the composition on the above line and its surroundings from the measurement of the melting point by DSC.
The amorphization sensitivity of the b ₂ Te ₃ system was investigated.

FIG. 6 shows the result. From this, the composition ratio x of GeTe is
In the region smaller than 0.8, it can be seen that even if the laser irradiation for a very short time of 50 ns, the amorphization is realized with the laser power of about 20 mW. Further
Amorphization has been realized with laser powers of 13, 15, and 17 mW for compound compositions with GeTe composition ratios of 33, 50, and 66%, respectively. These values are sufficiently low as compared with the case of GeTe alone being 30 mW or more, indicating that the amorphization sensitivity of the GeTe-Sb ₂ Te ₃ system is sufficiently higher than that of GeTe. Amorphization sensitivity is closely related to melting point
As it approaches Sb ₂ Te ₃ , it becomes higher, but with Sb ₂ Te ₃ alone, Tx is rather low as described above, and it is not practical. In the eutectic composition Ge ₄ Sb ₃₇ Te ₅₉ between GeTe and Sb ₂ Te ₃ , it has 593 ° C. and the lowest melting point of this system, so that high recording sensitivity and relatively stable amorphous phase can be obtained at the same time. .

Example 2 Next, the crystallization process of a Ge-Sb-Te ternary thin film was examined using X-ray diffraction and DSC. A plurality of test pieces were prepared on a quartz glass having a thickness of 0.3 mm and a square of 20 mm each having several compositions including the composition on the line deposited by about 100 nm. The X-ray diffraction patterns of the untreated composition and the composition annealed in argon gas for about 10 minutes were examined. As the annealing temperature, a transformation point such as a crystallization temperature was examined in advance using DSC, and the temperature immediately above the transformation point was used. As a result, a crystallization process as shown in Table 1 was found.

That is, from this table, 1) the untreated state as deposited is an amorphous state. 2) In the above composition on the line, first, a NaCl-type metastable phase appears as the initial phase. 3) From the line It can be read that a hexagonal stable phase appears from the beginning with a deviated composition, and it can be seen that the rapid crystallization in the composition on the line corresponds well with the appearance of the metastable phase described above.

The DSC showed that both the exothermic peak due to crystallization and the rapid heat peak due to melting were narrow and steep in the on-line composition, confirming that these systems were single phase.

Example 3 With respect to each composition point corresponding to Examples 1 and 2, an optical disk was prototyped and its dynamic characteristics were examined. The disk is Zn on a PMMA resin substrate with a diameter of 130 mm and a thickness of 1.2 mm equipped with a light guide groove.
The S, Ge-Sb-Te ternary film and ZnS were sequentially laminated, and the same PMMA plate as the substrate was laminated as a protective layer using an ultraviolet effect resin on it. The thickness of each layer is about 800A, 100 from the bottom.
0A, 1600A, designed to enhance the light absorption effect in the recording layer. The dynamic tester (deck) has a single laser spot that is narrowed down to a circular shape with a diameter of 0.9 μm (1/2 intensity) for both recording and playback, and for erasing. The laser output is high during recording. This is a test for so-called overwrite recording in which the old signal is overwritten with the new signal by lowering it during erasing. The disc rotation speed was set to 20 m / sec as a reference, and recording (overwriting) was alternately performed at two frequencies of 5 MHz and 7 MHz to examine the repeated life. The life limit is defined as the number of times the initial C / N is reduced by 3 dB, and the following conclusions (1)-(3) are obtained.

(1) With the composition on the line, 15-24mW when recorded, 6 when erased
Overwriting is possible in the power range of -12mW, and C / N of 50dB or more is obtained. It can be repeated more than 1 million times. In addition, overwriting was confirmed at a maximum of 30 m / sec (rank 1).

(2) As the composition on the line deviates, the erasing speed decreases, and the old signal cannot be erased and the quality of the new signal deteriorates. In this case, lower the recording frequency and reduce the rotation speed to 10
By slowing down to m / sec or 5 m / sec, overwriting can be performed as in the case of the composition on the line. However, if the deviation is too large, the rotation speed cannot cope. In addition, noise that is thought to be caused by phase separation generated by repetition is likely to occur.

(3) As shown in Fig. 6, the allowable range of composition deviation is + 10at% in the Sb direction and +10 in the Te direction as seen from the above line.
It is about at%, and in this range, it can be repeated 10,000 times at a rotation speed of 5 m / sec or less (rank 4). As well
In the range of + 7at% in the Sb direction and + 5at% in the Te direction
It can be repeated 100,000 times at a rotation speed of 15 m / sec or less (rank 3). Furthermore, in the range of + 5at% and + 3at%, it is possible to repeat 1 million times at a rotation speed of 25m / sec or less (rank 2). Table 2 shows composition regions corresponding to the above corner ranks.

As a matter of course, these disks can be repeatedly recorded / erased by using a plurality of laser spots as in the conventional case, and rather, the condition is that the degree of freedom in selection is large and easy.

Example 4 Environmental tests of the disks in Examples 1 and 2 were performed. Leave each disk in the environment of 80 ℃, 80RH% for 1 month,
The reflectance was monitored, but no change was observed in the disk having the composition having Tx of 140 ° C. or higher.

EFFECTS OF THE INVENTION As described above, according to the present invention, (1) the information transfer rate is extremely large, such as several megabytes per second.

(2) Overwriting is possible with a single laser beam.

(3) An optical information recording medium having a long repeated life is provided.

[Brief description of drawings]

FIG. 1 shows Ge-Sb-Te applied to the optical information recording medium of the present invention.
FIG. 2 is a composition diagram showing a composition region of a ternary recording film, FIG. 2 is a sectional view showing an embodiment of a recording medium in an example of the present invention, and FIG. 3 is a center of a Ge-Sb-Te ternary composition. GeTe-Sb ₂
Te ₃ pseudo-binary phase diagram, FIG. 4 shows an example of measurement of crystallization transition temperature Tx and its composition dependence, and FIG. 5 shows Ge-S of the present invention.
FIG. 6 is a diagram showing the composition dependence of the amorphization sensitivity of the b-Te ternary recording film, and FIG. 6 shows the laser irradiation time required for the Ge-Se-Te ternary recording film of the present invention to start crystallization. FIG.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Susumu Sanai 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP 61-89889 (JP, A) JP 61-66696 (JP, A) JP 62-53886 (JP, A)

Claims (4)

[Claims] 1. A recording thin film layer sandwiching dielectric thin film layers on both sides of the substrate is provided, and a reversible phase change between amorphous and crystal is caused in the recording thin film layer by irradiation of a laser beam. A rewritable phase change recording medium for recording and erasing information by utilizing the accompanying change in optical properties, wherein the recording thin film has a stable stoichiometric ternary compound composition Ge ₂ Sb ₂ Te ₅ , GeSb ₂ Te ₄ or GeSb ₄ Te ₇ , or a solid solution composition between them, which is characterized in that the recording thin film layer is converted to a crystalline phase in the initial state. Information recording medium. 2. The optical information recording according to claim 1, further comprising a reflecting layer formed on the dielectric thin film layer on the laser beam emitting side among the dielectric thin film layers sandwiching the recording thin film layer. Medium. 3. A recording thin film layer sandwiched between dielectric thin film layers on both sides of the substrate, wherein a reversible phase change between amorphous and crystalline is caused in the recording thin film layer by irradiation with a laser beam, and A rewritable phase change recording medium for recording and erasing information by utilizing the accompanying change in optical properties, wherein the recording thin film has a eutectic composition between GeTe and Sb ₂ Te _3. An optical information recording medium, wherein the recording thin film layer is converted into a crystalline phase in an initial state. 4. The optical information recording according to claim 3, wherein a reflective layer is further formed on the dielectric thin film layer on the laser beam emitting side among the dielectric thin film layers sandwiching the recording thin film layer. Medium.