JP2574325B2 - Optical information recording medium - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium capable of recording and reproducing information signals at high speed and with high density using means such as a laser beam and rewritable.

2. Description of the Related Art A technique for recording and reproducing information at a high density by irradiating a laser beam onto a thin film of a metal, a dye, or the like to cause a local change is known, and a type capable of additionally recording is a so-called write-once ( WRITE-ONCE) type optical disk device. On the other hand, the rewritable type is still in the research stage, but until now, Te, Se
Such as chalcogen or its compound (chalcogenide)
A material using a material thin film containing as a main component has been proposed. In these substances, especially in a system mainly composed of Te, it is possible to relatively easily cause a reversible phase change between the amorphous phase and the crystalline phase, and the optical constant changes greatly between them. Various compositions have been studied.

Te has absorption in the infrared region of light, melting point 400 ° C
Phase change type, such as low viscosity and high viscosity due to its basic structure consisting of two-coordinated chain-like atomic bonds, and easy formation of an amorphous phase when cooled from a liquid phase. It has characteristics desirable as a rewritable recording material. However, Te alone has a low crystallization temperature (Tx) and cannot obtain a stable amorphous phase at room temperature. Attempts have been made to obtain a stable amorphous phase by adding various additive elements to Te, or to control the rate of liquid crystal formation.

For example, Ge ₁₅ Te ₈₁ Sb ₂ S ₂ (JP-B-47-26897), Te
₉₂ Ge ₂ As ₅ (Applied Physics Letters (APPL
IED PHYSICS LETTERS), 18 (1971) P254), Te ₈₇ Ge ₈ Sn ₅
(APPLIED PHYS Letters
ICS LETTERS), 46 (1985) P734). This stabilizes the Te amorphous phase by adding Ge. Attempts to increase the crystallization rate have been made using TeGeAu-based alloys (Japanese Patent Laid-Open No. 61-219692) and TeGeSnAu-based alloys (Japanese Patent Laid-Open No. 61-270).
No. 190), and succeeded in increasing the diffusion rate of atoms by adding Au to break the Te chain structure.

Problems to be Solved by the Invention It is an object of the present invention to perform recording and erasing at a much higher speed than 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 for this is that crystallization is a high-speed process such as annealing of an amorphous phase or slow cooling from a liquid phase, while amorphousizing is a rapid process such as rapid cooling from a liquid phase. In other words, the recording time can be increased because the irradiation time of the laser beam can be shortened.

However, a new signal is recorded while erasing the already recorded information signal. When performing so-called simultaneous recording, this crystallization speed also needs to be sufficiently high. That is, the time required for crystallization (erasing) must be shortened to the same extent 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 has a problem in that the optical system is complicated in designing the device. In addition, if the rewriting speed is further increased from the speed of at most several hundred kilobytes / sec as before, for example, when recording at several megabytes / sec like a magnetic disk, the relative speed between the light spot and the recording medium becomes several The actual irradiation time becomes extremely short at several tens of nanoseconds.
In this case, the method of supplementing with the above-described optical system cannot cope with the problem, and a material having a truly high crystallization rate is required.

Means to solve the problem A ternary thin film of Te, Ge, Bi is adopted for the recording layer, and the composition range is particularly adjusted to a stoichiometric compound composition or a single-phase composition equivalent thereto. Choose to be.

Action In the ternary system Ge—Bi—Te, there is a ternary stoichiometric composition such as Ge ₃ Bi ₂ Te ₆ , GeBi ₂ Te ₄ , GeBi ₄ Te ₇ between GeTe and Bi 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. Furthermore, this ternary system has a relatively low melting point and is likely to form an amorphous phase. Excellent characteristics as an optical recording medium can be exhibited, for example, a sufficiently stable amorphous phase can be obtained due to a high crystallization temperature.

Embodiment The optical information recording medium of the present invention is, as shown in FIGS. 2a to 2c, on a substrate 1 having a smooth surface such as a resin such as PMMA and polycarbonate, a metal such as aluminum and copper, and a glass.
It is formed by forming a recording layer 2 sandwiched by derivatives 3 such as SiO ₂ , Al ₂ O ₃ and ZnS. The derivative layer is not necessarily required for the present invention, and is effective for reducing thermal damage to the resin base material due to repeated irradiation of a laser beam, or 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 present invention is characterized by the composition of the recording layer. The recording layer is composed of ternary elements Te, Bi, and Ge.
As shown in FIG. 1 (b), the binary stoichiometric compound Ge
It is located on a line connecting two composition points representing Te and Bi ₂ Te ₃ and can be represented by the following composition formula.

xGeTe · (1-x) Bi2Te3 (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 its concentration will be described.

As described above, when Te is contained in a Te-based recording medium in excess of that capable of forming a compound with another additive, this causes a limitation on the erasing speed. 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 with only one element added, such as CdTe, SnTe, PbTe, Sn ₂ Te ₃ , Bi ₂ T
e ₃ , GeTe, AuTe _2, etc., have the following reasons: 1) The melting point is too high, and if the irradiation time of the laser beam is short (recording pulse), it cannot be easily melted. 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.

It is unsuitable as a recording layer for any one or both reasons.

Among them, GeTe has a stable amorphous phase and a relatively low melting point of 725 ° C. Considering that the output of a very practical laser diode at present is about 30-40 mW at most, it becomes amorphous. It is not easy and has the disadvantage of low moisture resistance.

Therefore, the present inventors studied a ternary compound containing Te.
It has been found that a ternary compound of Ge-Bi-Te is 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 of all, there are multiple stoichiometric compounds between GeTe and Bi ₂ Te ₃ , namely Ge ₃ Bi ₂ Te ₆ , GeBi ₂ Te ₄ , GeBi ₄ Te
As shown in FIG. 7, a ternary compound phase exists between the binary compound phases at both ends. FIG. 3 shows a quasi-binary phase diagram of GeTe-Bi ₂ Te ₃ . As described above, the stoichiometric compound phase has a low free energy in the crystalline state and a large difference in energy level from the amorphous phase, 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. In particular, GeTe-Bi ₂ T
_On the line of e3, characteristics which are not different from the above three stoichiometric compound compositions were obtained.

The third point is that the crystallization temperature is sufficiently high, so that a stable amorphous phase is obtained. FIGS. 4 (a), (b) and (c) in Example 1 show GeTe-Bi ₂ T
A contour diagram shows a measurement example of the crystallization temperature of the Ge—Bi—Te ternary thin film including the composition on the line e ₃ and the result of examining the composition dependence. This shows that in the GeTe-Bi ₂ Te ₃ pseudo-binary system, a crystallization temperature sufficiently higher than room temperature can be secured over a wide range.

The fourth point was found during the crystallization process.
That is, in the GeTe-Bi ₂ Te ₃ pseudo-binary system, the stable crystal phase at room temperature is hexagonal, but as the initial phase of crystallization, first, a single plane consisting of the ternary Ge, Bi, Te It was found that a metacubic phase of the heart cubic type appeared. 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 ones in the crystal system. The metastable phase is likely to occur when heating and cooling are performed relatively quickly, such as laser irradiation. For the above reasons, it can be seen that the Ge—Bi—Te based thin film is suitable as an 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 evaporation and sputtering. For sputtering, it is also possible to use an alloy target calculated 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. The degree of vacuum during the deposition is from 10 ^-4 Torr to 10 ^-7 To
rr is enough.

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

Example 1 Test pieces of Ge-Bi-Te ternary recording media of various compositions were prepared by the above-described vacuum deposition method, and the characteristics thereof were examined.
The characteristics were evaluated from three points: 1. phase transformation temperature Tx 2. minimum laser irradiation power P required for amorphization 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 was defined as the temperature at which the chemical transmittance of the as-depo sample started to change when the sample was gradually heated. The temperature was raised at a rate of 1 ° C./s, and the change in the optical transmittance during that time was measured using He-Ne.
Monitoring was performed using a laser to detect inflection points. Thereby, the thermal stability of the amorphous phase can be evaluated.

For the measurement of the amorphization sensitivity P and the crystallization rate 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 of ZnS and 100 nm of Ge-Bi-Te ternary system were recorded thereon. Film, 200nm ZnS deposited sequentially, PMMA same as substrate
A plate obtained by bonding plates using an ultraviolet curing resin was used. P is a value obtained by irradiating a laser beam having a certain pulse width on a recording film surface in a crystalline state while irradiating the laser beam, and measuring an irradiation power required to start amorphousization. In this case, each sample is preliminarily blackened (crystallized) by preliminarily irradiating a laser with a power of 4 mW and a length of 10 μs, and then fixing the irradiation pulse width to 50 ns and controlling only the laser output. The irradiation power was measured by changing the irradiation power so that the amorphous state could be confirmed as a change in the optical reflectance. Thereby, the recording sensitivity is evaluated. Further, d is an irradiation time required for crystallization to start when light from a laser diode is irradiated as a spot having a diameter of about 1 μm on a recording film in an as-depo state by a lens system. In this case, the irradiation power is 2-25 mW and the irradiation time is 10-1000
The fastest crystallization conditions were determined by changing the range of ns.
As a result, the erasing speed can be evaluated.

Figure 4 (a) is an example of measurement of GeTe-Bi ₂ Te ³ system Tx.
It can be seen that the transmittance sharply decreased at a certain temperature and crystallization occurred.

(B) is an equivalent temperature diagram obtained by plotting Tx corresponding to each composition point on the ternary composition diagram of Ge-Bi-Te and connecting points at 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 and Tx of GeTe with respect to the composition on the line connecting the composition points of GeTe and Bi ₂ Te ₃ . xGeTe · (1-x) Bi 2 Te 3 (0 <x <1) phase transformation temperature of the system, x> exist stably high enough the amorphous phase with respect to room temperature and any case 100 ° C. or more 0 You can see that Further, in the region of x> 0.1, Tx was 140 ° C. or higher, and it was found that an extremely stable amorphous state was obtained.

FIG. 5 shows the results of measuring the laser irradiation time required for starting crystallization for the same system. In the figure, each curve represents a crystallization start pulse width d of a ternary composition of Ge-Bi-Te when the laser power is set to 8 mW, corresponding to each composition point, and is connected to a point at the same temperature. FIG. Than this,
GeTe-Bi 100n approximately 30ns in the composition on the ₂ Te ₃ line
It can be seen that crystallization starts in an extremely short irradiation time of s, and that the irradiation time required for crystallization increases as the line goes off the line. However, when the deviation from the line composition is not so large, crystallization can be started by laser irradiation of about 200 ns. The same operation was performed by further increasing the laser power, and it was confirmed that the laser irradiation time for starting crystallization was reduced to about 14 mW.

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. From the measurement of the melting point by DSC, it was found that the melting point of the Ge-Bi-Te ternary film in the vicinity of the line was relatively close to the composition on the line and its surroundings.
The amorphization sensitivity of -Bi ₂ Te ₃ system was investigated.
FIG. 6 shows the results. From this, the composition ratio x of GeTe is 0.
It can be seen that in a region smaller than 85, amorphousization is achieved with a laser power of about 20 mW even by laser irradiation for a very short time of 50 ns. Further Ge
In a compound composition with a composition ratio of Te of 33,50,75%,
Amorphization has been achieved with the laser power of 13,15,17mW. These values indicate that the GeTe-Bi ₂ Te ₃ system has a sufficiently high amorphization sensitivity that is sufficiently higher than that of GeTe alone, which is 30 mW or more. Amorphization sensitivity is closely related to melting point, and Bi
_As it approaches ₂ Te ₃ , it becomes higher, but Bi ₂ Te ₃ alone has a slightly lower Tx as described above and is not practical. The eutectic composition between GeTe and Bi ₂ Te ₃ , Ge ₄ Bi ₃₇ Te ₅₉ has the lowest melting point of 593 ° C and this system makes it possible to simultaneously obtain a high recording sensitivity and a relatively stable amorphous phase. it can.

Example 2 Next, the result of examining the crystallization process of a Ge—Bi—Te ternary thin film using X-ray diffraction and DSC will be described. 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 samples and the samples annealed in argon gas for about 10 minutes were examined for each composition. 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) that the untreated state as deposited is an amorphous state, and 2) in the above composition on the line, first, a NaCl-type metastable phase appears as the initial phase. 3) It can be seen that a hexagonal stable phase appears from the beginning in the composition deviating from the line, and that the high-speed crystallization in the composition on the line corresponds well with the appearance of the metastable phase described above. Recognize.

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 An optical disk was experimentally manufactured for each composition point corresponding to Examples 1 and 2, and its dynamic characteristics were examined. The disc is mounted on a 130 mm diameter, 1.2 mm thick PMMA resin substrate with light guide grooves.
A ternary film of ZnS, Ge-Bi-Te, and ZnS were sequentially laminated, and the same PMMA plate as the substrate was laminated thereon as a protective layer using an ultraviolet curable resin. The thickness of each layer is about 800 from the bottom
A, 1000A, 1600A, designed to enhance the light absorption effect in the recording layer. The dynamic tester (deck) has a single laser spot narrowed to a 0.9um (1/2 intensity) diameter circle for recording and playback, and also for erasing. At the time of erasing, a low signal is used to test an overwrite recording in which an old signal is overwritten with a new signal. Based on the rotation speed of the disk of 20 m / sec, recording (overwriting) was alternately performed at two frequencies of 5 MHz and 7 MHz, and the repetitive life was examined. Lifetime limit is 3dB from initial C / N
This was defined as the number of times of reduction, and the following conclusions 1) to 3) were obtained.

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

2) As the composition deviates from the composition on the line, the old signal cannot be completely erased due to a decrease in the erasing speed, and the quality of the new signal deteriorates. In this case, lower the recording frequency and set the rotation speed to 10m /
Overwriting can be performed in the same manner as in the case of the composition on the line by making the delay time as slow as 5 m / sec. However, if the deviation is too large, the rotation speed cannot cope. Also, noise likely to be caused by phase separation caused by repetition is likely to occur.

3) As can be seen from FIG. 6, the allowable width of the composition deviation is +10 at% in the direction of Bi and +10 at% in the direction of Te when viewed from the above line.
%, And within this range, 10,000 repetitions are possible at a rotation speed of 5 m / sec or less (rank 4). Similarly Bi
15m in the range of + 7at% in the direction of + and + 5at% in the direction of Te
It can be repeated 100,000 times at a rotation speed of less than / sec (rank 3). Furthermore, in the range of about + 5at% and + 3at%
One million repetitions is possible at a rotation speed of 25 m / sec or less (rank 2). The composition regions corresponding to the respective ranks are shown in Table 2.

As a matter of course, these discs can be repeatedly recorded / erased by using a plurality of laser spots as in the prior art. .

Example 4 An environmental test of the disks in Examples 1 and 2 was performed.
Each disk was left in an environment of 80 ° C. and 80% RH%, and the reflectance was monitored for one month, but no change was observed for the disk having a composition of Tx of 140 ° C. or more. No rust or the like was generated.

Effects of the Invention According to the present invention, 1) an information transfer rate is extremely large at several megabytes per second 2) overwriting is possible with a single laser beam 3) An optical information recording medium having a long repetitive life is provided. You.

[Brief description of the drawings]

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

Claims (4)

(57) [Claims] 1. A recording material layer whose optical properties are reversibly changed by means of light, heat or the like on a substrate, and the change is used to record, reproduce and erase information. An information recording medium, wherein the recording material layer is a solid solution xGeTe (1- 1) between stable stoichiometric compounds GeTe and Bi ₂ Te _3.
x) An optical information recording medium characterized by Bi ₂ Te ₃ (0 <x <1). 2. A recording material layer comprising a stable stoichiometric compound Ge
Solid solution between Ge and Bi ₂ Te ₃ xGeTe (1-x) Bi ₂ Te ₃ (0.1 <
x <0.85). Claim 1
The optical information recording medium described in the item. 3. The composition of a recording material layer is a stable stoichiometric ternary compound Ge ₃ Bi ₂ Te ₆ , GeBi ₂ Te ₄ or GeBi ₄ Te ₇ or a solid solution composition therebetween. The optical information recording medium according to claim 1, wherein: 4. The optical information recording medium according to claim 1, wherein the composition of the recording material layer is a eutectic composition between GeTe and Bi ₂ Te ₃ .