JPH0514629B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a recording material belonging to the field of optical recording systems, and in particular to a rewritable optical recording medium.

BACKGROUND ART Recording methods include (a) magnetic recording, (b) magneto-optical recording, and (c) optical recording. In the field of (a) magnetic recording methods, magnetic powder is applied to the tape and recorded in the in-plane direction of the tape (longitudinal recording), high-density longitudinal recording that further improves the recording density, and magnetization perpendicular to the tape. There is a perpendicular magnetic recording method. The magneto-optical recording method (b) is a method that utilizes the optical force effect. In the optical recording method (c), a: holes are made with light and recording is performed depending on whether or not they are formed.

b: Record the difference in light transmittance between the amorphous phase and the crystalline phase. c: There are methods such as recording the difference in light reflectance between the amorphous phase and the crystalline phase.

The middle (c) optical recording method has attracted attention due to the development of the first information society and the need to develop systems that can process large amounts of information at high speed, and high-density, large-capacity optical disk memories are being developed.

Optical disk memory is high speed (450~1800rpm)
A laser beam is focused on a rotating disk, recording bits of information on the thin memory film, and reading the information written with the same laser without damaging it.

Unlike playback-only optical video disks, this optical disk memory has functions such as instant recording, playback, and high-speed random access, and this rewritable optical disk system is expected to replace magnetic disks in the future. .

However, optical recording has both advantages and disadvantages.

First, the advantages are that recording and playback can be performed without contact, it can be used even if there is some dust or scratches, it is easy to handle, and unlike perpendicular magnetic recording, the S/N ratio decreases when separated from the recording medium. The size is half that of a floppy disk, and the recording density is one to two orders of magnitude higher than that of a magnetic disk.
The price is the same as a floppy disk, and the price per bit is comparable to magnetic tape.
etc.

On the other hand, rewritable half-discs can read the difference in light reflectance due to the transformation of the amorphous phase and crystalline phase of TeOx and FeGe-based chalcogenide, but chalcogen thin films are sensitive to temperature and oxidation-resistant. The transmittance changes over time due to blackening due to long-term temperature rises and falls.
Access time and data transfer speed are inferior to magnetic disks. This is because the rotation speed cannot be increased to the same level as that of a magnetic disk due to the relationship between the sensitivity of the recording medium and the output of the semiconductor laser.The bit error rate is 10 ^-7 to 10 ^-8 of that of a magnetic disk (before correction).
Compared to that, it is around 10 ^-6 for optical disks, which is 1 to 2
An order of magnitude lower. However, this is due to deterioration in quality when manufacturing optical disks (defects such as foreign objects, pinholes ^, and minute scratches during the manufacturing process), and this can be improved by correcting errors.
10, which is comparable to magnetic tape.

Problems to be Solved by the Invention The present invention solves the disadvantages of the conventional optical recording system described above.
That is, it attempts to improve such points as changes in transmittance over time, slow access time and data transfer rate, and high bit error rate.

Means for Solving the Problems The present invention provides a thin film layer made of a shape memory metal material whose austenite phase and martensite phase are reversibly transformed by temperature and stress, and whose light reflectance changes depending on the undulations of the surface shape. An optical recording material characterized by having:

The most fundamental feature of the reversible transformation of the austenite phase and martensitic phase used in the present invention is that it is a "shear deformation lattice transformation based on the coordinated movement of atoms." A typical metal in which this transformation occurs reversibly is a shape memory metal. The shape memory effect is generally said to be a phenomenon in which once an alloy is made to memorize a certain shape, even if it is deformed, it will return to its original memorized shape simply by heating it above a certain temperature.

The cause of the shape memory effect is stress-induced martensite and its reverse transformation process,
The main causes include the interdigitation of crystallographically equivalent sibling phases of martensite under stress and its reverse transformation process, and the movement of twin interfaces within martensite crystals. The main cause differs depending on the alloy system, and also varies depending on the temperature and stress conditions.

In the case of NiTi, the matrix CsCl type crystal (bcc) structure that exists at high temperatures transforms into another monoclinic martensitic phase when cooled below the transformation temperature. As a result of the martensitic transformation, a pattern with surface undulations appears on the surface tissue, such as a bamboo leaf (lens) shape, a nullus shape, a butterfly shape, and a short fence shape.

Other systems in which similar transformations are observed include Au
−Cd, In−Tl, Cu−Al−Ni, Cu−Zn, Ni−
There are many types such as Al and Cu-Al-Zn, but currently only the Ni-Ti and Cu-Al-Zn systems are actually used. Among them, the Cu-Al-Zn system is considered to have a wide range of uses because it is much cheaper than Ni-Ti, is much easier to process and manufacture, and can be rolled and wire-drawn. It has the disadvantage that it cannot prevent damage from damage.

The present invention records changes in light reflectance as a signal due to surface undulations of a thin film of a material that undergoes austenite-martensite reversible transformation as described above,
The signal is then erased by reading it, regenerating it, and flattening the surface undulations using reversible transformation.

To form the above thin film, sputtering method,
Various film forming methods such as vacuum evaporation, chemical vapor deposition, molecular beam epitaxy, plasma spraying, cluster ion beam method, etc. are used as the ion plating method.

The thin film production method will be explained below using a typical Rf sputtering device.

As shown in FIG. 1, the Ar gas pressure in the container 1 was set to 1 to 5 x 10 ^-2 Torr, and the target 2 and the substrate 3 were fixed at an interval of 40 to 50 mm.

As target 2, NiTi with adjusted composition is used.
The ingot base material was refined in an arc melting furnace and cast into a copper mold having an appropriate shape.

The substrate 3 on which the sputtered film is deposited includes:
Glass, quartz, acrylic substrate, Al, Cu thin film,
Various materials such as SiO ₂ are used. In order to improve adhesion, it is suitable that the material has a coefficient of thermal expansion close to that of the target and contains the same elements as the target.

Heating of the substrate 3 is necessary for crystallization of the sputtered film, and is heated from the bottom by a high frequency power source 4.

Of course, it is possible to crystallize a film with an amorphous structure by sputter-depositing it and then subjecting it to vacuum heat treatment. ,
There is a compositional deviation due to oxidation and carbonization of Ti, Zn, and Al.
Austenite, martensitic transformation temperature (Ms
Temperature) changes by several tens of degrees Celsius.

Figure 2 shows the time dependence of the deposited film thickness using a 145φ NiTi (50:50) target at an output of 0.8kW, and the film deposition rate has a linear relationship with time.

Figure 3 shows 80φNiTi with a lower area ratio per output.
(50:50) The relationship between target output, crystal structure, and deposition rate is shown for a sputtering time of 5 hours. In the case of an 80φ target, the structure changes from amorphous to crystalline at 0.6KW.

Next, the transformation temperature of the obtained thin film was investigated. Figure 4 shows the results of low-temperature DSC measurements of a film created by sputtering at 0.8 KW x 10 hours using an 80φ target. The film itself undergoes an austenite-martensitic transformation, but the DSC results do not show clear peaks for martensitic transformation and its reverse transformation, but rather a broad peak with a very small width. Of course, it is not possible to create an integral bulk for measurement purposes, and the diameter is 50 μm.
A number of layers of approximately 100% of the film are stacked and used for measurement.
Therefore, temperature followability may be poor,
One of the characteristics of sputtered film is that it does not have a clear DSC peak. Furthermore, the peak temperature difference between heating and cooling is more than 50°C, which is different from normal NiTi. In other words, when Ni and Ti atoms are deposited separately on the substrate from the NiTi target, columns grow, but C, O, and Ar enter between the columns and bond with Ti. Therefore, if the effective amount of metallic Ti, which influences the martensite initiation temperature (Ms), is different in the vicinity of the column and in the center, it will not appear as a clear peak, but as a dispersed peak. Along with this, the difference in temperature between martensite and its reverse transformation increases by more than 50°C.

FIG. 5 shows the change in optical reflectance due to thermal cycles when the temperature of the entire sputtered film was changed. The sputtered film thickness was 3 μm, and the film itself was crystallized. If the maximum reflectance is 100% and the minimum reflectance is 0%, the hysteresis temperature difference is
It has a temperature of about 100℃.

The present invention records changes in light reflectance due to surface undulations of austenite and martensitic transformations as signals of surface undulations, reads and reproduces them with light, and further utilizes reversible transformation. , canceling the signal by flattening the surface undulations.

Specifically, this method of recording, reproducing, and erasing is performed by preparing the recording surface of a substrate having a coefficient of thermal expansion similar to that of the recording medium with a surface accuracy of 0.1 μm. Thereafter, a recording medium is deposited to a thickness of 1 to 2 μm and crystallized to form a film in which austenite←→martensite transformation occurs.

Information is written using thermal stress, and 0.5
When irradiated with a focused laser beam of ~1 μm, the film undergoes transformation and surface undulations occur. This record is stable unless heat or stress is applied. Next, a low-output light source is used to read out the information, keeping the temperature of the film at a level that does not rise, and reading information based on changes in light reflectance. During erasing, the temperature is raised to such an extent that the surface state does not change, the martensite → austenite transformation occurs, and the transition is also erased, thereby making the recording medium repeatable.

Example A heat-resistant resin with an outer diameter of 100 mm and a thickness of 1.0 mm is used for the substrate, a light guide track is provided, a transparent nitride having the same coefficient of thermal expansion as that of the recording medium is formed by sputtering, and then layered. and deposit the recording medium. Furthermore, to protect this recording medium,
The above nitride is further formed by sputtering.

The recording head uses a laser with a relatively long wavelength of about 1.2 μm, focused on a spot with a diameter of about 0.6 μm, and irradiates the recording thin film with an electric signal of varying intensity.

By the above method, the disc rotation speed is 2500 rpm,
As a result of repeated recording and erasing with a recorder of 80 mm and a laser power of 8 mW for recording and 20 mmW for erasing, it was possible to repeat over 100,000 times with an initial C/N of 57 dB or more.

Effects of the Invention The present invention is resistant to oxidation due to temperature rises and falls because it is a metallic material and is a transformation of martensite and austenite phases between crystalline phases. Of course, the light reflectance changes somewhat due to the accumulation of dislocations during metamorphosis. The speed of martensite-austenite transformation is fast, so writing speed can be increased. Access time and transfer speed are equivalent to optical recording, but when using a semiconductor laser, recording may change due to heat.
In the case of a reproducing device other than a laser that uses a low amount of heat, it is possible to increase the reproducing speed. Since the manufacturing method is similar to that of optical recording, the bit error rate is at approximately the same level.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of the Rf sputtering device used in the present invention to form thin films, Figure 2 is a graph showing the relationship between deposited film thickness and time, and Figure 3 is a graph showing the relationship between output, crystal structure, and deposition rate. Graph, Figure 4 shows the low temperature of thin film.
A graph showing the results of DSC measurement, and FIG. 5 is a graph showing the relationship between the temperature of the thin film and the reflection intensity. 1... Container, 2... Target, 3... Substrate, 4... High frequency power supply.

Claims (1)

[Claims] 1. An optical recording material characterized by having a thin film layer made of a shape memory metal material whose austenite phase and martensite phase are reversibly transformed by temperature and stress and whose light reflectance changes depending on the undulations of the surface shape.