JPH05109113A - Phase transition type optical disk - Google Patents

Abstract

PURPOSE:To improve a jitter characteristic and recording density by using laser wavelengths lambda1 and lambda2 at the time of reproducing and at the time of recording and erasing and selecting layer thicknesses so that there is no difference in absorbance at lambda1 between crystalline and amorphous states and a difference in reflectivity is large at lambda2. CONSTITUTION:Protective films 2, 4 consisting of SiO2 a recording film 3 consisting of a GeSbTe material and a reflection film 5 consisting of Al are laminated at prescribed film thicknesses and in prescribed lamination order on a glass substrate 1. Lasers of the wavelengths lambda1, lambda2, varying at time of reproducing and at the time of recording and erasing are used. The absorbance of the recording film is constant with the lambda2, according to this constitution and, therefore, there are substantially no distortions in the bit shape at the time of overwriting and the increase in the jitters of the reproduced signals is suppressed to a lower level. The diameter of the laser beam spot is reduced by setting the lambda1, lambda2, by which the resolving power characteristic of the reproduced signals is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase change type optical disk for recording information by using reversible phase change by laser light irradiation.

[0002]

2. Description of the Related Art An optical disk recording system using laser light is capable of large-capacity recording and can be accessed at high speed in a non-contact manner.
Optical discs are classified into read-only types known as compact discs and laser discs, write-once types that can be recorded by the user himself, and rewritable types that can be repeatedly recorded / erased by the user. The write-once / rewritable optical disc is about to be used as an external memory of a computer or as a document / image file. Rewritable optical disks include phase-change optical disks that utilize the phase change of the recording film and magneto-optical disks that utilize the change of the magnetization direction of the perpendicular magnetization film. Among them, the phase change type optical disk is expected to become the mainstream of the rewritable type optical disk in the future because it does not require an external magnetic field and can be easily overwritten.

Conventionally, a so-called phase change type optical disk is known, which is rewritable by using a recording film which causes a phase change between a crystal and an amorphous by irradiation with a laser beam. In a phase change type optical disk, a high power laser beam spot corresponding to the information to be recorded is applied to the recording film to locally raise the temperature of the recording film to cause a phase change between crystalline and amorphous. Reproduction is performed by recording and reading the change in the optical constants accompanying this as a reflected light intensity difference with a low power laser beam. For example, in a phase change type optical disc using a recording film having a relatively slow crystallization time, the disc is rotated and the recording film formed on the disc is irradiated with laser light to raise the temperature of the recording film to a temperature equal to or higher than the melting point. Then, after the laser light passes, the portion is made into an amorphous state by rapid cooling, and recording is performed. At the time of erasing, the recording film is crystallized by holding the temperature of the recording film within a temperature range which is equal to or higher than the crystallization temperature and is equal to or lower than the melting point, which is sufficient for crystallization.
As a method for this purpose, there is known a method of irradiating a long elliptical laser beam in the laser beam traveling direction. When performing pseudo overwrite by two beams for recording new information while erasing already recorded data, it is arranged so that the elliptic laser light for erasing is irradiated prior to the circular laser light for recording. ..

On the other hand, a disk using an information recording film capable of high-speed crystallization uses a single laser beam focused in a circular shape. The conventionally known method is to increase the power of laser light to 2
Crystallization or amorphization is performed by changing between two levels. That is, by irradiating the recording film with a power laser beam capable of raising the temperature of the recording film to the melting point or more, most of the part becomes an amorphous state when cooled, while the recording film temperature is crystallized. A portion of the power irradiated with the laser light that reaches a temperature equal to or higher than the melting point and equal to or lower than the melting point becomes a crystalline state. The recording film of the phase-change optical disk is a chalcogenide-based material such as GeSbTe-based or I-based.
nSbTe system, InSe system, InTe system, AsTeGe
System, TeO _x -GeSn system, TeSeSn system, SbSe
Bi-based material, BiSeGe-based material, or the like is used, and both are formed by a film-forming method such as a resistance heating vacuum evaporation method, an electron beam vacuum evaporation method, or a sputtering method. The state of the recording film immediately after film formation is a kind of amorphous state, and an initialization process in which the entire recording film is made crystalline in order to perform recording on this recording film to form an amorphous recording portion. Is done. Recording is accomplished by forming an amorphous portion within this crystallized state.

[0005]

Conventionally, the laser used for recording / erasing and the laser used for reproducing are the same, and the same optical head has been used. Since the reproduction signal is obtained from the reflectance difference between the crystal and the amorphous, the optical disc is designed so that the reflectance difference between the crystal and the amorphous is large in order to obtain a good reproduction signal. As shown in FIG. 2, the structure of a normally used optical disk is a four-layer structure in which a first protective film 2, a recording film 3, a second protective film and a reflective film 5 are sequentially formed on a substrate 1. Disk reflectivity of Figure 3,
As shown in the absorptivity characteristics, when trying to increase the reflectance difference between the crystal and the amorphous in order to obtain a good reproduction signal, there is a drawback that the absorptivity difference between the crystal and the amorphous becomes large. .. If there is a difference in absorption rate, when overwriting is performed, the absorbed amount of irradiation power differs depending on whether the recording power irradiation part was crystalline or amorphous before overwriting, and the bit shape formed as a result is distorted. It had the drawback of coming out. In the so-called bit width recording system in which the signal is reproduced by the bit edge, this distortion has a drawback that the reproduced signal has an increased jitter. It is an object of the present invention to provide a novel phase change type optical disk which solves the above-mentioned drawbacks, improves the jitter characteristic of the reproduction signal of the phase change type optical disk, and enables high density recording.

[0006]

The present invention has a laser having at least a recording film and at least one protective film on a substrate, and a reversible phase change between crystalline and amorphous in the recording film. A phase-change type optical disc that records, reproduces, and erases information by light irradiation, using a laser with a different wavelength during reproduction and recording and erasing, and is crystalline and amorphous for the recording and erasing laser wavelength. The film thicknesses of the protective film and recording film are set so that there is no difference in the absorption rate at the laser and there is a difference in the reflectance between the crystal and the amorphous for the reproduction laser wavelength. It is a changeable optical disc.

[0007]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 2 is a diagram showing the configuration of the optical disc according to the present invention. A first protective film 2, a recording film 3, a second protective film 4, and a reflective film 5 are sequentially formed on a substrate 1. A disk-shaped glass or plastic is used for the substrate 1. Materials such as SiO ₂ , Si ₃ N ₄ , AlN, TiO ₂ , and ZnS are used for the first protective film 2 and the second protective film 4. The recording film 3 is a chalcogenide-based material, such as GeSbTe-based material, InSbTe-based material, or I-based material.
nSe system, InTe system, AsTeGe system, TeOx-G
eSn system, TeSeSn system, SbSeBi system, BiSe
Ge system or the like is used. The reflective film 5 includes Al, Au,
Metals such as Cu, Ag and Ti are used. The feature of the present invention lies in the reflectance and absorptivity characteristics of this disc. FIG. 1 shows the wavelength dependence of reflectance and absorptance of the phase-change optical disk according to the present invention. At the wavelength λ ₁ used for recording / erasing, there is no difference in absorptivity between crystals and amorphous, and at the wavelength λ ₂ used for reproduction, there is a large difference in reflectance between crystals and amorphous so that the film thickness of each layer is large. Is set. Since there is no difference in the absorption rate between crystalline and amorphous for the wavelength used for recording and erasing, there is no difference in the absorption amount of irradiation power due to the difference in the phase state, and as a result, the recording bit edge part No bit shape distortion occurs. This effectively works to suppress the increase in jitter of the reproduced signal. Recording / erasing wavelength λ ₁ during playback
Since a different wavelength λ ₂ is used, a signal can be reproduced well if there is a sufficient difference in reflectance between crystalline and amorphous at this wavelength.

Next, the recording / erasing laser wavelength is set to 83
A disk was prepared by setting 0 nm and a reproducing laser wavelength to 670 nm, and determining the disk configuration so that the absorptivity characteristics and reflectance characteristics were obtained. The substrate has a diameter of 130 m
A polycarbonate substrate with m and a thickness of 1.2 mm with a pre-groove was used. A ZnS + SiO ₂ mixed film was used for the protective film, GeSbTe was used for the recording film, and Al was used for the reflective film, and they were continuously formed by the magnetron sputtering method. The film thickness of each layer was the first protective film 250 nm, the recording film 25 nm, the second protective film 40 nm, and the reflective film 50 nm. In this disc, the absorptance at a wavelength of 830 nm is 74% for crystals and 74% for amorphous materials.
The reflectance in nm was 32% for crystals and 10% for amorphous.

Next, the above-mentioned disc was overwritten and the reproduction signal characteristics were evaluated. 8 wavelength for recording / erasing
An optical head equipped with a 30 nm semiconductor laser was used, and an optical head equipped with a 670 nm wavelength semiconductor laser was used for reproduction. Rotation speed of the disk after initialization processing is 3600
Rotate at rpm, 8.4 on a track with a radius of 30 mm
After recording the signal of MHz (Duty 50%), the signal of 2.2 MHz (Duty 50%) was overwritten on the same track. The recording power and the erasing power are 13 mW and 7 mW, respectively, so that the second harmonic distortion of the reproduction signal is minimized.
Set to. Next, this track was reproduced and the reproduction signal jitter observed at the recording bit edge was measured. 2.2MHz
The signal jitter was 3.1 ns at the first recording without overwriting, whereas after the overwriting,
Almost no change was seen at 3.2 ns. For comparison, the jitter after overwriting of the conventional disk to which the present invention was not applied was measured. Recording / erasing / reproducing was performed by an optical head equipped with a semiconductor laser having a wavelength of 830 nm. The jitter when the 2.2 MHz signal was overwritten after the recording of the 8.4 MHz signal was 5.8 ns, which was significantly larger than the jitter of 3.0 ns at the time of the initial recording of 2.2 MHz. 8.4MH at overwriting
Since the erasing rate of the z signal is 28 dB, the increase in jitter observed here is due to the bit edge distortion caused by the difference in the absorptivity between the crystal and the amorphous of the disk.

[0010]

As described above, in the present invention, since the absorptance of the recording film is constant with respect to the recording / erasing laser wavelength, there is almost no bit shape distortion at the time of overwriting, and the reproduction signal jitter is small. The effect is that the increase can be suppressed small. Furthermore, by setting the reproducing laser wavelength shorter than the recording / erasing laser wavelength, the reproducing laser spot diameter can be made smaller than the recording / erasing laser spot diameter, so that the reproduction signal resolution characteristic can be improved. There is.

[Brief description of drawings]

FIG. 1 is a diagram showing laser wavelength dependence of reflectance and absorptance of a phase-change optical disc of the present invention.

FIG. 2 is a cross-sectional view showing a configuration of a phase change type optical disc of the present invention.

FIG. 3 is a diagram showing the laser wavelength dependence of reflectance and absorptance of a conventional phase change optical disc.

[Explanation of symbols]

 1 substrate 2 first protective film 3 recording film 4 second protective film 5 reflective film

Claims (1)

[Claims] 1. A recording / reproducing of information by irradiating a laser beam having a recording film and at least one protective film on a substrate, and utilizing reversible phase change between crystal and amorphous in the recording film.・ A phase-change optical disc that performs erasing, uses different wavelength lasers during reproduction and recording / erasing, and there is no difference in the absorption rate between crystalline and amorphous for the recording / erasing laser wavelength. A phase-change type optical disc, wherein the thicknesses of the protective film and the recording film are set so that there is a difference in reflectance between crystalline and amorphous with respect to the reproducing laser wavelength.