JPH06195747A - Optical disc - Google Patents

Abstract

PURPOSE:To obtain an optical disc not deteriorating the erasing characteristic thereof even when it is used under a light source of a short wavelength or it is rotated at high speeds. CONSTITUTION:An Si3N4 layer 3 is formed adjacent to a recording layer 4 so as to facilitate the crystallization of the recording layer and a substrate 1. The crystallizing speed of the recording layer depends not only on the composition of the recording layer, but on the kind of the layer adjacent to the recording layer. Since the layer 3 is formed adjacent to the recording layer to facilitate the crystallization, the crystallizing speed of the recording layer is enhanced and the erasing characteristic 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 disc for recording information using a reversible phase change upon irradiation with a laser beam, and particularly to a high speed erasing type optical disc suitable for a high linear velocity and short wavelength laser. Recording medium

[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.
Rewritable optical disks include phase-change optical disks that utilize the phase change of the recording layer and magneto-optical disks that utilize the magnetization direction of the perpendicular magnetization film. Among them, the phase change type optical disc is
It is expected that it will become the mainstream of rewritable optical discs in the future, since it requires no external magnetic field and can be easily overwritten.

In the phase change type optical disc, the recording phase is irradiated with a laser high spot of high power corresponding to the information to be recorded,
By locally increasing the temperature of the recording phase, the crystal-
Recording is performed by causing a phase change between amorphous materials, and the reproduction is performed by reading the change in the optical constants accompanying the phase change as a reflected light intensity difference by a low power laser beam. Generally, the amorphous state is used as the recording state. In the case of erasing information, the recording phase is crystallized by irradiating with a laser beam having a power required to keep the recording phase at a crystallization temperature or higher and a melting point or lower.

[0004]

In the phase change type optical disk, when erasing is performed, the recording layer must be kept at the crystallization temperature or higher for the time required for the recording phase to crystallize. For example, in a GeSbTe-based recording layer that is well known as a phase change type optical disc material, the recording layer must be kept at a crystallization temperature or higher for 50 to 150 ns or more.

On the other hand, the optical disc is required to have a higher speed and a higher density. It is effective to shorten the wavelength of the light source used for the optical head for high density, but the reduction of the focused spot size with the shortening of the wavelength is the time when the recording layer is kept above the crystallization temperature, that is, the holding time. It will reduce the time. For example, as shown in FIG. 2, when the focused beam diameter is halved, the holding time is halved. Further, an increase in the number of rotations of the disk for speeding up also leads to a decrease in the holding time. The reduction of the holding time lowers the crystallization rate of the recording layer, that is, the erasing rate, which leads to the deterioration of the error rate. Therefore, it is necessary to increase the crystallization speed of the recording layer under the use of a short wavelength light source or under high speed rotation of the disk.

As a means for increasing the crystallization speed of the recording layer, a crystal nucleation layer having a crystal phase in the same phase as that of the recording layer, as described in JP-A-1-92937, is provided adjacent to the recording layer. It is known that the structure is provided. However, when a crystal nucleation phase having a crystal phase in the same phase as the recording layer is provided adjacent to the recording layer, the recording layer and the crystal nucleation layer react with each other to form an alloy as information is rewritten many times. Therefore, there is a problem that the function of the crystal nucleation layer is deteriorated.

Therefore, an object of the present invention is to solve the above-mentioned problems and to provide an optical disk which does not reduce the erasing rate even when a short wavelength light source is used or a disk is rotated at a high speed. .

[0008]

In order to achieve the above object, the present invention provides an optical disc for recording / reproducing / erasing information by inducing a reversible phase change of a recording layer by laser irradiation. , A lower protective layer on the substrate, a crystallization promoting layer,
An information recording layer, an upper protective layer, and a reflective layer, which undergo a reversible layer change, are laminated in this order, the crystallization promoting layer is Si ₃ N ₄ , and the lower and upper protective layers are ZnS—SiO ₂ layers. It is characterized by

[0009]

In the phase change type optical disc, as shown in FIG.
A structure in which a lower layer 2, a recording layer 4, an upper protective layer 5, and a reflective layer 6 are laminated in this order on a substrate 1 is generally used. As a result of research, we found that the crystallization rate of the phase change recording layer depends not only on the composition of the recording layer but also on the type of the protective layer adjacent to the recording layer. In the GeSbTe recording layer, S
By using i ₃ N ₄ as the protective layer, the crystallization speed of the recording layer can be increased. Considering the temperature change of the recording layer at the time of erasing, as shown in FIG. 4, the time for the substrate side of the recording layer to reach the crystallization temperature faster than the reflection layer side and be kept above the crystallization temperature. Is also long. Therefore, crystallization during erasing proceeds from the substrate side, and it is more effective to provide the crystallization promoting layer on the substrate side.

On the other hand, during recording, after the temperature of the recording layer exceeds the melting point, the crystallization temperature on the reflective layer side reaches the crystallization temperature faster than on the substrate side. When the crystallization promoting layer is provided between the recording layer and the reflecting layer, recrystallization is promoted and an amorphous state cannot be formed, which makes it difficult to perform good recording. Therefore, it is desirable to provide the crystallization promoting layer only adjacent to the recording layer and between the substrate and the substrate.

As described above, according to the present invention, Si ₃ N _{4 is provided} as a crystallization promoting layer between the recording layer and the substrate adjacent to the recording layer.
By forming the layer, the crystallization rate of the recording layer can be increased, and the erasing rate can be secured even when a short wavelength light source is used or the disk is rotated at high speed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 shows the structure of a phase change type optical disc which is an optical information storage medium according to the present invention. In the figure, a lower protective layer 2, a crystallization promoting layer 3, a recording layer 4, an upper protective layer 5, and a reflective layer 6 are sequentially laminated on a phase change type optical disc substrate 1.

The substrate 1 is a polycarbonate substrate with a pregroup. The lower protective layer 2 is made of ZnS-SiO ₂ of 20.
The reflective layer was 0 nm thick, and Al was laminated by sputtering to a thickness of 60 nm. In addition, for comparison, Zn
S-SiO ₂ 200 nm, Ge ₁ SbTe ₇ recording layer 20 nm, ZnS-SiO ₂ 20 nm, Al 60 n
m, a disk laminated in this order from the substrate side was evaluated.

By using an optical head having a wavelength of 830 nm, the signal of 2.12 MHz, duty = 50% and the signal of 8.47 MHz, duty = 50% are alternately overwritten by changing the linear velocity, and each signal is overwritten. The C / N and erasing rate were measured.
As shown in FIG. 5, in the disk formed with Si ₃ N ₄ for promoting crystallization, the linear velocity is 11.3 m / s to 22 m / s.
Until then, the erasing rate at 8.47 MHz is almost constant, and is about 25 dB at a linear velocity of 22 m / s. On the other hand, in the case of the conventional disk having only ZnS-SiO2, the erasing rate starts to drop from around the fast line of 15 m / s, and at the linear velocity of 22 m / s, the erasing rate is 18.47 MHz.
It was a low value of dB.

FIG. 6 shows the relationship between the number of repeated overwrites and the erasure rate of a disk having Si ₃ N ₄ formed as a crystallization promoting layer. Even after rewriting information 10 ⁵ times, the erase rate at 8.47 MHz at a linear velocity of 22 m / s is 25 dB.
It remained.

[0017]

As described above, according to the present invention, the crystallization speed of the recording layer can be increased, and the erasing characteristics can be improved even when the disk rotates at high speed or when a short wavelength light source is used. Further, in the present invention, since the dielectric layer that does not react with the recording layer is used as the crystallization promoting layer, the crystallization promoting function does not deteriorate even after the information is rewritten many times.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of an optical information recording medium that realizes the present invention.

FIG. 2 is a diagram showing a relationship between condensing beam size holding times.

FIG. 3 is a cross-sectional view showing a conventional configuration example of an optical information recording medium.

FIG. 4 is a diagram showing a temperature history of a recording layer.

FIG. 5 is a characteristic diagram showing a relationship between an erasing rate and a linear velocity.

FIG. 6 is a characteristic diagram showing the relationship between the number of repetitions and the erase rate.

[Explanation of symbols]

1 Substrate 2 Lower protective layer 3 Crystallization promoting layer (Si ₃ N ₄ ) 4 Recording layer 5 Upper protective layer 6 Reflective layer

Claims (1)

[Claims] 1. An optical disc for recording, reproducing, and erasing information by inducing a reversible phase change of a recording layer by irradiating a laser beam, a ZnS-SiO ₂ layer, and Si ₃ layer on a substrate.
N ₄ layer, recording layer that causes reversible phase change, ZnS-Si
An optical disc having an O ₂ layer and a reflective layer sequentially stacked.