JPH04251452A - Optical information recording medium - Google Patents

Abstract

PURPOSE:To provide the optical information recording medium which has excellent durability and allows high-speed recording and high-density recording. CONSTITUTION:A protective layer, a recording layer and a protective layer are formed in this order on a substrate and the protective layers are formed of tantalum oxide having >=7.25g/cm<3> density, by which the film deformation at the time of repeating recording and erasing is suppressed and the desired recording medium is obtd.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium capable of recording, erasing, and reproducing information at high speed and high density by irradiation with a light beam such as a laser beam.

0002

2. Description of the Related Art In recent years, optical discs using laser beams have been developed as recording media that meet the demands for increased information content and higher density and faster recording and reproduction. There are two types of optical discs: write-once type, which allows recording only once, and rewritable type, which allows recording and erasing any number of times. Examples of rewritable optical disks include magneto-optical recording media that utilize the magneto-optical effect and phase change media that utilize reversible changes in crystalline state. Phase change media do not require an external magnetic field and can be recorded and erased simply by modulating the power of laser light.

Furthermore, it has the advantage that one-beam overwriting, in which erasing and re-recording are performed simultaneously with a single beam, is possible. In a phase change recording system capable of one-beam overwriting, recording bits are generally formed by making the recording film amorphous and erased by crystallizing it. As the recording layer material used in such a phase change recording method, a chalcogen alloy thin film is often used. For example, Ge-Te system, Ge-Te-
Examples include Sb-based, In-Sb-Te-based, and Ge-Sn-Te-based alloy thin films.

Generally, in a rewritable phase change recording medium,
Two different laser light powers are used to achieve different crystal states. This method will be explained by taking as an example a case where recording and erasing are performed using amorphous bits and a crystallized erase/initial state. Crystallization is performed by heating the recording layer to a temperature that is sufficiently higher than the crystallization temperature of the recording layer and lower than its melting point. In this case, the recording layer is sandwiched between dielectric layers or an elliptical beam that is long in the direction of beam movement is used so that the cooling rate is slow enough to achieve sufficient crystallization.

On the other hand, amorphization is carried out by heating the recording layer to a temperature higher than its melting point and rapidly cooling it. in this case,
The dielectric layer also functions as a heat dissipation layer to obtain a sufficient cooling rate (supercooling rate). Furthermore, in order to prevent deformation due to melting and volume change of the recording layer during the heating and cooling process, thermal damage to the plastic substrate, and deterioration of the recording layer due to moisture, the dielectric layer is The protective layer consisting of the body layer is important. The material of the protective layer material must be optically transparent to laser light;
They are selected from the viewpoints of high melting point, softening point, and decomposition temperature, ease of formation, and appropriate thermal conductivity.

[0006]

[Problem to be Solved by the Invention] However, due to reasons such as the protective layer not having sufficient heat resistance and mechanical strength, the protective layer, recording layer, and substrate deform during repeated recording and erasing. There are problems such as peeling occurs at the interface of the dielectric protective layer, and the C/N ratio decreases as the number of recording/erasing cycles increases. Whether or not the film has excellent physical properties as a protective layer depends largely on the film formation conditions in addition to the material. The present inventors used tantalum oxide as the dielectric protective film, and as a result of conducting studies under various film formation conditions, they discovered a dielectric thin film with excellent heat resistance and mechanical strength, and arrived at the present invention.

[0007]

[Means for Solving the Problems] The present inventors have discovered that an excellent dielectric protective film can be obtained when a specific tantalum oxide is used as the above-mentioned dielectric protective layer, and have completed the present invention. That is, the gist of the present invention is to provide an optical information recording medium in which information can be reversibly recorded and erased by using a phase transition between an amorphous state and a crystalline state by irradiation with a laser beam, in which at least a tantalum oxide protective layer is provided on a substrate. , a recording layer and a tantalum oxide protective layer are laminated in this order, and the tantalum oxide protective layer is substantially transparent to the laser beam and has a density of 7.25 g/cm3 or more. optical information recording medium.

[0008] The substrate used in the present invention is as follows:
Examples include transparent resins such as polycarbonate, acrylic, and polyolefin, and glass. The protective layers are made of tantalum oxide and have a thickness of 100 Å to 5000 Å.
It is desirable that it be in the range of Å. If the thickness of tantalum oxide is less than 100 Å, the effect of preventing deformation of the substrate and recording film will be insufficient, and it will not serve as a protective layer. on the other hand,
When using a plastic substrate, at a thickness of 5000 Å or more,
The internal stress of tantalum oxide itself and the difference in elastic properties between it and the substrate become significant, making cracks more likely to occur. Two protective layers are provided at positions directly or indirectly sandwiching the recording layer.

A chalcogen alloy thin film is often used for the recording layer. Its thickness is usually chosen in the range of 100 Å to 1000 Å. If the thickness of the recording layer is less than 100 Å, sufficient contrast cannot be obtained, while if it exceeds 1000 Å, cracks are likely to occur. Note that the thicknesses of the recording layer and the protective layer are selected in consideration of the interference effect due to the multilayer structure so that the laser light absorption efficiency is high and the amplitude of the recorded signal, that is, the contrast between the recorded state and the unrecorded state is large.

Furthermore, an optical reflective layer and a hard coat layer for preventing thermal deformation may be provided on the opposite side of the recording layer to the substrate. An adhesive layer may be provided in place of the hard coat layer and bonded to the protective substrate or another recording medium. As the optical reflective layer, a metal thin film such as Al, Au, Ag, or Ni, which has a high reflectance, is usually used. In this case, the thicknesses of the recording layer and the protective layer are determined in consideration of interference effects including the reflective layer. Note that the reflective layer also has the effect of promoting diffusion of thermal energy absorbed by the recording layer.

The recording layer, protective layer, and reflective layer are formed by sputtering or the like. In order to prevent oxidation and contamination between each layer, it is desirable to form the film using an in-line device in which a recording film target, a protective film target, and, if necessary, a reflective layer material target are placed in a continuous vacuum chamber. It is also excellent in terms of productivity. The tantalum oxide thin film is formed by reactive sputtering using a Ta target and applying direct current or high frequency power in a mixed gas atmosphere of oxygen gas and inert gas. Alternatively, it is formed by sputtering using a Ta2O5 target and applying high frequency power.

The physical properties of the film change depending on the pressure inside the vacuum container, gas flow rate, discharge power, etc. during film formation. As a result of repeated studies, the present inventors have found that by increasing the density, it is possible to obtain a tantalum oxide film that is excellent as a protective layer in terms of heat resistance and mechanical strength. The reason is not necessarily clear, but the density is large and the bulk value (8.7
This is thought to be because a film close to 8 g/cm3) is dense and has a small porosity in the film, which suppresses film deformation during repeated recording and erasing.

The density of the tantalum oxide film is mainly determined by the total pressure of the inert gas or mixed gas of inert gas and oxygen gas during film formation. That is, the lower the total pressure is, the higher the density of the film can be obtained, but in general, a thin film can be obtained with a density of about 90% of the bulk value. Note that even if the pressure inside the vacuum vessel during film formation is the same, the density of the film can be increased by decreasing the oxygen gas flow rate ratio and increasing the discharge power. However, depending on the film formation conditions, the density may increase due to oxygen vacancies, and in this case, no improvement in characteristics is observed during repeated recording and erasing.

A film whose density has increased due to oxygen vacancies has a large extinction coefficient. Therefore, by increasing the density of the tantalum oxide film and making the extinction coefficient substantially zero, a film excellent as a protective layer can be obtained. Note that the deposition rate of tantalum oxide formed by this reactive sputtering method is 2 to 5 times higher than that of other oxide films, nitride films, etc., and has the advantage of excellent productivity. The density of the tantalum oxide protective layer that can be used in the present invention is 7.2
It needs to be 5 g/cm 3 or more and to be substantially transparent, that is, to substantially transmit laser light. Generally, this can be done by visually checking the presence or absence of coloring.

[0015] Furthermore, as the density of the film increases, the Knoop hardness, which is considered to represent mechanical strength, also increases. especially,
When the Knoop hardness is 500 or more, an excellent dielectric protective film is obtained in which the C/N ratio and the like decrease little due to repeated recording and erasing. In addition, the compressive stress of the tantalum oxide film provided on the upper part of the recording layer is 2.0×109 dyn/cm2 or more and 4.0×
When it is 109 dyn/cm2 or less, it has excellent stability over time. Regarding internal stress, the lower the total pressure during film formation, the greater the compressive stress. If the stress is large, cracks or bulges may occur in the film, and if multiple layers are stacked like the disk used in the present invention, separation may occur at the interface due to the difference in stress between the layers. In order to obtain a thermally and mechanically strong film, the pressure during film formation should be lowered as described above, but at the same time, the compressive stress increases, making peeling more likely. In the recording medium of the present invention, especially if the compressive stress of the protective layer on the upper part of the recording layer is too large, peeling is likely to occur. Therefore, the compressive stress of the upper dielectric layer is 4.0×109dyn/c.
It is desirable that it be less than m2.

[0016]

[Operation] A high-density tantalum oxide protective film is a protective film with excellent heat resistance and mechanical strength, and a recording medium capable of high-density recording, erasing, and reproduction can be obtained.

[0017]

[Examples] The present invention will be explained in more detail with reference to Examples and Comparative Examples below, but the present invention is not limited to the following Examples. In Examples and Comparative Examples, the density of tantalum oxide was calculated by dividing the weight change before and after film formation by the volume of the film. The stress was determined by using a stress gauge (30114 manufactured by Ionic Systems) to determine the change in warpage of the substrate before and after film formation when a film was formed on a Si substrate. In addition, the presence or absence of absorption was determined by visual inspection of the membrane. If absorption is observed, the color will be brown. Further, Knoop hardness was determined using a hardness meter (MVK1S manufactured by Akashi Co., Ltd.) according to a test method based on JIS Z-2251.

Example 1 A polycarbonate substrate was introduced into a sputtering apparatus.
After exhausting to below ×10-4 Pa, Ar gas was
sccm, O2 gas was introduced at 48 sccm and the total pressure was reduced to 0.
．． A tantalum oxide film was deposited by sputtering a Ta target at 28 Pa and a DC current of 1.3 A.
000 Å was formed. The density of this tantalum oxide film is 7.8
5g/cm3, and the hardness was 563. After evacuating the chamber, 150 sccm of Ar gas was introduced, the total pressure was 0.4 Pa, the discharge current was 0.8 A, and Te54Ge was heated.
A recording layer having a thickness of 700 Å was formed by sputtering a 12Sb34 target.

After evacuating the chamber again, the Ar gas flow rate was set to 145 sccm, the O2 gas flow rate was set to 48 sccm,
The total pressure was set to 0.28 Pa, the DC current was set to 1.3 A, and the T
A tantalum oxide film having a thickness of 1500 Å was formed by sputtering the a target. The density of this protective layer is 7.
85g/cm3, hardness is 563, compressive stress is 3.7×
It was 109 dyn/cm2 and transparent to the naked eye. After evacuating the chamber again, Ar gas was
50 sccm was introduced, the total pressure was set to 0.55 Pa, and the DC current was set to 1.3 A, and an Al alloy target was sputtered to form an Al alloy film with a thickness of 500 Å. After forming the above three layers, a 4 μm thick hard coat layer made of UV curable resin was provided.

Deterioration of characteristics such as C/N due to repeated recording and erasing of the disc thus prepared was measured. The measurement conditions are as follows. The disk was rotated at a linear velocity of 10 m/s, and the same track was repeatedly irradiated with a 4 MHz, 50% duty laser beam consisting of a recording power of 15 mW and an erasing power of 8 mW, and characteristics such as the C/N ratio were measured. As a result, C/
N ratio 55.5dB, C/N after 105 repetitions
A ratio of 55.1 dB was obtained. In addition, this disk was heated to 85
When an accelerated test was carried out under an environment of 85% RH and humidity for 50 hours, no peeling was observed.

Example 2 The protective layer, recording layer, reflective layer, and hard coat layer close to the substrate were formed in the same manner as in Example 1, and for the protective layer far from the substrate, the Ar gas flow rate was 145 sccm, and the O2 gas flow rate was 48 sccm, total pressure 0.4 Pa, DC current 1.
A tantalum oxide film with a thickness of 1500 Å was formed by sputtering a Ta target under a setting of 3A. The density is 7.25g/cm3, the hardness is 516, and the compressive stress is 2.
It was 0×10 9 dyn/cm 2 and transparent to the naked eye. As a result of the same evaluation as in Example 1, the C/N ratio after one repetition was 54.6 dB, and the number of repetition was 10.
After 5 times, a C/N ratio of 53.5 dB was obtained. When the same accelerated test as in Example 1 was conducted, no peeling was observed.

Comparative Example 1 The protective layer, recording layer, reflective layer, and hard coat layer close to the substrate were formed in the same manner as in Example 1. For the protective layer far from the substrate, the Ar gas flow rate was 145 sccm, and the O2 gas flow rate was 48 sccm. , total pressure 0.5 Pa, DC current 1.3 A
A tantalum oxide film with a thickness of 1500 Å was formed by sputtering a Ta target. The density is 6.
95g/cm3, hardness 495, compressive stress 0.6x
It was 109 dyn/cm2 and transparent to the naked eye. As a result of the same evaluation as in Example 1, the C/N ratio after one repetition was 54.1 dB, but after 105 repetitions, the C/N ratio decreased to 20.5 dB.

Comparative Example 2 The protective layer, recording layer, reflective layer, and hard coat layer close to the substrate were formed in the same manner as in Example 1. For the protective layer far from the substrate, the Ar gas flow rate was 145 sccm, and the O2 gas flow rate was 48 sccm. , total pressure 0.65Pa, DC current 1.3
A and sputtering a Ta target to form a tantalum oxide film with a thickness of 1500 Å. Density is 6
．． 56g/cm3, hardness is 439, compressive stress is 0.1
×10 9 dyn/cm 2 and was visually transparent. As a result of the same evaluation as in Example 1, the C/N ratio after one repetition was 57.1 dB, but after 105 repetitions, the C/N ratio decreased to 23.6 dB.

Example 3 The protective layer, recording layer, reflective layer, and hard coat layer close to the substrate were formed in the same manner as in Example 1. For the protective layer far from the substrate, the Ar gas flow rate was 150 sccm and the total pressure was 0. 28P
a. A tantalum oxide film with a thickness of 1500 Å was formed by sputtering a Ta2O5 target with the applied power set at 500W. The density is 7.95g/cm3,
Hardness is 667, compressive stress is 7.2 x 109 dyn/cm
It was 2. Although brown coloring was observed visually, Example 1
As a result of the same evaluation as above, the C/
The N ratio was 56.7 dB, and the C/N ratio after 105 repetitions was 52.5 dB.

Comparative Example 4 The recording layer, the protective layer far from the substrate, the reflective layer, and the hard coat layer were formed in the same manner as in Example 1. For the protective layer close to the substrate, the Ar gas flow rate was 145 sccm, and the O2 gas flow rate was 48 sccm. , total pressure 0.65Pa, DC current 1.3
A tantalum oxide film with a thickness of 1000 Å was formed by sputtering a Ta target. Density is 6
．． It was 56 g/cm3 and transparent to the naked eye. As a result of the same evaluation as in Example 1, the number of repetitions was 1.
The C/N ratio after the repetition was 48.1 dB, but after 105 repetitions, the C/N ratio decreased to 14.0 dB.

Comparative Example 5 The recording layer, reflective layer, and hard coat layer were formed in the same manner as in Example 1, and for the protective layer close to the substrate and the protective layer far from the substrate, the Ar gas flow rate was 145 sccm, and the O2 gas flow rate was 48sccm, total pressure 0.65Pa, DC current 1
．． 3A and sputtering a Ta target to form tantalum oxide films of 1000 Å and 15 Å, respectively.
00 Å was formed. Both layers have a density of 6.56g/cm3
, hardness 439, compressive stress 0.1×109dyn/c
m2 and was visually transparent. As a result of the same evaluation as in Example 1, the C/N ratio after one repetition was 45.7 dB, but after 104 repetitions, the C/N ratio decreased to 21.5 dB.

Example 4 The recording layer, reflective layer, and hard coat layer were formed in the same manner as in Example 1, and for the protective layer close to the substrate and the protective layer far from the substrate, the Ar gas flow rate was 72 sccm, and the O2 gas flow rate was 24sccm, total pressure 0.2Pa, DC current 1.3
A tantalum oxide film with a thickness of 1000 Å and 1500 Å, respectively, was formed by sputtering a Ta target.
A was formed. Each layer has a density of 7.75 g/cm3,
Hardness 640, compressive stress 5.1 x 109 dyn/cm2
and was transparent to the naked eye. As a result of the same evaluation as in Example 1, the C/N ratio after one repetition was 5.
8.7dB, C/N ratio after 105 repetitions is 5
7.3dB was obtained. When the same accelerated test as in Example 1 was performed, some peeling was observed.

[0028]

[Effect of the invention] As described above, the density of the protective layer is 7.2.
When a transparent tantalum oxide film with a density of 5 g/cm 3 or more is used, an excellent protective film can be obtained, and an optical information recording medium that is excellent in durability and capable of high-speed recording and high-density recording can be provided.

Claims (1)

[Claims] Claim 1: An optical information recording medium in which information can be reversibly recorded and erased by using phase transition between amorphous and crystalline materials by laser beam irradiation, comprising at least a tantalum oxide protective layer and a recording layer on a substrate. and a tantalum oxide protective layer are laminated in this order, and the tantalum oxide protective layer is substantially transparent to the laser beam and has a density of 7.
An optical information recording medium characterized by having a density of 25 g/cm3 or more.