JPH07105571A - Optical information recording medium - Google Patents

Abstract

PURPOSE:To increase reflectance and contrast by a relatively simple structure of layers and an easy sputtering method. CONSTITUTION:At least an intermediate layer formed by laminating two or more thin films different from each other in refractive index, a phase shift type optical recording layer, an upper dielectric layer and a reflecting layer are successively laminated on a substrate. The reflectance of the resulting medium is >=65% when the recording layer is in a crystalline state and it is <35% when the recording layer is in an amorphous state. A high refractive index thin film as one of the thin films constituting the intermediate layer is made of a semiconductor having a specified complex index of refraction to the wavelength of light for recording and reproduction. The absolute value of the imaginary part of the complex index of refraction is <0.1 and the real part is >=2.5.

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 by irradiation with laser light or the like. In particular, the present invention relates to an optical information recording medium on which information can be recorded and erased and which can be reproduced by a conventional CD reproducing device.

[0002]

2. Description of the Related Art Optical discs include a read-only type, an optical recordable type, and a rewritable type. The read-only type has already been put into practical use as a video disc, an audio disc, and a disc memory for a large-capacity computer. There is. Among these, for audio playback of music etc.,
Compact discs (CDs) are widespread. A compact disc (CD) is a CD-formatted EFM (Eight to Fourteen Mod).
(Translation) signal holes (pits) are transferred to a plastic substrate, and a reflective film and a protective layer made of a metal such as aluminum are provided thereon.

Information is read from a CD by irradiating a semiconductor laser beam from the substrate side and irradiating the optical disc, and a CD format signal or the like is read by a change in reflectance depending on the presence or absence of pits. At this time, the conventional CD is characterized by having a high reflectance of 65% or more and a modulation degree of 60% or more. However, this reproduction-only CD cannot record / edit or rewrite information.

Also, files such as software, data files, and still images can be read from a CD-ROM (Read
only memory) or CD-I (int
There is a demand for an optical recordable / erasable optical disc for rasive. On the other hand, typical examples of the optical recordable type are a hole-forming / deformable type, a magneto-optical type, and a phase change type. Drilling
As the deformable type, a recording layer made of a low melting point metal such as Te or a dye is used and locally heated by laser light irradiation to form a hole or a recess.

In practice, such a perforation mold must have voids on the recording layer. For this reason, two discs are made to face each other and are pasted together by using a spacer so that a gap is provided between recording layers. As a matter of course, a disc having such a laminated structure cannot be mounted on a CD drive which is currently popular. The magneto-optical type performs recording and erasing according to the direction of magnetization of the recording layer, and performs reproduction by the magneto-optical effect, so that reproduction cannot be performed by a conventional CD drive that utilizes the difference in reflectance.

As a disc for recording a CD format signal, an optical disc having a recording layer made of a dye or a polymer containing the dye on a substrate, and an optical information recording method using the optical disc have been proposed (Japanese Patent Laid-Open Publication No. Sho. 61-237239, 61-233943),
These optical discs cannot be rewritable. On the other hand, the phase-change type utilizes the fact that the reflectance changes before and after the phase change, and is common to the CD in that reproduction is performed with a different reflectance without the need for an external magnetic field.

Further, there is an advantage that recording / erasing can be performed only by modulating the power of the laser beam, and one-beam overwriting in which erasing and re-recording are simultaneously performed by a single beam is also possible. In the one-beam overwritable phase change recording method, it is general that the recording film is made amorphous to form a recording bit and is crystallized to erase.

As a recording layer material used in such a phase change recording system, a chalcogen alloy thin film is often used. For example, Ge-Te system, Ge-Te-Sb
System, In-Sb-Te system, Ge-Sn-Te system alloy thin film, and the like. Usually, a dielectric protective layer is provided above and below the recording layer in order to protect the recording layer from deformation and the like, to protect it from deterioration such as oxidation, and to utilize the interference effect. A layer structure in which a reflective layer is provided on the dielectric layer above the recording layer is often used in order to adjust the cooling rate and effectively use the interference effect.

That is, a layer structure in which a dielectric protective layer, a phase transition type recording layer, a dielectric protective layer, and a reflective layer are sequentially provided on a substrate is common. A write-once type phase change medium can also be realized by using almost the same material and layer structure as the rewritable type. In this case, since there is no reversibility, information can be recorded and stored for a longer period of time, and in principle, it can be stored almost permanently.

[0010]

However, the phase change optical disk having the above-mentioned general layer structure has a low reflectance and cannot be reproduced by a CD drive. It is possible to increase the reflectance by changing the film thickness of the dielectric layers above and below the recording layer, but at that time, a sufficient degree of modulation cannot be obtained.

Further, in order to reproduce with a CD drive, it is necessary for the unrecorded portion to have a high reflectance, so that in the case of a Ge-Te or Ge-Sb-Te phase change medium that requires initialization. It is necessary to increase the reflectance in the crystalline state, but at a high reflectance, the reflectance in the amorphous state with a small absorption coefficient (imaginary part of complex refractive index) tends to increase. There has been proposed an attempt to increase the reflectance by forming two dielectric layers below the recording layer (Japanese Patent Laid-Open No. 1-27324).

The dielectric material is determined in consideration of the refractive index, thermal conductivity, chemical stability, mechanical strength, adhesiveness, etc.,
Generally, oxides, sulfides, which have high transparency and high melting point,
Nitride and fluorides such as Ca, Mg and Li can be used. In order to simplify the manufacturing process, it is preferable that the number of dielectric intermediate layers is small in order to reduce peeling at the interface, and in order to achieve high reflectance and high contrast with a small number of layers, the refractive index It is preferable to use two or more types of dielectrics having a large difference.

Ca having a relatively small refractive index is Ca.
F₂, MgF₂, Na₃AlF₆, SiO₂ Etc., refraction
TiO has a relatively high rate.₂, ZnS, T
a₂O _Five , ZnS and SiO₂There is a mixture of. High reflectance
・ To obtain high contrast, light between the recording layer and the substrate
Alternating low-refractive index and high-refractive index dielectric films with a film thickness of λ / 4
The so-called dielectric mirror laminated on the
However, as the number of laminated layers increases, or the refraction of both dielectrics
It is known that the larger the index difference, the higher the reflectance.
(JP-A-3-91128, JP-A-5-151614)
etc).

However, as long as the above-mentioned oxides, sulfides, nitrides and fluorides used for ordinary dielectric mirrors are used, the high refractive index is at most about 2.5 of TiO ₂ and the low refractive index is. , MgF ₂ , CaF _2, etc. is about 1.3 as the lower limit. Therefore, in order to obtain a sufficient dielectric mirror effect, it is necessary to stack 3 to 6 layers with only the dielectric mirror layer. However, when the number of layers is too large, it is not practical considering the manufacturing cost and the difficulty of ensuring reproducibility.

There is also a problem that defects and peeling occurring at the interface increase as the number of layers increases.

[0016]

DISCLOSURE OF THE INVENTION As a result of studying the layer constitution and materials, the present inventors have found that an intermediate layer in which at least two kinds of thin films having different refractive indexes are laminated on a substrate, and a phase transition type optical recording layer. In an optical information recording medium in which an upper dielectric layer and a reflective layer are sequentially laminated, the medium has a reflectance of 65% or more when the recording layer is in a crystalline state, and a reflectance of 35 when the recording layer is in an amorphous state. %, And the absolute value of the imaginary part of the complex refractive index with respect to the recording / reproducing light wavelength is less than 0.1. It has been found that by using a semiconductor having a real part of 2.5 or more, it is possible to manufacture an optical information recording medium having a large reflectance and contrast with a relatively simple layer structure.

By using a semiconductor thin film having a high refractive index and a low absorptivity as the high refractive index thin film, the absorption for the recording / reproducing light wavelength can be substantially ignored, and an efficient dielectric mirror can be formed, and the recording layer crystal can be formed. High reflectance of 65% or more in the state, 35% or less in the amorphous state,
High contrast media can be achieved with a relatively small number of layers.

The semiconductor thin film preferably has a bandgap equal to or larger than the wavelength of the recording / reproducing light, for example, a wavelength of 780 in an ordinary compact disc.
For nm, amorphous silicon, germanium, Si
C, Sb ₂ S _{3 and the} like can be mentioned. It is also a characteristic of semiconductor thin films that the desired optical characteristics can be obtained by controlling the bandgap by amorphizing, hydrogenating or fluorinating even if the absorptance in the crystalline state is large. Among them, hydrogenated or fluorinated amorphous silicon has a band gap of 1.6 to 2.0 electron volts and 780.
Wider than nm (about 1.5 electron volts),
Complex refractive index for 780 nm is about 3.5-0.1i
(I is an imaginary number) and shows a good value, and the crystallization temperature is 400 ° C.
It is an excellent material in that it is also thermally stable.

Furthermore, by using at least one gas selected from hydrocarbon gas, NH ₃ , nitrogen, oxygen gas, etc. mixed with hydrogen gas during reactive sputtering, various hydrogenated amorphous SiC, SiN, SiCN, etc. can be obtained. An amorphous film based on Si is obtained. These are effective for controlling the optical constants of the film, the mechanical strength, and the adhesiveness with other films.

On the other hand, as the low refractive index film, Mg, Ca, S
r, Y, La, Ce, Ho, Er, Yb, Zr, Hf,
An oxide such as V, Nb, Ta, Zn, Al, Si, Ge, or Pb, a sulfide, a nitride, or a fluoride can be used. A heat-resistant organic thin film (sputtered film of polyimide, polyfluorocarbon, etc., plasma-polymerized film, vapor-deposited polymerized film, etc.) can also be used especially when the heat damage due to recording / erasing is small or the number of times of repeated recording may be small .

It is not always necessary that these dielectric films have a stoichiometric composition, and it is also effective to control the composition or mix them for controlling the refractive index and the like. Note that the film thickness of the dielectric layer does not necessarily have to be an optical film thickness of λ / 4, and by adjusting the film thickness from this film thickness, the reflectance, the wavelength dependence of the reflectance, and the contrast can be adjusted.

The refractive index nL of the low refractive index thin film is preferably nH / nL of 1.2 or more, where nH / nL is the refractive index of the high refractive index thin film. In the present invention, since the semiconductor thin film which is a high refractive index film has a high refractive index of 2.5 or more, it is usually regarded as a high refractive index.
A mixed film of S, ZnS and SiO2 (both have a refractive index of about 2.0) can also be used as a low refractive index film. In this case, since there is an advantage that the range of usable dielectric films is widened, it is possible to select a material that places importance on thermophysical properties and mechanical strength.

The reason why the real part of the complex refractive index with respect to the wavelength of the recording / reproducing light is 2.5 or more is used as the high refractive index thin film, but the absolute value of the imaginary part of the complex refractive index is 0.
If it is 1 or more, the absorptance becomes too large and a good reflectance cannot be obtained. Further, by using the oxide or nitride of the high refractive index semiconductor film as the low refractive index film, the adhesion between the respective layers can be improved, and it is effective in preventing deterioration such as interfacial peeling due to repeated recording and erasing.

In addition, using the same semiconductor target, Ar
If a semiconductor film is formed by sputtering in gas and then reactive sputtering is performed by introducing oxygen or nitrogen, an oxide film or a nitride film of the above semiconductor can be formed. Both oxygen and nitrogen may be introduced to adjust the refractive index. In any case, a laminated film having excellent adhesion at the interface can be easily obtained by turning on / off the introduction of oxygen or nitrogen gas.

In some cases, by gradually turning on or off the oxygen or nitrogen gas, it is possible to produce a graded composition at the interface and further improve the adhesion. Further, if a reactive gas in which both oxygen and nitrogen are mixed is used, the refractive index can be controlled within the range of about 1.5 to 2.0. In particular, since the oxide film of silicon has a low refractive index of 1.5, an efficient dielectric mirror can be obtained by using it together with the above-mentioned hydrogenated or fluorinated amorphous silicon. Hydrogenated or fluorinated silicon also has the property that it is difficult to oxidize,
It is also effective in preventing deterioration due to diffusion from silicon oxide and has excellent stability over time.

Usually, the phase change recording layer is a GeSbTe system, I
An nSbTe-based material or the like is used, but a material having a large change in refractive index is preferable for increasing the contrast. GeS
Regarding the bTe system, a composition on the Sb ₂ Te ₃ —GeTe line or a composition obtained by adding a small amount of Sb or the like to this composition is suitable for phase change recording, but in terms of refractive index change, it is better to be close to GeTe.

Therefore, a material based on GeTe is preferable, and it is possible to add a third element to improve the bit shape, crystal grain shape, stability over time, recording sensitivity, optical physical properties and the like. Sb, Sn, In, T as the third element
e, Ge, Pb, As, Se, Si, Bi, Au, T
You may add i, Cu, Ag, Pt, Pd, Co, Ni, etc. The composition ratio of GeTe is 50 at. %(atom%)
To ± 5 at. There is no problem with a deviation of about%.

As the substrate of the recording medium in the present invention,
It may be any of glass, plastic, glass provided with a photocurable resin, etc., but a polycarbonate resin is preferable from the viewpoint of CD compatibility. As the reflective layer, Au, Ag, Cu or Al, or an alloy film containing these as main components is used. Further, if a protective layer made of a photo- or thermosetting resin or a dielectric film is further provided thereon, corrosion resistance is improved and deterioration due to repeated recording / erasing can be suppressed.

The recording layer, the dielectric layer and the reflective layer are formed by a sputtering method or the like. It is desirable to perform film formation by an in-line apparatus in which the target for recording film, the target for protective film, and the target for reflective layer material, if necessary, are installed in the same vacuum chamber in order to prevent oxidation and contamination between the layers. It is also excellent in terms of productivity.

[0030]

The present invention will be described in detail below with reference to examples. Example 1 A substrate is a polycarbonate resin, a reflective layer is gold, and a recording layer is
Ge ₄₆ Sb ₈ Te ₄₆ (atomic%), and hydrogenated amorphous silicon (n = 3.5 to 0.05i) as a high refractive index dielectric
Alternatively, using Sb ₂ S ₃ (n = 2.8 to 0.05i),
As a low refractive index film (ZnS) ₈₀ (SiO ₂ ) ₂₀ (refractive index 2.1), SiO ₂ (refractive index 1.5), Si ₃ N ₄ (refractive index 1.7), Ta ₂ O ₅ (refractive index) Table 1 shows the measurement results of the reflectance of the medium in the amorphous state and the reflectance of the medium in the crystallization state when discs were manufactured with various layer configurations using the index 2.1).
Put together.

About the reflectance, a value almost as calculated is obtained.
Was given. Also GeTe, Ge₄₆Te ₅₄, Ge₅₄Te₄₆,
Ge₄₇Se₆Te₄₇, Ge₄₇Bi₆Te₄₇, Ge₄₇In₆
Te_{Four 7}Complex refractive index of Ge₄₆Sb₈Te₄₆And both
Crystallized 5.1-4.5i, amorphous 4.2-1.2i
There was no difference. Recording layer in crystalline state and amorphous silicon
Calculate the ratio of the amount of light absorbed in the light source layer with the amount of incident light as 1.
The value obtained by
It is 0.05 to 0.06 and the absorption rate to the recording layer is 0.25.
Met.

1.4 m / s for the disk of Example 1
When a signal of 196 kHz, duty 50%, 19 mW was recorded at ec, C / N> 50 dB was obtained and 10 mW
When the DC laser of No. 2 was projected, an erasing rate of 25 dB or more was obtained. The laser wavelength is 780 nm and the aperture ratio NA is 0.55. When it was reproduced under the same conditions as the reproduction of the CD, the reproduction was excellent. [Comparative Example 1] The recording layer is made of Ge ₄₆ Sb ₈ Te having a film thickness of 200 liters.
₄₆ is used, a high-refractive-index dielectric is made of high absorption amorphous silicon (3.5 to 0.2i), and a low-refractive-index dielectric is SiO ₂ (refractive index 1.5). Table 1 shows the measurement results of the reflectance of the medium in the amorphous state and the reflectance of the medium in the crystallization state when the disk was manufactured with almost the same layer structure.

A value obtained by calculating the ratio of the amount of absorbed light in the recording layer and the amorphous silicon layer in the crystalline state with the amount of incident light being 1 indicates that the absorption rate in the amorphous silicon layer is 0.17.
.About.0.18 and the absorption rate to the recording layer is 0.17 to 0.1
It was 8. The reflectance was almost as calculated. In Comparative Example 1, absorption to the amorphous silicon layer is too large, and sufficient reflectance cannot be obtained in the crystalline state.

Further, since the light absorption in the recording layer and that in the amorphous silicon layer are almost the same, the light energy is not effectively utilized in the recording layer, and the recording sensitivity is significantly lowered. When the CD was reproduced under the same conditions, the reproduction was impossible. [Comparative Example 2] All the dielectric intermediate layers between the substrate and the recording layer were made of insulators such as SiO ₂ (refractive index 1.5) and (Zn
S) ₈₀ (SiO ₂ ) ₂₀ (refractive index 2.1), and the thickness of each layer is λ / 4, 130 n each
m, 93 nm is required.

On the other hand, with amorphous silicon, 56 nm is sufficient as in the embodiment. For example, if the layer structure shown in Table 1 is adopted, the crystalline state reflectance of the recording layer can be 70% or more and the amorphous state reflectance can be less than 35%, but the overall film thickness is increased.
In addition, these insulator layers generally have a low film formation rate and a large required film thickness, so that the production rate becomes slow. Further, when the film thickness of each layer is increased, the influence of the internal stress of the film is increased, and problems such as film peeling are likely to occur.

[0036]

[Table 1]

[0037]

[Table 2] In the table-is as follows: hydrogenated amorphous _{silicon: SiO 2: Sb 2 S 3} : Ta 2 O 5: (ZnS) 80 (SiO 2) 20: Si 3 N 4: Ge 46 Sb 8 Te 46 : Au

[0038]

EFFECT OF THE INVENTION By using the optical recording medium of the present invention, an optical information recording medium capable of achieving high reflectance and contrast can be prepared by a relatively simple layer structure and a simple sputtering method. .

Claims (3)

[Claims] 1. A substrate having at least two different refractive indexes.
Reflection of a medium when the recording layer is in a crystalline state in an optical information recording medium in which an intermediate layer in which thin films of at least one kind are laminated, a phase transition type optical recording layer, an upper dielectric layer, and a reflective layer are sequentially laminated. The reflectance is 65% or more, the reflectance in the amorphous state is less than 35%, and the thin film having a high refractive index among the two or more thin films having different refractive indexes forming the intermediate layer is a recording / reproducing light wavelength. The absolute value of the imaginary part of the complex index of refraction is 0.
An optical information recording medium comprising a semiconductor having a real part of less than 1 and a real part of 2.5 or more. 2. The optical information recording medium according to claim 1, wherein the thin film having a low refractive index among the two types of thin films forming the intermediate layer is an oxide or a nitride of the semiconductor film having a high refractive index. . 3. A thin film having a high refractive index is amorphous silicon and a thin film having a low refractive index is silicon oxide, silicon nitride or silicon oxynitride among the two or more thin films forming the intermediate layer. The optical information recording medium according to claim 2, which is characterized in that.