JPH08273198A - Optical recording medium - Google Patents

Abstract

PURPOSE: To improve recording sensitivity by laminating a first reflection film, first interference film, recording film, second interference film, second reflection film and protective film in this order successively from the substrate side on a transparent substrate and specifying the reflectivity of the unrecorded parts at a reproducing laser wavelength to specific % or above. CONSTITUTION: The thin films are laminated in order of the first reflection film 2, the first interference film 3, the recording film 4, the second interference film 5, the second reflection film 6 and the protective film 7 successively from the substrate side on the transparent substrate 1 and the reflectivity of the unrecorded parts at the reproducing laser wavelength is specified to >=65%. Further, the film thickness of the second reflection film 6 is specified to 30 to 45nm. As a result, the laser power required for recording of a reloadable optical recording medium CD-E of >=65% in the reflectivity of the unrecorded parts at the reproducing laser wavelength is lowered. The recording sensitivity is greatly improved while the optical characteristics required for the CD-E are satisfied by specifying the second reflection film thickness to 30 to 45nm.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a rewritable optical recording medium,
In particular, the present invention relates to a rewritable optical recording medium that is optically compatible with a read-only optical disc whose reflectance in an unrecorded portion is 65% or more. In the present invention, a read-only optical disc is a typical read-only optical disc, a CD-ROM, a write-once optical disc is CD-R, and a rewritable optical disc is C.
In the present invention, the reflectance of the unrecorded portion is 65, which is expressed as DE.
Any rewritable optical recording medium that is optically compatible with a read-only type optical disc of which the content is at least 100% is applicable, and is not particularly limited to CD-E.

[0002]

2. Description of the Related Art A CD-ROM has a large capacity and a low production cost in mass production, and is therefore used for distributing software for computers. However, since the data cannot be rewritten, it cannot be used as a data file of user data like a floppy disk or a magneto-optical disk. Therefore, the user needs to prepare a recording / reproducing apparatus for a floppy disk or a magneto-optical disk in addition to the CD-ROM reproducing apparatus. However, the CD
-By providing the ROM with a rewriting function, it becomes possible to obtain software and data files using one CD-ROM recording / reproducing device.

Rewritable C that meets these needs
Research on D-ROM (CD-E) has been made. CD-
The first technical problem of the E development is to satisfy the reflectance standard of CD-R (reflectance of unrecorded portion is 65% or more, modulation degree is 60% or more). This type of optical disc is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 6-36342 and 6-162563.

As a means for solving this problem, a thin film is laminated in the order of a first reflective film, a first interference film, a recording film, a second interference film and a second reflective film from the substrate side on a transparent substrate. Structure CD-E has been developed (Page 1014 (1993) of Proceedings of the 40th Joint Lecture on Applied Physics, 1993,
And page 9 of the proceedings of the 5th Phase Change Research Workshop
Page 14 (1993) and Optical Data Storage
Topical Meeting Technical Digest pages 59-6
0 (1994)).

[0005]

The output of a semiconductor laser developed for an optical disk is 70 mW even with a high output. Since the laser efficiency of the optical head is generally about 45% at maximum, the maximum output of this semiconductor laser on the disk is about 32 mW. However, the rewritable optical recording medium has a reflectance of 7 in the unrecorded area.
Since it is 0% or more, the absorptance of the unrecorded portion is 30% or less, and as a result, there is a problem that the recording laser power needs to be as high as 50 mW or more on the disk. It is difficult to obtain such a high-power semiconductor laser, and there is also a problem that the life of the semiconductor is shortened because it is used at a high output.

Further, since a noble metal such as Au is used for the reflection film, there is a problem that the material cost required for the reflection film increases. The essential cause of the low recording sensitivity of CD-E is that the reflectance is as high as 70%, so that most of the irradiated laser beam is reflected and the amount of light absorbed by the recording film is reduced. However, there are several other causes. One of them is to use a high reflectance metal such as Au as the metal for the reflective film. There is a reason for this. In order to achieve a high reflectance and a high degree of modulation, it is necessary to use a metal having a reflectance as high as possible for the reflective film.
However, high reflectance metals (Au, Ag, Cu) have a very high thermal conductivity of 200 (W / m · K) or more. As a result, heat is rapidly diffused from the recording film through the reflective film, so that the recording film is hard to reach the recording temperature. Then, as a result of earnest research, the inventors have paid attention to the fact that the specific resistance of a metal thin film depends on the film thickness, and when a high reflectance metal is used as a reflective film for CD-E, the recording sensitivity and the optical characteristics are improved. The optimum film thickness range that satisfies both of the above was found.

Therefore, an object of the present invention is to reduce the laser power required for recording CD-E, that is, C
It is to improve the recording sensitivity of DE and further reduce the material cost.

[0008]

A rewritable optical recording medium according to the present invention comprises a first reflection film, a first interference film, a recording film, a second interference film and a second reflection film from at least the substrate side on a transparent substrate. Thin films are laminated in this order on the film, the reflectance of the unrecorded portion at the reproduction laser wavelength is 65% or more, and the film thickness of the second reflective film is 30 to 45 nm. Further, the recording film is a rewritable optical recording medium in which information is recorded by utilizing a change in optical constant between crystalline and amorphous, and the recording film in the unrecorded portion is crystalline. .

[0009]

In the rewritable / writable type optical disk having the above-mentioned laminated structure, the second reflective film thickness is set to 30 to 45 nm, thereby satisfying the optical characteristics required for the CD-E and greatly improving the recording sensitivity. Can be made. Also,
By making the recording film in the unrecorded (high reflectance) part of the optical disk in a crystalline state, the thermal conductivity can be lowered and the recording sensitivity can be improved.

[0010]

First, the principle of the present invention will be described. Au in Figure 1
A measurement example of the relationship between the film thickness and the specific resistance is shown (cited from page 166 of Applied Physics Selection Manual 3, "Thin Film"). Au film thickness is 50
The specific resistance is almost constant above nm, but A
When the u film thickness is 45 nm or less, the film thickness increases rapidly. Wie
According to the Demann-Franz law, the thermal conductivity of a metal is inversely proportional to the specific resistance. Therefore, if the Au film is 45
It was considered that the thermal conductivity can be lowered by making the film thickness to be less than nm. Also, of course, A
By reducing the thickness of the u film, the amount of electrons per unit area that serves as a medium for heat conduction also decreases. As a result, the amount of heat diffused through the second reflective film is reduced and the recording sensitivity is improved. However, thinning the second reflective film may have an optical adverse effect. Therefore, as shown in the examples, an optical simulation was performed to find the allowable thinnest film thickness. As a result, it was found that the optical characteristics were not affected if the second reflective film thickness was at least 30 nm.

This effect is particularly great when the first reflective film is present between the transparent substrate and the first interference film. Further, the effect of the present invention becomes remarkable when the state of the recording film in the unrecorded (high reflectance) portion is crystalline. In the above description, the case where Au is used as the metal for the second reflective film is described, but other high reflectance metals (for example, Ag and C) are used.
The same characteristics are exhibited when u) or an alloy containing them as a main component is used. However, in particular, when Au having excellent corrosion resistance is used as the metal for the second reflective film, even if the second reflective film is thinned, the optical characteristics and recording characteristics do not change for a long period of time, so that a highly reliable CD −
E can be provided. Further, since the thickness of the second reflective film thickness is as thin as about half that of the conventional one, the material cost can be reduced.

As described in detail above, by setting the second reflection film thickness to 30 to 45 nm, the recording sensitivity can be greatly improved while satisfying the optical characteristics required for CD-E. . As an example in which the reflective film is thinned to improve the characteristics, for example, a structure as shown in pages 21 to 22 of Optical Memory Symposium '92 is known. This is to reduce the jitter of the reproduced signal when overwriting data. In addition, the structure of this conventional example does not have the first reflection film, and the film thickness of the Au film is too thin as 10 to 20 nm, so that the optical characteristics required for the CD-E cannot be satisfied. Therefore, it goes without saying that the idea of this conventional example is basically different from that of the present invention.

(Embodiment 1) FIG. 2 shows the structure of an optical disk which was subjected to an optical simulation in order to obtain the allowable thinnest thickness of the second reflective film. Au as the first reflection film 2 and ZnS as the first interference film 3 on the polycarbonate substrate 1.
-SiO2, GeTe as the recording film 4, ZnS-SiO2 as the second interference film 5, Au as the second reflective film 6, and a UV protective film 7 are sequentially laminated.

FIG. 3 shows the dependency of crystal and amorphous reflectance and transmittance on the second reflection film thickness, and FIG. 4 shows the dependency of transmittance on the second reflection film thickness. The optical constants used in the simulation are shown in the table of FIG. Further, the first reflection film is 16 nm, the first interference film is 30 nm, and the recording film is 1 nm.
The second interference film has a thickness of 6 nm and the second interference film has a thickness of 30 nm.

As is apparent from the figure, the second reflection film thickness is 3
When the thickness is 0 nm or more, the reflectance level hardly changes in both crystal and amorphous. As far as the reflectance is judged, it can be seen that the thickness of the second reflective film can be reduced to 30 nm. On the other hand, regarding the transmittance, the crystal transmittance is 30 n.
Almost no change occurs at m or more, but the amorphous transmittance greatly increases at 30 nm. This suggests that when the thickness of the second reflective film is set to 30 nm, the absorptivity in the amorphous state may decrease, which may cause a problem, but it does not actually cause a problem. That is,
The parameters that directly affect the recording sensitivity are the reflectance and the transmittance in the crystalline state. The case where the highest output is required is the case where recording is performed on the recording film in the crystalline (high reflectance) state. Because it is. Therefore, even if the absorptivity of amorphous is lowered, it does not have a great influence on the recording sensitivity. Therefore, the allowable thinnest thickness of the second reflective film is 30 nm.

(Embodiment 2) In the structure having the first reflection film, the first interference film, the recording film, the second interference film and the second reflection film to which the present invention is applied, the effect of the present invention is particularly great. In order to show that, only the Au film was formed on a polycarbonate substrate, and the Au film thickness dependence of reflectance and transmittance of a simple structure in which a UV protective film was applied was calculated, and the result is shown in FIG. . In the case of the structure having the first reflection film, the first interference film, the recording film, the second interference film, and the second reflection film, the reflectance and the transmittance when the recording film which has a large influence on the recording sensitivity is the crystal are When the two-reflection film thickness was 30 nm or more, it was at a sufficient level. But,
In the structure in which only the Au film and the protective film are laminated on the polycarbonate substrate, it cannot be said that the reflectance and the transmittance are sufficiently saturated when the Au film thickness is 30 nm. In particular, since the transmittance is as high as about 13%, even if recording is performed in this state, since the amount of transmitted laser light is large, the amount of absorbed light is reduced and it is difficult to improve recording sensitivity.

As described above, in the structure having the first reflection film, the first interference film, the recording film, the second interference film and the second reflection film, the reason why the effect of the present invention is particularly large is that the first reflection film is present. This is because the recording film in the unrecorded (high reflectance) state is crystalline. That is, the presence of the first reflective film,
Since the recording film has a property that it is difficult for light to pass in the crystalline state, the incident laser light is reflected to the substrate side before reaching the second reflective film. As described above, the effect of the present invention is that the thin film is laminated in this order from the substrate side on the transparent substrate to the first reflection film, the first interference film, the recording film, the second interference film, and the second reflection film. In particular, when the state of the recording film in the unrecorded (high reflectance) state is crystalline, it becomes particularly remarkable.

(Embodiment 3) Several discs having different second reflection film thicknesses were actually prepared, and the reflectances of crystal and amorphous were measured and the results are shown in FIG. 7. As shown in the simulation, each reflectance level has hardly changed. In addition, these discs have a disc linear velocity of 5.6.
Rotation was performed at m / s, a signal having a frequency of 0.8 MHz was recorded, and the optimum recording power was measured. FIG. 8 shows the relationship between the optimum recording power and the second reflective film thickness when the second reflective film thickness of 70 nm, which is the conventional structure, is standardized to 1. When the second reflection film thickness is 45 nm or less, the recording power reduction effect is remarkable. Further, when the cases where the second reflection film thickness is 70 nm and 30 nm are compared, the second reflection film thickness is 30 n.
At m, the recording power was reduced by about 20% and the recording sensitivity was improved.

(Embodiment 4) In the optical simulation of Embodiment 1, calculation was performed for the case where Au was used for the first and second reflective films.
An example using an alloy in which a small amount of element is added to u will be described.
Specifically, several discs having different second reflection film thicknesses were prepared by using an alloy obtained by adding 2% to 3% of Ti, Cr, Co, and Ni to Au as the first and second reflection films. The reflectance level was measured. Also in this case, the reflectance level hardly changed when the second reflection film thickness was 30 nm or more.

Further, the disk was rotated at a linear velocity of 5.6 m / s, a signal having a frequency of 0.8 MHz was recorded, and the optimum recording power was measured. As a result, the recording power reduction effect is remarkable when the second reflection film thickness is 45 nm or less. Appeared in. In this case, as compared with the case where the second reflective film is Au and the film thickness is 70 nm, the recording power is 25 to 3 when the second reflective film thickness is 30 nm.
The recording sensitivity was improved by about 5%. Au as the second reflective film
The reason why the effect of improving the recording sensitivity is larger than that in the case where the heat conductivity of Au is large is that the thermal conductivity of Au is lowered because a small amount of element is added to Au. As described above, the present invention is effective even when an alloy obtained by adding a small amount of element to Au is used as the first and second reflective films.

[0021]

As described in detail above, by using the rewritable optical recording medium of the present invention, the first reflective film, the first interference film, the recording film, the second film, and the second reflective film are formed from the substrate side on the transparent substrate. A thin film is laminated in this order on the interference film and the second reflective film, and the laser power required for recording on the rewritable optical recording medium CD-E having a reflectance of 65% or more at the unrecorded portion at the reproduction laser wavelength is reduced, Further, the material cost can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the film thickness of an Au thin film and the specific resistance.

FIG. 2 is a diagram showing a structure of an optical disc according to an embodiment of the present invention.

FIG. 3 is a diagram showing a result of an optical simulation according to an example of the present invention.

FIG. 4 is a diagram showing a result of optical simulation according to an example of the present invention.

FIG. 5 is a chart showing optical constants used for simulation.

FIG. 6 is a diagram showing a result of an optical simulation according to an example of the present invention.

FIG. 7 is a diagram showing a relationship between a second reflective film thickness and reflectance.

FIG. 8 is a diagram showing a relationship between a second reflective film thickness and recording power.

[Explanation of symbols]

 1 Polycarbonate Substrate 2 First Reflection Film 3 First Interference Film 4 Recording Film 5 Second Interference Film 6 Second Reflection Film 7 UV Protection Film

Claims (2)

[Claims] 1. A thin film is laminated in the order of a first reflective film, a first interference film, a recording film, a second interference film and a second reflective film from at least a substrate side on a transparent substrate, and an unrecorded portion at a reproduction laser wavelength. Is a rewritable optical disk having a reflectance of 65% or more, and a film thickness of the second reflective film is 30 to 45 nm. 2. The rewritable optical disc according to claim 1, wherein information is recorded as a recording film by utilizing a change in crystalline and amorphous optical constants, wherein the recording film in the unrecorded portion is crystalline. A rewritable optical recording medium characterized by: