JP2001189035A - Zinc sulfide based thin film and optical recording medium usiing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protective layer of a phase change type optical recording medium by obtaining a thin film having a refractive index higher than that of zinc sulfide, to provide a material for forming the thin film having a resistivity lower than that of conventional one by securing making the film thin and thermal stability thereby, to especially provide the protective layer used for the phase change type optical recording medium and to enhance productivity of the thin film. SOLUTION: The thin film is formed using a zinc sulfide based material containing zinc sulfide as a main component and niobium oxide and especially the niobium oxide content in the zinc sulfide based thin film is specified to be 10-30 wt.% expressed in terms of Nb2O5 to impart the high refractive index to the thin film and especially the refractive index of the light having 400 nm wavelength is specified to be 2.5 or greater. A raw material sintered compact having a low electric resistivity is obtained by adding the niobium oxide to the zinc sulfide and the sintered compact is easily formed in the thin film shape by a direct current sputtering method. Such a thin film as this is utilized for the protective layer for applying and protecting an alloy phase recording layer on which the irradiation signal of a laser beam is recorded as a phase change in an optical storage medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zinc sulfide thin film, and more particularly to an optical recording medium using a protective layer made of a zinc sulfide thin film.

[0002]

2. Description of the Related Art A zinc sulfide-based thin film is used as a protective layer for protecting a recording layer in a phase change type optical recording medium using an alloy of Te or Sb for the recording layer. This medium is a CD-RW, DVD-RAM, DVD-RW,
Although known as an optical disk such as a DVD + RW, FIG. 4 shows a basic structure of the optical disk. A first protective layer 2 (21) is formed on a disk-shaped substrate 10, and a recording layer 3 is formed thereon. On the recording layer 3, a second protective layer 2 (22)
And a reflective layer 4 made of aluminum, gold, silver, or an alloy containing the same as a main component is formed thereon.

The compact disk is irradiated with a laser beam from the disk-shaped substrate 10 side.
The recording layer 3 is irradiated with the light passing through the protective layer 2 (21, 22) and is reflected by the reflective layer 4 so that the recording layer 3 and the protective layer 2
It returns to the irradiation side via (21, 22) and is detected as an electric signal. During recording, the phase-change type optical recording medium is irradiated with a laser beam modulated in accordance with the signal intensity, and changes the phase of the recording layer 3 by its thermal energy (for example,
The alloy thin film of the recording layer is changed into a crystalline phase and an amorphous phase reciprocally), signal information is recorded as this phase change, and at the time of reproduction, a laser beam is irradiated by irradiating a laser beam and a laser beam accompanying the phase change in the recording layer 3 is generated. Is detected as a signal.

The protective layer 2 transmits a laser beam and contacts the recording layer 3 to protect both surfaces thereof. For example, a composite material of ZnS-SiO ₂ is known. For example, this ZnS—SiO ₂ material has a molar ratio of 8
A composition having a composition of 0 mol% ZnS-20 mol% SiO ₂ is known.

[0005] These two protective films 2 (21, 22)
It is formed into a thin film by an RF sputtering method using a ZnS-SiO ₂ sintered material as a sputtering target. Such a sintered body is disclosed in JP-A-11-27.
No. 8936 and JP-A-7-138071. In this thin film forming process, a disk-shaped substrate 10 and a target are arranged opposite to the disk-shaped substrate 10 in an RF sputtering apparatus, and high-frequency plasma is generated in a high-vacuum, dilute Ar atmosphere. A film is formed on it. Further, even after the formation of the recording layer 3, a film was formed on the recording layer 3 in an RF sputtering apparatus.

In a write-once optical disk, the protective layer 2 (21, 22) is instantaneously heated in the temperature range of 400 ° C. to 700 ° C. during recording / erasing by laser irradiation, and a large temperature change occurs. receive. For this reason, a ZnS film having high heat resistance has been used for the protective layer 2 (21, 22).
nS has a problem that crystal grain growth occurs. The above-mentioned 80 mol% ZnS-20 mol% SiO ₂ -based thin film was prepared by adding SiO ₂ to suppress crystal grain growth due to repetition of laser, thereby forming a fine structure having a small film crystal grain size.

Further, the recording layer 3 undergoes a temperature change due to laser irradiation of a large output at the time of writing, and is heated and cooled, so that the protective layer 2 which is directly in contact with the recording layer 3 is written.
(21, 22) also requires that the alloy material used in the recording layer has little chemical reaction in a temperature range from room temperature to a maximum of 700 ° C.

[0008]

SUMMARY OF THE INVENTION Conventional ZnS-SiO
_The protective layer 2 (21, 22) made of a _two- system thin film has a high electrical resistance due to its sintered material, so that the disk-shaped substrate 10
Although the thin film was formed on the upper surface by RF sputtering, this method had a low efficiency of film formation, and it was necessary to apply a DC sputtering or other highly efficient deposition method. In particular, in the case of a large-capacity optical disc that requires a high recording density for an optical recording medium, in the production process, it is necessary to increase the film forming rate and increase the productivity of the thin film.

Further, ZnS alone has a relatively high refractive index, the refractive index of the formulation of SiO _2, a protective layer 2 made of ZnS-SiO ₂ based thin film (21, 22) is ZnS
It was lower than the simple substance. If the refractive index of the protective layer 2 (21, 22) is much lower than the refractive index of the recording layer 3, for example, the crystalline phase and the amorphous phase of the Ge—Te—Sb alloy layer,
The optimum thickness of the protective layer 2 (21, 22) cannot be too small in order to optimize the reproduction signal intensity ratio by the laser signal. If the thickness is large, ZnS of the protective layer 2 (21, 22)
-The SiO ₂ -based thin film has a low thermal conductivity, so that the protective layer 2 (21, 2
The heat in 2) was not easily dissipated, and easily peeled off as the temperature rose.

As described above, the protective layer 2 (21, 22) used for the optical recording medium has a lower electric resistance and a higher optical characteristic as compared with the conventional mixed material of ZnS and SiO _2. If a stable mixed material having a refractive index can be obtained, it is expected that the performance of the optical recording medium is improved, the efficiency of the manufacturing process is improved, and the usefulness is high in terms of cost.

Further, in order to increase the density of the optical disk,
The wavelength of the laser beam has been shortened along with the high-speed rotation of the disk. However, conventionally, for example, Ge-Te-Sb has been used for 400 nm wavelength light compared to 830 nm or 780 nm light.
The rate of change of the complex refractive index associated with the phase change in the recording layer 3 such as an alloy layer becomes small (the S / N ratio of the signal becomes small),
As the wavelength becomes shorter, a higher refractive index is also required for the conventional protective layer 2 (21, 22) in order to improve the S / N ratio. Zinc sulfide alone has a refractive index n of about 2.35 for 400 nm wavelength light, but ZnS-Si
The O ₂ -based thin film is further reduced by the addition of SiO ₂ , and from this point, a higher refractive index of n = 2.50 or more is required.

Further, in the phase change type optical recording medium, the thickness of the polymer substrate 10 of the conventional optical recording medium is 1.2 mm in the conventional CD specification, while the thickness is 0.6 m in the DVD specification.
m. The ZnS—SiO ₂ based thin film has a low refractive index and a low thermal conductivity, so that an internal stress is easily generated. Therefore, a residual stress is generated in the thinned substrate 10 or its optical recording medium, and the stress causes optical recording. The reliability of the medium could be impaired.

In view of the above problems, the present invention provides a thin film having a higher refractive index than a thin film of zinc sulfide alone, provides a protective layer for a phase-change optical recording medium, and reduces the thickness of the protective layer. The thermal stability is thereby ensured to improve the reliability of the optical recording medium. The present invention also provides a material for forming a thin film having a lower resistivity than in the past, and in particular, provides a protective layer used for a phase-change optical recording medium,
The purpose is to increase the productivity of the thin film.

The present invention further relates to a conventional ZnS-SiO
An object of the present invention is to provide an optical recording medium having a protective layer that is thermally stable as compared with the _two- system thin film and is not easily damaged even by repeated laser irradiation during writing.

[0015]

According to the present invention, a zinc sulfide-based thin film containing niobium oxide in zinc sulfide is provided.
By increasing the refractive index, the refractive index is 400-
A high refractive index of 2.25 or more can be realized in a wavelength region of 800 nm.

In particular, by making niobium oxide into zinc sulfide in an amount of 10 to 30% (% by weight, hereinafter the same) in terms of Nb ₂ O ₅ , the refractive index is particularly reduced to 2.50 for light having a short wavelength of 400 nm. The above is the description.

In the present invention, the form of niobium oxide in the thin film and during sintering for forming the thin film is not necessarily clear, but niobium oxide contains a large number of oxides having a lower niobium oxidation number. In the following description, niobium oxide is, for convenience,
Although represented by Nb ₂ O ₅ , the present invention is not limited to this.

Since the zinc sulfide-based thin film of the present invention has a refractive index higher than that of zinc sulfide alone, it protects the recording layer in a phase-change optical recording medium using a Te or Sb alloy layer for the recording layer. Used as a high refractive index layer for the ZnS-based protective layer to optimize the film thickness to a small film thickness in the range of 10 to 500 nm, enhance the heat dissipation of the thin film of the protective layer, To prevent physical failure.

Further, the present invention reduces the electrical resistivity of a zinc sulfide-based material by increasing the rate of thin film formation from a gas phase by forming a thin film obtained by mixing zinc sulfide with niobium oxide. is there. In particular, niobium oxide is added to zinc sulfide.
By setting the content to 0 to 30% (% by weight, the same applies hereinafter), the sintered body can have a low surface resistance of 10Ω / □ or less.

The zinc sulfide-based thin film of the present invention has a low electric resistance even in a sintered body of zinc sulfide and niobium oxide. (DC) sputtering can be adopted,
This makes it possible to increase the sputtering power and increase the thin film forming speed in forming the protective layer of the optical recording medium, thereby improving the productivity of the thin film.

The zinc sulfide-based thin film of the present invention can contain zinc sulfide and niobium oxide and can be made stable and amorphous. Even in the repetition of laser irradiation, crystallization or crystal grain growth of the protective layer due to heating and cooling is suppressed, thereby preventing a reduction in the S / N ratio. Furthermore, this amorphous protective layer is stable and prevents crystallization,
Since the internal residual stress of the thin film can be kept small, the protective layer is prevented from being peeled or damaged, which is effective for extending the life of the optical recording medium.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The zinc sulfide-based thin film of the present invention contains zinc sulfide as a main component and contains niobium oxide.
The thin film has a higher refractive index than zinc sulfide alone and is used as a high refractive index thin film. First, the addition of niobium oxide is performed by using Zn
In order to increase the refractive index of the S-based thin film, the refractive index of the thin film becomes 10% in terms of Nb ₂ O ₅ with the increase of niobium oxide.
(% By weight, the same applies hereinafter), and increases more rapidly than 10%. Therefore, the content of niobium oxide, calculated as Nb ₂ O ₅ is preferably in the range of 10-50%. In this range, for 400 nm wavelength light, 2.
A high refractive index in the range of 5-2.7 is obtained.

The zinc sulfide-based thin film of the present invention increases the refractive index while maintaining the high heat resistance of the ZnS-based thin film. In particular, a phase change type light using an alloy containing Te or Sb for the recording layer is used. In a recording medium, it can be suitably used as a protective layer.

The protective layer of the optical recording medium is composed mainly of ZnS and contains 10 to 30% of niobium oxide (in terms of Nb ₂ O ₅ ).
Those formed by addition are preferred. The addition of Nb ₂ O ₅
This has the effect of inhibiting the crystal growth of ZnS when S is heated and cooled in a wide temperature range from room temperature to 700 ° C. or lower. The zinc sulfide-niobium oxide-based thin film is generally amorphous when formed from a gas phase within the above range of the Nb ₂ O ₅ composition,
Compared to the crystallized zinc sulfide-based thin film, the chemical, optical, and mechanical properties of the thin film are homogeneous, especially when the internal residual stress of the thin film is small, and when used as the above-mentioned optical recording medium, The deformation of the disk-shaped substrate caused by the thin film can be prevented.

In the application of this protective layer, when Nb ₂ O ₅ exceeds 30%, it is heated to about 700 ° C., although momentarily, by repeated irradiation of laser light, and zinc sulfide or niobium oxide is repeatedly heated. Depending on the conditions of the crystal growth, the crystal grains may further grow, and the optical film characteristics may be degraded, causing stress in the thin film and the deformation of the disk-shaped substrate.

The thickness of the thin film as the protective layer is optimized depending on the refractive index of the protective layer so that the ratio of the laser reflection intensity between the crystalline phase and the amorphous phase of the alloy recording layer is maximized. In the invention, since the refractive index of the thin film is increased, the thickness of the thin film can be reduced. However, from the property of the thin film, the thickness of the thin film of the protective layer is preferably in the range of 10 to 500 nm, and particularly preferably 50 to 300 nm.
If it is less than 10 nm, the thermal intensity is insufficient, and if it exceeds 500 nm, the intensity ratio of the laser decreases (the S / N ratio decreases).
Then, there is a fear that the thin film is peeled off.

In the present invention, reducing the thickness of the protective layer can reduce the internal stress depending on the thin film of the protective layer itself as a material, thereby improving the thermal stability of the optical recording medium and reducing the thickness. By doing so, the time required for forming the protective layer can be shortened, which is useful for improving the productivity of the thin film.

The zinc sulfide-niobium oxide thin film of the present invention comprises:
Vapor deposition method, for example, vacuum evaporation method, sputtering method,
It is formed by a plasma CVD method, an optical CVD method, an ion plating method, an electron beam evaporation method, or the like.

However, in the present invention, the addition of niobium oxide to zinc sulfide makes it possible to add zinc sulfide for forming a thin film.
The electrical resistivity of the niobium oxide sintered body can be reduced.

In the present invention, the content of niobium oxide in ZnS is preferably in the range of more than 5% and 50% or less in terms of Nb ₂ O _{5 in} order to lower the electric resistivity. Thereby, a low resistance having a surface resistance value of 100Ω / □ or less can be obtained. In particular, by setting the content of niobium oxide in the range of 10 to 30% in terms of Nb ₂ O ₅ , the surface resistance value becomes 10%.
A low resistivity of Ω / □ or less can be obtained.

By utilizing the low resistivity zinc sulfide-niobium oxide sintered body of the present invention, when forming a thin film from the sintered body,
A thin film can be efficiently formed by applying a thin film forming method of flowing a current to a sintered body as a target, for example, a DC sputtering method. Conventional ZnS-Si
Although the DC sputtering method could not be applied to the protective layer made of the O ₂ -based thin film, the zinc sulfide-niobium oxide-based thin film of the present invention uses the DC sputtering method to achieve a higher thin film forming rate than conventional RF sputtering. Can be obtained. Thereby, the ZnS—Nb ₂ of the present invention is obtained.
In the step of forming a protective layer of an optical recording medium using a zinc sulfide-niobium oxide sintered body for an O ₅ -based thin film, thin film productivity can be enhanced by using a DC sputtering method.

In order to use such a zinc sulfide-based thin film for the protective layer of the optical recording medium, a sintered body having a predetermined shape is produced from a mixed powder containing ZnS and Nb ₂ O ₅ , A thin film is formed by a vapor deposition method using the body as a target. In the optical recording medium, as shown in FIG. 4, a thin film is formed from a zinc sulfide-niobium oxide sintered body on a polymer disk-shaped substrate 10 by a vapor deposition method including a DC sputtering method. Is formed. After the recording layer 3 is separately formed on the protective layer 21, a thin film is again formed on the recording layer 3 from a zinc sulfide-niobium oxide sintered body by a vapor deposition method, and the second protective layer 22 is formed thereon, and a vapor deposition film of Al, Au, or the like is separately formed as the reflection film 4 to form a main structure of the optical recording medium.

[0033]

EXAMPLES Example 1 As raw materials, hexagonal zinc sulfide powder having an average particle diameter of 4 μm and a purity of 99.9%, and an average particle diameter of 0.1 μm were used.
3 μm niobium oxide powder having a purity of 99.9% was used. These zinc sulfide powders and niobium oxide were mixed to form a mixture. The content of niobium oxide in the mixture is 0.5 to 30.
%.

Each sample mixture was dry-mixed for 24 hours using a zirconia ball in a resin pot. After granulation,
The temperature is raised to 900 ° C at 3 ° C / min by hot pressing.
While pressurizing at a surface pressure of 250 kg / cm ² in an r atmosphere,
Sintering was performed for 3 hours to obtain a disc-shaped sintered zinc sulfide-niobium oxide body having a diameter of 50 mm and a thickness of 5 mm. The surface resistance value of the zinc sulfide-niobium oxide sintered body was measured by a four probe method. Table 1 shows the measurement results.

[0035]

[Table 1]

From Table 1, it can be seen that the surface resistance of the ZnS-Nb ₂ O ₅ sintered body is not particularly changed when the Nb ₂ O ₅ content is 5.0% or less, but 5.0%. Beyond 10
%, The surface resistance decreases to 10 Ω / □, and at 20 to 30%, it decreases to 1 Ω / □ or less.

Example 2 Using the sintered body obtained in the above example, the sputter rate was measured by the RF sputtering method and the DC sputtering method. The conditions for the RF sputtering method are as follows: input power 800 W, Ar gas 1 P
a was selected and formed on a quartz substrate to a film thickness of 100 nm. In the other DC sputtering method, the measurement conditions were such that a DC input was 2 KA and an Ar gas pressure was 1 Pa on a glass substrate so as to have a film thickness of 100 nm under a thin film formation condition of 1 Pa. The test results are summarized in Table 2.

[0038]

[Table 2]

From Table 2, it can be seen that the sputtering rate is approximately twice as high according to the DC sputtering method as compared to the RF sputtering method. In this way, in terms of improving the efficiency of thin film production, the conventional ZnS—SiO ₂ thin film
Can not be formed only by an RF sputtering method, but the sputtering rate is low, it can be seen that increasing the sputtering rate by ZnS-Nb ₂ O ₅ based thin film. In addition, conventional ZnS
-SiO ₂ based thin film, and immediately after the start of use of the sputtering target by sputtering, in the case that severe late target consumable used, but there is the optical characteristics of the thin film is changed, ZnS-Nb ₂ O ₅ based thin film It has been confirmed that the optical characteristics do not change.

Example 3 The relationship between the refractive index and the composition of a ZnS—Nb ₂ O ₅ -based thin film was determined by the method used in Example 2 on a quartz glass substrate and the composition of Nb ₂ O _{5 in} zinc sulfide. Different ZnS-based thin films were formed, and the complex refractive indexes of the thin films were measured.

FIG. 1 shows the measurement results of the complex refractive index at 400 nm wavelength light. When the content of niobium oxide is up to 10% in terms of Nb ₂ O ₅ , the refractive index sharply increases as the content increases. However, when the niobium oxide content exceeds 10%, the rate of increase in the refractive index with respect to niobium oxide decreases. From this figure, it can be seen that a refractive index for light with a wavelength of 400 nm of 2.5% to 2.7 can be obtained at a value exceeding 10%.

FIG. 2A shows the relationship between the complex refractive index of a ZnS-based thin film containing 20% Nb ₂ O ₅ and 15% Nb ₂ O ₅ and the wavelength.

In order to confirm the reproducibility of the optical characteristics of the zinc sulfide-niobium oxide-based thin film and the dependence on the manufacturing method, in addition to the above-mentioned RF sputtering method and DC sputtering method, vacuum evaporation method, sputtering method, plasma CVD method, optical CVD method Method, an ion plating method, an electron beam evaporation method, a thin film was formed by another thin film forming method, and the refractive index was measured. In any method, in a wavelength region of 400 nm,
It was found that a high refractive index was stably obtained at 2.57 to 2.59, and that almost the same thin film could be produced irrespective of the film forming method.

In addition, the thin film of the present invention has a high refractive index of 2.29 or more at a wavelength of 800 nm or less, thereby providing an optical recording medium having a phase change type recording layer of an alloy containing Te or Sb. A protective layer of the medium can be formed, and the protective layer can be made thinner by using a high refractive index, thereby ensuring thermal stability and enabling production of an optical recording medium having a small stress. The reliability of the optical recording medium can be improved.

Example 4 Next, with respect to the ZnS-based thin film structure, niobium oxide was 10% and 30% (in terms of Nb ₂ O ₅ ).
A ZnS thin film was formed on a glass substrate to a thickness of 100 nm at room temperature by a sputtering method in the same manner as in Example 2. After heating this thin film sample at 700 ° C., the crystal orientation of the thin film was examined by XRD. As shown in FIGS. 3A and 3B, it can be seen that the thin film has no crystal diffraction peak and is amorphous. Even if heating at 700 ° C., Nb
_{Since 2} O ₅ itself does not crystallize, it is less likely to crystallize even by laser irradiation, and is expected to prevent stress from being generated in the thin film and peeling of the thin film and deformation of the substrate.

[0046]

As described above, according to the zinc sulfide-based thin film of the present invention, niobium oxide is contained with respect to zinc sulfide as a main component, so that a higher refractive index than zinc sulfide alone can be obtained. Heat resistance and heat dissipation can be improved. In particular, a zinc sulfide-based thin film containing 10% by weight or more of niobium oxide in terms of Nb ₂ O ₅ can obtain a high refractive index of 2.5 or more for 400 nm wavelength light.

Further, niobium oxide is converted into Nb ₂ O ₅ by 10 to 10%.
Since the zinc sulfide-based thin film containing 30% by weight is very stable against heat, it can be used even when heated up to 700 ° C. and then rapidly cooled during erasing and recording of the optical recording medium.
Since the ZnS particle growth and the mixed material itself are not crystallized, the amorphous state can be stably maintained and can be suitably used as a protective layer for protecting the recording layer of the optical recording medium.

Further, the zinc sulfide based thin film of the present invention is not limited to a conventional RF sputtering method as a method for obtaining a thin film, but may be a vacuum deposition method, a plasma CVD method, or an optical CVD method.
Method, an ion plating method, a film forming method such as an electron beam evaporation method can be used, and the thin film has stable thermal characteristics and a lower electric resistance compared to a ZnS-SiO ₂ based thin film. Because of its characteristics, direct current sputtering can be used to form a thin film from the zinc sulfide-niobium oxide sintered body, and by increasing the power granularity, the sputtering speed is increased to increase the protective layer of the optical recording medium. In this case, the productivity of the thin film can be improved.

[Brief description of the drawings]

FIG. 1 shows the effect of the addition amount of niobium oxide on the refractive index of a zinc sulfide-niobium oxide thin film according to an example of the present invention.

FIG. 2 is a diagram showing the relationship between the refractive index and the wavelength of light of a zinc sulfide-niobium oxide-based thin film according to an embodiment of the present invention;
Indicates a sample having a niobium oxide content of 20% by weight, and (B) indicates a sample having a niobium oxide content of 20% by weight (both in terms of Nb ₂ O ₅ ).

FIG. 3 shows an XRD measurement chart for examining the crystal orientation of a zinc sulfide-niobium oxide-based thin film according to an example of the present invention. (A) is a niobium oxide content of 10% by weight, (B)
Indicates a sample having a niobium oxide content of 30% by weight (both in terms of Nb ₂ O ₅ ).

FIG. 4 is a partial cross-sectional view of an optical recording medium including a phase-change type recording layer and a protective layer in contact therewith.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical recording medium 10 Disk substrate 2 Protective layer 3 Recording layer 4 Reflective layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yukio Noguchi 6F, Takeda Toba-cho, Fushimi-ku, Kyoto, Kyoto Prefecture F-term in Kyocera Corporation (Reference) 4K029 AA08 AA09 BA43 BA51 BC08 BD12 CA05 DC09 DC34 DC35 5D029 LA14 LA15 LA17 LA19 LB01 LB07 LC06 LC21

Claims (6)

[Claims] 1. A zinc sulfide thin film comprising zinc sulfide as a main component and containing niobium oxide. 2. The zinc sulfide-based thin film according to claim 1, wherein the content of said niobium oxide is 10 to 30% by weight in terms of Nb ₂ O ₅ . 3. The zinc sulfide-based thin film according to claim 1, wherein the zinc sulfide-based thin film has a refractive index of 2.5 or more with respect to light having a wavelength of 400 nm. 4. An optical recording medium comprising a recording layer of an alloy phase for recording an irradiation signal of a laser beam as a phase change and a protective layer for covering and protecting the recording layer, wherein the protective layer comprises An optical recording medium which is a zinc sulfide-based thin film containing zinc as a main component and containing niobium oxide. 5. The optical recording medium according to claim 4, wherein the content of said niobium oxide is 10 to 30% by weight in terms of Nb ₂ O ₅ . 6. The zinc sulfide-based thin film of the protective layer is 400
The refractive index for light having a wavelength of nm is 2.5 or more.
Or the optical recording medium according to 5.