JP2886883B2 - Light disk - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk for recording / reproducing or erasing information by laser light, and more particularly to an optical disk having high corrosion resistance and controlling the recording sensitivity on the disk. The present invention relates to a suitable disk.

[Prior Art] With the recent development of a highly information-oriented society, needs for high-density and large-capacity file memories are increasing, and optical disks have been attracting attention in order to meet this demand. Write-once optical disks that can be written only once have already been put into practical use, and magneto-optical recording is at the stage near the stage of practical application as a reversible optical disk that can be rewritten as many times as possible. I have.

In optical disks, metal films have been used as reflective layers or as protective films in order to improve the use efficiency of information recording films and light. These metallic materials are vulnerable to corrosion, and the reliability of the information recording film has been improved by adding an element for improving corrosion resistance. But,
It is considered that there is a limit to the type and concentration of elements that can be added to the reflective film in consideration of the reflectance. Known examples of this are disclosed in JP-A-57-169996 and JP-A-61-848.
03 can be given.

[Problems to be solved by the invention]

In the above prior art, sufficient consideration has not been given to the improvement of the corrosion resistance of the metal material used as the recording medium of the optical disk, and the metal material film is corroded and degraded with time, and the information is lost by recording. In some cases, the price of reliability was met. Among the corrosions occurring in these metallic materials, a particular problem is that the metallic film is locally corroded, such as pitting, and a hole is formed in the metal film.

That is, the metal layer provided on the optical disk is often active against water or oxygen in the atmosphere and easily corroded. In particular, in the case of local corrosion represented by pitting corrosion, recorded information is destroyed and reliability is reduced. One problem of practical use is to solve this problem. The metal material in the recording medium is largely governed by physical properties, and must satisfy both corrosion resistance and physical properties, which are in conflict.

Therefore, an object of the present invention is to improve the corrosion resistance of a metal material used in an optical disk recording medium without deteriorating the optical or magnetic properties, and to use this material to have high reliability. To provide an optical disk.

[Means for solving the problem]

In the present invention, as a method for simultaneously satisfying both of the above characteristics, nitrogen is mixed into a metal material. Specifically, by forming the film in a nitrogen atmosphere or a nitrogen-containing atmosphere, nitrogen can be mixed into the metal layer. For example, a nitrogen gas or a nitrogen-containing atmosphere is used as a discharge gas when a film is formed by a sputtering method. The metal film thus formed is resistant not only to wet corrosion but also to pitting corrosion, and can significantly improve corrosion resistance.

[Action]

In forming the metal film provided on the uppermost layer of the optical disk, by including nitrogen in the metal layer, diffusion of oxygen into the film can be suppressed, and local corrosion such as pitting corrosion, particularly corrosive ions , It has high corrosion resistance and acts as a protective film. This is useful as a protective film for an optical recording medium. Further, by controlling the concentration of nitrogen contained in the metal layer, the thermal conductivity of the film can be changed, and as a result, the recording sensitivity to the disk can be freely selected. Thereby, the consistency between the disk drive and the disk can be easily stopped. In addition, the metal film containing nitrogen has a state in which internal stress is free, and peeling of the film and generation of cracks are significantly suppressed,
The reliability of the disk is improved.

In a disk having a structure such as a substrate / first dielectric film / recording film / second dielectric film / metal protective film using the film of the present invention as a protective film, the outermost metal protective film has high corrosion resistance. Therefore, the three layers below the film are not eroded at all. This is confirmed by the fact that the bit error rate or defect rate does not increase.

The present invention is useful not only for improving the reliability of the optical recording medium, but also for facilitating the control of the recording sensitivity of the disk. That is, as the concentration of nitrogen contained in the metal increases, the thermal conductivity decreases. By utilizing this property, the recording sensitivity of the disc can be arbitrarily selected, and the consistency with the disc drive can be easily obtained. In particular, the fact that the characteristics of the film can be controlled by the process control such as the control of the discharge gas as described above has an advantage that the consistency with the disk drive can be enhanced without lowering the productivity of the disk.

Further, in the metal film of the present invention, the internal stress of the film can be reduced to a substantially free state, the mechanical characteristics of the substrate can be improved, particularly the vertical vibration acceleration can be reduced, and the substrate and the recording medium can be further reduced. Peeling and cracking can be suppressed.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to Examples 1 and 2.

Example 1 FIG. 1 is a schematic cross-sectional view of an optical disk manufactured in this example. First, a silicon nitride film was formed as a first dielectric film 2 on a disk substrate 1 having a guide groove or a pit (not shown) on the surface thereof by a sputter method under the following conditions.

· Tagetsuto: Si ₃ N ₄ · discharge gas: Ar-N ₂ mixed gas _{(Ar / N 2 = 80/} 20) · discharge gas ^{pressure: 1 × 10 -2 (Torr)} · input RF Voltage Density: 4.5 (W / cm ² ) The obtained film 2 had a thickness of 850 ° and a refractive index of n = 2.05. To form the Tb ₂₄ Fe ₆₁ Co ₁₀ Nb ₃ having the composition information recording layer 3 thereon. The conditions of the spatter are as follows.

・ Target: Tb ₂₄ Fe ₆₁ Co ₁₀ Nb ₃ alloy target ・ Discharge gas: Ar (purity of 99.999% or more) ・ Discharge gas pressure: 5 × 10 ^-3 (Torr) ・ Input RF power density: 4.5 W / cm ² The thickness of the film 3 is 300 °, the magnetic characteristics are Kerr rotation angle: θ _k : 0.33 °, Curie temperature: T _C = 200 ° C., compensation temperature: T
_CoMP = 80 ° C., the coercive force: a H _C = 10KO _e. Subsequently, a nitrogen silicon film was formed as the second dielectric film 4 by a sputter method. The condition of the spatter in this case is that the discharge gas is pure.
The conditions were the same as for the first dielectric film 2 except that Ar was used and the pressure was 2 × 10 ⁻² Torr. The thickness of the obtained film is
200 °, refractive index: n = 2.15.

Finally, a metal film 5 was formed by a sputter method. The spatter conditions at that time are as follows.

・ Target: Al ₈₅ Ti ₁₅・ Discharge gas: Ar / N ₂ (= 90/10) ・ Discharge gas pressure: 1 × 10 ^-2 (Torr) ・ Input RF power density: 4.5 W / cm ² The metal film is amorphous and has a reflectance of 82.
%. Further, the nitrogen concentration in the film could be controlled by changing the N ₂ concentration in the discharge gas.

First, a reliability test of the disk manufactured as described above was performed by the following method. That is, the change with time of the defect rate when the disk was left in an environment of 80 ° C. and 85% RH (85% relative humidity) was measured. The reason why the error rate is not used is that not only errors caused by corrosion such as pitting corrosion but also bit shifts and errors on the apparatus are included, and it is not possible to clearly distinguish between the two.

The result is shown in FIG. In the disk of the comparative example, pure Ar containing no nitrogen was formed using a discharge gas when the metal film 5 was formed. As a result, in the disk of this example using a metal film containing 20% nitrogen and formed in an Ar atmosphere, no change in the defect rate was observed even after being left in this boundary for 3000 hours. On the other hand, the disk of the comparative example started to increase at around 1500 hours after standing, and showed a sharp increase after 2000 hours.

The cause of the defect was mainly pitting corrosion generated in the metal film except for the defect of the substrate and the defect generated in the manufacturing process. A film formed in a nitrogen-containing atmosphere has particularly high corrosion resistance against local corrosion typified by pitting corrosion, and greatly contributes to high reliability of an optical disk. In particular, this effect largely depends on the concentration of nitrogen in the metal film. Ar containing 0.5 to several% of nitrogen
On the contrary, the film formed in the atmosphere has a phenomenon that the film is weak against local corrosion, and the nitrogen concentration is particularly preferably 5% or more.

FIG. 3 shows the recording laser output dependence of the reproduction output on several disks obtained by changing the nitrogen concentration in the discharge gas (corresponding to the control of the nitrogen concentration in the film). The minimum recording laser power, when the N ₂ concentration of 0%, Atsuta at 5.2 mW.
On the other hand, as the nitrogen concentration was increased to 10%, 20%, and 30%, the sensitivity was improved to 4.0 mW, 3.7 mW, and 3.5 mW. As described above, according to the present invention, the recording sensitivity can be arbitrarily selected by controlling the gas composition at the time of film formation, which is effective for fine adjustment of the sensitivity when matching with the disk drive. This is the atmosphere N ₂
This is because controlling the concentration changes the thermal conductivity of the metal film.

Figure 4 shows the results of measuring the internal stress of the metal film, was examined the relationship between the N ₂ concentration. According to this, the internal stress sharply decreases as the nitrogen concentration increases, and becomes almost stress-free at a nitrogen concentration of 10% or more. By reducing the internal stress, an effect is obtained in that separation or cracks between layers or between the medium and the substrate due to the stress can be suppressed.

The disk was examined for peeling and cracking when left in 60 ° C.-95% RH. Here, as a comparative sample, a disk of a metal film manufactured in an atmosphere containing no nitrogen was used. As a substrate, a polycarbonate substrate was used for each disk. As a result, the disk manufactured by using the present invention showed no peeling of the medium and the substrate and no generation of cracks even after being left in the above environment for 2000 hours. In contrast,
In the disk of the comparative example, peeling began to occur in the flat part of the inner peripheral part and the outer peripheral part from around 1500 hours, and thereafter, the peeling progressed with the passage of time. As described above, the internal stress of the film is one of the causes of peeling, and the occurrence of the peeling can be suppressed by making the film stress-free by implementing the present invention.

In addition to this effect, in addition to the Al film containing Ti, Ta, Cr, Ni, Z
Similar effects can be obtained by using an Al alloy to which r and Cu are added. It should be noted that it is important to control the N ₂ concentration and the input RF power in consideration of the reactivity between each metal element and nitrogen. That is, when the metal becomes nitride, the absorption coefficient decreases, the reflectance decreases, and the transmittance increases, which causes a change in performance. The addition of elements such as Ti and Ta to the base elements Al, Cr and Cu is not only to improve the corrosion resistance, but also to control the thermal conductivity of the metal layer and to make the recording / erasing characteristics of the disc optional. It is also to make it possible to select.

Embodiment 2 This embodiment is a case where Cu, Cr or Al alone is used for the metal film 5. In order to confirm the effectiveness of the metal film 5 as a protective film, a Cu, Cr or Al film is formed on a glass disk by a spatter method, and the sample is immersed in a 1N aqueous sodium chloride solution. The change with time of the potential was measured. This immersion potential corresponds to a change in the surface state, for example, dissolution of the film, and serves as a guideline for evaluating corrosion resistance. FIG. 5 shows a schematic diagram of the measuring device. Using Ag / AgCl electrode as reference electrode,
The potential between the electrode and the sample was measured by an electrometer, and the result was output to a recorder. The results are shown in FIG. This figure shows the case of an Al film.

Here, the condition of the spatter is as follows, and the thickness is 500 mm.

· Tagetsuto: Al (purity 99.999% or higher), the discharge _{gas: Ar / N 2 = 95/} 5 · applied RF electrode: 4.2 W / cm ² and discharging gas pressure: 1 × 10 ^-2 Torr Also, samples for comparison The conditions were the same as above except that pure Ar was used as the discharge gas. The thickness of the formed film is 500 mm.

First, in the film manufactured using the present invention, a slight potential change was observed at the beginning of immersion. However, no potential fluctuation was observed after 12 minutes of immersion. Microscopic observation revealed that the potential fluctuations at the initial stage of immersion corresponded to the occurrence of pitting caused by foreign matter such as dust that had entered during film formation. In contrast, a film was formed in an atmosphere containing no N ₂
In the Al film, the change in the potential is the initial stage of immersion (up to about 15 minutes)
And 30 minutes later. The initial potential fluctuation corresponded to the occurrence of pitting with foreign matter as a nucleus, as in the case of the above embodiment. However, the degree of fluctuation is larger in the comparative example, indicating that the corrosion resistance is weak. Corrosion occurring after 30 minutes of immersion corresponded to the occurrence of pitting corrosion without nuclei. As described above, the metal film manufactured by using the present invention has high corrosion resistance and is useful as a protective film.

Next, the relationship between the N ₂ concentration in the discharge gas at the time of film formation and the thermal conductivity of the film was examined. In general, the thermal conductivity can be approximated to be inversely proportional to the specific resistance of the film. Therefore, here, the relative change in the thermal conductivity was determined from the change in the specific resistance. FIG. 7 shows the above measurement results. According to this figure, the specific resistance sharply increases by slightly including N ₂ in the discharge gas. That is, it was found that the thermal conductivity decreased as the N ₂ concentration increased. This tendency is the same when using any material of Al, Cr and Cu. From this, the thermal conductivity of the film can be controlled by controlling the N ₂ concentration. Therefore, when the film is used as the fourth metal film 5 (FIG. 1) of the disk,
Since the recording / erasing characteristics of the disk can be arbitrarily changed, it is easy to achieve consistency with the disk drive.

〔The invention's effect〕

According to the present invention, by including nitrogen in the metal film, the resistance to wet corrosion and oxidation is improved, and the corrosion resistance to local corrosion typified by pitting is significantly improved. Significantly improved. The effect is
It is sufficient that a nitrogen concentration of a certain amount or more is secured, and the discharge gas concentration is about 5% or more.

Further, by controlling the nitrogen concentration, the thermal conductivity of the metal film can be changed.
The erasing sensitivity can be arbitrarily selected, and it is easy to achieve consistency with the disk drive.

Further, by containing nitrogen, the internal stress of the film can be reduced, so that cracks and peeling of the film due to the stress can be suppressed.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of an optical disk manufactured in an embodiment of the present invention, FIG. 2 is a measurement diagram of the change over time in the defect rate indicating the corrosion resistance of the disk of the embodiment of the present invention, and FIG. FIG. 4 is a characteristic diagram showing the recording laser output dependency of the reproduction output of the optical disk according to the embodiment, FIG. 4 is a measurement diagram showing the nitrogen concentration dependency of the internal stress, FIG. 5 is a schematic diagram of an immersion potential measuring device, FIG. 6 is a measurement diagram of the change with time in the immersion potential, and FIG. 7 is a measurement diagram of the nitrogen concentration dependence of the specific resistance. DESCRIPTION OF SYMBOLS 1 ... board | substrate, 2 ... 1st dielectric film, 3 ... information recording film, 4 ... 2nd dielectric film, 5 ... metal film.

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Norio Ota 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Inside the Hitachi, Ltd. Central Research Laboratory (56) References JP-A-63-69049 (JP, A) JP-A-62 -180538 (JP, A) JP-A-64-53360 (JP, A)

Claims (3)

(57) [Claims] 1. An optical disc on which recording, reproduction, or erasing is performed using a laser beam, a first dielectric film on a substrate,
An information recording film on the first dielectric film, a second dielectric film made of a nitride film on the information recording film, and a metal reflection film on the second dielectric film; An optical disc characterized by being mixed with nitrogen. 2. An optical disk on which recording, reproduction, or erasure is performed using a laser beam, wherein a first dielectric film is formed on a substrate.
An information recording film on the first dielectric film, a second dielectric film made of a nitride film on the information recording film, and a metal reflection film on the second dielectric film; Is a metal material layer formed by mixing nitrogen, wherein the metal material layer contains at least one element selected from Al, Cu, and Cr. 3. An optical disk on which recording, reproduction, or erasing is performed using a laser beam, wherein a first dielectric film is formed on a substrate.
An information recording film on the first dielectric film, a second dielectric film made of a nitride film on the information recording film, and a metal reflection film on the second dielectric film; Is a metal material layer obtained by mixing nitrogen, the metal material layer includes at least one element selected from Al, Cu, and Cr, and an element other than the element and Al, Cu, Cr, Nb, An optical disc containing at least one element selected from Ti, Ta, Ni, and Zr.