JP2815977B2 - Protective film for optical recording media - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protective film in an optical recording medium that records and erases information by irradiating a laser beam and causing an optical change in an irradiated portion. is there.

(Prior Art) Recently, a rewritable optical disk that records and erases information by irradiating a thin film medium on a substrate with a condensed laser beam to cause an optical change in the thin film has been developed. Is attracting attention as a new technology that makes it possible. This rewritable optical disk includes a magneto-optical disk and a phase-change optical disk. The magneto-optical disk uses the magnetic Kerr effect to record and erase information by rotating the plane of polarization of laser light.

On the other hand, a phase-change optical disk performs recording by heating and quenching a thin-film optical recording medium to a melting point or higher by laser light, thereby making the laser-irradiated portion amorphous and recording the amorphous portion with laser light. Heating to a temperature higher than the crystallization temperature and annealing to return to a crystalline state and perform erasure.

A transparent dielectric protection film is added to the front and back surfaces of the thin film recording layer of the phase change optical disk in order to prevent the deformation of the recording layer and to provide an optical enhancement effect. Also recently,
In many cases, a metal reflective layer is added to the uppermost layer in order to further enhance the enhanced effect. As a material of the protective film, SiO ₂ , ZnS, SiN, etc. is conventionally used, and usually,
It is produced by a vapor deposition method or a sputtering method.

The development of the characteristics of such a phase change optical disk has been focused on improving the characteristics of the recording film itself, such as improvement of the erasing speed and improvement of the data storage life. recently,
A recording film capable of high-speed erasing of 100 ns or less and having a data storage life of 10 years or more has been obtained, and so-called one-beam overwriting has also been realized. Therefore, the emphasis on development is shifting to improvement of recording sensitivity and repetition durability, and optimization of a medium configuration method of a phase change optical disk, particularly, optimization of a dielectric protection film is becoming a major issue. And
Despite the fact that the recording sensitivity and repetition characteristics strongly depend on the material of the dielectric protective film, no firm guideline has been obtained regarding the effect of the difference in the material of the protective film and the manufacturing method on the characteristics of the optical disk. It is the current situation.

Needless to say, a magneto-optical disk also requires a dielectric protection film in order to protect the recording film and enhance the Kerr effect.

The drawbacks of the protective film of the conventional phase change optical disk are as follows.

If the dielectric protective film is formed by sputtering or vapor deposition, the film deposition rate is low, so that the deposition time is long and the rate of optical disk production is determined.

If and residual film stress of the protective film is large, when the protective film and the recording film, the recording film and adhesion to the substrate is weak, when repeatedly recording and erasing, induces irreversible deformation at about 10 ⁴ times In many cases, the characteristics deteriorated.

In order to improve the recording / erasing repetition characteristics, for example, a literature (Ohta et al .; Jpn. J. Appl. Phys. 28, (1989), Suppl.
28-3, pp. 123-128), when a medium having a quenching structure is used, the recording sensitivity is reduced, and the recording power is insufficient during high-speed rotation.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned drawbacks of the protective film of an optical disk, and is intended to protect a high-sensitivity optical recording medium capable of preventing deterioration of the disk performance due to repeated recording / erasing. It is to provide a membrane.

(Means for Solving the Problems) In order to achieve the above-mentioned problems, the present inventors aimed at a phase change optical disk, and applied for Japanese Patent Application Nos. 1-201417 and 1-20.
In 2918, we devised a method for fabricating a protective film by chemical vapor deposition (CVD) using electron cyclotron resonance (ECR) plasma, and realized that a phase-change optical disk with excellent repetition characteristics and recording sensitivity could be realized. Indicated.

That is, in the ECR plasma, the conventional RF plasma (13.56
MHz), which has a feature that a high activity (decomposition / excitation) and high ionization rate plasma can be stably generated at a low gas pressure of about 10 ^-4 Torr due to the presence of a large amount of high-energy electrons that move in a circular motion compared to that of doing.

Accordingly, the reaction gas is efficiently decomposed, so that high-speed deposition is possible (high-speed deposition of a protective film for an optical disk). A good thin film with strong adhesiveness can be formed without heating (a low-temperature process that can be applied to PC substrates, and a protective film for optical discs with a good protective effect can be realized), unlike the sputtering method Since the gas is dissociated by plasma and a thin film is formed by gas phase reaction, the reaction gas flow rate,
Film quality greatly depends on manufacturing conditions such as reaction gas ratio and input power. For example, film stress can be controlled in a wide range from compressive force → stress free → tensile stress (realization of protective film with high repetitive durability), SiH _{(4) Since} H atoms generated by the dissociation of the gas are mixed into the obtained thin film, the thin film contains hydrogen, and the thermal conductivity of the film can be changed by the difference in the hydrogen content (according to the hydrogen content). (Improvement of recording sensitivity utilizing thermal conductivity reduction).

Due to these effects, conventional sputtering and RF plasma
It is possible to provide an optimal thin film as a protective film for an optical disk that cannot be obtained by the CVD method.

The present invention proposes a method of forming an a-SiN: H film having a low internal conductivity by reducing the thermal conductivity by increasing the hydrogen content, and having a low internal stress.
This demonstrates that high recording sensitivity and high repetition characteristics can both be achieved using this thin film.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a view for explaining one embodiment of the present invention, and FIG.
The relationship between the hydrogen content of a SiN thin film formed by the R plasma CVD method and the recording sensitivity is shown. The medium configuration of the optical disk used was a PC (polycarbonate) grooved substrate / SIN undercoat layer / Sb-Te based recording layer / SiN overcoat layer / Au metal reflection layer / epoxy resin layer for sealing. The recording layer and the metal reflection layer were formed by RF sputtering, and the film thickness of both was 40
nm. The sealing epoxy resin layer is formed by spinner coating and has a thickness of about 10 μm. SiN undercoat layer is 100nm, SiN overcoat layer is 200nm, ECR
It was produced by a plasma CVD method. Protective film (SiN
The hydrogen content of the undercoat, overcoat) was fixed at 500 W microwave power and the reaction gas ratio was SiN ₄ : N ₂
The control was performed by changing the total flow rate of the reaction gas while keeping the ratio constant at = 1: 3. Table 1 shows the relationship between the production conditions and the hydrogen content.

FIG. 1 shows that the recording sensitivity is improved as the hydrogen content is increased. Hydrogen content 3.20 × 10 ²² H atoms / cm
^When the protective film of No. 3 was used, 11 mW was obtained as a value which can obtain a C / N value of 52 dB or more under a recording condition of a linear velocity of 10 m / s and 5 MHz (50 ns). The recording sensitivity of a conventional phase change optical disk (sputtered SiN protective film not containing hydrogen in Table 1) was about 17 under the same recording conditions.
Since it is で あ 18 mW, it is understood that a medium having excellent recording sensitivity can be realized by the present invention.

FIG. 2 shows the repetitive recording / erasing characteristics of the phase change optical disk having a high recording sensitivity of 11 mW. The measurement was performed in an overwrite mode with a linear velocity of 10 m / s and recording conditions of 5 MHz (50 ns) -11 mw / 5.5 mw (recording power / base erase power). Each of the recording level, the erasing level, and the noise level is shown in dB from the top in the figure.

Than this, C even in repeated recording and erasing of more than 10 ⁶ times
It can be seen that the / N value is maintained at 50 dB or more and the erasure rate is maintained at 27 dB or more. That is, hydrogen-containing SiN by ECR plasma CVD
It has been demonstrated that excellent repetition characteristics and high recording sensitivity can be simultaneously secured by using a protective film. The high repetition characteristics show that the protective film formed by the ECR plasma CVD method has a strong adhesive force and a small stress, so it is unlikely to be deformed even when the heat cycle of recording and erasing is repeated,
This is because the effect of protecting the recording film is large. In fact, the stress of the high hydrogen content SiN film used in this example was 1.09 × 10 ⁹ dyn / cm ²
The adhesive strength was as good as about 80 kg / cm ² .

Example 2 SiN dehydrogenated 2.08 × 10 ²² H atoms / c as protective film material
Using m ³ , the same repeated evaluation as in Example 1 was performed.
The medium configuration is the same as in the first embodiment. Measurement is at a linear velocity of 10m / s,
Recording was performed in an overwrite mode of 5 MHz (50 ns) -14 mw / 7 mw. The result is shown in FIG.

From this, it can be seen that the initial properties are maintained even at 10 ⁶ or more times the recording after erasing. That is, since the hydrogen content is small as shown in Example 1, recording sensitivity is slightly lowered, but recording is possible at 15 mw and repetition characteristics are good. Also the protective film formed under the conditions shown in Table 1 other than the above, recording and erasing Again characteristics were investigated, C / N value of 50 for both 10 ⁶ repetitions
The erasure rate was maintained at 27 dB or more, and good repetition durability was exhibited. This is a low residual stress and high adhesion thin film (stress; tensile 8 ×) formed by ECR plasma CVD.
By using 10 ⁸ to 1.5 × 10 ⁹ dyn / cm ² and an adhesive force of 80 kg / cm ² or more, it is demonstrated that excellent repetition characteristics can be secured.

Regarding the composition of the protective film, a film composition in which Si is richer than Si _0.6 N _0.4 loses the transparency of the film and has a wavelength of 830 nm.
And thus became unsuitable as a protective film for an optical disk. Further, those having a film composition in which N was richer than Si _0.4 N _0.6 had weak film quality and could not be used as a protective film.

When the hydrogen content is less than 1 × 10 ²² atoms / cm ³ , the recording sensitivity is 17 to 18 nm, and the Si
The effect of improving the recording sensitivity of the N protective film is small. Further, when the hydrogen content is larger than 4 × 10 ²² atoms / cm ³ , the thermal stability of the protective film and the adhesive force of the film are deteriorated, and the recording / erasing repetition characteristics tend to be reduced to 10 ⁴ to 10 ⁵ times. there were.

The film stress, compressive stress 3 × 10 ⁹ dyn / cm ² or more, and a tensile stress 1 × when using 10 dyn / cm ² or more protective films, easy peel and local stress release of the protective film occurs, recording -The erase repetition characteristic was as low as about 10 ⁴ to 10 ⁵ times.

Regarding the adhesion to the substrate, the number of recording / erasing repetitions of an optical disk using a protective film having an adhesion of 50 kg / cm ² or less was as low as 10 ⁴ to 10 ⁵ times.

When a sputtered SiN protective film was used, since it did not contain hydrogen, the recording sensitivity was poor at 18 nm, the stress and adhesion were inferior to those of the ERC-SiN film, and the repetition characteristics were 10%.
Only about ^four times.

As described above, specific results have been shown regarding the simultaneous securing of high repetition durability and high recording sensitivity by the protective film formed by the ECR plasma CVD method. The cause of the improvement in repeated durability is
The use of a thin film with low residual stress means that there is almost no release of thermal stress due to repeated recording / erasing, irreversible deformation is suppressed, and the adhesion and tightness of the thin film are improved by moderate ion bombardment. It is considered that this contributes to the improvement of the mechanical strength of the film itself and the improvement of the adhesion at the interface between the protective layer and the recording film.

It is considered that the improvement in the recording sensitivity is due to the fact that the thermal conductivity decreases due to hydrogen in the thin film, and the input laser power efficiently contributes to the temperature rise of the recording film. To further state,
The main point of the present invention is to show that the two characteristics are not contradictory but compatible.

In this example, an example in which the protective film was applied as a protective film for a phase-change high recording medium was shown. However, a protective film for a photomagnetic medium or a photon mode optical recording medium using a photochromic compound was also formed by an ECR plasma CVD method. It is clear that the protective film has the effect of improving the repetition durability and the weather resistance due to the good adhesion and the tightness, and at the same time improves the recording sensitivity by the effect of the hydrogen content.

(Effects of the Invention) As described above, the protective film for an optical recording medium of the present invention
The hydrogen-containing SiN protective films provided on both sides of the thin-film recording layer are fabricated by using the ECR plasma CVD method, and by using a protective film with low residual film stress, high adhesion, and high density, recording and erasing can be performed. The effect of preventing deterioration of disk performance due to repetition, achieving high sensitivity by controlling thermal conductivity by containing hydrogen, etc. is obtained, and the effect of contributing to high performance of phase change optical disk media is very large. .

In addition, since the recording sensitivity can be controlled by changing the hydrogen content of the protective film, the protective film can be used not only for high-speed media with a linear velocity of 30 m / s or more, but also for specifications in the medium and low linear velocity regions. Can be realized without changing the material system.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the hydrogen content and the recording sensitivity of a SiN protective film produced by the ECR plasma CVD method, and FIG. 2 is a graph showing the overwrite repetition characteristics (hydrogen content 3.20 × 10 3) when using the SiN protective film. ²² H atoms / cm ² ; recording sensitivity
11mw), and FIG. 3 shows overwrite repetition characteristics (hydrogen content 2.08 × 10 ²² H atoms / cm ² ; recording sensitivity when using a SiN protective film).
14mw).

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Susumu Fujimori 1-6-1, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-63-162048 (JP, A) JP-A Sho 62-54855 (JP, A) JP-A-2-123533 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 7/24

Claims (2)

(57) [Claims] 1. An electron suncrotron resonance (ECR) plasma C
In a protective film for an optical recording medium in which protective films are formed on both sides of a thin film recording layer of an optical recording medium using a VD method, the material of the protective surface is from Si _x N _1-x : (0.4 ≦ X ≦ 0.6). A protective film for an optical recording medium, characterized by comprising hydrogen in the range of 1 × 10 ²² to 4 × 10 ²² atoms / cm ³ . 2. A protective film for an optical recording medium according to claim 1, wherein the residual film stress of said protective film is 3 × compression.
An optical recording medium having a range of 10 ⁹ dyn / cm ² to a stress of 0 to a tensile force of 1 × 10 ⁹ dyn / cm ² and an adhesive force of the protective layer to the polycarbonate substrate of 50 kg / cm ² or more. Protective film.