JPH064905A - Optical information recording medium - Google Patents

Abstract

PURPOSE:To increase the quantity of reflected light at the time of reproducing and to improve recording density by setting the reflectivity of the reflected film in the pits of the recessed parts of a base body higher than the reflectivity in the circumferential parts. CONSTITUTION:Ruggedness is formed on the substrate 7 and the recessed parts are formed as the recording pits 9. The reflected film 8 consisting of Al, etc., and an over coat 4 are laminated on the substrate 7. The reflected film 8 in the pits 9 is formed thicker than in the circumferential parts so that the reflectivity of the reflected film is increased in these parts. as a result, pit information is well reproduced even where the spot light for reproducing is made small at the time of reproducing the pits by the reproducing light. The optical information recording medium which can be increased in the recording density and is exclusively used for neproduction is thus obtd.

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, particularly an optical disc, which can reproduce previously recorded information by using a laser beam or the like.

[0002]

2. Description of the Related Art A technique for converging a laser beam by a lens system to form a small light spot whose diameter is on the order of the wavelength of the light and reproducing a minute signal pit provided on a substrate is known. It has been put to practical use as a dedicated optical disc, for example, a compact disc (CD) or a laser video disc (LVD).

In a read-only type optical disc, a signal track having small concave and convex pits is provided in advance on a transparent disc substrate made of resin or the like, and a reflective film such as Al is provided thereon.

When reproducing a signal, the signal track is scanned with a light spot formed by condensing a laser beam. The laser light is reflected as it is in the place where there is no pit and reaches the photodetector, but in the place where there is a pit, the laser light is diffracted by the pit, so it hardly comes to the photodetector. Therefore, the presence or absence of the signal pit can be detected in accordance with the magnitude of the reflected light amount.

[0005]

The limit of the recording density of a read-only type optical disc is determined by how small a signal pit can be detected with a light spot narrowed down to the wavelength limit. The size of the light spot is determined by the wavelength of the laser light used and the numerical aperture (NA) of the objective lens for narrowing the laser light. The shorter the wavelength of the laser light and the larger the aperture ratio of the objective lens, the smaller the light spot can be focused.

Further, in order for the laser light to be sufficiently diffracted by the recording pit, it is necessary that the amount of reflected light from the recording pit and the amount of reflected light from its surroundings be substantially equal.
This determines the minimum size of the recording pit.

Therefore, the recording density can be improved by developing a short wavelength laser and increasing the numerical aperture of the objective lens, but these require a drastic technological innovation and cannot be easily realized. There is.

[0008]

Means for Solving the Problems An optical information recording medium in which uneven pits are formed on a base material and a reflective film is placed on the pits to preliminarily record a signal, the reflectance inside the pits is determined by the surroundings. Higher than the reflectance of.

[0009]

With the above structure, the amount of light reflected from the recording pit and the amount of light reflected from the surroundings can be made substantially equal with a recording pit smaller than the conventional one. That is,
Even if the size of the light spot is the same, it is possible to reproduce a recording signal formed with smaller pits than the conventional one, and therefore the recording density can be improved.

[0010]

FIG. 2 shows an example of the structure of a conventional read-only recording medium. (A) is sectional drawing, (b) is a top view.
A signal is written as concave and convex recording pits 2 on a substrate 1, a reflective film 3 of Al or the like is formed thereon, and an overcoat 4 for protecting a signal recording surface is provided.

Reproduction of a signal is performed by scanning a recording pit with a laser spot 6 in which a laser beam 5 is narrowed down to a wavelength limit. That is, in the area where there is no recording pit, the reflected light from the recording medium returns directly to the photodetector, but when the recording pit exists, the reflected light from the recording pit and the reflected light from the periphery of the recording pit have different phases. Therefore, the reflected light from the recording pit is diffracted and does not return to the photodetector. That is, since the amount of reflected light reaching the photodetector differs depending on the presence or absence of the recording pit, the recording signal can be detected.

The diffraction effect of the reflected light by the recording pit largely depends on the depth and shape of the recording pit.

When the depth of the recording pit is changed, the phase difference of the reflected light changes depending on the presence or absence of the recording pit. When the wavelength of the laser light is λ, the phase difference of the reflected light that gives the greatest diffraction effect is λ / 2, and therefore the pit depth is λ / 4. For example, the laser wavelength is 780 nm
In this case, considering the refractive index of the substrate (for example, 1.58), the pit depth is 780 nm ÷ 1.58 ÷ 4 = 123 nm, and the largest amplitude is obtained at this time. However, in an actual optical disc, the laser spot follows the recording pit string, and in consideration of the tracking performance, about 10
The depth is near 0 nm.

The size of the recording pit is determined by the maximum diffraction effect obtained when the amount of reflected light from the recording pit and the amount of reflected light from its surroundings are substantially the same.

The present invention has been made paying attention to this point, and achieves higher density by increasing the reflectance inside the recording pits higher than the reflectance around the recording pits.

That is, by increasing the reflectance inside the recording pits higher than the reflectance around the recording pits, the amount of reflected light from the recording pits and the amount of reflected light from the surroundings can be made substantially the same with a recording pit smaller than before. That is, even if the laser spot diameter is the same, a recording mark smaller than the conventional one can obtain the same signal amplitude as the conventional one, and as a result, it is possible to reproduce the signal from the medium recorded with higher density.

The present invention will be described in detail below with reference to the specific drawings. (Embodiment 1) FIG. 1 is a sectional view showing an example of the structure of an optical disk according to the present invention. The substrate 7 has a diameter of 120 mm and a thickness of 1.
2mm polycarbonate substrate with a depth of about 1 on the surface
A recording pit 9 for a signal of 10 nm is formed, and further Au
A reflective film 8 made of is provided. The feature of this disk is that the film thickness of the reflective film inside the recording pit is thicker than the film thickness around it.

FIG. 3 shows A at a wavelength of 780 nm when an Au thin film is formed on a planar polycarbonate substrate.
The relationship between the film thickness of u and reflectance is shown. The reflectance monotonically increases as the film thickness of Au increases.

In this embodiment, the film thickness inside the recording pit is set to 45 in order to increase the reflectance inside the recording pit from the surrounding area.
nm (reflectance about 90%), the film thickness around the recording pit is 13
nm (reflectance about 45%).

In order to examine the signal reproduction characteristics of such an optical disc, the pitch of the recording pits is set to 2 μm:
Optical discs having different recording pit lengths were manufactured, and the relationship between the recording pit length and the reproduction signal amplitude was investigated. Here, in order to make a comparative study with the conventional example, an optical disk having an Au reflective film with a uniform film thickness of 45 nm was also manufactured and the reproduction characteristics were obtained. The laser wavelength of the signal reproducing device was 780 nm, and the relative speed of the laser beam and the optical disk was constant at 5 m / s.

FIG. 4 shows the dependence of the reproduced signal amplitude on the recording pit length. The reproduction signal amplitude was standardized by the value when the recording pit length was 1 μm. In the conventional structure, the reproduction signal amplitude sharply decreases as the recording pit length becomes shorter, but it can be seen that the optical disk according to the present invention can obtain a large reproduction signal amplitude even with smaller recording pits.

That is, according to the present invention, a reproduction signal equivalent to that of the conventional example can be obtained even with a signal recorded with a recording pit smaller than the conventional one, so that it is possible to reproduce a signal recorded at a higher density than the conventional one.

In this embodiment, Au is used as the reflective film material.
However, metals such as Al, Ti, Ni, Cr and Ta or alloys thereof may be adopted.

Further, in this embodiment, in order to increase the reflectance inside the recording pits higher than the reflectance around the recording pits, the reflection film thickness inside the recording pits is made thick. It can also be realized by adopting different reflective film materials in and around it. For example,
The inside of the recording pit is Au: 50 nm (reflectance of about 93
%), And the surrounding area is NiCr: 50 nm (reflectance about: 55
%), The same effect can be obtained.

(Embodiment 2) FIG. 5 shows a recording medium having another structure according to the present invention. The substrate 10 is a flat glass substrate having a diameter of 120 mm and a thickness of 1.2 mm. ZnS on it
After forming the dielectric film 11 of, the recording pit 1 corresponding to the signal
No. 2 was prepared by forming a recess in the ZnS film. Zn
The formation of the recesses on the S film is performed by first applying a photoresist on the ZnS film, exposing the ZnS film with a laser beam modulated by a recording signal, and then removing the photoresist in that portion and further removing ZnS by dry etching. Using. continue,
Au was deposited thereon with a uniform film thickness to form a reflective film. The film thickness was 56 nm for ZnS and 14 nm for Au. In this medium, at the wavelength of 780 nm, the reflectance inside the recording pit is about 50%, the reflectance around the recording pit is about 32%, and the phase difference between the reflected light inside and around the recording pit is about λ.
/ 2.

The same experiment as in Example 1 was conducted to examine the signal reproduction characteristics of this optical disk. That is, the interval between recording pits was kept constant at 2 μm, and an optical disc in which the length of the recording pit was changed was produced, and the relationship between the length of the recording pit and the reproduction signal amplitude was investigated. The laser wavelength of the signal reproducing device is 780 nm, the relative speed of the laser beam and the optical disk is 5
It was constant at m / s.

FIG. 6 shows the recording pit length dependence of the reproduced signal amplitude. The reproduction signal amplitude was standardized by the value when the recording pit length was 1 μm. Also in this case, as in the case of the first embodiment,
It can be seen that a large reproduction signal amplitude can be obtained even with a small recording pit.

That is, even with the optical disc having such a structure, a large reproduction signal can be obtained even with a signal recorded in a small recording pit, and therefore it can be seen that the optical disc can have a high density.

In this embodiment, when the pits are formed in ZnS, the ZnS is etched to the substrate side, but the structure may be such that the etching is finished before the substrate reaches and the ZnS remains in the pits.

[0030]

According to the optical information recording medium of the present invention, even if the reproducing light spot size is the same, it is possible to reproduce a signal recorded with a recording pit smaller than the conventional one, and therefore the recording density is improved. Can be achieved.

That is, according to the present invention, the recording density of the read-only recording medium put to practical use as a CD or LVD is increased, and it becomes possible to record for a longer period of time than before.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a configuration of a conventional example.

FIG. 3 is a characteristic diagram showing a relationship between Au film thickness and reflectance.

FIG. 4 is a characteristic diagram showing recording pit length dependency of reproduced signal amplitude in the configuration of the first embodiment.

FIG. 5 is a schematic diagram showing a second configuration according to the present invention.

FIG. 6 is a characteristic diagram showing recording pit length dependency of reproduced signal amplitude in the configuration of the second embodiment.

[Explanation of symbols]

 1, 7, 10 Substrate 2, 9 Recording pit 3, 8, 12 Reflective film 4 Overcoat 5 Laser light 6 Laser spot 11 Dielectric film

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noboru Yamada 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Nobuo Akabira, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (4)

[Claims] 1. An optical information recording medium used in a reproducing apparatus using a reproducing laser beam having a predetermined frequency, wherein signals are formed by forming concave and convex pits on a base material and installing a reflective film on the pits in advance. An optical information recording medium which is recorded and has a reflectance inside the pit higher than that around the pit at the predetermined frequency. 2. The optical information recording medium according to claim 1, wherein the reflection film thickness inside the pit is thicker than the reflection film thickness around the pit. 3. An optical information recording medium used in a reproducing apparatus using a reproducing laser beam having a predetermined frequency, wherein a signal is formed by forming uneven pits on a base material, and then reflecting signals of different materials inside and around the pits. An optical information recording medium, wherein a film is provided and pre-recorded, and the reflectance inside the pit is higher than the reflectance around the pit at the predetermined frequency. 4. An optical information recording medium used in a reproducing apparatus using a reproducing laser beam of a predetermined frequency, wherein a signal forms uneven pits on a base material with a substance having a refractive index different from that of the substrate, An optical information recording medium, which is prerecorded with a reflection film provided thereon, and in which the reflectance inside the pit is higher than the surrounding reflectance at the predetermined frequency.