Optical recording medium with a substrate of two different resin layers

What is claimed is:

1. An optical recording medium including a substrate, and a reflective layer, a recording layer and a transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal, the substrate comprising: a first resin layer forming a surface of the substrate on which the reflective layer is formed; a second resin layer formed on the first resin layer having a higher flexural modulus than the first resin layer, a first dielectric layer is located between the reflective layer and the recording layer, and a second dielectric layer is located between the recording layer and the transparent layer, to control the optical and thermal characteristics of the optical disc.

2. The optical disc according to claim 1, wherein the substrate further comprises: a third resin layer formed on a surface opposing to the surface on which the first resin layer is formed and made from the same resin as that of the first resin layer.

3. The optical disc according to claim 1, wherein the flexural modulus of the resin of the second resin layer is 29,000 kgf/cm.sup.2 or more.

4. The optical disc according to claim 1, wherein the resin of the second resin layer has a higher hardness and a higher rigidity than the first resin layer.

5. The optical disc according to claim 1, wherein the resin of the second resin layer comprises a resin which additionally contains a suitable amount of at least one of a heterogeneous resin, fibers, fillers, talc powder, mica powder, and carbon fillers, added to provide a flexural modulus of at least 29,000 kgf/cm.sup.2.

6. An optical disc, comprising: a substrate, said substrate including a core layer and a skin layer, said skin layer being located on said core layer; a reflective layer; a recording layer; and a transparent layer; wherein a first dielectric layer is located between the reflective layer and the recording layer, and a second dielectric layer is located between the recording layer and the transparent layer, to control the optical and thermal characteristics of the optical disc.

7. The optical disc according to claim 6, wherein the resin of said core layer has a higher hardness and a higher rigidity than the skin layer.

8. The optical disc according to claim 6, wherein the core layer comprises a resin having at least one of heterogeneous resins, fibers, fillers, talc powder, mica powder, and carbon fillers, added to provide a flexural modulus of at least 29,000 kgf/cm.sup.2.

9. An optical recording medium including a substrate, and a reflective layer, a recording layer and a transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal, the substrate comprising: a first resin layer forming a surface of the substrate on which the reflective layer is formed; a second resin layer formed on the first resin layer having a higher flexural modulus than the first resin layer; and a third resin layer formed on a surface opposing to the surface on which the first resin layer is formed and made from the same resin as that of the first resin layer.

10. An optical recording medium including a substrate, and a reflective layer, a recording layer and a transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal, the substrate comprising: a first resin layer forming a surface of the substrate on which the reflective layer is formed; and a second resin layer formed on the first resin layer having a higher flexural modulus than the first resin layer, wherein the flexural modulus of the resin of the second resin layer is 29,000 kgf/cm.sup.2 or more.

11. An optical recording medium including a substrate, and a reflective layer, a recording layer and a transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal, the substrate comprising: a first resin layer forming a surface of the substrate on which the reflective layer is formed; and a second resin layer formed on the first resin layer having a higher flexural modulus than the first resin layer, wherein the resin of the second resin layer comprises a resin which additionally contains a suitable amount of at least one of a heterogeneous resin, fibers, fillers, talc powder, mica powder, and carbon fillers, added to provide a flexural modulus of at least 29,000 kgf/cm.sup.2.

12. A method of manufacturing, by injection molding, an optical recording medium including a substrate, a recording layer and a light transparent layer, a light being incident from the light transparent layer to write and/or read an information signal, the substrate being formed by two-color molding steps of: forming a first resin layer at least on a side of the substrate on which the recording layer is formed; and forming a second resin layer on the first resin layer from a resin having a higher flexural modulus than the resin of the first resin layer.

13. The method according to claim 12, wherein the two-color molding is done in a same mold.

14. The method according to claim 12, wherein the first resin layer is formed and then the second resin layer is formed on the first resin layer.

15. The method according to claim 12, wherein the resin for the first resin layer injected into a mold at a slower speed than the resin for the second layer, whereby the resins for the first and second resin layers, respectively, are injected simultaneously into the same mold.

16. The method according to claim 12, wherein the resin for the first resin layer is injected into a mold and then the resin for the second resin layer is injected into the same mold while at least a part of the resin for the first resin layer is still in the slush state.

17. An optical disc, comprising: a substrate; a reflective layer; a recording layer; and a transparent layer; wherein said substrate includes a core layer and a skin layer, said skin layer formed on a side of the substrate on which the reflective layer is formed, said core layer formed on a side of the substrate opposite to the reflective layer and having a higher flexural modulus than the resin of the skin layer, wherein the resin of the core layer comprises a resin which additionally contains a suitable amount of at least one of heterogeneous resins, fibers, fillers, talc powder, mica powder, and carbon fillers, added to provide a flexural modulus of at least 29,000 kgf/cm.sup.2.

18. An optical disc, comprising: a substrate; a reflective layer; a recording layer; and a transparent layer; wherein said substrate includes a core layer and a skin layer, said skin layer formed on a side of the substrate on which the reflective layer is formed, said core layer formed on a side of the substrate opposite to the reflective layer and having a higher flexural modulus than the resin of the skin layer, wherein the resin for the core layer has a flexural modulus of 29,000 kgf/cm.sup.2 or more.

19. An optical disc, comprising: a substrate; a reflective layer; a recording layer; and a transparent layer; wherein said substrate includes a core layer and a skin layer, said skin layer formed on a side of the substrate on which the reflective layer is formed, said core layer formed on a side of the substrate opposite to the reflective layer and having a higher flexural modulus than the resin of the skin layer, wherein the resin for the core layer has a flexural modulus of 34,000 kgf/cm.sup.2 or more.

20. An optical disc, comprising: a substrate; a reflective layer; a recording layer; and a transparent layer; wherein said substrate includes a core layer and a first skin layer, said skin layer formed on a side of the substrate on which the reflective layer is formed, said core layer formed on a side of the substrate opposite to the reflective layer and having a higher flexural modulus than the resin of the skin layer a second skin layer formed on a surface of the substrate opposite to that on which the first skin layer is formed.

21. The optical disc according to claim 20, wherein the second skin layer is made from the same resin as that of the first skin layer.

22. An optical disc, comprising: a substrate; a reflective layer; a recording layer; and a transparent layer; wherein said substrate includes a core layer and a skin layer, said skin layer formed on a side of the substrate on which the reflective layer is formed, said core layer formed on a side of the substrate opposite to the reflective layer and having a higher flexural modulus than the resin of the skin layer, wherein a first dielectric layer is located between the reflective layer and the recording layer, and a second dielectric layer is located between the recording layer and the transparent layer, to control the optical and thermal characteristics of the optical disc.

23. The optical disc according to claim 22, wherein the resin of the core layer has a higher hardness and a higher rigidity than the skin layer.

24. The optical disc according to claim 22, wherein the resin of the core layer comprises a resin which additionally contains a suitable amount of at least one of heterogeneous resins, fibers, fillers, talc powder, mica powder, and carbon fillers, added to provide a flexural modulus of at least 29,000 kgf/cm.sup.2.

25. The optical disc according to claim 22, wherein the resin of the core layer has a higher hardness and a higher rigidity than the skin layer.

26. The optical disc according to claim 22, wherein the resin of the core layer comprises a resin which additionally contains a suitable amount of at least one of heterogeneous resins, fibers, fillers, talc powder, mica powder, and carbon fillers, added to provide a flexural modulus of at least 29,000 kgf/cm.sup.2.

27. An optical disc, comprising: a substrate; a reflective layer; a recording layer; and a transparent layer; said substrate includes a core layer and a skin layer, said skin layer formed on a side of the substrate on which the reflective layer is formed, said core layer formed on a side of the substrate opposite to the reflective layer and having a higher flexural modulus than the resin of the skin layer, wherein a first dielectric layer is located between the reflective layer and the recording layer, and a second dielectric layer is located between the recording layer and the transparent layer, to control the optical and thermal characteristics of the optical disc.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical recording medium comprising a substrate, and at least a recording layer and light-transparent layer formed on the substrate, and a method of manufacturing the optical recording medium. More particularly, the present invention concerns an optical recording medium having a multilayered substrate structure contributing to an improved strength and durability and also capable of recording data with a high density, and a method of manufacturing the optical recording medium.

2. Description of Related Art

Optical and magnetic recording media are widely used to record various kinds of information such as audio, video and other information. More particularly, they are generally classified into an optical disc in which information signals are previously written in the form of embossed pits, a phase-change optical disc in which information signals are written utilizing the phase change of its recording layer, a magnetooptic optic in which information signals are written utilizing the magneto-optical effect of its recording layer, and a magnetic disc in which information signals are magnetically written.

These disc-like recording media use a resin-made substrate in which phase pits, pre-grooves, and so forth are formed as data, tracking signals, and so forth in the form of delicate grooves and lands.

Conventionally, such a resin-made disc substrate is molded using an injection mold as shown in FIG. 1. FIG. 1 is a sectional view of the injection mold.

The injection mold is generally indicated with a reference 100. It consists essentially of a fixed mold 101a to form one of the main surfaces of a disc substrate, a movable mold 101b disposed opposite to the fixed mold 101a to form the other main surface of the disc substrate, and a mold 112 to form a periphery of the disc substrate. The fixed mold 101a and movable mold 101b are provided each with a stamper having formed thereon grooves and lands corresponding to a desired groove/land pattern indicative of information signals, and so forth.

The movable mold 101b is moved toward and away from the fixed mold 101a by a driving mechanism (not shown). When clamped together, the fixed mold 101a, movable mold 101b and peripheral mold 112 define together a cavity 111.

The fixed mold 101a has provided therein and positioned in the center of the cavity 111 a nozzle 114 to fill, by injection, the cavity 111 with a molten synthetic resin into the cavity.

In the injection mold 100 having the above configuration, the movable mold 101b is first moved toward the fixed mold 101a (clamping) by the driving mechanism not shown) to define the cavity 111. Next, the molten synthetic resin is filled into the cavity 111 by injection through the nozzle 114.

Then, the injected synthetic resin is cooled by a thermoregulator (not shown) to a slush state. The movable mold 101b has a first ejecting member 116 disposed therein. A punch 117 is thrust from the central hole of the ejecting member 116 toward the fixed mold 101a to make a hole which will be a center hole in a disc substrate. Thereafter, in the injection mold 100, the injected synthetic resin is hardened by cooling by the thermoregulator (not shown).

Then, in this injection mold 100, the movable mold 101b is moved away from the fixed mold 101a (mold opening) by the driving mechanism (not shown). Finally, the disc substrate formed in the cavity 111 is taken out by a stripping mechanism (not shown).

Thereafter, a recording layer, reflective layer, protective layer, and so forth for example, are formed on the resin-made disc substrate thus molded to produce an optical disc.

However, the above-mentioned injection molding used to produce an optical disc is disadvantageous in that at the step in which a molten resin is filled, by injection, in the injection mold, a change of the injecting pressure, change of the injecting temperature and a friction between the molten resin and mold will cause stresses in the molten resin in the cavity.

More particularly, such stresses will take place at the following steps. First, during injection of the molten resin into the injection mold 100, the molten resin injected in the cavity 111 will flow to cause a shear stress. Next, when the injection of the molten resin into the cavity 111 is complete, a screw (not shown) for injecting the resin abruptly stops moving and also the molten resin stops flowing abruptly. Thus their respective inertia will take place as stress. Also in the process up to gate sealing for injection of the molten resin, the molten resin is pressurized to prevent the molten resin from flowing and a sink from taking place due to a volumetric shrink of the molten resin. An uneven pressure distribution in the entire disc substrate will result in a stress. Especially when cooling to harden the molten resin, an uneven temperature distribution will take place in an outer portion 120a (see FIG. 2) of the molten resin in contact with the fixed mold 101a and movable mold 101b as well as in an inner portion 120b (also see FIG. 2) not in contact with the molds, thus causing a stress.

A part of such stresses will be partially relaxed in the process until the molten resin is cooled and hardened in the mold, but the majority will reside, not relaxed, as a residual stress in the molded disc substrate.

As a result, the molded disc substrate 120 will be subject to deformations such as partial warpage 121 and sink 122 and an uneven distribution of birefringence or double refraction as shown in FIG. 2.

The disc substrate molded from a resin by injection molding is unavoidably shrunk in the molding process, especially at the cooling step. More particularly, the shrinkage is different in the outer periphery from in the inner periphery of the disc substrate in many cases. The outer periphery of a disc substrate 130 warps, resulting in a dish-like deformation as shown in FIG. 3.

Thus, to minimize the deformation of the disc substrates 120 and 130, it is required for the conventional injection molding that mold clamping should be done with a reduced force and injection be made at a slower speed to reduce the packing rate, thereby reducing the pressure inside the resin. However, such measures taken in the conventional injection molding for manufacture of disc substrates are very troublesome but cannot practically attain any completion elimination of the deformation of the disc substrates 120 and 130.

Because of such deformation of the disc substrates 120 and 130, a predetermined land/groove pattern cannot be formed on the disc substrates 120 and 130 with a high accuracy but the disc substrate 120 incurs a poor stamping 123 of the land/groove pattern as shown in FIG. 4. As the result, the optical disc produced using such disc substrate 120 or 130 is disadvantageous in that its signal characteristic is not satisfactory.

The recording density of an optical disc depends upon a diameter of a laser spot focused on a recording layer of the optical disc. That is, the smaller the laser spot diameter, the higher the recording density is. The laser spot diameter is proportional to a product .lambda./NA (.lambda.: laser wavelength and NA: numerical aperture) of a reading/writing optical system. For an increased recording density in an optical disc, it is necessary that a laser having a shorter wavelength .lambda. and an objective lens having an increased numerical aperture NA should be used.

However, such an increased NA of the objective lens will raise a problem of coma aberration because the coma is proportional to ([skew angle].times.NA.sup.3.times.[optical disc thickness through which a laser light passes]). To cope with this coma problem, it has been proposed to reduce the thickness of the transparent substrate for optical disc.

Normally, however, there is a relationship that the strength of an optical disc is proportional to a cube of the thickness of the disc. A conventional optical disc including a substrate of which the thickness is reduced for an increased recording density is disadvantageous in that the mechanical properties such as bending strength, and so forth are inferior and a bimetallic deformation is very easily arisen due to a moisture absorption, and so forth in addition to the aforementioned stresses.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome the above drawbacks of the prior art by providing an optical recording medium having a sufficient strength and durability without deformations such as warpage, sink, and so forth and capable of recording data with a high density, and a method of manufacturing the optical recording medium.

According to the present invention, there is provided an optical recording medium including a substrate, and a recording layer and transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal, the substrate comprising: a first resin layer forming a surface of the substrate on which the recording layer is formed; and a second resin layer formed on the first resin layer from a resin having a higher flexural modulus than that of the resin of the first resin layer.

In the optical recording medium having the above-mentioned structure according to the present invention, the substrate is comprised of the first resin layer forming the surface of the substrate on which the recording layer is formed, and the second resin layer formed from the resin having the higher flexural modulus, namely, a higher rigidity, than the resin of the first resin layer. Therefore, the substrate has an increased strength to minimize the deformation of the substrate. As the result, the optical recording medium according to the present invention is enhanced in strength, thus hardly subject to deformations such as warpage, sink, and so forth and has an outstanding durability. Since in this optical recording medium according to the present invention, a laser light is incident from the transparent layer side, the structure of the substrate composed of the first and second resin layers will not adversely affect the optical writing and/or reading.

Further, the first resin layer forming the surface of the substrate on which the recording layer is formed is formed from the resin having the smaller rigidity, namely, a better fluidity, than the second resin layer. Therefore, the first resin layer has an excellent stampability, and thus the substrate has a superior stampability.

According to the present invention, there is provided a method of manufacturing, by injection molding, an optical recording medium including a substrate, and a recording layer and transparent layer formed in this order on the substrate, a light being incident from the transparent layer to write and/or read information signal.

In the optical recording medium manufacturing method according to the present invention, the substrate is formed by two-color molding steps of forming a first resin layer at least on a side of the substrate on which the recording layer is formed, and a second resin layer on the first resin layer from a resin having a higher flexural modulus than the resin of the first resin layer.

In the optical recording medium manufacturing method according to the present invention, the substrate is formed by forming, by the two-color molding, the first resin layer on the surface of the substrate on which the recording layer is formed, and the second resin layer formed from the resin having the higher flexural modulus, namely, a higher rigidity, than the resin of the first resin layer. Therefore, the substrate has an increased strength to minimize the deformation of the substrate. As the result, the method according to the present invention can be used to manufacture an optical recording medium enhanced in strength, thus hardly subject to deformations such as warpage, sink, and so forth and having an outstanding durability.

Further, in the method according to the present invention, the first resin layer forming the surface of the substrate on which the recording layer is formed is formed, from the resin having the smaller rigidity, namely, a better fluidity, than the second resin layer. Therefore, the first resin layer has an excellent stampability, and thus the substrate has a superior stampability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an injection mold used for forming a conventional disc substrate;

FIG. 2 is a sectional view, enlarged in scale, of an example of conventional injection mold;

FIG. 3 is a sectional view, enlarged in scale, of an example of conventional disc substrate;

FIG. 4 is a sectional view, enlarged in scale, of another example of conventional disc substrate;

FIG. 5 is a sectional view of an embodiment of optical disc according to the present invention;

FIG. 6 is a sectional view of another embodiment of optical disc according to the present invention;

FIG. 7 is a sectional view of an embodiment of injection molding machine;

FIG. 8 is a sectional view of a mold used to manufacture the optical disc shown in FIG. 5;

FIG. 9 is a sectional view of the mold, showing the mold geometry in the step of forming a skin layer in the process of manufacturing the substrate for the optical disc according to the present invention;

FIG. 10 is a sectional view of the mold, showing the mold geometry in the step of defining a cavity for the core layer by moving the molded skin layer along with a second movable mold;

FIG. 11 is a sectional view of the mold, showing the mold geometry in the step of forming a core layer in the process of manufacturing the substrate for the optical disc according to the present invention;

FIG. 12 is a sectional view of a substrate molded following the molding steps in FIGS. 9 to 11;

FIG. 13 is a sectional view, enlarged in scale, of a heating cylinder used in the injection molding machine to form the substrate for the optical disc according to the present invention;

FIG. 14 is a sectional view, enlarged in scale, of the mold, showing the mold geometry when the substrate for the optical disc according to the present invention is formed in the mold;

FIG. 15 is a sectional view of the substrate for the optical disc according to the present invention; and

FIG. 16 is a graph showing the results of physical property evaluation effected on embodiments 1 to 10 and an comparative example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It should be noted that the optical disc according to the present invention may be a read-only one having a reflective metal layer formed on a substrate on which pits corresponding to information are formed, the pits and reflective metal layer forming together a signal recording layer of the optical disc.

FIG. 5 is a sectional view of an embodiment of optical disc as an optical recording medium according to the present invention.

As shown in FIG. 5, the optical disc 1 comprises a substrate 2 having pre-grooves 2a formed thereon, a reflective layer 3, recording layer 4 and a transparent layer 5 formed in this order on the substrate 2.

The substrate 2 includes a core layer 6 and skin layer 7, formed from resins different in flexural modulus from each other, respectively.

The skin layer 7 is provided on a side of the substrate 2 on which the recording layer 4 is formed, namely, on the signal recording surface of the optical disc 1. It is formed on the core layer 6. The skin layer 7 has formed thereon phase pits for data information, tracking servo signal, and so forth and pre-grooves 2a having a predetermined pattern of delicate pits and lands formed therein. The skin layer 7 should preferably have a thickness of 100 .mu.m or more.

The skin layer 7 is formed from a material having a good fluidity. Therefore, the skin layer 7 has such an excellent stampability that the predetermined pattern of pits and lands can be formed with a high accuracy. More particularly, the skin layer 7 is formed from a synthetic resin such as polycarbonate or ZEONEX (thermoplastic olefin resin by Nihon Zeon).

The core layer 6 is formed on a side of the substrate 2 opposite to the signal recording surface and has the skin layer 7 disposed on the surface thereof. The resin of the core layer 6 has a higher flexural modulus than the resin of the skin layer 7. Therefore, the core layer 6 has a higher hardness, namely, a higher rigidity, than the skin layer 7.

The resin of the core layer 6 may be a highly rigid one prepared by adding to a commonly used resin a heterogeneous resin, fibers, fillers or the like to increase the hardness of the resin. More particularly, the resin for this core layer 6 should desirably be a one having a flexural modulus of 29,000 kgf/cm.sup.2 or more, and preferably of 34,000 kgf/cm.sup.2 or more. For example, the resin may be a one which additionally contain a talc powder, mica powder, carbon fibers or the like. The core layer 6 should have a thickness equal to 50% or more of that of the entire substrate, and preferably of 70%.

In the optical disc 1 of the present invention, the skin layer 7 and hard core layer 6 forming together the substrate 2 contribute to an increased strength of the substrate 2. Thus, the substrate 2 can be extremely restrained from being subjected to deformations such as warpage, sink, and so forth. Therefore, the optical disc 1 according to the present invention has an enhanced strength and is little subjected to deformations such as warpage, sink, and so forth. Thus, the optical disc 1 is oustandingly durable in strict conditions in the manufacturing process as well as in operating ambient conditions, and thus has a high reliability.

Also, since the skin layer 7 having an excellent stampability is used in the substrate 2, the optical disc 2 having the substrate 2 can have stamped thereon with a high accuracy a predetermined pattern of pits and lands corresponding to information signals, and so forth. Namely, the optical disc 1 has an excellent stampability and thus it is capable of recording information with a further increased density. It will thus have superior writing and reading characteristics.

Furthermore, because of the excellent durability, the optical disc 1 is little subjected to deformations caused by the conditions at manufacturing site and operating ambient conditions. Thus, it is not necessary to forming on a to-be-read surface other than the signal recording surface a translucent layer, moisture-proof sheet or the like through which light can pass, in order to prevent the optical disc from being deformed, as in the case of the conventional optical discs. Therefore, the optical disc 1 according to the present invention can be manufactured more easily and efficiently than the conventional optical discs, so that the manufacturing process can be simplified and the optical disc 1 can be produced with an improved yield percentage.

Also, the optical disc 1 according to the present invention is designed so that information signal is written and/or read with a laser light incident from the transparent layer 5. Thus, the optical write and/or read are not influenced by the double-layer structure of the substrate 2 consisting of the skin layer 7 and core layer 6. On the contrary, the conventional optical discs such as compact disc (CD), digital versatile or video disc (DVD), and so forth are designed so that an information signal is written and/or read with a laser light incident from a disc substrate which is transparent. Therefore, the disc substrate must be of a uniform optical structure and thus cannot be multilayered as in the case of the two-layered substrate 2 of the optical disc according to the present invention.

Thus, according to the present invention, the disc substrate 2 having a marble or mottle pattern can be produced by the two-color molding using different resins for the skin and core layers 7 and 6, respectively.

Note that the reflective layer 3 is formed on one main surface 6a of the substrate 2 on which the pre-grooves 2a are formed. The reflective layer 3 reflects a light having passed through the recording layer 4 while serving as a heat sink to prevent heat confined excessively in the recording layer 4.

The reflective layer 3 should desirably be formed from a metal element, semi-metal element, semiconductor element or their compound singly or in combination.

The reflective layer 3 should preferably be formed from a material containing Si in 0.4-0.8% by weight, Fe in 0.7% or less by weight, Cu in 0.15-0.40% by weight, Mn in 0.15% or less by weight, Mg in 0.8-1.2% by weight, Cr in 0.04-0.35% by weight, Zn in 0.25% or less by weight, Ti in 0.15% or less by weight and A1 as remainder (base material). When this material is used, the reflective layer 3 is formed to have a thickness of 50 to 200 nm.

The reason for the above content of the material is that a phase-change recording layer 4 formed on the reflective layer 3 will hardly be influenced by a crystalline structure of the reflective layer 3 and an interfacial shape defined by the material grain size of the reflective layer 3. Thus, the recording layer 4 will accurately reflect the surface shape of the substrate 2.

Further, the reflective layer 3 is formed from the above-mentioned material on the substrate 2 by ion beam sputtering, DC sputtering or RF sputtering, of which the ion beam sputtering method should preferably be adopted.

The recording layer 4 is an optical recording layer to or from which an information signal can be written or erased by irradiation of a laser light. The recording layer is a layer of a phase-change material showing a reversible change between a crystalline phase and an amorphous phase, a magneto-optically recording layer which loses the coercive force when its temperature rises above the Curie temperature and shows a reversal of magnetization in the direction of external magnetic field, or the like.

The phase-change layer is formed from a simple calcogen or a calcogen compound. More particularly, the simple calcogen is Te or Se, and the calcogen compound is a calcogenite material such as Ge--Sb--Te, Ge--Te, In--Sb--Te, In--Se--Te--Ag, In--Se, In--Se--TI--Co, In--Sb--Se, Bi.sub.2 Te.sub.3,BiSe, Sb.sub.2 Se.sub.3 or Sb.sub.2 Te.sub.3.

The magneto-optically recording layer is formed from a vertically magnetized film having a magnetooptic characteristic such as Kerr Effect, or Faraday Effect, including an amorphous alloy film of Tb--Fe--Co or the like.

The transparent layer 5 is formed on the recording layer 4. For writing and/or reading information signal or the like into the optical disc 1, a laser light is irradiated onto the transparent layer 5. The laser light having passed through this transparent layer 5 is incident upon the recording layer 4 to write and/or read the information signal. Also the transparent layer 5 protects the reflective layer 3 and recording layer 4 from external shock, and also serves to prevent the layers 3 and 4 from getting into contact with an corrosive factor such as moisture or the like.

The transparent layer 5 is formed from a material through which the laser light can be transmitted. For example, the transparent layer 5 is formed by applying an ultraviolet-curable resin to the recording layer 4 by spin-coating or other method and irradiating ultraviolet rays to the resin applied to the recording layer 4. It should be appreciated that the transparent layer 5 may be formed by bonding to the recording layer 4, with a transparent adhesive such as ultraviolet-curable resin or the like, a transparent sheet of glass or an acrylic resin, polyolefinic resin or the like, for example.

The transparent layer 5 has a limited thickness of 0.3 mm or less because this layer thickness can considerably suppress a coma aberration taking place due to a tilt of the optical even if the numerical aperture (NA) of an objective lens included in the writing and/reading optical system is increased up to 0.8.

The basic configuration of the optical disc 1 according to the present invention has been described in the foregoing. It will be appreciated, however, that in addition to the layers included in the optical disc 1, dielectric layers may be provided to control the optical and thermal characteristics of the optical disc 1. In this case, the reflective layer 3, first dielectric layer, recording layer 4, second dielectric layer, and the transparent layer 5 are formed in this order on the substrate 2. In the optical disc of this construction, a cross protection between the recording layer 4, reflective layer 3 and dielectric layers will control the optical characteristics such as reflectivity, and especially the reflective layer 3 and second dielectric layer will control the thermal characteristic.

In the foregoing, an optical disc having the two-layered substrate 2 (will be referred to as "two-layered optical disc" hereinunder) has been described as an embodiment of the optical disc according to the present invention. However, it should be noted that the present invention can provide an optical disc having a substrate consisting of two skin layers provided at opposite sides of the substrate and a core layer provided between the skin layers as shown in FIG. 6. FIG. 6 is a sectional view showing another embodiment of the optical disc according to the present invention.

An optical disc according to the second embodiment of the present invention is generally indicated with a reference 10 in FIG. 6. It is composed of a substrate 11 and a reflective layer 12, recording layer 13 and a transparent later 14 formed in this order on a substrate 11.

The substrate 11 is comprised of a core layer 15 formed from the aforementioned material, and skin layers 16 and 17 formed on the opposing sides of the core layer 15.

It should be noted that the reflective layer 12, recording layer 13 and transparent layer 14 in this optical disc 10 are similar to those in the optical disc 1 having been described with reference to FIG. 5.

The optical disc 10 has the substrate 11 consisting of the highly hard core layer 15 and the skin layers 16 and 17 having an excellent stampability and provided on the opposite sides of the core layer 15 (this optical disc will be referred to as "sandwich type optical disc" hereinunder). The increased strength of the substrate 11 and the same nature of the skin layers 16 and 17 provided on the opposite sides of the substrate 11 contribute to more effective prevention of the substrate 11 from being warped and sunk as well as to a high reliability of the optical disc 10.

Next, the method of manufacturing the two-layered optical disc shown in FIG. 5 will be described. An injection molding machine used for manufacture of the substrate of the two-layered optical disc shown in FIG. 5 will be described with reference to FIG. 7. FIG. 7 is a sectional view of that injection molding machine for injection-molding of the disc substrate 2.

In FIG. 7, the injection molding machine is generally indicated with a reference 20. The injection molding machine 20 includes a mold 21 which is clamped and filled with a molten resin to mold the molten resin, a heating cylinder, 22 connected to an inlet provided on the mold 21 to fill, by injection, the molten resin into the mold 21, a hopper 23 connected to the heating cylinder 22 and into which a resin is introduced, a hydraulic motor 24 to drive a screw 25, and a clamping cylinder 26 to open and close the mold 21.

The hopper 23 has a generally cylindrical shape and is provided with a material inlet 23a for connection to the heating cylinder 22.

The screw 25 is provided in the heating cylinder 22, connected to the hydraulic motor 24 and driven to rotate by the latter. A heater (not shown) is provided around the heating cylinder 22. In the heating cylinder 22, a resin introduced in the hopper 23 is kneaded by the screw 25 and fed along the groove of the screw 25 to the end of the heating cylinder 22 while being heated by the heater.

The hydraulic motor 24 is connected to the screw 25 by means of a hydraulic cylinder 24a. The hydraulic motor 24 drives the screw 25.

The mold 21 used in the present invention is shown in detail in FIG. 8. It consists of a fixed mold 27 having an inlet 27a and nozzle 27b through which the resin is injected, a first movable mold 28 disposed opposite to the fixed mold 27 and which is moved toward and away from the fixed mold 27, a second movable mold 30 disposed inside a cavity 29 defined between the fixed mold 27 and first movable mold 28 when clamped and which is moved toward and away from the fixed mold 27, and an outer-peripheral ring 31 interlocked with the second movable mold 30.

As mentioned above, the first movable mold 28 is moved by the clamping cylinder 26 toward and away from the fixed mold 27.

The inlet 27a of the fixed mold 27 is connected to the heating cylinder 22 and the molten resin is injected from the inlet 27a. From the inlet 27a, the nozzle 27b is penetrated through the fixed mold 27. The molten resin is filled, by injection, into the cavity 29 through the nozzle 27b from the inlet 27a. Th