Optical Recording Element Having a Plurality of Thin Film Filtering Layers and Optical Recording Element Having an Electrically Conductive Layer

The present invention relates to an optical recording element for use in optically recording, erasing and playing back information from optical disks, optical memory cards and the like.

While there has been an increasing need for high density memory systems capable of storing large amounts of information and accessible at high speeds optical recording elements capable of optically recording, erasing and playing back information have begun to attract attention. Research and development especially aimed at improving DRAW (Direct Read After Write) type optical disks, rewritable optical disks and portable memory cards are expedited in order to find wide applications for the optical recording elements.

Conventionally, a multiple-layer construction having anti-reflection properties is widely employed in such optical recording elements. With reference to FIGS. 5(a)-5(c), optical recording elements of multiple-layer construction, are known and including (i) optical recording elements comprising a recording medium 20 formed on one face of a transparent substrate 14, the recording medium 20 including a recording layer 11, transparent dielectrical layer 12 and reflective film layer 13 (see FIG. 5(a)); (ii) optical recording elements comprising a recording medium 21 dormed on one face of the transparent substrate 14, the recording medium 21 including two transparent dielectric layers 15 and 16, and the recording layer 11 (see FIG. 5(b)); and (iii) optical recording elements comprising a recording medium 22 formed on one face of the transparent substrate 14, the recording medium 22 including two transparent dieletric layers 17 and 18, the recording layer 11, and the reflective film layer 13 (see FIG. 5(c)). These types of optical recording elements have anti-reflection properties exhibited by the interference effect of the transparent dielectric layers 12, 15, 16, 17 and 18, respectively.

In DRAW type optical recording elements, e.g. punching type optical recording elements, the anti-reflection properties acheived by the multiple-layer construction is essential for enhancing the contrast in the reproduced information and obtaining high recording sensitivity. Also, the above anti-reflection properties are required in a magneto-optical recording element, in order to improve the element's playback performance by enhancing the magneto-optical effects.

The aforesaid transparent substrate 14 is made of a plastic such as polycarbonate resin, glass ot the like. In particular, it is possible to economically produce large quantities of the substrates 14 made of a plastic at low cost by injection molding or similar methods, and therefore such substrates 14 are widely used. The recording layer 11 is comprised of, for example, an amorphous rare earth-transition metal thin film such as GdTbFe which is suitable for magneto-optical recording.

Although the recording and playback performances of the optical recording elements can be improved by the conventional constructions, the amount of reflected light from the recording layer 11 is inevitably decreased since it is required to effectively cause the interference of light at the abovementioned transparent dieleletric layers 12, 15, 16, 17 and 18. Therefore, the optical recording element having any of the above constructions has a dark tone appearance, and shows up badly in comparison with disks in which a material having metalic luster is used for obtaining a high reflective factor such as compact disks and video disks for playback use only; and magnetic cards having a colorful design provided thereon such as cash cards, credit cards and various prepaid cards.

The transparent substrate 14 composing the conventional optical recording element tends to generate a static charge, as apparent from the fact that the optical recording element having the transparent substrate 14 of polycarbonate resin shows a specific resistance of approximately 1.times.10.sup.16 .OMEGA..multidot. cm on the surface thereof. Since the surface of the transparent substrate 14 easily gets dust because of the static charge, noise is generated deteriorating the reliability of the optical recording element.

It is an object of the present invention to provide an optical recording element in which a reflective material is used for reflecting lights having wavelengths in a specified visible region of the spectrum, in order to make the appearance of the optical recording element colorful.

It is another object of the present invention to provide an optical recording element having improved reliability with the surface which is not hardened even if it comprises a transparent substrate made of a resin material.

It is still another object of the present invention to provide an optical recording element capable of eliminating a static charge generated on the surface thereof in order to prevent the adhesion of dust to the surface, thereby increasing the reliability of the optical recording element.

In order to achieve the above objects, an optical recording element according to the present invention is characterized in comprising: (i) a transparent substrate through which light passes; (ii) a recording medium formed on one face of the transparent substrate; and (iii) an optical thin film composed of at least one layer, formed on the other face of the transparent substrate so as to transmit light beams used for recording, erasing and playback, and reflect lights which have wavelengths different from the wavelength of the above light beams and are within a specified visible region of the spectrum. With the above arrangement, lights having wavelengths in a specified visible region of the spectrum are reflected at the surface of the optical recording element so that the surface appears to be colored according to the wavelengths of the reflected lights, and the disadvantage of a dark appearance can be thus overcome. On the other hand, light beams used for the recording, erasing and playback of information pass through the optical recording element, and therefore there is no fear of damaging the functions of recording, erasing and playback.

In order to achieve the above objects, another optical recording element according to the present invention is characterized in comprising: (i) a transparent low-resistance substrate having a specific resistance at the surface thereof in the range from more than 0 .OMEGA..multidot.cm to 1.times.10.sup.6 .OMEGA..multidot.cm; (ii) a recording medium formed on one face of the transparent low-resistance substrate, the face being opposite to the side on which a light beam is incident; and (iii) a protective layer formed on one face of the recording medium, the face being opposite to the side at which the transparent low-resistance substrate is formed.

Having a low specific resistance, i.e., 1.times.10.sup.6 .OMEGA..multidot.cm or less, the transparent low-resistance substrate in the above arrangement is electrically conductive. More specifically, when a static charge is generated on the optical recording element, the transparent low-resistance substrate can transmit the static charge outward thereby to prevent the accumulation of static charges and the adhesion of dust to the surface of the transparent low-resistance substrate. Consequently, noise can be reduced.

This specification also discloses the manufacturing method of the optical recording element having a low specific resistance at the surface thereof.

The above and other objects of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.

FIGS. 1 to 3 respectively show one embodiment of the present invention.

FIG. 1 is a sectional view of the essential parts of an optical recording element according to the present invention.

FIG. 2 is a graph showing the dependence of the reflectivity on the wavelength at the optical recording element in the respective cases the optical recording element has a transparent dielectric thin film laminated layer of bi-layer construction and it has a transparent dielectric thin film laminated layer of quadri-layer construction.

FIG. 3 is a graph showing the dependence of the reflectivity on the wavelength at the surface of the optical recording element having a transparent dielectric thin film laminated layer of the quadri-layer construction when the only thickness of a low refractive index dielectric thin film is changed more or less from the value of the thickness thereof obtained by the equation (1).

FIG. 4 is a vertical sectional view showing the structure of an optical recording element according to another embodiment, which has a low specific resistance.

FIGS. 5(a) to (c) are sectional views respectively showing the multiple-layer construction of a prior art optical recording element.

FIG. 6 is directed a further preferred embodiment of the invention having a protective layer disposed on one side of the optical recording element.

FIG. 7 is directed another preferred embodiment of the invention having protective layers disposed in the optical recording element.

Referring to FIGS. 1 to 3, one embodiment of the present invention will be hereinafter described in detail.

The optical recording element of the present invention, as shown in FIG. 1, comprises a transparent substrate 1 through which light beams utilized for the recording, erasing, playback of information pass; recording medium 2 formed on one face of the transparent substrate 1; and transparent dielectric thin film laminated layer 3 formed on the other face of the transparent substrate 1. Like conventional optical recording elements, the recording medium 2 has a construction for anti-reflection, in which transparent dielectric layers, a reflective film layer and a recording layer are formed in combination.

The transparent dielectric thin film laminated layer 3 includes high refractive index dielectric thin films 3a which are optical thin films having a refractive index higher than that of the transparent substrate 1, and low refractive index dielectric thin films 3b having a refractive index lower than that of the transparent substrate 1. These high refractive index dielectric thin films 3a and low refractive index dielectric thin films 3b are alternately laminated forming N layers, such that the first layer, as viewed from the side on which light beams used for the recording, erasing and playback are incident, is the high refractive index dielectric thin film 3a. Accordingly, the high refractive index dielectric thin film 3a is formed at the odd layers and the low refractive index dielectric thin film 3b at the even layers. The thicknesses of the high refractive index dielectric thin film 3a and the low refractive index dielectric thin film 3b are respectively given by: ##EQU1## where n.sub.k : the refractive index of the k-th transparent dielectric thin film as counted from the side on which a light beam is incident.

d.sub.k : the thickness of the k-th transparent dielectric thin film.

.lambda.: the wavelength of light beams used for recording, erasing and playing back information.

With the above arrangement, due to the interference effect of the multiple-layers, the reflectivity of the optical recording element of multiple-layer construction is equivalent to that of the optical recording element wherein neither the high refractive index dielectric thin film 3a nor low refractive index dielectric thin film 3b are provided. The reflectivity for the incident light beam of the wavelength .lambda. is the minimum value. In other words, the amount of reflected light becomes large when light having other wavelengths than the above wavelength .lambda. is incident on the optical recording element, and therefore the optical recording element has colors corresponding to the wavelengths of the reflected light.

More concretely, when employing ZnS (the refractive index n=2.39) as the high refractive index dielectric thin film 3a; MgF.sub.2 (the refractive index n=1.39) as the low refractive index dielectric thin film 3b; polycarbonate (the refractive index n=1.585) as the transparent substrate 1, and setting the wavelength .lambda. of the light beams used for recording, erasing and playback to 780 nm, the thicknesses of the ZnS layer and MgF.sub.2 layer are respectively obtained using the above equation (1). The thickness t.sub.zns of the ZnS layer and the thickness t.sub.MgF2 of the MgF.sub.2 layer are approximately 163 nm and 281 nm respectively. Under the above condition, the dependence of the reflectivity on the wavelength at the surface of the optical recording element is observed in the respective cases that the transparent dielectric thin film laminated layer 3 is composed of two layers (i.e. in the case of N=2 in FIG. 1) and composed of four layers (i.e. in the case of N=4 in FIG. 1). As shown in FIG. 2, in both cases, the as shown reflectivity is the minimum value R.sub.0 at the wavelength of 780 nm, and is high at the wavelength which is in the vicinity of 520 nm (the reflectivity of the optical recording element having no transparent dielectric thin film is also the value R.sub.0 at the wavelength of 780 nm). Hence, the light beams used for recording, erasing and playback pass through the transparent substrate 1 without being reflected so as to reach the recording film, while lights (in this case, the lights whose wavelength is in the vicinity of 520 nm) having other wavelengths than that of the above light beams do not pass through the transparent substrate 1 but are reflected thereat. As a result, an optical recording element whose appearance is in yellow tone can be obtained without damaging the recording, erasing and playback functions of the optical recording element. It often occurs that the surface of the transparent substrate 1 is hardened by incident light beams, when the transparent substrate 1 is of a resin material, but such a drawback can be avoided in this embodiment wherein an inorganic transparent dielectric thin film is laminated on the surface of the transparent substrate 1.

When the thickness of the MgF.sub.2 layer serving as the low refractive index dielectric thin film 3b more or less differs from the value obtained by the foregoing equation (1) (e.g. t.sub.MgF2 =100 nm) and the transparent dielectric thin film laminated layer 3 is of quadari-layer construction (i.e. in the case of N=4 in FIG. 1), the dependence of the reflectivity on the wavelength at the surface of the optical recording element is observed. As shown in FIG. 3, not only is the wavelength region at which the reflectivity of the optical recording element is high wider compared with that in the first example shown in FIG. 2, but the reflectivity at the wavelength of 780 nm is lower than the minimum value R.sub.0. Therefore, the adverse influence of light reflected at the surface of the optical recording element in the course of recording, erasing or playback operation can be reduced.

In the above examples, transparent dielectric thin films which are optical thin films are laminated to form a multiple-layer construction, whereby the light beams used for recording, erasing and playback pass therethrough due to the interference effect, while lights having wavelengths within a specified visible region of the spectrum, different from the wavelength of the above light beams are reflected thereat. In lieu of the aforesaid transparent dielectric thin film of multiple-layer construction, a single optical thin film of mono-layer construction may be employed. Also, it is to be understood that the transparent dielectric thin film laminated layer 3 is not limited only to the alternate layers of ZnS and MgF.sub.2, but other materials may be employed in combination selecting from materials such as Sb.sub.2 S.sub.3, TiO.sub.2, CdS, CeO.sub.2, PbCl.sub.2, WO.sub.3, SiO, Al.sub.2 O.sub.3, SiO.sub.2, CaF.sub.2, LiF, NaF, AlN, SiN, etc. so as to satisfy the aforementioned relation between two materials regarding the refractive index. Further, in the foregoing embodiment, the transparent substrate 1 is provided with the transparent dielectric thin film laminated layer 3 at the surface opposite to the surface on which the recording medium 2 is formed, but it also may be positioned between the transparent substrate 1 and the recording medium 2 for example.

In the case of a conventional optical recording element having the transparent substrate made of polycarbonate resin or the like, the specific resistance at the surface of the transparent substrate is very high i.e. 1.times.10.sup.16 .OMEGA..multidot.cm and therefore a static charge is likely to be generated, causing dust to adhere to the surface thereof. This is one of the reasons for deteriorating the reliability of the optical recording element. In order to overcome such a drawback, another optical recording element according to the present invention, as shown in FIG. 6 comprises (i) the transparent substrate 1; (ii) the recording medium 2 formed on one face of the transparent substrate 1; (iii) a protective layer 9 for protecting the recording medium 2, formed on the surface of the recording medium 2, the surface being opposite to the side at which the transparent substrate 1 is formed; and (iv) a coating layer 4 formed on the other face (on which a light beam is incident) of the transparent substrate 1. Incidentally, in this embodiment, parts that are substantially equivalent or function substantially similarly to those in the first embodiment are indicated by the same reference numerals for convenience of explanation and the description thereof is omitted.

The material of the transparent substrate 1 is not limited, but any material so long as it is light-permeable like plastics such as polycarbonate resin, and glass. The recording medium 2 may have at least one of functions such as optical recording, playback and erasing operation. The coating layer 4 comprises a transparent conductive coating of a transition element oxide such as SnO.sub.2, In.sub.2 O.sub.3 and CdO, and is 50 to 600 nm in thickness. This coating layer 4 permits the transparent low-resistance substrate 5 composed of the transparent substrate 1 and the coating layer 4 to have a specific resistance of 1.times.10.sup.6 .OMEGA..multidot.cm or less which is lower compared with the case in which no coating layer 4 is provided. In the case of the optical recording element having the construction shown in FIG. 1, the coating layer 4 may be formed on the face of the transparent dielectric thin film laminated layer 3, the face on which a light beam is incident. In the case of the optical recording element having the transparent dielectric thin film laminated layer 3 (shown in FIG. 1) formed between the transparent substrate 1 and the recording medium 2, the coating layer 4 may be formed on the face of the transparent substrate 1, the face on which a light beam is incident.

The following description describes the manufacturing method of the optical recording element having the coating layer 4.

For producing the optical recording element, the coating layer 4 of SnO.sub.2 or the like having a thickness of about 100 nm is deposited on one face of the transparent substrate 1 made of polycarbonate resin (or the transparent dielectric thin film laminated layer 3) by means of sputtering or other method. Then, the other face of the transparent substrate 1 is provided with the recording medium 2 composed of an amorphous rare earth-transition metal thin film such as GdTbFe and the protective layer 9 is further covered thereon.

The method for forming the coating layer 4 is not limited to the sputtering but may be a heat decomposition method for instance, so long as the coating layer 4 having a thickness from 50 nm to 600 nm can be obtained with it. Also, the coating layer 4 may be formed after depositing the recording medium 2 and the protective layer 9.

As described above, since the transparent low-resistance substrate 5 comprises the coating layer 4 formed on one face of the transparent substrate 1, the coating layer 4 being composed of the transparent conductive coating, and the specific resistance at the surface of the transparent low-resistance substrate 5 is not more than 1.times.10.sup.6 .OMEGA..multidot.cm, even if a static charge is generated on the optical recording element, the static charge will be eliminated outward by the transparent low-resistance substrate 5. Accordingly, the optical recording element is less likely to generate a static charge and get dust. As a result, the surface of the transparent low-resistance substrate 5 can be kept clean, thereby reducing noise to be generated when recording, playing back or erasing information.

In this embodiment, the transparent low-resistance substrate 5 comprises the transparent substrate 1 and the coating layer 4 composed of the transparent conductive coating, the coating layer 4 being formed on the transparent substrate 1, but the transparent low-resistance substrate 5 is not limited to the above construction. It may be composed of conductive glass which contains V.sub.2 O.sub.3, WO.sub.3, MoO.sub.3, TiO.sub.2, Fe.sub.2 O.sub.3 or others. More specifically, the noise can be reduced so long as the transparent low-resistance substrate 5 is light permeable and the specific resistance at the surface thereof is in the range from more than 0.OMEGA..multidot.cm to 1.times.10.sup.6 .OMEGA..multidot.cm.

FIG. 7 shows the optical recording element of FIGS. 1 and 6 having a combination of protective layers 4 and 9. A first protective layer 4 is disposed between the plurality of dielectric layers 3 and the substrate 1. The protective layer 4 protects the transparent substrate 1 from light which passes through the plurality of dielectric layers and is incident upon the surface fo the transparent substrate 1. The second protective layer 9 prevents against the adhesion of dust on the surface of the recording layer due to static electricity. Both protective layers and the basic structure of the optical recording element have previously been discussed with respect to FIGS. 1 and 6.

As mentioned above, the optical recording element of the present invention is characterized in comprising: (i) a transparent substrate through which light passes; (ii) a recording medium formed on one face of the transparent substrate; and (iii) an optical thin film composed of at least one layer, formed on the other face of the transparent substrate so as to transmit light beams used for recording, erasing and playback, and reflect lights which have wavelengths different from the wavelength of the above light beams and are within a specified visible region of the spectrum. With the above arrangement, lights within a specified visible region of the spectrum are reflected, whereby the optical recording element appears to be colored according to the wavelengths of the reflected lights and the disadvantage of a dark appearance can be eliminated. Furthermore, such a problem in an optical recording element having a transparent substrate made of a resin material that the surface of the optical recording element is hardened by light beams, can be overcome.

Another optical recording element of the present invention comprises: (i) a transparent low-resistance substrate having a specified resistance at the surface thereof in the range from more than 0.OMEGA..multidot.cm to 1.times.10.sup.6 .OMEGA..multidot.cm; (ii) a recording medium formed on one face of the transparent low-resistance substrate, the face being opposite to the side on which a light beam is incident; and (iii) a protective layer formed on one face of the recording medium, the face being opposite to the side on which the transparent low-resistance substrate formed. In such an arrangement, even if a static charge is generated on the optical recording element, the static charge can be eliminated out of the optical recording element by the transparent low-resistance substrate which is conductive. Accordingly, the optical recording element is less likely to generate a static charge thereon and get dust. Consequently, the surface of the transparent low-resistance substrate can be kept clean, thereby to reduce noise generated when recording, playing back and erasing information and improve the reliability thereof. FIGS. 4, 6 and 7 show embodiments of the present invention with like numerals representing the same elements in the previously described figures.

The invention being thus described, it may be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention.

There are described above novel features which the skilled man will appreciate give rise to advantages. These are each independent aspects of the invention to be covered by the present application, irrespective of whether or not they are included within the scope of the following claims.