1. Field of the Invention
The present invention relates to an optical recording disc and a method of the production thereof. The recording disc comprises a transparent disc substrate on one surface of which a resin layer and a recording layer are formed, more particularly, to an optical recording disc and a method of the production thereof having an improved signal to noise ratio.
2. Description of the Prior Art
There is provided an optical recording disc comprising at least one transparent disc substrate made of glass or plastic material, on one surface of which a resin layer has preformatted pits corresponding to information signal and guiding grooves corresponding to a tracking signal is formed, with a recording layer formed on the surface of the resin layer. In a dual sided optical recording disc, a pair of optical transparent disc substrates are bonded together through spacers so that each of the resin layers and each of the recording layers face each other. On the other hand, in a single sided optical recording disc, one optical transparent disc substrate and a reinforcement plate in a similar shape as the optical transparent disc substrate are bonded together through spacers so that the resin layer and the recording layer face the inside of the optical recording disc.
In this sort of the optical recording disc, when a laser light is radiated onto the top surface of the transparent disc substrate, where the resin layer and to a recording layer through a substrate, information can be written in the disc or read from the disc. Therefore, the transparent disc substrate is required having a high transparency, a smooth surface, a uniformity of thickness and composed of homogeneous material. In particular, the roughness of the surface of the transparent disc substrate 1 onto which the laser light is injected is a significant factor when an information signal is written and read into and from the recording layer of the optical recording disc, and the height h between a peak and a bottom of the rough surface 1b is referred to hereinafter as the roughness of the surface of the transparent disc substrate as shown in FIG. 3. A distance l between the peaks of the rough surface 1b is also a significant factor as shown in FIG. 3. In an optical recording disc comprising a transparent disc substrate having a relatively rough surface whose roughness h is larger than 0.02 .mu.m, a scattering and a diffraction of the laser light occurs on the rough surface, thereby the signal to noise ratio become smaller than a predetermined signal to noise ratio when an information signal is written and read into and from the recording layer, resulting in that it is difficult for the information signal to be written and read properly. Accordingly, it is necessary for the roughness h to be smaller than 0.02 .mu.m in order to properly write and read an information signal into the recording layer.
On the other hand, the roughness of the surface of the transparent disc substrate where the resin layer is formed is also generally a significant factor, and it is known that relatively small roughness h leads to a larger signal to noise ratio when an information signal is written and read into the recording layer, therefore, it is necessary for the roughness h of the surface where the resin layer is formed to be smaller than 0.02 .mu.m.
In general, the aforementioned transparent disc substrate for an optical recording disc is made by the following methods.
In a first production method of the transparent disc substrate, a mold is formed in a predetermined shape and a predetermined volume of the transparent disc substrate. Melted material made of glass or plastic material is injected into the inside of the mold, then the melting material is solidified, resulting in the transparent disc substrate being formed in the shape of the mold. In the transparent disc substrate made by the aforementioned production method, the surface of the substrate not contacting the surface of the mold during the solidification process, corresponding to the surface solidified by the exposure to the air, becomes relatively smooth surfaced, since the surface is formed independent of the surface of the mold during the above solidification, resulting in a relatively very smooth surface having a roughness h smaller than 0.02 .mu.m ; However, the other surface of the disc substrate contacting the inside surface of the mold during the above solidification becomes a relatively rough surface, since the surface of the mold is transferred to the surface of the disc substrate, resulting in a relatively rough surface having a roughness h larger than 0.02 .mu.m to 0.03 .mu.m and smaller than 0.1 .mu.m, even though the surface of the mold is precisely polished.
In another production method of the transparent disc substrate, the disc substrate is formed by cutting a glass plate being on the market in a predetermined shape. The glass plate is made by the Colburn method or the floating method. In the Colburn method, as shown in FIG. 1, glass material 11 is melted in a furnace 10 and is pulled upward by a pulling roller 12. The pulled glass material 11 is pulled up in the horizontal direction. Then, the pulled semi-melted glass material 11 is pulled out being solidified by the transferring roller 15 arranged in a slow cooling tank 14 following by the furnace 10, resulting in a glass substrate obtained by cutting the solidified glass material in a predetermined length. In the floating method, as shown in FIG. 2, after a thickness of a glass material 11 melted in a furnace 10 is adjusted to a predetermined thickness by a thickness adjustment roller 16, the glass material 11 is slowly flowed onto a top surface of melted tin 18 in a melting metal tank 17. Then, the semi-melting glass plate is pulled out and solidified by a transferring roller 20 arranged in a slow cooling tank 19 following the melted metal tank 17.
In the glass plate made by the aforementioned production methods, the surface of the glass plate, not contacting with the bending roller 13 and the transferring roller 15 in the Colburn method or the melted tin 18 and the transferring roller 20 in the floating method, become relatively smooth having a roughness h smaller than 0.02 .mu.m.
The opposite surface of the solidified glass plate contacting with the bending roller 13 and the transferring roller 15 in the Colburn method or the melted tin 18 and the transferring roller 20 in the floating method becomes relatively rough, since the surface is formed dependent upon the surfaces of the rollers 13, 15 and 20 and the melted tin 18. The relatively rough surface is quickly cooled by contacting with the surfaces of the rollers 13, 15 and 20 and the melted tin 18 with the relatively rough surface having a roughness h in the range of 0.02 .mu.m to 0.03 .mu.m.
Accordingly, as described above, in an optical recording disc comprising the transparent disc substrate having a relatively rough surface whose roughness is larger than 0.02 .mu.m, the scattering and the diffraction of the laser light occurs on the rough surface, thereby a signal to noise ratio becomes smaller and it is difficult for an information signal to be written and read into and from the recording layer. The transparent disc substrate made by the aforementioned method can not be used as a transparent disc substrate for an optical recording disc. Therefore, in general, at least one rougher surface of the transparent disc substrate must be precisely polished, thereby the roughness h of both sides of the transparent substrate is smaller than 0.02 .mu.m.
However, the aforementioned process of polishing precisely has the disadvantages as follows.
(1) The polishing process cost is very expensive, since high skill is necessary.
(2) The production efficiency is very small, since it takes a long time to perform the polishing process.
(3) The polished surface is easily marred during the transferring process to the fabrication process of the optical recording disc after the polishing process, resulting in a decreased percentage of conforming articles. Therefore, the production cost of the conventional optical recording disc comprising the polished transparent disc substrate is very expensive. In case a mold is used during the production of the transparent disc substrate, in addition, it is necessary to form a smooth inside surface of the mold having a roughness h smaller than 0.02 .mu.m and to maintain the smoother surface, resulting in production costs and maintenance costs that are very expensive. Moreover, in case the resin layer onto which the information pattern is transferred is formed onto the precisely polished relatively smooth surface of the transparent disc substrate, there is a problem that the resin layer can be undesirably stripped from the transparent disc substrate, since the transparent disc substrate and the resin layer are not strongly bonded together.