1. Field of the Invention
The present invention relates to a method of manufacturing an optical disc such as digital audio disc which is conventionally called "compact disc" or an optical video disc.
2. Description of the Prior Art
The digital audio disc, called "compact disc" is commentated, for example, in "BROADCAST TECHNOLOGY" (pp. 316-317, April 1981) and "NIKKEI ELECTRONICS" (pp. 187-206, Aug. 17, 1981).
FIG. 1 is the schematic sectional view of a conventional compact disc. Information is recorded by pits on one surface of disc substrate 1 made from polycarbonate resin, and in addition, reflection film 2 made from aluminum is coated on the information recorded surface. Furthermore, protection layer 3 made from ultraviolet-hardening plastic on which label 4 is silk-screen printed, is formed on aluminum film 2. Total thickness "t" of the compact disc is about 1.2 millimeters.
Since information can be reproduced from the optical disc without using any mechanical means, users expect that the service life of any optical disc will last semi-permanently. However, there is still much uncertainty about the actual service life of those optical discs in the market. Any optical disc is carved to make a large number of minute pits densely on the plastic surface, and thus, extremely precise processing technology is required. This means that even the slightest invisible deformation degrades physical property of the optical disc. In particular, compact discs are expected to replace conventional magnetic-tape car audio system. However, there is a problem ought to be solved that the probability of the compact disc deformation caused under the mounted condition on the car audio playback system. Above all, warping of the compact disc is a critical problem.
FIG. 2 is the partial sectional view of a compact disc. It is essential for the audio system to satisfactorily reproduce information by controlling warp angle .theta. to a maximum of 1.6.degree., where warp angle .theta. is generated by light Bout which is reflection of light Bin perpendicularly incident upon the basic surface of the compact disc, i.e., the surface of the compact disc in contact with the turntable. Any conventional compact disc warps if it is exposed to high temperature, e.g., 100.degree. C. for a certain while, and yet, once the disc warps, its surface cannot recover perfect flatness any more even under room temperature.
Next, process for forming warp prevention layer on the surface of disc substrate opposite from the information-recorded surface using a conventional compact disc manufacturing equipment is described below. First, reflection film 2 is formed for protecting the information-recorded surface 1a of the disc substrate 1. Next, label printing is executed on the protection film 3 to complete the production of a conventional compact disc before eventually forming warp prevention layer on the surface of the disc substrate opposite from the information-recorded surface 1a.
However, while forming reflection film 2 and prevention film 3 based on those sequential processes mentioned above, dust easily adheres to one surface of the disc substrate 1 opposite from the information-recorded surface 1a before eventually forming warp prevention layer. If the warp prevention layer were formed on the dust-deposited surface, a huge number of convex swells will be generated owing to dust particles. Even when no dust adheres to the surface of disc substrate 1 without recorded information, wettability of the surface on which supposed to form warp prevention layer becomes poor as time passes by. This causes to remain foam between the said surface 1 and the warp prevention layer when forming warp prevention layer, thus generating convex swells all over the warp prevention layer.
Since the compact-disc playback system reproduces information signals using light that enters from the warp prevention layer side and reflects from the substrate surface, presence of those convex swells affects adversely the optical information reproducing ability. Consequently, it is extremely important for the information signal reproducing system to prevent from occurring convex swells as much as possible.