1. Field of the Invention
The present invention relates to a substrate for an information recording medium in which information is optically recorded and reproduced as well as to a method of producing the same.
2. Description of the Related Art
There have hitherto been methods of reading of signals from optical cards. In one of these methods, signals are detected from the phase difference between beams of laser reflected from recorded and unrecorded portions. In another method, signals are detected using the difference between the amount of beams of laser reflected from recorded and unrecorded portions. In both methods, an incident laser beam passes through a substrate protection layer and the substrate, reaches the surface of a recording medium, passes again through the substrate and the protection layer and reaches a reader.
Thus, it is important that the optical card be devoid of dust, cracking, optical distortion or the like so that signals can be read precisely. In particular, the substrate surfaces of optical cards which used portably are frequently susceptible to such damage. In order to prevent this type of damage, U.S. Pat. No. 4,673,626 discloses an optical recording card in which the surface hardness is increased (by forming a cured surface layer 36 on a substrate 35, as shown in FIG. 5, inter alia, column 16, line 36 to column 17, line 18).
A known method of providing such a hard coating layer involves dissolving a hard coating material in a solvent such as ethyl acetate, toluene, Cellosolve acetate.sup..RTM. (ethylene glycol monoethyl ether acetate manufactured by the Union Carbide Corp.) or the like (referred to as "hard coating solvent" hereinafter), and applying the resultant solution to the surface of a substrate by a spinner method, roll coater method, dipping method, spray method or the like. However, when these hard coating layers are provided on a transparent substrate, the hard coating solvent partially dissolves the surface of the substrate or the creates microcracks roughening the substrate surface. Disadvantages are therefore encountered in that the reflectance ratio and the level of the tracking servo signal are reduced, noise occurs during recording and reproducing, and recording and reproducing will thus become impossible.
Japanese Patent Laid-Open No. 92450/1986 discloses a substrate for an optical recording medium which is formed by providing a cured coat of ultraviolet curable resin on the specular surface of a casting mold. The cured coat is then transferred to the surface of a cured resin obtained by casting and curing epoxy resin. This method, however, requires forming a transfer coated resin layer in the specular mold of a casting apparatus for each molding and is thus unsuitable for mass production. Moreover, since this method uses a resin having a high glass transition temperature to obtain suitable heat resistance, recording sensitivity is significantly reduced. That is, since information recording is performed by evaporating and melt deforming the recording layer by laser beam absorption, if the substrate in contact with the recording layer has a high glass transition temperature, the recording sensitivity decreases due to the fact that deformation of the recording layer is inhibited.