1. Field of the Invention
This invention relates to an optical recording medium and a process for its production. More particularly, it relates to an optical recording medium comprising a substrate and an inorganic dielectric film formed thereon, and a process for its production.
2. Related Background Art
In recent years, there is an increasing demand for recording media having a large recording capacity and enabling a high-speed data transfer for use as computer memories and image information memories. As recording media that can answer such a demand, optical recording media for recording and reproducing information by the use of laser light as exemplified by magneto-optical recording media and phase change recording media are energetically being developed.
More specifically, the magneto-optical recording media have a system in which information is recorded and reproduced by utilizing the characteristic that the polarization plane of laser light being reflected has a rotational direction which differs depending on the direction of magnetization when the polarized laser light is shed on the surface of a recording layer containing a magnetic material (i.e., the magnetic Kerr effect).
As for the phase change recording media, they have a system in which information is recorded and reproduced by utilizing the reversible phase transformation between an amorphous state and a crystalline state, caused when, for example, laser light is shed on a recording layer containing Te, and different optical properties are shown in the respective states.
These optical recording media employ such layer configuration that in order to obtain reproducing signals having a greater C/N ratio, for example, an inorganic dielectric film is provided between a recording film and a substrate so that the reproducing signals can be amplified utilizing an optical interference effect (i.e., the enhancement effect), or, as shown in FIG. 4A, a recording film 401 is held between inorganic dielectric films 402 and 403 so that the reproducing light can be amplified and, at the same time, they can serve as protective films of the recording film 401. Another configuration is also known in which, as shown in FIG. 4B, a reflective layer 404 is additionally provided on the inorganic dielectric film 403 in order to improve the interference effect.
To form such inorganic dielectric films, thin films of, for example, Si.sub.3 N.sub.4, SiC, SiO, amorphous Si (a-Si), AlN, Al.sub.2 O.sub.3, TiO.sub.2, Ta.sub.2 O.sub.5 and ZnS have been conventionally used.
Performances as exemplified by C/N ratios of reproducing signals and stability with the passage of time of recording films of the magneto-optical recording media and phase change optical recording media making use of such inorganic dielectric films greatly depend not only on the properties of recording films as previously stated, but also on the properties of the inorganic dielectric films. Accordingly, in domestic and foreign research institutions, research and development are energetically pursued on materials and production processes for dielectrics having a great enhancement effect on reproducing signals and a superior protective performance on recording layers so that the optical recording media can be made to have a higher performance. For example, it is attempted to form inorganic dielectric films for optical recording media by forming AlN or SiN by reactive sputtering carried out in an atmosphere of N.sub.2 using an Al target or Si target as disclosed in Japanese Patent Publication No. 2-15929, or by ECR plasma-assisted CVD as disclosed in Japanese Patent Application Laid-open Nos. 3-66043, 3-69033 and 4-30343.
In recent years, however, in the trend toward the lower-cost production of optical recording media, it is sought to improve the film forming rate for the formation of inorganic dielectric films.
Under such circumstances, in the above film forming processes for the inorganic dielectric films, the reactive sputtering must be greatly improved in the power to be applied to the target, in order to improve the film deposition rate. In such a case, however, abnormal discharge onto the target tends to occur because of the concentration of charges on the target, and hence it has been difficult to stably form high-quality inorganic dielectric films for optical recording media. On the other hand, the plasma-assisted CVD makes it possible to form films at a higher rate than the reactive sputtering. The former, however, has its own limit in the film formation rate in order to form inorganic dielectric films with good enhancement effect and recording layer protective performance without causing inclusion of a stress that may cause a deformation of optical recording media, and has been unsatisfactory for achieving the lower-cost production of optical recording media.
Moreover, in the conventional plasma-assisted CVD, a uniform plasma density can only be attained in a limited region inside a reaction chamber, and it becomes necessary to make a film forming apparatus larger when uniform inorganic dielectric films are simultaneously formed on a plurality of substrates, where it has been not easy to achieve the low-cost production of optical recording media when the plasma-assisted CVD is used.