Improvements of optical, mechanical, and geometrical properties of DVD-RAM substrate

It is necessary to improve optical, geometrical, and mechanical properties in the optical disk substrates as the information storage devices with high storage density using short wavelength lasers are being developed. However, the conventional injection molding process is no longer appropriate for producing the high-density information storage optical disk substrates with superb optical, geometrical, and mechanical properties. It is, therefore, inevitable to develop new concepts for the molding processes. In the present study, DVD-RAM substrates were fabricated by injection-compression molding, which is regarded as the most suitable process to manufacture optical disk substrates. The effects of various processing conditions on the birefringence distribution, the land-groove structure including the groove depth and the surface roughness of the land, and the radial tilt were examined experimentally. It was found that the birefringence, which is regarded as one of the most important optical properties for optical disk, was very sensitive to the mold wall temperature history. Also, the integrity of the replication, represented by the land-groove structure, and the radial tilt were influenced mostly by the mold temperature and the compression pressure.