Plastic Substrate for Optical Memory Disk

Polymethylmethacrylate(PMMA) and Polycarbonate(PC) was usually used for the materials for the substrate of optical disks. PMMA is used for the commercialized disk substrates larger than 20cm in diameter because of the excellent optical properties, especially low level of birefringence. But the substrate of PMMA has a fault, that warping takes place more easily than PC by heat and water absorption. On the other hand PC is not liable to warp but has generally poorer moldability and higher birefringence. Thus PC is used only for compact disk of 12cm diameter. PC is superior to PMMA if the birefringence can be decreased. We reported on the topical meeting at Montrey last year that 20cm diameter PC substrate with low birefringence was obtained by optimum selection of conditions for injection molding. It was possible to keep the retardation of the recordable area of PC 20cm diameter substrates within 40nm(double pass). Furthermore we investigated the molding conditions for PC 30cm diameter substrates and succeeded in molding substrates of low birefringence(retardation) as shown in Fig.1. We used a special mold which has several pressure sensors in the cavity and optimized the molding conditions, using the correlation between pressure distribution and birefringence. The environmental durability of optical disks is very important from the viewpoint of actual usage of optical disks as optical recording media. We found that the depth of tracking pregrooves engraved on plastic substrates by injection molding changes depending on environmental conditions. Generally, the substrates for optical disk have groove of submicron size. In the process of injection molding of disk substrate molten polymer is injected into mold cavity with high flow rate and pressed on the stamper by cavity pressure. Then pregrooves and pits on the stamper is replicated to molten polymer. The change of cylinder position, ram pressure and pressure of mold cavity during injection is shows in Fig.2. The time for the process from the injection of the hot molten polymer to the rise of pressure in a mold cavity is only a few milliseconds.