Magneto-Optical Memory Disk Using Plastic Substrate

Characteristics are described on erasable magneto-optical(M0) memory disks using some plastic substrates of 200 mm 4id 120 mm with a 2.5 um pitch pregrooved. Static recording test was conducted for TbFeCo films deposited on plastic substrates with a He-Ne laser (A =633 nm) at a pulse duration of 1 us. In the case of PC (polycarbonate) substrate disk, a threshold power was 0.8 mW for recording a bit of 0.6 um in diameter. Recorded bit was smaller than in the case of PMMA(polymethyl methacrylate) substrate disk at the same power. This result can be considered that MO disk of PC substrate should be feasible for a high bit density memory disk. Following that, dynamic characteristics were also examined. The mean value of C/N (carrier to noise ratio) of 48dB was obtained at 1 MHz (z1f: 30kHz) for TbFeCo thin films on PMMA disks. Furthermore, TbFeCo MO thin film with a dielectric layer exhibited the value of 52 dB. In order to estimate the life time of MO disk, two type of disks were exposed under at constant temperature of 60 °C, and in constant moisture of 90% RH. Type (A) was constructed with [substrate/MO film/protector] and (B), [substrate/intermediate/ MO film/ protector] . In type (B) of PMMA disk, C/N measured by semiconductor laser (A =830 nm ) at 1 MHz, decreased by 5 dB compared to the initial state after a few days. In the case of PC substrate MO disk, on the other hand, type (A) disk exhibited long life time as compared with PMMA substrate. C/N ratio of type (B) of PC substrate disk had kept the initial state for two months. These results strongly suggest that PC substrate obtained from our new injection molding method should be suitable for MO disk substrate.