Kazuhiro Nishimura

Ultrahigh-density recording using overwritable phase-change optical disk

The possibility of ultrahigh density recording has been shown using an overwritable phase change optical disk with a Ge-Te-Sb recording film. It was found that recording marks of 0.2 micrometers length can be formed stably and that high contrast ratio can be obtained in the wide range of wavelength. From these results, about six times as high linear density as the existing format is possible by using mark-edge recording or MCAV method. It was also found that the crosstalk of PCE optical disk is so small that double track density recording is possible by track pitch reduction or groove/land recording. These results show a possibility of large capacity of 10 GB for 5.25 inch diameter disk.

Disk structure and writing method for high-performance phase-change erasable optical disk

Correlation of overwriting performance and disk structure of phase change erasable optical disk was investigated using Ge-Te-Sb recording material. It was found that quadrilayered structure is superior in cyclability to trilayered structure because molten material is liable to flow along the track in the case of the trilayered structure. A million cycle BER (bit error rate) stability was achieved by applying new writing method to the quadrilayered structure.

High-density recording using phase-change optical disk

We have measured linear velocity dependency of overwriting erasability and jitter. It was found that overwriting jitter correlates with overwriting erasability which reflects the degree of mark distortion. A 5-layered disk was developed whose absorptivity of crystalline state was larger than that of amorphous state. This disk showed small jitter, which was less than 16% and 15m/s when the bit length was 0.45 micrometers /bit without applying a writing compensation. And we have studied land/groove recording at relativley low linear velocity of 4.5 m/s. Crosstalk both from land and groove were lower than -26db in the recording pitch of 0.7 micrometers , which is sufficiently small for practical use. The bit length of 0.45 micrometers /bit and the recording pitch of 0.7 micrometers realize 2.6 Gbytes in 120 mm, supposing that user data capacity is three quarters of the total capacity.

BASIC STUDY ON HIGH DENSITY RECORDING BY USING OVERWRITABLE PHASE CHANGE OPTICAL DISK

High density recording characteristics has been studied by using an overwritable phase change optical disk with a Ge-Te-Sb recording film. CNR of more than 45 dB was obtained at 0.42 micron mark length by using a laser diode with 680 nm wavelength and a objective lens of 0.6 NA. And it was observed by TEM microscopy that 0.2 micron marks can be formed. Track pitch dependence of crosstalk was measured. Small crosstalk of less than -26 dB was obtained at the track pitch of 1.0 micron in the wide range of writing power. We also discuss the reason why small crosstalk can be obtained by phase change disk.

Stability of phase-change optical disks produced by a pass-through type sputtering system

Mass production technology of phase-change rewritable disks can be achieved by using a pass-through type in-line sputtering system. The qualities of the mass-produced PCR disk were high enough to satisfy all the specifications for this product. We optimized the composition of the recording layer (SbTeGe alloy) of the PCR disk. It was found that a Ge content of more than 20 at% in the recording layer is necessary to achieve a long archival life. The mass-produced PCR disks were very stable in the accelerated aging test. The BER stayed at nearly the same value for 2000 hrs at 90 degree(s)C, 80% RH and 4000 hrs at 80 degree(s)C, 80% RH. The life of the PCR disks was estimated to be greater than 50 years at 30 degree(s)C, 80% RH. The activation energy for the PCR disks obtained from Arrhenius plots was 1.11 eV.