Kenichi Nagata

High Speed Overwritable Phase Change Optical Disk Material

It was found that GeTe-Sb2Te3 pseud-binary amorphous alloy films showed remarkably fast switching properties to laser irradiation. By the static laser irradiation test, the film whose composition corresponded to stoichiometric compound of GeSb2Te4 were crystallized within 50ns of pulse duration at power of 8mW, whilst they could be amorphized with the same pulse duration at power of 20mW. Direct overwriting cycle test was performed on the revolving disk system for 105 times using single laser beam. CNR of more than 50dB and erasability of -22dB were obtained for linear velocity of 22m/s and overwriting frequencies of 5 and 7 MHz. The laser powers were 22 mW for recording and 10 mW for erasing. These materials will be applicable to high data rate direct overwritable disk media.

Phase Change Disk Media Having Rapid Cooling Structure

It is found that a rapid cooling structure is effective to improve overwrite characteristics of phase change type optical disk media. For a rapid cooling structure, thin A1N dielectric layer, 30 nm, with high thermal conductivity is suitable. The layer is adopted between the active layer and the reflective metal layer in quadrilayer disk structure. The cooling rate of the disk is calculated to be 12°C/nsec. This disk structure shows clear amorphous marks. A thin active layer disk structure using GeTe–Sb2Te3–Sb ,alloy of 20 nm has produced two million cycle stable bit error rate characteristics.1) And the erasability of the disk having a rapid cooling structure goes up more than 30 dB and shows rather wide plateau region of power dependency.

Rewritable Dual-Layer Phase-Change Optical Disk

This paper describes the development of a rewritable phase-change optical disk with a dual-layer structure. To obtain equivalent signal amplitude and recording sensitivity from the medium located on the beam incident side (the first medium) and the medium beneath (the second medium), we developed a high transmittance first medium featuring a very thin recording film (7 nm) and no reflection layer, and a high recording sensitivity and high reflectivity second medium with a semi-transparent front layer and a reflection layer. We confirmed a jitter of approximately 10% for both media at a recording density corresponding to 8.5 GB on a 120 mm disk. The laser power required to record on the first medium was 12 mW, and the second medium, 13 mW. It was also confirmed that the new rewritable dual-layer phase-change optical disk possesses the potential for operation with a blue laser.

Overwritable phase-change optical disk recording

Three key technologies have overcome the cycle issue of phase-change optical disks. The interchangeability testing data for Multimedia PD by different media makers has verified the values specified in ECMA-240 which has become a worldwide standard. Wide wavelength response will extend the capacity of 120 mm phase-change optical disk to 15 GB/side.

Accelerated aging studies for phase-change-type disc media

The excellent stability of GeTe-Sb2Te3-Sb phase change disc media was demonstrated in accelerated aging studies. The disc was composed of the pseudo ternary alloy active layer of GeTe-Sb2Te3-Sb, ZnS-SiO2 dielectric layers and Al quadrilayer rapid cooling structure. This structure leads high performance C/N ratio of 55dB and more than 30dB eras ability with wide plateau power dependency. We measured the archival and the shelf characteristics of C/N and bit error rate (BER) as a function of exposure time for discs exposed to 80 degrees C 80% RH, and 90 degrees C 80% RH. From the Arrhenius plot, the life time of the phase change type disc media were expected to be longer than 60 years in 32 degrees C 80% RH environment.

Advanced 4.7-GB DVD-RAM with a 4X data transfer rate

We demonstrated the possibility of high data rate recording on a DVD-RAM disk which utilizes Ge-Sb-Te phase-change materials. To ensure high transfer rate overwriting on the DVD, quadruple speed (44Mbps) recording at a linear velocity of 16.4 m/s was tested using a Sn-added Ge-Sn-Sb-Te material as the recording layer. Double speed (22Mbps) recording on the present 4.7GB DVD-RAM at a linear velocity of 8.2 m/s was also tested. A CNR of more than 53 dB and an erasability of more than 30 dB were obtained at each double, triple and quadruple speeds. In addition, by recording via 8-16 random modulation signals, a jitter of 9 percent or less and a direct overwrite performance of 100,000 cycles were confirmed.

Phase-change optical disk with nitride interface layers

Two marked effects are obtained by forming a Ge-N interface layer on either side of Ge-Sb-Te recording layer. One effect is a suppression of atomic diffusion between Ge-Sb-Te layer and protective layers, ZnS-SiO2 representatively, which leads to a significant improvement in overwrite cyclability, and the other is the acceleration of crystallization process which leads to higher speed optical disks. A rapid-cooling type experimental disk with Ge-N layers on both sides of the Ge-Sb-Te recording layer proved to be capable of exceeding 105 cycle overwrites and a recording data rate 40 Mbps at linear velocity 12 m/s. The recording conditions: bit length 0.28 micrometer and track pitch 0.6 micrometer (L/G method) using laser source with a wavelength 658 nm and a numerical aperture 0.6 correspond to a capacity 4.7 GB/(phi) 120 mm.

Edge detection readout signal and cross talk in phase-change optical data storage

Readout signal, noise, and cross-track cross talk were investigated for edge detection in a phase-change optical data storage system. Both theoretical and experimental results indicate that edge detection has a performance superior to the conventional detection of reflectance variations, especially when the amorphous marks are shorter than the size of the focused spot. More than 50 dB of carrier to noise ratio for marks of 0.36 μm in length is obtained using light at a wavelength of 690 nm and an objective lens of 0.6 numerical aperture. Diffraction analysis on the cross talk has shown that, in the scheme of land-groove recording, there is no optimum groove depth which can cancel the cross talk from adjacent tracks.

Optical characterization of multilayer stacks for phase-change media

We report results of measurements of the optical constants of the dielectric layer, reflecting layer, and phase-change layer used as the media of phase-change optical recording. The refractive index and the absorption coefficient k of these materials vary to some extent with the film thickness and with the film deposition environment. We report the observed variations of optical constants among samples of differing structure and among samples fabricated in different laboratories.