Tetsuya Akiyama

Rewritable Dual-Layer Phase-Change Optical Disk Utilizing a Blue-Violet Laser

We have demonstrated for the first time the feasibility of using a rewritable dual-layer phase-change optical disk utilizing a blue-violet laser. For the first medium, we adopted a very thin recording layer with a new phase-change material Ge–Sn–Sb–Te, and a 10-nm-thick silver-alloy reflective layer to obtain a large transmittance and high-quality signals. For the second medium, we optimized the thickness of each layer to obtain both a large optical absorption of the recording layer and a small heat capacity. Carrier-to-noise ratios of more than 50 dB, erasability of more than 30 dB and recording powers of 8 mW for the first medium and 11 mW for the second medium were obtained under typical recording conditions corresponding to a capacity of 27 GB per one side of a 120 mm disk and a user data transfer rate of 33 Mbps.

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.

Phase-change material for use in rewritable dual-layer optical disk

A thin film of Sn-doped and GeTe-rich GeTe-Sb2Te3 shows characteristics that make it suitable for use in rewritable dual-layer optical disks employing a violet laser. By increasing the GeTe component form Ge2Sb2Te5 to Ge4Sb2Te7, and Ge8Sb2Te11, optical changes were increased. By substituting Sn for a proposition of Ge in these compositions, crystallization rates are greatly increased and even a 5 nm-thick film showed a very short laser-crystallization time of less than 50 ns. The material film was successfully applied to Layer 0 of rewritable dual-layer disk: capacity of 27 GB and a 33 Mbps data transfer rate were confirmed for a disk using a conventional 0.6 mm substrate, and 45 GB capacity and the same data transfer rate were obtained for another disk using thin cover layer 0.1 mm thick.

Rewritable Dual-Layer Phase-Change Optical Disk with a Balanced Transmittance Structure.

A rewritable dual-layer phase-change optical disk with a balanced transmittance structure for the layer located at the laser beam incident side (Layer 0) was developed. In this disk structure, transmittance of Layer 0 is almost constant, whether the layer is recorded or not. This structure was realized by adopting Ge–Sb–Te film that has appropriate optical constants, and optimizing the thickness of the dielectric layers. It is proved that this disk structure is effective in suppressing the influence of the recording state of Layer 0 on the layer beneath (Layer 1). Practical performances of the disk with this structure were confirmed for both layers. The feasibility of up to 55 GB capacity was examined for this rewritable dual-layer phase-change optical disk.

Effect of laser pulse width on overwrite cycle characteristics of phase-change disk media

It is found that an equivalent temperature profile and mark size produces one million cycle stable overwrite characteristics. The linear velocity at constant angular velocity (CAV) disk rotation mode varies by a factor of two between the outermost track and the innermost track. Varying the laser pulse width can achieve an equivalent mark formation process at both inner and outer track positions. The configuration of the phase change disk media features a quadrilayer rapid cooling structure. Our disk has a new structure, consisting of a dielectric layer combination of a ZnS-SiO2 mixture layer and an SiO2 layer. The equivalent thermal profile was determined using a heating and cooling process simulation method. A large pulse width of 70 ns on the inner track gives the equivalent temperature profile of a smaller 50 ns pulse width on the outer track. This method produces million-cycle-overwrite characteristics through the whole linear velocity range on the disk in a large capacity MCAV system.

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.

Dual-layer write-once media for 1x-4x speed recording based on Blu-ray Disc format

We have developed dual-layer write-once media with Te-O-Pd based recording films on Blu-ray (BD) format. Recording capacity was 50GB with dual layers on a disk of 120mm in diameter. Rear and Front layers showed jitters of 5.8% and 7.7% at 1x speed, and 6.0% and 8.0% at 2x speed, respectively, which were good enough to satisfy the BD format. Evaluations were carried out with blue-violet laser of 405nm wavelength, objective lens NA of 0.85. Recording linear velocities were 4.92m/s at BD 1x (36Mbps), and 9.84m/s at BD 2x (72Mbps). Characteristics at 4x speed recording were also examined, and it was revealed that carrier to niose ratio at high recording linear velocity of 19.7m/s, which corresponds to BD 4x (144Mbps), was alomst as same as those of 1x and 2x. Recording mechanism was discussed and proposed a model that Te-O-Pd films were not crystallized directly through solid process, but crystallized through melting.

Accelerated aging studies of million cycle overwrite phase change media

The environmental stability of phase change optical disk media having new disk structure was demonstrated in accelerated aging studies. The disk consists of GeTe-Sb2Te3-Sb active layer, ZnS-Si02 mixture layer. SIO2 layer and Al alloy reflective layer. The accelerated aging test conditions were 90°C. 80%RH, for 600 hours. The archival and shelf characteristics of phase jitter were stable. The life time of the disk is expected to be longer than 60 years in a 32°C, 80%RH environment.

Performances of pregrooved phase-change rewritable media in the 680-nm laser diode system

The performances of the pregrooved Phase Change Rewritable (PCR) media have been measured by using an test drive which optical head have a 680 nm laser diode and focus lens with numerical aperture of 0.55. The write/read characteristics and groove parameters have been measured. The linear density of 1.42 Kbits/mm by using Pulse Position (PPM) recording method and track pitch of less than 1 micrometers are expected to be realized.