Shigeaki Furukawa

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.

Million Cycle Overwritable Phase Change Optical Disk Media

GeTe-Sb2Te3-Sb pseudo ternary components alloy shows laser induced rapid crystallization and amorphous change phenomena. Films of non stoichiometric GeTe-Sb2Te3-Sb can be crystallized using laser pulses less than 100ns duration. The crystalline structure shows the cubic structure. The time it takes to laser crystallize varies Sb concentration. And increasing the Sb concentration leads to increased crystallization temperature. The crystallization temperature of 180 C and more than 1200h stable of acceleration test of 80C 80% RH environment. Thin film disk structure of 20nm active layer produce more than million cycle over-write BER(bit error rate) stability. I would like to discuss the degradation model of pinhole generation in phase change disk media.

4.7 GB Phase-Change Optical Disk for an Authoring System of Digital Versatile Disc

A phase-change optical disk that has an optical phase-shift signal reproducing structure and the same physical specifications as the 4.7 GB read-only digital versatile disc (DVD) was developed. A minimum jitter of 9% and a wide tilt tolerance for the jitter were obtained under DVD reproducing conditions of λ=650 nm and NA=0.60. This new medium can be used as an inspection tool for DVD authoring. It was also confirmed that this optical disk had a potential for use as a rewritable 4.7 GB DVD medium that has compatibility with the read-only DVD.

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.

Thermally balanced structure of phase-change optical disk for high speed and high density recording

A new “distortion model” is proposed to explain why the erasability at direct overwrite mode decreases as the disk speed or the recording mark density increases. It insists that the thermal characteristics difference between the amorphous and crystalUne films causes of the mark shape distortion and the distortion involves the old mark information. The model is experimentally proved on quadri-layer optical disks having a recording layer of Ge2Sb2Te5 alloy; in which the thermal characteristics difference: especially the heat of fusion is optically compensated by having the optical absorbance in the crystalline film. Aery, larger than that in the amoohous film, Aamo. It is confirmed on an optimized disk (Acry/Aamo=1.3) that 55dB of C/N and >30dB of erasability are obtainable over 10-40m/s of disk speed (mark-length=0.9pm) . The erasability value is kept constant (>35dB) during the mark length becomes shorter from 1.2 to 0.6pm to 780nm laser beam (N.A.=0.55) at 30m/s.

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.

High Speed and High Density Recording in a Phase Change Rewritable Disk System

A Phase Change Rewritable optical disk system with a high density of 0.57 μm/bit and high transfer rate up to 46.3 Mbps is presented. In this system, the Mark Edge Recording method is used to attain the total capacity of 19 GB in a double-sided 300 mm-diameter disk. Since this method requires precise recording mark length, a write compensation method, disk media and read data detection method suitable for a wide range of scanning linear velocities ranging from 10.7 to 27.4 m/s have been developed.