Motoyasu Terao

Nanometer-sized Phase-Change Recording using a Scanning Near-Field Optical Microscope with a Laser Diode.

We present for the first time a nanometer-sized phase-change recording using a scanning near-field optical microscope (PC-SNOM recording). The recording experiments were performed with a SNOM using a 785-nm-wavelength semiconductor laser diode, shear force detection for gap control and reflected light detection for observing the domains (reading). The recording media of ZnSSiO2(20 nm)/GeSbTe(30 nm)/ZnSSiO2(150 nm)/polycarbonate substrate were used. The writings were done at laser powers of 8.4–7.3 mW in the probe for pulse widths of 5 or 0.5 ms. As a result, we obtained a minimum recorded domain size of 60 nm in diameter. This size shows a potential to achieve an ultrahigh density PC-SNOM recording with about 170 Gb/in2. A possibility of achieving high speed readout for the future data storage is also discussed.

Phase change recording using a scanning near‐field optical microscope

The formation and observation, with reflected light, of 60‐nm‐diam phase‐changed domains in a thin GeSbTe film using a scanning near‐field optical microscope with a 785 nm wavelength laser diode is demonstrated. The dependence of the domain size on incident laser power was obtained, and the size changed from 150 to 60 nm in diameter with incident power of 8.4–7.3 mW in the probe. At the threshold power of 7.3 mW, the film temperature rose to around 180 °C to partially phase change the local area of the film from amorphous to crystalline. A detected reflectivity increase due to phase change in the formed domain was 8%–2%. The observing (reading) was performed with an incident laser power of 0.2 mW, which corresponds to 10−2–10−3 times less than in a magneto‐optical recording. The incident laser power shows that the phase change reading using the reflection scanning near‐field optical microscope has the potential to read the recorded bit at a speed over 10 MHz.

Chalcogenide thin films for laser‐beam recordings by thermal creation of holes

The shapes of holes thermally created by a laser beam in thin films of wide range of materials are studied. The most cleanly shaped holes, which measure up to recent requirements for direct‐read‐after‐write video recordings by submicron dots, are obtained in some compositions of amorphous chalcogenide thin films such as As‐Te system, Ge‐Te system, As‐Se system, Ge‐Se system, and Sb‐S system thin films. The experiments clarify several important conditions helpful in obtaining cleanly shaped holes. The most important condition is that the viscosity of the material in liquid or softened state should be high. The required incident laser‐beam power for recording in an As‐Te system thin film on a polymethyl methacrylate substrate rotating at 1800 min−1 is about 10 mW. Addition of Se to the As‐Te system thin films greatly increases film stability. A signal‐to‐noise ratio of about 45 dB is obtained in reproduced color video signals.

Electrical Phase-Change Memory: Fundamentals and State of the Art

Phase-change random access memory (PRAM) technology is reviewed. PRAM uses the phase change between the amorphous state and the crystalline state caused by Joule heating as its memory mechanism. A change in electrical resistance owing to a phase change is detected by a small electric current. The merits of this approach are that the resistance change is more than one order of magnitude, and its simple structure decreases the number of steps in the manufacturing process. Suppression of reset current for the change from the low-resistance crystalline state to the amorphous state and an improvement in durability against set-reset cycles and high-temperature operation will ultimately be achieved.

A GeSbTe phase-change memory cell featuring a tungsten heater electrode for low-power, highly stable, and short-read-cycle operations

This paper presents a GeSbTe memory cell with a tungsten heater electrode. The cell has the lowest reset current (50 /spl mu/A) ever reported for a phase-change memory device. The factors responsible for re-amorphization, which increased the instability of crystallization are shown. The GeSbTe cell in this work offers a read-time within 2 nsec, which allows 200 MHz-chip operation with negligible effects of read disturbance.

A Review of Optical Disk Systems with Blue-Violet Laser Pickups

We have applied the blue-violet laser in developing several technologies for the next-generation digital versatile disk (DVD). We are convinced that four items, i.e., large capacity, high data-transfer-rate, cyclability, and portability, are equally important. We have set targets for capacity and data-transfer-rate. Separate development processes have been carried out in the fields of optical heads, systems (servo-mechanisms, read/write channel and signal processing) and basic subjects and combined the results to achieve our targets. Basic subjects that cross these technical fields and have previously been neglected were studied to find solutions to new problems that arise with next-generation requirements.

Single‐beam overwrite experiment using In‐Se based phase‐change optical media

Single‐beam overwrite in an optical disk is reported using In‐Se‐Tl amorphous‐crystalline phase‐change recording films having very short erasing (crystallization) time of 0.2 μs. The lifetime of the amorphous state in an In‐Se‐Tl film is estimated to be longer than 10 years at 60 °C. This is due to its high crystallization temperature (135 °C) and high activation energy (2.6 eV). The phase‐change cycles can continue over 106 cycles in stationary state experiments using test samples. The possibility of single beam overwrite (rewriting without prior erasing as with a magnetic disk) is verified by using a 1.6‐μm‐diam round laser beam spot irradiated on a 5‐in.‐diam disk rotating at 2400 rpm.

A New Super-Resolution Film Applicable to Read-Only and Rewritable Optical Disks

A new super-resolution material for optical disks is proposed which contains cobalt oxide as a main component and shows a refractive index change upon irradiation of a laser beam. This new material is different from the ones already proposed for super-resolution in that it is durable against high-power laser irradiation and thus is applicable to rewritable optical disks as well as read-only (ROM) disks. This paper reports fundamental properties of this material, C/N improvement of ROM and phase-change disks with this material, and its durability against continuous many-time readout. The potential of this technique is discussed.

Doped In-Ge-Te Phase Change Memory Featuring Stable Operation and Good Data Retention

We have fabricated a phase change memory using doped In-Ge-Te to improve the data retention required for industrial and automotive use. This chalcogenide features higher thermal stability as well as denser texture and improved adhesion. The memory cell using doped In-Ge-Te provided a larger read margin and better data retention than conventional Ge2Sb2Tes, and we demonstrated 10-year retention at temperatures above 150degC, which is the highest temperature ever reported.

Oxygen-doped gesbte phase-change memory cells featuring 1.5 V/100-/spl mu/A standard 0.13/spl mu/m CMOS operations

We demonstrated the operation of phase-change memory cells that enabled 1.5-V/100-muA programming through a tungsten-bottom-electrode contact with a diameter of 180 nm. This is the lowest power ever reported. This was achieved with oxygen-doped GeSbTe, and resulted from the high electric resistance of the germanium oxides in this material. Germanium oxides were also estimated to restrain the growth of crystal in GeSbTe, and our cells maintained a 10-year thermal lifetime at 100 degC