Katsutaro Ichihara

The origin of the inhomogeneity of electrical resistivity in magnetron-sputtered indium tin oxide thin films

The intensity distribution of the ion flux on a substrate surface has been measured to examine the origin of the inhomogeneity of resistivity in magnetron-sputtered indium tin oxide (ITO) thin films. Ion flux intensity was determined as the product of ion saturation current Ii and floating potential Vf which were measured by a cylindrical Langmuir probe placed near a glass substrate. IiVf varied depending on the substrate position and showed a maximum near the upper position of the target erosion line. The resistivity of the ITO films showed a minimum at a certain oxygen flow rate. The oxygen flow rate where resistivity showed a minimum decreased as IiVf increased. Carrier mobility increased as IiVf increased under low oxygen flow rate conditions. Carrier concentration decreased as IiVf increased under high oxygen flow rate conditions. The film microstructure and composition were related to the electrical properties.

Recording Characteristics of Ge Doped Eutectic SbTe Phase Change Discs with Various Compositions and Its Potential for High Density Recording

We describe the recording characteristics of discs having Ge doped eutectic SbTe alloy recording films with various compositions. The recording characteristics are fairly sensitive on the film composition. For our recording condition, the Sb/Te ratio of around 4 and the Ge content of about 5 at.% were preferable to obtain good carrier to noise ratio (CNR). A disc with eutectic recording film of that composition is compared with one with a GeTe–Sb2Te3 pseudo-binary alloy recording film, varying the mark length and the track pitch. The eutectic disc showed better CNR for the shorter mark lengths than the pseudo-binary disc. The cross-erase ratio of the eutectic disc is substantially higher than that of the pseudo-binary disc. The result suggests that the eutectic disc is suitable for higher linear bit density and the pseudo-binary disc is suitable for higher track density.

Reactive ion etching of Co-Zr-Nb thin film using BCl3

Chlorine-based reactive ion etching (RIE) of a high-permeability Co-Zr-Nb film has been carried out. The etching rate was only about 10 nm/min in pure Cl 2 in the temperature range in which anisotropic ion etching was dominant. Addition of BCl 3 to the Cl 2 resulted in an increase in the etching rate at low temperature. The activation energy in BCl 3 -Cl 2 RIE was about 0.5eV, whereas that in pure Cl 2 RIE as 1.0 eV. The optimum BCl 3 content was around 30 vol.%. The etching rate reached 750 nm/min at 1.2 W/cm 2 RF power density and 300°C etching temperature, This improvement is due to the heavier mass and the higher ionization rate of BCl 3 than those of Cl 2 . The ratio of the side etching rate to the depth-etching rate was about 30%, when the RF power density was 1.2 W/cm 2 in BCl 3 -Cl 2 RIE. The side etching was suppressed by both an increase in the RF power and addition of SiCl 4 or SiCl 4 -O 2 to the BCl 3 -Cl 2 . Anisotropy was promoted by both an increase in the ion energy and sidewall protection by Si or Si-oxide. An etching rate of 200 nm/min at 150°C and 2 W/cm 2 RF power density in BCl 3 - Cl 2 -SiCl 4 RIE has been achieved. The etching selectivity vs SiO 2 was 20 and the side-etching rate was only 2% of the depth-etching rate.

Spin-Dependent Tunneling between a Soft Ferromagnetic Layer and Hard Magnetic Nanosize Particles

Spin-dependent tunneling has been investigated for ferromagnetic tunnel junctions with a new structure, which consists of a hard ferromagnetic insulating granular thin film sandwiched between two soft ferromagnetic layers, or soft ferromagnetic and non-ferromagnetic layers. The granular film, consisting of ferromagnetic Co80Pt20 nanosize particles embedded in a SiO2 matrix with 13 nm thickness, has a high coercive force. The film was prepared by co-sputtering of Co80Pt20 and SiO2 targets on a glass substrate with a Cr or a Co80Pt20/Fe underlayer as a bottom electrode. A ferromagnetic Co9Fe layer as a top electrode was deposited over the granular film to form cross pattern junctions of 0.01 mm2 junction area through a metal mask. The tunneling magnetoresistance up to 4.5% at RT was observed for a Co9Fe/Co80Pt20–SiO2/Co80Pt20/Fe junction at a low field.

Super-Resolution Effect of Semiconductor-Doped Glass

Semiconductor-doped glass is proposed as a super-resolution layer for future ultra-high-density optical disc systems. It was confirmed that this material system showed very fast response and large transmittance change by laser-beam irradiation when CdSSe-doped glass was used. The rise time of the transmittance change was less than 10 ns and the transmittance change reached 30%. These optical responses were obtained at a power density of the pumped laser beam of 1 MW/cm2. This power density corresponds to the readout power in digital versatile disk-read only memory (DVD-ROM) and digital versatile disk-random access memory (DVD-RAM) discs. This material system is regarded as a potential candidate for a super-resolution readout layer that is applicable to both ROM and RAM discs.

Dual-layer rewritable phase-change recording media for HD DVD system

We have investigated the recording characteristics of the dual-layer phase-change recording media for a system that uses the blue-violet laser (wavelength of 405 nm), with the numerical aperture of 0.65 and light incidence on 0.6-mm-thick substrate having the land and groove format (HD DVD). In order to achieve higher storage density for its beam spot size, a number of technologies have been incorporated into the media, such as the bismuth-substituted pseudobinary GeSbTe alloy recording film and hafnium oxide-based interface layer material. The feasibility of the user capacity of 36 GB has been successfully demonstrated.

Media Technologies of 20 GB Single-Layer Rewritable Phase-Change Disc for HD DVD System

Sufficient signal amplitude, erase ratio and low amount of cross-erase must be attained to achieve higher recording capacity of rewritable phase-change media. We have developed new media technologies, such as the GeTe-rich GeTe-Sb2Te3 pseudobinary alloy recording film partially substituted with bismuth and an additional layer to control thermal diffusion. The recording layer material enables a high erase ratio with a high signal amplitude. The additional layer also reduces the amount of cross-erase while maintaining a sufficient erase ratio. We have successfully demonstrated rewritable phase-change media having 20 GB capacity by using these media technologies for the HD DVD system with a blue-violet laser diode having the wavelength of 405 nm, a numerical aperture of the objective lens of 0.65 and light incident on a 0.6-mm-thick substrate. In this paper, we describe our new media technologies and the evaluation results, such as tilt margins, uniformity, overwriting cyclability and life durability.

High-Speed Etching of Indium-Tin-Oxide Thin Films Using an Inductively Coupled Plasma.

The dry etching of indium-tin-oxide (ITO) films using a high-density inductively coupled rf plasma is described. Various etching characteristics are examined for an etching gas of CH4/H2 mixture. A very high etching rate of ITO films of over 200 nm/min was obtained at 100% CH4, while the high etching rate of ~100 nm/min was achieved even at 100% H2 with the moderate dc self-bias voltage of 300 V. The etch selectivity of ITO to SiO2 and Si3N4 was over ~10. The dependences of etching rate on substrate temperature and self-bias dc voltage suggest that the high-flux ion bombardment given by the inductive rf plasma enhances the etching of ITO films. On the other hand, the crystallinity of ITO has a nominal effect on the etching characteristics of ITO films.

Advanced phase change media for blue laser recording of 18 GB capacity for 0.65 numerical aperture and 30 GB capacity for 0.85 numerical aperture

We attempted to improve phase change media to achieve larger capacity for systems with blue lasers using numerical apertures (NA) of both 0.85 and 0.65. It was found that the Bi-added GeTe-rich pseudo-binary composition promised high erase ratio even when we did not use any interface layers on a recording layer. The use of an absorption control layer (ACL) was also attempted to suppress the cross erase (XE). It was found that the ACL was very effective in suppressing the XE. The advanced phase change media that used the above novel technologies showed good analog performance. The advanced media also showed bER of less than 1.0×10-4, promising 18 GB capacity for an NA of 0.65 and 30 GB capacity for an NA of 0.85.

High-Density Recording Capability of Five-Layered Phase-Change Optical Disc

High-density recording capability of the five-layered phase-change disc has been examined from the viewpoints of optical simulation and disc performance. The optical simulation showed that the temperature rise in the amorphous state balanced that in the crystal state for the case when the thickness of the lower protective layer was adjusted from 70 nm to 120 nm. Carrier-to-noise ratio (CNR), overwriting erase ratio and jitter were measured by the optical recording system using a laser diode with a wavelength of 636 nm and an objective lens of 0.6 numerical aperture. Track-pitch was fixed at 0.6 µm and bit length was varied at a linear velocity of 6 m/s. CNR of 52 dB, overwriting erase ratio of more than 35 dB, and overwriting jitter of less than 10% were obtained on both land and groove at a bit length of 0.34 µm/bit.