Tsukasa Nakai

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.

Effect of Interface Layers on Phase-Change Recording Material Analyzed by Hard X-ray Photoelectron Spectroscopy Method

The influence of interface layers on the chemical and electronic states of a phase-change recording material, GeBiTe (GBT) alloy, used in high-speed rewritable HD DVD media was investigated for the first time by hard-X-ray photoelectron spectroscopy (HX-PES). The binding state of elements for the amorphous state of the phase-change recording film with interface layers is closer to that of the crystalline state than the amorphous film without interface layers. The density of states (DOS) for the valence band of the amorphous state without an interface layer was smaller than that of the crystalline state. The band-edge energy of the amorphous state without an interface layer was lower than that of the crystalline state by about 0.5 eV. On the other hand, the DOS and the band-edge energy of the amorphous state of GBT with interface layers were almost the same as those of the crystalline state. This result may lead to almost the same carrier density for electrical conduction for the crystalline state as the amorphous state, which is totally unexpected, thus very interesting, because the atomic arrangements should differ from each other. We speculate that these effects are a factor allowing high-speed crystallization.

High-Speed Recording Media for HD DVD Rewritable System

We have developed optical recording media for high-speed operation on the HD DVD rewritable (HD DVD-R) format, which uses a laser wavelength of 405 nm, a numerical aperture of the objective lens of 0.65 and a cover layer thickness of 0.6 mm. In this study, we optimized the amount of bismuth substitution for antimony in pseudo-binary GeTe–Sb2Te3 alloy recording film and inserted interface layers to achieve high-speed recording and sufficient thermal stability in our media. As the amount of bismuth substitution increased, the erase ratio of the media improved. The rewritable media in which all the antimony is substituted by bismuth, i.e., the media having GeTe–Bi2Te3 alloy recording film achieved a sufficient erase ratio at a linear velocity 2.4 times that of the previous study without sacrificing the good thermal stability of amorphous marks. We demonstrate the operation of the media at data transfer rates ranging from 36.55 to 87.72 Mbps.

Local Structure Analysis and Interface Layer Effect of Phase-Change Recording Material Using Actual Media

The influences of the interface layer on crystal structure, the local atomic arrangement, and the electronic and chemical structure of a GeBiTe (GBT) phase-change recording material have been investigated using X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), and hard X-ray photoelectron spectroscopy (HX-PES) methods using actual rewritable high-speed HD DVD media without special sample processing. XRD results showed that the crystal structure of laser-crystallized GBT alloy in the actual HD DVD media is the same as that of GeSbTe (GST) alloy, which has a NaCl-type structure. No differences between samples with and without interface layers were found. The lattice constant of GBT is larger than that of GST. Bi increases the lattice constant of GST with respect to the Bi substitution ratio of Sb. According to HX-PES, the DOS of in the recording film amorphous state with an interface layer is closer to that of the crystalline state than the recording film without an interface layer. From XAFS results, clear differences between amorphous (Amo.) and crystalline states (Cry.) were observed. The interatomic distance of amorphous recording material is independent of the existence of an interface layer. On the other hand, the coordination number varied slightly due to the presence of the interface layer. Therefore, the electronic state of the recording layer changes because of the interface layer, although the local structure changes only slightly except for the coordination number. Combining these results, we conclude that the interface layer changes the electronic state of the recording layer and promotes crystallization, but only affects the local structure of the atomic arrangement slightly.

The effect of boundary thermal resistance on HD DVD-ARW optical recording media

Thermal conductivities and boundary thermal resistances of thin films having the thickness of the order of ten nanometers were measured by using the thermo-reflectance method at room temperature. A thermal simulation of HD DVD-ARW (the next-generation advanced rewritable DVD) media was carried out to clarify the effect of boundary thermal resistance at the interface of those films. The thermal conductivity of thin films greatly depends on film thickness. The result of the thermal simulation depends significantly on whether the boundary thermal resistance is considered or not. Thus it is important to consider the boundary thermal resistances and using thermal properties of thin films to perform more accurate calculation for the phase change recording media. The results of the thermal simulation also suggested that the boundary thermal resistances dominate the thermal diffusion and response of the medium.

Dual-layer phase-change recording media for system with NA of 0.65 and light incidence on 0.6-mm-thick substrate

We have investigated the recording characteristics of the dual-layer phase change recording media for the sysetm with the NA of 0.65, the wavelength of 405 nm, and the light incidence on 0.6-mm-thick substrate having the land and groove format. For both L0 and L1 of the dual-layer disc, we have adopted bismuth substituted pseudo-binary GeSbTe alloy film as the recording layer and a novel interface layer material in order to improve overwriting characteristics. Bit error rate measurements have successfully demonstrated the feasibility of the user capacity of 36GB and confirmed 30GB with tilt margins.

High-Speed Deposition of New Low-Refractive-Index Dielectric Film "SiOC" for Rewritable HD DVD Media

A low-refractive-index film plays an important role in attaining a high density of rewritable HD DVD optical recording media, for which the blue-violet laser of 405 nm wavelength incident on a 0.6-mm-thick substrate is used. We have developed a new low-refractive-index material, SiOC, for the medium, which can be sputter-deposited at much higher rates than the conventional SiO2 film. The SiOC film is formed using a SiC target with an Ar and O2 gas mixture using either a RF or DC power supply. The film exhibited similar optical constants as SiO2, and DC sputtering in particular showed very low substrate temperature rise. The resultant deposition rate per sputter power is about three times higher than the rate of deposition using a SiO2 target. The obtained thin film consists of Si, O, and C. Although the film includes a substantial amount of carbon, it showed almost the same optical characteristics as SiO2. We demonstrated that the single-layer HD DVD rewritable medium with 20 GB capacity using SiOC film had satisfactory recording characteristics. We believe that this media can be manufactured at a cost similar to that of conventional rewritable DVDs in mass production.

XAFS study of phase-change recording material using actual media

The relation of the local structure of the phase-change recording material and the interface layer has not been clarified while we already reported that the interface layer affects the electronic state of recording material by using the HX-PES method. It is necessary to understand more detailed physical phenomenon for crystallization mechanism of recording layer in order to develop the high-speed and higher density rewritable optical recording media. The influence of the interface layer to the local structure for atomic arrangement of a GeBiTe phase-change material was investigated by using XAFS on the actual rewritable HD DVD media. The XAFS signal and EXAFS oscillation from the actual media is obtained nondestructively. It has been shown that the interface layer influence slightly the local atomic arrangement of the recording layer, while the electronic state of recording layer is changed by the presence of the interface layer.

Carrier Profiling Technology in Ten Nanometers Devices

The advanced carrier concentration evaluation scheme was proposed with combined higher precise scanning spreading resistance microscopy (SSRM) measurement and technology-CAD (TCAD) analysis in this paper. The cyclic contact (CC) method was applied to variation reduction of SSRM resistance to more accurately characterize for the device. The CC method suppresses dust generation. The variation of resistance with the CC method decreased drastically less than 10%. SSRM resistances were corrected to obtain the potential drop in the probe and other resistance component in the measurement system using TCAD analyses. The effective contact probe tip radius was derived from TCAD analysis. The optimization of both sample and measurement conditions were obtained before the actual measurement. The electrical phenomenon in the device measurement can be known by the TCAD analysis. The SSRM measurement of advanced flash memory was successfully demonstrated with adapted these technologies. This result suggests strongly that 10 nm order size device can be measured by using SSRM with the CC method.