Papers by Author: Osamu Sakata

Abstract: Using X-ray microdiffraction (XRMD) and transmission electron microscopy (TEM) techniques, we have investigated the microscopic structure of Si(011)/Si(001) direct silicon bonding (DSB) substrates. XRMD was performed to measure the local lattice spacing and tilting in the samples before and after oxide out-diffusion annealing. Diffraction analyses for (022) lattice planes with two orthogonal in-plane directions of X-ray incidence revealed anisotropic domain textures in the Si(011) layer. Such anisotropy was also confirmed by TEM in the morphology at the Si(011)/Si(001) bonded interface. The anisotropic crystallinity is discussed on the basis of interfacial defect structures which are proper to the DSB substrate.

Abstract: The use of Si(011)/Si(001) direct silicon bonding (DSB) substrates is a key element of future complementary metal-oxide-semiconductor device technology. In the conventional bonding process, it is necessary to remove interfacial SiO2 to achieve direct atomic bonding. In this study, using X-ray microdiffraction and transmission electron microscopy, we investigate the structural changes caused by oxide out-diffusion annealing (ODA). It is revealed that crystallinity of the bonded Si(011) layer is degraded after low temperature ODA and gradually recovered with an increase in the ODA temperature and annealing time, which is well correlated with the interfacial SiO₂/Si morphology. Characteristic domain textures depending on the ODA temperature are also detected.

Abstract: We have developed new microdiffraction system at the SPring-8. This system uses a focused beam produced using a phase zone plate combined with a narrow slit, which makes a small focused beam that has a small angular divergence. Furthermore, we can use the two-dimensional x-ray CCD detector, which enable us to measure local reciprocal space maps at many points in a sample, that is, the distribution of strain fields and lattice tilts can be revealed in high-angular- and high-spatial-resolution.

Abstract: The structures of Cr-N films deposited by arc ion plating on steel substrates were investigated using a synchrotron radiation system that emits ultraintense X-rays. The Cr-N films were found to be mainly composed of {110} oriented CrN crystals, but they also had a small component of randomly oriented Cr2N crystals. The CrN220 diffraction shifts to a high diffraction angle as the annealing temperature increases. In contrast, the peak position of the Cr2N211 diffraction hardly changes with an increase in the annealing temperature up to 873 K. The ratio of nitrogen and oxygen to chromium at the film surface and inside in the film was estimated by Auger electron spectroscopy. After annealing at 973 K, the surface layer was oxidized, but the composition inside the Cr-N films (N/Cr = 0.83) remained unchanged. The residual stress in a 1600-nm-thick as-deposited CrN layer was found to be -11.0 GPa. The residual stresses of Cr-N films relaxed to thermal stress levels on annealing. However, the residual stress in the Cr2N layer could not be evaluated.

Abstract: Residual stresses near the grain boundary of a bicrystal were measured by synchrotron radiation of SPring-8 at Japan Synchrotron Radiation Research Institute. A copper bicrystal specimen with a 90-degree tilt boundary was deformed 30% in tension. After the plastic extension, kink bands developed in a deformed matrix along the grain boundary. In this study, we focused on the residual stresses in the deformed matrix and the kink band. Residual stresses were evaluated by the X-ray single crystal measurement method. Stereographic projections were used to determine crystal orientations of deformed regions. Our observation showed that crystal orientations were different between the deformed matrix and the kink band. Residual stresses in the direction along the grain boundary in the deformed matrix and kink band were compressive. Residual stresses in the direction vertical to the grain boundary were seen opposite between the deformed matrix and the kink band.

Abstract: A visible four-layers structure with anomalous nano-sturucture was formed from a homogeneous -phase Bi3Pb7 intermetallic compound under a strong gravitational field (1.02x106 G, 130°C, 100 hours). In the 4th layer (lowest-gravity region), pure Bi particles precipitate. In the 2nd 3rd layers, composition graded structures, where Pb content increased along the gravity direction, were formed. It was found that the very broad XRD peak appeared in the 2nd layer, which indicated that an amorphous structure was contained.

Abstract: The high pressure simple hexagonal structure of silicon, which has not been synthesized, is quenched using femtosecond laser-driven shock wave. Any high-pressure phases of silicon do not remain after the pressure release in the case of the hydrostatic and conventional shock compression methods. We found the existence of the simple hexagonal structure after the intense femtosecond laser irradiation to silicon by analyzing the crystalline structures using a synchrotron grazing-incidence XRD method. Femtosecond laser-driven shock wave is a useful tool for the synthesis of non-equilibrium high-pressure phases.

Abstract: We synthesized polymorphic diamond directly from highly oriented pyrolytic graphite (HOPG) using femtosecond laser driven shock wave without catalyst. A femtosecond laser pulse (wavelength: 800 nm, pulse width: 120 fs, intensity: 2×1015 W/cm2) was irradiated onto the HOPG surface in air. Crystalline structures of HOPG after the laser irradiation were analyzed using the synchrotron X-ray at the BL13XU in the SPring-8. We found that the hexagonal diamond exists in the HOPG which was irradiated by the femtosecond laser normal to the basal plane.

Abstract: Stress-assisted atomic migration occurs in thin films due to thermal stress development, followed by hillock and void formation on a film surface. Relation between thermal stresses and hillock formation was investigated on copper films with and without passivation layer. Copper films with a thickness of 10, 50 and 100 nm on oxidized silicon wafer were prepared for investigating thermal stress and hillock formation. In-situ thermal stress observation by X-ray measurement revealed that compressive stresses develop in an early stage of heating followed by a sudden decrease in the temperature region between 100 and 200 deg. In a cooling stage, stresses in a film linearly changed with decreasing temperature to form a tensile residual stress state. Surface morphology is observed by optical microscope and SEM after the heat cycle as well as at elevated temperatures in a vacuum chamber. Dome-like swells were formed on an AlN passivation layer. Almost of all of the swells on 100 nm thick film collapsed after the heat treatment up to 350 deg whereas the swells on 10 nm thick film had no collapse excepting a few case. Comparing with the film without passivation, the swell is considered to be the result of atomic migration of copper film to form hillocks in the interface between copper film and AlN passivation film during heating. Atoms are considered to migrate reversibly into the copper film in the cooling stage, resulting to make vacant hall in the swell of AlN film and then collapse due to tensile stress development.

Abstract: The synthesis of the high-pressure ε phase of iron, which has not been observed under a conventional shock compression, was attained using a femtosecond laser. The lower pressure and temperature α phase (bcc) transforms to the γ phase (fcc) at higher temperatures and to the ε phase (hcp) at higher pressures. A shock induced α to ε phase transition in iron is one of the most famous transitions under high pressure. The induced high-pressure ε phase by a conventional shock loading returns to the α phase and it is not quenched after the shock release because this transition is considered to be diffusionless. Crystalline structures in a recovered iron sample after the femtosecond laser (800 nm, 120 fs, 1014 W/cm2) irradiation were determined using the electron diffraction and the synchrotron X-ray diffraction methods. These results show the existence of the ε phase and the fcc structure in the recovered iron. The femtosecond laser-driven shock wave may have the potential to synthesis high-pressure phases of other materials that has not been done using the conventional shock wave.