Naoki Saito

Effect of spatial gradient in fluid shear stress on morphological changes in endothelial cells in response to flow

Arterial bifurcations are common sites for development of cerebral aneurysms. Although this localization of aneurysms suggests that high shear stress (SS) and high spatial SS gradient (SSG) occurring at the bifurcations may be crucial factors for endothelial dysfunction involved in aneurysm formation, the details of the relationship between the hemodynamic environment and endothelial cell (EC) responses remain unclear. In the present study, we sought morphological responses of ECs under high-SS and high-SSG conditions using a T-shaped flow chamber. Confluent ECs were exposed to SS of 2-10Pa with SSG of up to 34Pa/mm for 24 and 72h. ECs exposed to SS without spatial gradient elongated and oriented to the direction of flow at 72h through different processes depending on the magnitude of SS. In contrast, cells did not exhibit preferred orientation and elongation under the combination of SS and SSG. Unlike cells aligned to the flow by exposure to only SS, development of actin stress fibers was not observed in ECs exposed to SS with SSG. These results indicate that SSG suppresses morphological changes of ECs in response to flow.

Evaluation of the crystallinity of grain boundaries of electronic copper thin films for highly reliable interconnections

The change of the crystallinity of grain boundaries of the electroplated copper thin films used for the various interconnections in electronic products was evaluated quantitatively by applying an electron back-scattering diffraction analysis. It was found that the crystallinity of the electroplated films varied drastically depending on the electroplating conditions such as the composition of solute, temperature of the plating, the current density during electroplating, the surface material on the substrate, and so on. It also changed after the annealing at temperatures higher than 200°C. The change of the crystallinity of the films caused the drastic variation of both the electrical and mechanical reliability of the films.

Production of Large-Diameter Microwave Plasma with a High-Permittivity Material Window

As a large-diameter plasma generation method for ultralarge-scale integrated (ULSI) circuit processes, microwave plasma production was studied by employing a high-permittivity material window. The plasma parameters were examined in conjunction with the permittivity of the dielectric window material. The results revealed that the alumina window with higher permittivity provided higher electron density in both Ar and O2 plasmas. Since this dependence was only seen in the regime above the cut-off density of 2.45 GHz microwave, it was concluded that the plasma production was due to the surface-wave mode. The method was emphasized to be promising in producing a large-diameter plasma for ULSI processes.

Micro texture dependence of the mechanical and electrical reliability of electroplated copper thin film interconnections

Both mechanical and electronic properties of electroplated copper films used for interconnections were investigated experimentally considering the change of their micro texture caused by heat treatment. Since those properties varied drastically depending on the electroplating conditions and thermal history after the electroplating, a novel evaluation method of the crystallinity of grains and grain boundaries of the electroplated copper thin films has been proposed by applying the conventional electron back scatter diffraction method. It was found the porous grain boundaries in the films cause brittle mechanical and electrical fractures of the films. The proposed method was effective for evaluating the crystallinity of grain boundaries quantitatively, and thus, estimating both the mechanical and electrical properties of the films used for mass production.

High Wall Shear Stress Gradient Suppress Morphological Responses of Endothelial Cells to Fluid Flow

Arterial bifurcations are known as the common sites for development of cerebral aneurysms. Although localization of aneurysms suggests that high wall shear stress (WSS) and high wall shear stress gradient (WSSG) occurring at the bifurcations may be crucial factors for endothelial dysfunction involved in aneurysm formation, the details of the relationship between the hemodynamic environment and endothelial cell (EC) responses still remain unclear. In this study, we observed morphological responses of ECs under high WSS and high WSSG condition using a T-shaped flow chamber and under high WSS without WSSG condition using a parallel plate flow chamber to evaluate the effects of WSSG to ECs, After 24 h exposure to flow, ECs under high WSS (10 Pa) without WSSG condition oriented perpendicular to the flow, whereas ECs at high WSS (10 Pa) with WSSG condition did not cause EC alignment. After 72 h exposure to flow, ECs exposed to WSSG were not polarized whereas ECs at high WSS without WSSG condition orientated and elongated to the direction of flow. These results indicate that a WSSG may suppress orientation of ECs to the flow direction.

Effect of Crystallographic Quality of Grain Boundaries on Both Mechanical and Electrical Properties of Electroplated Copper Thin Film Interconnections

Effects of crystallographic quality of grain boundaries on mechanical and electrical properties were investigated experimentally. A novel method using two parameters of image quality (IQ) and confidence index (CI) values based on electron back-scattering diffraction (EBSD) analysis was proposed in order to evaluate crystallographic quality of grain boundaries. IQ value was defined as an index to evaluate crystallinity in region irradiated with electron beam. CI value determined existence of grain boundaries in the region. It was found that brittle intergranular fatigue fracture occurred in the film without annealing and the film annealed at 200 °C because network of grain boundaries with low crystallinity remained in these films. On the other hand, the film annealed at 400 °C caused only ductile transgranular fatigue fracture because grain boundaries with low crystallinity almost disappeared. From results of measurement of electrical properties, electrical resistivity of copper interconnection annealed at 400 °C with high crystallinity (2.09 × 10−8 Ωm) was low and electron migration (EM) resistance was high compared with an copper interconnection without annealing with low crystallinity (3.33 × 10−8 Ωm). It was clarified that the interconnection with high crystallinity had superior electrical properties. Thus, it was clarified that the crystallographic quality of grain boundaries has a strong correlation of mechanical and electrical reliability.

Blue-violet laser write-once optical disc with spin-coated dye-based recording layer

Jitter of 9.5% with conventional equalizer and 6.1% with limit equalizer have been achieved on a 23.3GB blue-violet laser write-once optical disc with a recording laeyr formed by spin coating with a dye-based solution. Data transfer speed is 36Mbit/s. The disc is composed of a polycarbonate substrate, a reflective layer, a dye-coated recording layer, a bonding layer, and a polycarbonate protective sheet. Cavities formed in the marks during recording result in large changes in refractive index for signal amplitude. Recording and playback was successful even at 2x drive speeds with data transfer of 72Mbit/s.

Improvement of Crystallographic Quality of Electroplated Copper Thin-Film Interconnections for Through-Silicon Vias

The relationship between the electrical properties and crystallographic quality (crystallinity) of electroplated copper thin-film interconnections was investigated. The crystallinity of the grains and grain boundaries of the interconnections was evaluated on the basis of the image quality (IQ) value obtained by electron back-scatter diffraction (EBSD) analysis. The electrical properties of the interconnections vary markedly depending on their crystallinity. The crystallinity also changed markedly as functions of electroplating conditions and the annealing temperature after electroplating. Although the electro migration (EM) resistance of the annealed interconnection was improved, stress-induced migration (SM) was activated by a high residual stress after annealing. To improve electrical reliability without heat treatment after electroplating, the effects of the seed layer under the interconnections on the crystallinity were investigated. As a result, the crystallinity was improved by changing the seed layer from Cu to Ru. In addition, the decrease in current density during electroplating also improved the crystallinity. Therefore, both introducing the Ru seed layer and decreasing the current density during electroplating are effective for developing highly reliable copper interconnections.

Mechanical and electrical reliability of copper interconnections for 3DIC

The mechanical and electrical properties of electroplated copper thin films were found to vary drastically depending on their electroplating conditions and thermal history after electroplating. The change of their microstructure was the main reason for the variation. The crystallinity of the micro texture was evaluated quantitatively by applying an electron back-scattering diffraction method.

Stress-Induced and Electro-Migration of Electroplated Copper Thin Film Interconnections Used for 3D Integration

Electroplated copper thin films have started to be applied to not only interconnections in printed wiring boards, but also thin film interconnections and TSV (Through Silicon Via) in semiconductor devices because of its low electric resistivity and high thermal conductivity. Thus, the electrical reliability of the electroplated copper interconnections was investigated experimentally. Self-made electroplated copper thin film interconnections were used for the evaluation. Electroplating conditions are as follows. The thin film interconnections were made by damascene process for electromigration tests. The applied current density during the test was varied from 1 MA/cm2 to 10 MA/cm2 . Abrupt fracture caused by the local fusion was often observed in the as-electroplated interconnections within a few hours during the test. Since there were a lot of porous grain boundaries in the as-electroplated thin films, the local high Joule heating should have caused the fusion at one of the porous grain boundaries. Actually, it was confirmed that the failure rate increased linearly with the square of the amplitude of the applied current density. However, the diffusion of copper atoms caused by electromigration was enhanced significantly when the film was annealed at 400°C. Many voids and hillocks were observed on their surfaces. This change of the fracture mode clearly indicates the improvement of the crystallographic quality of the annealed film. It was also observed that the stress-induced migration was activated substantially in the annealed film. Large voids and hillocks grew during the custody of the film even at room temperature without any application of current. This stress-induced migration was caused by the increase of residual tensile stress of about 200 MPa in the annealed film. It was also found that sulfur atoms segregated in the grown hillocks, though no sulfur atoms were found by EDX in the initial as-electroplated interconnections or other area in the annealed thin film interconnections. Thus, the hillock formation in the annealed interconnections was enhanced by the segregation of sulfur atoms. These sulfur atoms should have been introduced into the electroplated films during electroplating. Therefore, it is very important to control the micro texture, the residual stress and the concentration of sulfur in the electroplated copper thin film interconnections to assure the stable life, in other words, to eliminate their sudden brittle fracture and time-dependent degradation caused by the residual stress in the thin film interconnections.Copyright