Electric current–assisted direct joining of silicon carbide

Conventional direct joining technologies are difficult to use with silicon carbide (SiC) materials, especially for fiber composite forms of SiC, because of the harsh conditions required. To reduce the temperature and/or process time required for the direct joining process, an electric current–assisted joining (ECAJ) method was studied. Joining of low–resistivity grade, nitrogen doped β-SiC was demonstrated at a relatively low nominal temperature of 1750 °C with a 10 min hold by enhancing the passage of current through the material. The joining mechanism is discussed in terms of localized overheating and accelerated self-diffusion at the interface. In the case of joining at 2160 °C for 1 min, rapid crystal growth of textured SiC was found at the interface. This study indicates that rapid ECAJ-based direct joining is a practical and appropriate method for joining SiC-based materials.     

Investigation of combined stress state failure criterion for glass fiber/epoxy interface by the cruciform specimen method

The interfacial failure criterion under combined stress state in a glass fiber/epoxy composite is investigated by the cruciform specimen method. Experiments were conducted by using specimens with a fiber whose angle from the loading direction is varied in order to make various stress state of normal and shear at the interface. Finite element analysis is performed to calculate the interfacial stress distribution. By combining the experimental measurement of the specimen stress at the interfacial debonding initiation and the finite element stress analysis, it is possible to obtain the interfacial stress state at interfacial failure. A method to determine the interfacial failure criterion and the interfacial failure initiation location simultaneously is proposed in the present study. We conclude the value of the interfacial shear strength is higher than that of the interfacial normal strength for the material system used in the present study.     

Hydration of ethene over W–P mixed metal oxide catalysts

W–P mixed metal oxide catalysts are active and selective for the gas-phase hydration of ethene to ethanol. The activity and selectivity of this catalytic reaction depend on the W/P atomic ratio. However, ethene conversion slightly decreases at higher W/(W + P) atomic ratio. The selectivity for ethanol increases with the W/P atomic ratio and reaches the highest value (92%) at W_0.81P_0.19O_x. The W_0.81P_0.19O_x catalyst is less active than the conventional H₃PO₄/SiO₂ catalyst, but the activity is maintained for a long time without the supply of any catalyst components. The reaction temperature does not affect substantially the rate of ethene hydration over the W_0.81P_0.19O_x catalyst. The H₂O/ethene molar ratio of 0.4 is the most appropriate for both reaction rate and selectivity. The active species of W–P mixed metal oxide are amorphous. But there is Keggin structure of W–P oxide species (PW₁₂O₄₀ ³⁻) in the presence of steam. And the species are the active sites for the hydration of ethene, confirmed by in situ Raman spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Determinant in human immunodeficiency virus type 1 for efficient replication under cytokine-induced CD4(+) T-helper 1 (Th1)- and Th2-type conditions

Cytokines are potent stimuli for CD4(+)-T-cell differentiation. Among them, interleukin-12 (IL-12) and IL-4 induce naive CD4(+) T cells to become T-helper 1 (Th1) or Th2 cells, respectively. In this study we found that macrophage-tropic human immunodeficiency virus type 1 (HIV-1) strains replicated more efficiently in IL-12-induced Th1-type cultures derived from normal CD4(+) T cells than did T-cell-line-tropic (T-tropic) strains. In contrast, T-tropic strains preferentially infected IL-4-induced Th2-type cultures derived from the same donor CD4(+) T cells. Additional studies using chimeric viruses demonstrated that the V3 region of HIV-1 gp120 was the principal determinant for efficiency of replication. Cell fusion analysis showed that cells expressing envelope protein from a T-tropic strain effectively fused with IL-4-induced Th2-type culture cells. Flow cytometric analysis showed that the level of CCR5 expression was higher on IL-12-induced Th1-type culture cells, whereas CXCR4 was highly expressed on IL-4-induced Th2-type culture cells, although a low level of CXCR4 expression was observed on IL-12-induced Th1-type culture cells. These results indicate that HIV-1 isolates exhibit differences in the ability to infect CD4(+)-T-cell subsets such as Th1 or Th2 cells and that this difference may partly correlate with the expression of particular chemokine receptors on these cells. The findings suggest that immunological conditions are one of the factors responsible for inducing selection of HIV-1 strains.     

Low-Resistance Cu-Sn Electroplated–Evaporated Microbumps for 3D Chip Stacking

Low-resistance copper-tin (Cu-Sn) microbumps, with sizes varying from 5 μm × 5 μm to 20 μm × 20 μm and formed by electroplating–evaporation bumping (EEB) technology for three-dimensional integration of large-scale integrated chips, have been evaluated for their microstructure and electrical resistance. It was inferred from x-ray diffraction data that the formation of low-resistance Cu₃Sn intermetallic compound (IMC) is facilitated at higher bonding temperature. Electron probe microanalysis mapping showed that, even before bonding, Cu-Sn IMCs were formed at the interface between Cu and Sn, whereas they were sandwiched between the Cu of the upper and lower microbumps after bonding. Electron backscatter diffraction analysis revealed that the crystal orientation of Sn grains was sharply localized in the (100) orientation for physical vapor deposited (PVD) sample, while electroplated Sn film exhibited a mixed crystal orientation in all (100), (110), and (001) axes. A resistance value of ~35 mΩ per bump was obtained for Cu-Sn microbumps with area of 400 μm², which is several times lower than the resistance value reported for Cu-Sn microbumps fabricated by a pure electroplating method. The low resistance value obtained for EEB-formed Cu-Sn microbumps after bonding is explained by (i) the reduced surface roughness for evaporated Sn, (ii) the high degree of crystal grain orientation resulting from layer-by-layer growth in the PVD Sn, despite their smaller grain size, and (iii) the absence of impurity segregation at grain boundaries.     

Automatic parameter optimization of the local model fitting method for single-shot surface profiling

The local model fitting (LMF) method is a single-shot surface profiling algorithm. Its measurement principle is based on the assumption that the target surface to be profiled is locally flat, which enables us to utilize the information brought by nearby pixels in the single interference image for robust LMF. Given that the shape and size of the local area is appropriately determined, the LMF method was demonstrated to provide very accurate measurement results. However, the appropriate choice of the local area often requires prior knowledge on the target surface profile or manual parameter tuning. To cope with this problem, we propose a method for automatically determining the shape and size of local regions only from the single interference image. The effectiveness of the proposed method is demonstrated through experiments.     

Stability of Cu(In,Ga)Se2 solar cells and evaluation by C–V characteristics

To confirm the long-term reliability of Cu(In,Ga)Se₂, (CIGS) solar cells, we investigated the I–V and C–V characteristics during tests under irradiation or dark condition. Under irradiation, the test samples showed a little increase in efficiency (η) and open-circuit voltage (V_oc) which showed their electrical durability to light irradiation. But the diode factor (n) and series resistance (R_s) showed large changes in value. Also, the built-in voltage (V_b) and density gradient (dN_A/d_x) in the CIGS layer calculated from the C–V characteristics showed distinct changes during the test. After 4 SUN irradiation, two samples in the same fabrication-lot showed new light absorption in the lower-energy range than sun the energy gap of CIGS. We explain the change of C–V characteristics for the samples under strong irradiation with a new model named “Junction retrograde” which can treat defect generation by irradiation to reduce the acceptor density in graded p-n junction. This model for C–V analysis can be used to investigate the long-term reliability of CIGS solar cells under irradiation.     

Influence of moisture on BCN (low-K) film for interconnection reliability

The integration of low dielectric constant (low-K) interlayers and Cu wiring is necessary to produce next-generation LSI devices. Low-K films of porous type have been investigated to reduce a dielectric constant (K value). However, most porous low-K materials have faced a serious problem of water incorporation during the wet processes used in making the interconnections. Thus, the influence of moisture on boron carbon nitride (BCN) films is important to investigate. To study water-treated BCN films, we measured the current-versus-voltage (I–V) and capacitance-versus-voltage (C–V) characteristics of BCN films, using an MIS (mercury electrode/BCN/Si substrate) structure. We found that both the leakage current and dielectric constant of BCN films decrease as the film's carbon-composition ratio increases. D₂O detection using thermal desorption spectroscopy (TDS) reveals that BCN films with carbon-composition ratios greater than 30% can suppress the incorporation of water into the film. Desorption of hygroscopic water from the BCN film occurred at temperatures as low as 390 °C, which is a normal LSI-production process temperature. The use of BCN film appears to resolve the serious problem of water incorporation into the porous low-K materials. In addition, we have investigated a conduction mechanism resulting from the water protons, based on the frequency dependence of the BCN film impedance before and after water treatment.