Takao Komiyama

High-Temperature Thick Al Wire Bonding Technology for High-Power Modules

In order to enhance the strength of thick Al wire bonds while eliminating damage, we have developed a new high-temperature thick Al wire bonding technology. The 300-µm-diameter Al wires were bonded to Al pads on an insulated gate bipolar transistor (IGBT) chip at varying substrate temperatures and ultrasonic powers. Al wire bonds joined at 423 K with 2.0 W ultrasonic power exhibited high bonding strength compared to those joined at room temperature with 5.0 W power. The main reason for the high bonding strength exhibited by the high-temperature bonding process with low ultrasonic power may be the ease of deformation of Al wires and Al electrode films which results in the enhancement of the true bonded area between Al wires and Al electrode films. We also confirmed that Si damage did not occur during this high temperature bonding process using low ultrasonic power. High-temperature thick Al wire bonding technology is considered to be a promising candidate for the production of reliable IGBT modules.

The Low Temperature Bonding between Si and Electrode Using Sputtered Ag Films

A low temperature Joining process using sputtered Ag films has been investigated. The surfaces of Ag films were cleaned by heating them to more than 523 K in air. After cleaning, the Si chip and the Mo can be joined using a Ag film under joining conditions as: temperature 503 K; compressive stress: 60 MPa; holding time: 0.5 h; atmosphere: air. A diode made with this low temperature joining process using Ag film exhibits an excellent low leakage current–voltage characteristics.

A Void Free Soldering Process in Large-Area, High Power Insulated Gate Bipolar Transistor Modules

We have developed a new void free process for making the solder joint between the chip mounted AlN substrate and the metal substrate in large-area, high power insulated gate bipolar transistor (IGBT) modules. This new process consists of two steps. First, Ar+ were used to clean the surfaces of Ni plated film on a metal and AlN substrates which were then coated with 0.5-µm-thick Ag film. Second, 50 wt% Pb–Sn solder was sandwiched between the two substrates and heated to 503 K in a vacuum for 5 min before being cooled in a N2 atmosphere. By using this process, the area percentage of voids in a soldering area up to 130×190 mm2 can be reduced to less than 0.1%. IGBT modules made by this process were also found to exhibit satisfactory current-voltage characteristics.

Grain-Oriented Ca3Co4O9 Thermoelectric Oxide Ceramics Prepared by Solid-State Reaction

We studied a method to enhance the degree of grain orientation of Ca3Co4O9 thermoelectric oxide ceramics. Ceramic specimens were prepared by solid-state reaction with different growth conditions. Large-grained Ca3Co4O9 powders were obtained by using “heavy-calcination” and “moderate-grinding” steps before pelletizing, and these large-grained powders contributed to the enhancement of the degree of orientation. Scanning electron microscopy (SEM) observation results showed that plate-like crystal grains were stacked up in layers for the heavily calcined ceramics, while no such anisotropic structure was found for those that were lightly calcined. x-Ray diffraction (XRD) analysis also indicated that the specimen obtained by heavy-calcination and moderate-grinding steps had a high degree of (002) orientation. The effect of the heavy-calcination and moderate-grinding steps was clearly evidenced by the electrical resistivity ρ. The electrical resistivity ρ at 700°C for the higher-oriented ceramics was 73% of that for the lower-oriented ceramics. Since ρ was reduced without deterioration of the Seebeck coefficient S, the power factor (S2/ρ) at 700°C for the former was increased by 29% compared with that for the latter.

Photoconductivity of the two polar surfaces of ZnO

Transient photoconductivity of the two polar surfaces of ZnO single crystal has been studied by using a fast-pulse technique. When ZnO crystal was irradiated by an ultraviolet (UV) light pulse, an obvious Dember effect was observed for both polar surfaces. The sign of the Dember effect indicated that the dominant photocarriers were electrons. Additionally, two distinct differences in UV photoconductivity were observed for the two polar surfaces. First, photocurrent Q was much larger for the O surface than for the Zn surface. The ratio of QO to QZn was about 100 when the applied electric field E was perpendicular to the c-axis and about 30 when E was parallel to it. Second, the excitation light intensity I dependence was quite different. While the photocurrent of the O surface was proportional to I, the photocurrent of the Zn surface was proportional to I1∕2. These results indicated that the recombination processes of photocarriers were quite different between O and Zn surfaces; namely, the direct recombin...

Magnetic properties of ZnO:V films formed by pulsed laser deposition with bias voltage application

Magnetic properties of ZnO films doped with V atoms (a 3d transition metal) were investigated by fabricating specimens using pulsed laser deposition (PLD) with bias voltage application. Electron concentrations of the ZnO:V films were changed by two methods: one was to dope with conventional n-type and p-type impurities, and the other was to change native (donor-type) defect concentrations by applying bias voltages during the film deposition. In M-H curves, smaller magnetization was observed for the film with a low electron concentration (5×1016 cm−3) and it increased linearly with the applied magnetic field, while larger magnetization was observed for the film with a medium electron concentration (5×1018 cm−3) and it exhibited a saturation at about 3 kOe. The saturation magnetization had maximum values for the electron concentrations of 1×1018–5×1018 cm−3 for the films formed by using either dopant impurities or PLD bias voltages. Anomalous Hall effect measurement of the films showed that the convergence ...

Hydrogen Annealing of ZnMgO Sol–Gel Films

ZnMgO films were formed on quartz substrates by a sol–gel technique using high (800 °C) and low (500 °C) temperature processes, followed by annealing in a hydrogen atmosphere at 420 °C. Photoluminescence (PL) measurements indicate that, for the low temperature process films, hydrogen annealing decreases the green band (500–565 nm) emission but it increases only slightly the near band edge (NBE) emission. For the high temperature process films, hydrogen annealing decreases the green band emission significantly and it also increases the NBE emission by three to four times. The effect of the hydrogen annealing on the PL property is explained by a conventional polycrystalline and grain boundary model. The above results indicate that the combination of the 800 °C crystallization process and the 420 °C hydrogen annealing process is very effective to obtain sufficiently strong NBE emission from the ZnMgO sol–gel films.

Thermoelectric Properties of Ca3Co4O9/[Ca2(Co0.65Cu0.35)2O4]0.624CoO2 Composites

Ca3Co4O9 is one of the most promising p-type thermoelectric materials because of its high dimensionless figure of merit ZT. However, polycrystalline Ca3Co4O9 ceramics shows lower ZT value than that for single crystal Ca3 Co4O9 due to its higher electrical resistivity ρ. Mikami et al. have reported that the addition of Ag to Ca3Co4O9 ceramics could successfully reduce ρ and enhance the power factor. On the other hand, Ohtaki et al. reported that a composite structure could be highly effective to reduce κ for ZnO dually doped with Al and Ga. In this work, we tried to enhance the power factor and reduce κ by forming Ca3Co4O9/[Ca2(Co0.65Cu0.35)2O4]0.624CoO2 composite structure. As a result, the ZT value for Ca3Co4O9/[Ca2(Co0.65Cu0.35)2O4]0.624CoO2 composites reached 0.164 at 700 °C, which was 40 % higher than the value for Ca3Co4O9.

Electroluminescence from ZnO/Si heterojunctions fabricated by PLD with bias voltage application

Electroluminescence (EL) for ZnO films has been investigated by fabricating n-ZnO/p-Si heterojunctions and changing the VI/II (O/Zn) ratio of the films. In the photoluminescence (PL) spectra, both the near band edge (NBE) emission and the defect-related emission were observed, while in the EL spectra only defect-related emission was observed. The EL spectra were divided into three components: green (550 nm), yellow (618 nm) and red (700 nm) bands; and their intensities were compared. As the VI/II (O/Zn) ratio was increased, the red band emission intensity decreased and the green band emission intensity increased. This implies that the oxygen and the zinc vacancies are related to the red and the green band emissions, respectively. Electron transitions from the conduction band minimum (Ec) to the deep energy levels of these vacancies are suggested to cause the red and the green luminescences while the energy levels of the Zn interstitials are close to the Ec in the band gap and no NBE emission is observed.