Kazuhiro Tsuruta

Control circuit in an in-pipe wireless micro inspection robot

We have been developing an in-pipe wireless micro robot for inspection on inner surface of pipes. The robot consists of a CCD camera, a locomotive device, a system control circuit and wireless energy supply and communication devices. The robot moves in a 10 mm diameter pipe without wire and observes the inner surface of the pipe using the installed CCD camera. We have developed a compact control circuit which controls all the devices installed in the robot by commands from outside and transmits the image data from the CCD camera. As for the control circuit, the power consumption and the size are greatly restricted in order to be installed in the robot. In order to reduce the size of the circuit, we have newly developed an image data communication LSI based on a new architecture. The LSI is made of 0.35 /spl mu/m CMOS technology and has the size of 3.9 mm by 3.9 mm. To make the control circuit compact, we used a flip chip assembly for the LSI and eight more ICs in the robot. Through a fabricated prototype of the micro robot, we have successfully confirmed the wireless image data communication of 2.27 frames per second and control of the robot by microwave technology.

Mechanical Deformation of Sintered Porous Ag Die Attach at High Temperature and Its Size Effect for Wide-Bandgap Power Device Design

The mechanical properties of sintered Ag paste with microporous structure have been investigated by tensile and shear tests, focusing on the temperature-dependent plastic deformation at various temperatures from 25°C to 300°C, corresponding to the target operating temperature range of emerging wide-bandgap semiconductor devices. Specimens were prepared by sintering hybrid Ag paste consisting of microflake and submicron spherical Ag particles, simulating a typical bonding process for power semiconductor die attach. Mechanical tests revealed that the unique microstructure caused a brittle-to-ductile transition at temperature of around 160°C, remarkably lower than that of bulk Ag. The obtained Young’s modulus and shear modulus values indicate obvious softening with increasing temperature, together with a remarkable decrease in Poisson’s ratio. These plastic behaviors at elevated temperature can be explained based on Coble creep in the microporous network structure. Fracture surfaces after tensile and shear tests indicated unique features on scanning electron microscopy, reflecting the variation in the ductile behavior with the test temperature. Furthermore, these temperature-dependent mechanical parameters were employed in three-dimensional finite-element analysis of the thermomechanical stress distribution in wide-bandgap semiconductor module structures including Ag paste die attach of different sizes. Detailed thermal stress analysis enabled precise evaluation of the packaging design for wide-bandgap semiconductor modules for use in high-temperature applications.

6-in-1 Silicon carbide power module for high performance of power electronics systems

The excellent characteristics (low power loss, high speed/high temperature operation) of SiC semiconductors can contribute to realizing smaller power converter with a higher power output. Using our own packaging technology of double-sided cooling and SiC devices, we have developed the new, small 6-in-1 power module with high output power density. If the inductance of the main circuit is large, it will cause a large surge voltage when switched. Therefore, we have incorporated the optimum low inductance structure into this module, and have made it possible to drive the SiC device at high speed. Using this module, we have built a prototype of inverter unit with 75 kW output power, and have achieved an efficiency of 99% and a power density of 100 kW/L.

Reverse Electrical Characteristics of 4H-SiC JBS Diodes Fabricated on In-House Substrate with Low Threading Dislocation Density

The impacts of threading dislocations, surface defects, donor concentration, and schottky Schottky barrier height on the reverse IV characteristic of silicon carbide (SiC) junction barrier schottky Schottky (JBS) diodes were investigated. The 100 A JBS diodes were fabricated on 4H-SiC 3-inch N-type wafers with two types of threading dislocation density. The typical densities are were 0.2×104 and 3.8×104 cm-2, respectively. The improvement of vIt was found that variations in the leakage current and the high yield of large area JBS diodes werecould be were obtained improved by using a wafer with a low threading dislocation density. In the range of low leakage current, the investigation shows showed a correlation between leakage current and threading dislocation density.

Self-healing of cracks in Ag joining layer for die-attachment in power devices

Sintered silver (Ag) joining has attracted significant interest in power devices modules for its ability to form stable joints with a porous interconnection layer. A function for the self-healing of cracks in sintered porous Ag interlayers at high temperatures is discovered and reported here. A crack which was prepared on a Ag joining layer was closed after heating at 200 °C in air. The tensile strength of pre-cracked Ag joining layer specimens recovers to the value of non-cracked specimens after heating treatment. Transmission electron microscopy (TEM) was used to probe the self-healing mechanism. TEM images and electron diffraction patterns show that a large quantity of Ag nanoparticles formed at the gap with the size less than 10 nm, which bridges the crack in the self-healing process. This discovery provides additional motivation for the application of Ag as an interconnection material for power devices at high temperature.

Low-Stress Design for SiC Power Modules with Sintered Porous Ag Interconnection

The mechanical properties of sintered porous Ag-paste are investigated by tension test and shear test in the temperature range from 25 °C to 300°C. Stress-strain curves of sintered porous Ag-paste are measured at different temperatures. The Poissons ratio, which is calculated by Youngs modulus and shear modulus, decreased from 0.31 at 25 °C to 0.11 at 300 °C. In addition, 3D finite element model (FEM) is constructed for six types of module structures under thermal cycling analysis, which focuses particularly on the stress of sintered porous Ag-paste at different temperatures to find an optimized low-stress structure for the long-term reliability. The obtained results in this study suggested that the sintered porous Ag-paste may survive longer and continue to functions at high temperature variations because of the large plastic deformation.

High-Temperature Die Attachment Using Sn-Plated Zn Solder for Power Electronics

Pure Zn is a Pb-free die-attach material considered suitable for application in high-temperature electronics such as widebandgap semiconductor devices, as it has both the requisite electrical/thermal conductivity and an excellent thermal shock resistance. The high melting point of Zn (419.5 °C), however, means that die-attachment using pure Zn would require higher bonding temperatures (>430 °C), whereas a lower temperature is more desirable for mass production. This paper, therefore, presents a bonding method based on using Sn-plated Zn solder (Sn/Zn/Sn structure) to achieve a lower bonding temperature than pure Zn die attachment. The resulting shear strength of this bonding exceeds 25 MPa at 350 °C for 30 min, which is attributed to a uniform and complete Ni-Zn IMC (γ-Ni5Zn21) reaction layer. Furthermore, this bonding strength is retained beyond 25 MPa without serious degradation, even after thermal shock testing within a temperature range of -50 °C-300 °C; and thus, the Sn-plated Zn solder proposed has great potential as a die-attach material for high-temperature applications such as power devices.

Low-pressure sintering bonding with Cu and CuO flake paste for power devices

Low-temperature sintering bonding has been proposed as an alternative technique for the soldering to overcome such high operating temperature in wide-gap semiconductor power devices. Ag nanoparticle sintering is one of the candidates in die-attach bonding, but there are certain obstacles for mass production mainly due to the high cost of silver. In addition, metal nano-particle paste including Ag nanoparticle paste bonding needs to apply certain high pressure of MPa order. For mass productions, it is necessary to decrease the applying pressure during the bonding process. In the present study, the authors make flake-shaped Cu based particles by using mechanical milling for improving the contact area between the particles to decrease the required pressure. The die-bonding with Cu flake pastes was carried out at 300 °C with a formic acid. Resulting die-shear strength exceeds 15 MPa for bonded at 300 °C for 60 minutes low pressure (0.4 MPa). Moreover, Cu flake pastes with polyethylene glycol (PEG) solvent showed solid interface layer like bulk Cu. Thus, the Cu flake PEG paste is one of the most promising bonding materials with the remarkably high strength of the sintered bonding.

Impact of Surface Morphology above Threading Dislocations on Leakage Current in 4H-SiC Diodes

The impact of threading dislocation density on the leakage current of reverse IV characteristics in 1.2 kV Schottky barrier diodes (SBDs), junction barrier Schottky diodes (JBSDs), and PN junction diodes (PNDs) was investigated. The leakage current density and threading dislocation density have different positive correlations in each type of diode. For example, the correlation in SBDs is strong, but weak in PNDs. The threading dislocations were found to be in the same location as the current leakage points in the SBDs, but not in the PNDs. Nano-scale inverted cone pits were observed at the Schottky junction interface in SBDs, and it was found that leakage current increases in these diodes due to the concentration of electric fields at the peaks of the pits. These nano-scale pits were also observed directly above threading dislocations. In addition, this study succeeded in reducing the leakage current variation of 200 A-class JBSDs and SBDs by eliminating the nano-scale pits above the threading dislocations. As a result, a theoretical straight-line waveform was achieved.

Effect of Damage Removal Treatment after Trench Etching on the Reliability of Trench MOSFET

Guaranteeing the reliability of gate oxides is one of the most important topics to realize regarding the SiC power MOSFET. In the case of trench MOSFET, since the gate oxides are formed on the trench sidewall, the damage and roughness on the trench sidewall can affect the lifetime of the gate oxides. Generally speaking, damage removal treatment is processed after trench dry etching in most cases. In Si processes, sacrificial oxidation, H2 anneal and CDE (Chemical Dry Etching) are adopted commonly. In the case of SiC processes, sacrificial oxidation, H2 anneal, and SiH4/Ar anneal have been reported. Neverthless CDE which applied to SiC trench MOSFET has few precedents. We clarified the effect of CDE as a damage removal process. CDE has the effect of flattening the trench sidewall, and CDE makes the lifetime of gate oxides improve. CDE is an effective process for the reliability of SiC trench MOSFET.