Yoshiyuki Nagatomo

Scanning electron microscope observation of dislocations in semiconductor and metal materials

Scanning electron microscope (SEM) image contrasts have been investigated for dislocations in semiconductor and metal materials. It is revealed that single dislocations can be observed in a high contrast in SEM images formed by backscattered electrons (BSE) under the condition of a normal configuration of SEM. The BSE images of dislocations were compared with those of the transmission electron microscope and scanning transmission electron microscope (STEM) and the dependence of BSE image contrast on the tilting of specimen was examined to discuss the origin of image contrast. From the experimental results, it is concluded that the BSE images of single dislocations are attributed to the diffraction effect and related with high-angle dark-field images of STEM.

Atomic structures of a liquid-phase bonded metal/nitride heterointerface

Liquid-phase bonding is a technologically important method to fabricate high-performance metal/ceramic heterostructures used for power electronic devices. However, the atomic-scale mechanisms of how these two dissimilar crystals specifically bond at the interfaces are still not well understood. Here we analyse the atomically-resolved structure of a liquid-phase bonded heterointerface between Al alloy and AlN single crystal using aberration corrected scanning transmission electron microscopy (STEM). In addition, energy-dispersive X-ray microanalysis, using dual silicon drift X-ray detectors in STEM, was performed to analyze the local chemistry of the interface. We find that a monolayer of MgO is spontaneously formed on the AlN substrate surface and that a polarity-inverted monolayer of AlN is grown on top of it. Thus, the Al alloy is bonded with the polarity-inverted AlN monolayer, creating a complex atomic-scale layered structure, facilitating the bonding between the two dissimilar crystals during liquid-phase bonding processes. Density-functional-theory calculations confirm that the bonding stability is strongly dependent on the polarity and stacking of AlN and MgO monolayers. Understanding the spontaneous formation of layered transition structures at the heterointerface will be key in fabricating very stable Al alloy/AlN heterointerface required for high reliability power electronic devices.

Fracture Process of Aluminum/Aluminum Nitride Interfaces during Thermal Cycling

Al circuit substrates, which are composed of a sintered AlN plate and pure Al plate joined to both sides of the AlN plate, are used for semiconductor power devices. It is important to prevent fracture of the Al/AlN interface to ensure normal and stable device operation. In this study, the fracture process of Al/AlN interface during thermal cycling was investigated using advanced scanning electron microscopy (SEM). Al circuits joined to an AlN plate were plastically deformed with thermal cycling. Al grains were divided with the formation of sub-boundaries due to the plastic deformation. After 2000 thermal cycles, a crack was generated at edges of the Al/AlN interface and propagated gradually to the center of the substrate. Cross-sectional observation, using an angle selective backscattered electron detector (AsB), revealed that the Al grain size near the Al/AlN interface decreased to 3 m or less, and the crack proceeded along the Al grain boundaries. To clarify the temperature dependence of the fracture process, a repeated bending test was performed at various temperatures. Shear strains were induced at the Al/AlN interface by the repeated bending. The rate of crack propagation tends to be higher at higher temperatures for bending test. In substrates bent at 373 K or higher, the crack proceeded after the Al grains had been refined. These results indicate that fine-grained Al resulting from thermal cycling is formed by creep deformation and recrystallization at higher temperatures. Thus, improving the creep strength of the Al plate is thought to be effective for prevent cracking during thermal cycling. The effect of additive elements in the Al plate was also discussed in this study.

Novel silver die-attach technology on silver pre-sintered DBA substrates for high temperature applications

Abstract Currently, Ag die-attach techniques, using nano-silver particles, are of high interest for manufacturing of wide-band-gap (WBG) power module due to their high-temperature operation capability. However, the high cost of silver and complicated processing requirements are the main driving force in the search for simpler and more cost-effective attached technologies. In this study, a new die-attach technique based on silver die-attach, without conventional Ag-paste, for high-temperature applications is developed. Glass containing Ag paste was pre-sintered on the DBA substrates, and later on, semiconductor dies were simply placed on this pre-sintered Ag layer and attached under heat and pressure. The samples were tested under shear and thermal cycling loadings (−45 °C/250 °C) to evaluate the quality and reliability. Destructive and non-destructive analysis methods, such as Scanning Acoustic Tomography and cross-section observation, were used to identify fracture modes. The samples demonstrated sufficient shear strength and high thermal reliability. Furthermore, the effects of Ag recrystallization, grain growth and rearrangement of the voids are considered to be the main fracture factor of conventional Ag die-attach joints based on samples cross-sections.