Toshiyuki Nagase

Development of a surface-treatment method for AIN substrates to improve adhesion with thick-film conductors

A surface-treatment method for AIN substrates to improve adhesion with thick-film conductors has been developed based on the interfacial reaction mechanism between 96% alumina and glasses. The surface-treated AIN substrates were prepared through a combination of oxidation and sol-gel processes. An oxidation atmosphere with high PO2 and low PH2O, which produced the densest Al2O3 layer on AlN, was selected as the oxidation conditions for the surface treatment. A SiO2 layer was deposited on the Al2O3 layer, using the dipping technique with a sol-gel solution. The surface-treated AlN substrates with adequate thicknesses of Al2O3 and SiO2 as the surface-treatment layers showed a good adhesion strength with conventional thick-film conductors. This is the same result as with the 96% alumina substrates, although the adhesion strength of the conductors with bare AlN substrates was poor. These results suggest that glasses in the thick-film conductors react with the surface-treated AlN substrates in the same way as ...

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.