Microstructural Investigation of Interfacial Features in Al 
Wire Bonds
In the present study the microstructure of ultrasonically bonded Al wires on AlSiCu and AlSi metallization was investigated by means of scanning electron microscopy, electron back-scattered diffraction, and high-resolution transmission electron microscopy techniques. Detailed microstructural investigations were conducted on samples in the as-bonded condition, subsequent to power cycling tests, and after long-time thermal exposure to reveal the temperature-dependent evolution of the interfaces and the metallization layer. Typical interfacial features were found to be ultrafine and nanoscaled grains of Al and Al2O3, amorphous Al oxide particles, voids, and pores, with regions of high density of dislocations and dislocation loops within the larger grains of the wire and metallization. The observed interface features confirm the suggested mechanism of formation of bonding interface by emergence of submicron grains at the thin interfacial boundary between the metallic pair as a result of dynamic recrystallization and interdiffusion. While isothermal and/or thermomechanical cycling lead to strong grain growth in the metallization layer and the Al wire, the nanostructured interfacial regions mainly remain, indicating a high thermal stability and strength of the interface. Furthermore, evaluation of a large number of wire bonds prepared using standard bonding conditions showed the presence of a certain percentage of nonbonded areas and microstructural variations between the interconnects processed under nominally identical conditions. However, it was found that, if a sufficient effective bonding interface is provided, the long-time reliability of Al wire bonds is maintained due to the stability and strength of the nanostructured interface.