Transmission electron microscopy of Au-Al wire bonded interface

In order to develop a microstructure observing technique for bonded interfaces, which is important in the IC industry, thin films of wire-bonded Au-Al samples were prepared by ion milling and microtome techniques and observed by transmission electron microscopy. The effect of heating on the microstructure and residual stress was also investigated by electron and X-ray diffraction. The results obtained are as follows. (1) In the films prepared with a microtome no amorphous phase is observed, whereas in the films prepared by ion milling a large part of the intermetallic compound formed on the interface is in the amorphous state. (2) The A1₂Au₅ phase grows predominantly among the five intermetallic compounds which can be formed on the interface. (3) Voids are produced by heating along the Au-Al₂Au₅ boundary. (4) A larger compressive residual stress is produced after heating on the Au side than on the Al side. From these results, a suitable technique for microstructure observation of the interface and the deterioration mechanism of the bonded interface are discussed.