FAILURE OF AGED CU-AU THIN-FILMS BY KIRKENDALL POROSITY

Failure after accelerated aging of thermocompression-bonded thin films involving a CuAu couple is shown to be due to the Kirkendall effect. Plated Cu-Au thin films were heated at 350°C in either air or hydrogen in order to represent the case of a perfect sink or a perfect barrier. Film cross sections were studied with a scanning electron microscope and intermetallic phases were detected with a Seeman-Bohlin X-ray diffractometer. Kirkendall voids approximately 0.5 μm in diameter formed and coalesced within 1 h at 350°C to produce a fissure. The detection of voids at much lower temperatures than have been previously reported for CuAu is thought to be due to a high density of grain boundaries which act as sources and sinks for vacancies. Voids in the air-annealed films had twice the volume of voids in the hydrogen-annealed samples. This weak dependence on the ambient shows that grain boundary diffusion contributed to the Kirkendall effect. © 1979.