Fatigue of copper films subjected to high-strain rate thermo-mechanical pulsing

During service in power electronic applications, microelectronic chips encounter rapid changes in temperature, which are manifested by sub-millisecond-short heat pulses with amplitudes of a few hundred kelvin. When being subjected to such conditions repeatedly, in the long term, material fatigue of integrated metallization layers is caused, representing an important reliability issue. Within this work, dedicated test chips with integrated heating (poly-heaters) have been utilized to emulate aforementioned application-related loading conditions. Hence, the thermo-mechanical fatigue behavior of a 20-μm-thick copper metallization on silicon and its dependency on the two variation parameters starting base temperature and heating rate is studied. Monitoring of the electrical resistance and scanning electron microscopy image acquisitions have been performed in situ during the course of the stress tests comprising 12,000 heat pulses. For obtaining a more comprehensive picture of the investigated degradation behavior, focused ion beam inspections and roughness characterizations have been carried out at the end of the tests. For 0.2-ms-short heat pulses, distinct intergranular cracking is observed at base temperatures ≥95 °C. For longer heat pulses, a change in the dominant degradation mechanism from cracking to more and more pronounced dislocation-driven plastic deformation manifested in roughening is found.