Thermal Fatigue Failure Analysis of Copper Interconnects under Alternating Currents

In this paper, we present transmission electron microscopy (TEM) investigations of a potential failure mode for Cu lines subjected to alternating thermal loads. Wide Cu lines with 200 nm thickness were produced on a Si/SiO₂/SiN_x wafer by a series of conventional microfabrication steps. Alternating currents (AC) with a frequency of 100 Hz was then applied to the Cu lines and produced temperature cycles with a range of 190degC due to Joule heating. The cyclic temperature change gave rise to a cyclic strain in the Cu line due to the difference in the thermal expansion coefficient between the metal line and the wafer. The thermally-induced mechanical cyclic loading leads to the formation of severe damage in the Cu lines, such as thinning along the twins and wrinkles in the grain. Eventually the AC loading leads to local melting and electrical failure of the Cu line. TEM investigations revealed that constrained diffusional creep and the interaction of dislocations with twins and other interfaces play an important role in the development of damage. The results for Cu lines tested under thermal fatigue conditions are compared with the microstructure and damage morphology of films tested under pure mechanical fatigue and the important differences and the corresponding damage mechanisms are discussed