Current-Induced Fatigue in Chip Level Interconnects

Aluminum interconnects on silicon-based substrates have been tested using high current density a.c. cycling. Joule heating within the lines induced temperature cycling over ranges of approximately 29-177 K during low frequency (100 Hz) testing at rms current densities in the range 6-14 MA/cm². Differential thermal expansion between the aluminum and the substrate resulted in a cyclic total strain amplitude in the range 0.06-0.35%, and corresponding biaxial cyclic stress amplitudes of approximately 61-368 MPa. We will present observations of damage and lifetime behavior in these interconnects that are partially consistent with conventional fatigue approaches to damage evolution. For instance, damage during early stages of cycling showed site selectivity, with surface offsets being confined to individual grains. Continued cycling increased the severity of damage, as well as the proportion of surface area damaged, with final failure occurring in the form of open circuit. This happened at locations where the interconnect thinned down excessively, presumably due to localized plasticity and melting. Some observations are not easily described by conventional fatigue concepts as applied to bulk metals, such as lack of lower energy dislocation arrangements after millions of cycles, absence of microcracking, and the formation of whiskers. Effects of current density, interconnect geometry, and encapsulating materials will be discussed. We suggest that thermomechanical fatigue may pose an important reliability problem for copper-low-k dielectric interconnect systems in the context of low frequency operation, energy-saving modes, and power cycling.