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Electromigration experiments were conducted to investigate the thresholds required for electromigration-induced extrusion failures in Cu/low-k interconnect structures. Extrusions at the anode were observed after long periods of void growth. Characterization of failure sites was carried out using scanning and transmission electron microscopy, which showed that failures occurred through delamination at the interface between the silicon-nitride-based capping layer diffusion barrier and the underlying Cu, Ta liner, and interlevel dielectric (ILD) materials. This interface is subjected to near tensile (mode I) loading with a mode mixity angle between 4° and 7°, estimated using finite-element-method analysis, as electromigration leads to a compressive stress in the underlying Cu. Comparisons of the fracture toughness for interfaces between the capping layer and individual underlayer materials indicate that the extrusion process initially involves plane-strain crack propagation. As Cu continues to extrude, the crack geometry evolves to become elliptical. An analysis of the critical stress required for extrusions based on these observations leads to a value of approximately 710 MPa, which agrees well with the value determined through estimation of the volume of material extruded and the required stress to accomplish this extrusion. The analysis of the critical stress required for extrusion formation also indicates that sparsely packed, intermediate to wide interconnect lines are most susceptible to electromigration-induced extrusion damage, and that extrusion failures are favored by ILDs with low stiffness (low elastic moduli) and thin liners, both of which are needed in future interconnect systems.

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Abstract--The effects of interconnect length on the reliability of dual-damascene Cu metallization have been investigated. As in Al-based interconnects, the lifetimes of Cu lines increase with decreasing length. However, unlike Al-based interconnects, no critical length exists, below which all Cu lines are 'immortal'. Furthermore, we found multi-modal failure statistics for long lines, suggesting multiple failure mechanisms. Some long Cu interconnect segments have very large lifetimes, whereas in Al segments, lifetimes decrease continuously with increasing line length. It is postulated that the large lifetimes observed in long Cu lines result from liner rupture at the bottom of the vias, which allows continuous flow of Cu between the two bond pads. As a consequence, the average lifetimes of short lines and long lines can be higher than those of lines with intermediate lengths.

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