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To determine the effects of grain structures on the rate of electromigration-induced failure of Cu interconnects, scanned laser annealing (SLA) has been used to produce Cu interconnects with very different grain structures. SLA, in which a moving hot-zone induces local grain growth, can be used to produce interconnects with fully bamboo grain structures that have bamboo grain lengths up to ten times the interconnect width. Electromigration experiments have been carried out on interconnects with very-long-grained bamboo structures, as well as on interconnects with polygranular structures in which the average grain size is less than the linewidth. Such differences are known to lead to orders of magnitude changes in lifetimes for Al-based interconnects. However, no significant differences in the failure rates were found for these Cu interconnects. This result supports earlier work that suggested that electromigration in Cu interconnects with now-standard liners and interlevel diffusion-barrier layers occurs by mechanisms that are faster than grain boundary self-diffusion.

We have conducted electromigration experiments and modeling on Cu Damascene structures surrounded by different interlevel dielectric ILD and Cu-cap materials. We have determined the mechanical properties of the surrounding ILD and Cu cap to play a key role in the critical stress change to void nucleation (Δσcrit), which is one of the critical parameters in determining electromigration lifetime or any other void-limited lifetime. Specifically, we found that Δσcrit decreases as the Young’s modulus of the interlevel dielectric decreases, which is the case with low-k materials. In order to compensate for the lower threshold to void nucleation in low-k materials, a stronger emphasis needs to be placed on the quality or adhesion of the Cu∕cap interface, which is currently the preferred site for void nucleation, so that interconnects fabricated in low-k materials continue to meet the ever-increasing electromigration reliability requirements. Finally, the methodology developed in this study, which is based on experiment and modeling, can be used to determine Δσcrit, and therefore the critical jL product, for any combination of ILD and Cu-cap materials.

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Plastic behavior has previously been observed in metallic interconnects undergoing high-current-density electromigration (EM) loading. In this study of Cu interconnects, using the synchrotron technique of white-beam x-ray microdiffraction, we have further found preliminary evidence of a texture correlation. In lines with strong (111) textures, the extent of plastic deformation is found to be relatively large compared with that of weaker textures. We suggest that this strong (111) texture may lead to an extra path of mass transport in addition to the dominant interface diffusion in Cu EM. When this extra mass transport begins to affect the overall transport process, the effective diffusivity, D eff , of the EM process is expected to deviate from that of interface diffusion only. This would have fundamental implications. We have some preliminary observations that this might be the case, and report its implications for EM lifetime assessment herein.

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.

A method has been developed for inducing controlled microstructural evolution in thin films patterned into lines, using scanned laser annealing (SLA). Experimental studies show at least three distinct types of microstructures resulting from SLA, depending on the scan rate and laser power. Starting with a polygranular initial microstructure, scanned laser annealing leads to either a large-grained polygranular structure, a bamboo structure, or an agglomerated structure. Microstructural evolution induced by SLA was found to lead to different evolution than conventional annealing, as well as to produce unique large-grained “bamboo” structures. Simulations of SLA further suggest the possibility of producing near-single crystal microstructures under properly controlled conditions.