The effect of interlevel dielectric on the critical tensile stress to void nucleation for the reliability of Cu interconnects

Abstract

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.