Large-scale statistical analysis of early failures in Cu electromigration, Part II: Scaling behavior and short-length effects

The first part of this study, presented in a separate paper, focused on the early failure mechanisms in down-flow electromigration. Since bimodality can occur at very small percentage levels, specific test structures were designed based on the Wheatstone Bridge technique. The use of these structures enabled a tested sample size past 800,000 for the 90 nm technology node, allowing a direct analysis of electromigration failure mechanisms at the single-digit ppm regime. The activation energy for the down-flow early failure mechanism was determined to be 0.83±0.01 eV, significantly lower than the usually reported activation energy of about 0.90 eV for electromigration-induced diffusion along Cu/SiCN interfaces. Very short experimental lifetimes due to small, slit-shaped voids under vias were found to control the chip lifetime at operating conditions. In this second part of our large-scale, statistical study, we will discuss the electromigration scaling behavior across 90, 65, and 45 nm technologies. Results indicate that the early failure mechanism follows the expected dependency, i.e., the lifetimes scale with the interconnect line height and the critical void size. The slitlike character of the early failure void morphology also raises concerns about the validity of the short-length effect for this mechanism. A very small amount of Cu depletion may cause failure even before a stress gradient is established. We therefore conducted large-scale statistical experiments close to the critical current density-length product (jL)^*. The results indicate that at very small failure percentages, the critical product extrapolates to about 2100±300 A/cm for SiCOH-based dielectrics in 90 nm technology. This value represents a decrease from the previously determined (jL)^* product of about 3000±500&#x- - 2002;A/cm for the same dielectric material and technology node, acquired with single link interconnects. Utilizing the advantages of the Wheatstone Bridge technique, the total sample size encompassing 90, 65, and 45 nm technologies was increased past 1.2×10⁶.