Blech Effect and Lifetime Projection for Cu/Low-K Interconnects

Electromigration in short Cu interconnects was studied under a number of conditions in Cu/Low-K technologies. Observed failure distributions displayed increasing dispersion of the failure times, characterized by the lognormal ¿, as the Blech critical threshold, (jL)c, was approached. Simulations show that the apparent increase in ¿ can be correlated to variations in current density which arise from variation in the interconnect cross section inherent to any manufacturing process. As a consequence, the median time to failure, t50, for a short line at a given current may be immortal, but failures may still occur at lower percentiles. Because of the significant ¿ variation (or deviation from lognormal distribution), the choice of failure time percentile results in different calculated (jL)c, which in turn will have significant impact on lifetime projections and circuit design applications. In addition, the (jL)c calculated from L/tinc vs jL is essentially independent of temperature, and the slope can be used to derive the activation energy. The derived activation energy for short interconnects agrees well with that calculated from Black´s law using 200um long lines with jL ≫≫ (jL)c.