The effect of low-k ILD on the electromigration reliability of Cu interconnects with different line lengths

We have compared the electromigration performance of Cu electromigration structures with varying line lengths imbedded in two different ILD materials. In the regime of high jL where there is no significant back-stress, we observed that three of the key electromigration parameters (i.e., MTF, n, and σ) are constant and approximately equivalent between the two materials. In the regime of lower jL where there is significant back-stress, both materials exhibit similar trends, however, the Cu with low-k material performed relatively worse in terms of MTF and similarly in terms of n and σ. That is, while the MTF of Cu with both materials increased with decreasing jL, the MTF of Cu with low-k material was less than that of the Cu with SiO₂-based material due to lower back-stress at a given jL. Further, while a regime of complete immortality was observed for the SiO₂-based material, no regime of immortality was observed for the low-k material. The values of n and σ were comparable for both materials, and were constant in the absence of significant back-stress but increased in the presence of significant back-stress. Due to the higher MTFs in the regime of high backstress, MTF is more sensitive to j and L, thereby increasing n as per Black´s Law. The increase in σ is a consequence of heightened sensitivity to process variations such as via barrier integrity and CD variation.