Current Crowding Impact on Electromigration in Al Interconnects

Abstract

The current flow through interconnects can be locally crowded and its impact on electromigration (EM) has not been studied well. We design test structures with different levels of current crowding and test its impact on EM Lifetime. We also simulate the current crowding of each case and correlate them with the observed EM lifetimes. Even if there is localized hot spot having very high current density but the region is small, the impact on EM lifetime is small. For example, EM lifetime decreases less than 4% for 24% local current density increase. The extent of impact is a function of length of the crowding as well as the local current density. Including both the current density and the length effects on EM, we propose a quantifiable local current crowding factor (LCCF). The EM lifetime depends on the power of the LCCF with variable power exponents. The power exponent increases with LCCF and reaches a maximum value before starting to decrease to zero. At very large LCCF, when currents are crowded over long length, the impact is not localized anymore and the additional lifetime reduction by additional current crowded space is zero. So the power exponent is zero for the case. Utilizing the LCCF provides a practical method for how to address the inevitable local current crowding and enhance the EM check accuracy especially for the analog semiconductor circuits.