De-Coupling Thermo-Migration from Electromigration Using a Dedicated Test Structure

Using a test structure with copper (Cu) injectors directly connecting a tungsten (W) line, we quantify the impact of thermal gradients on the lifetime of Cu interconnects. The W-line average self-heating is assessed experimentally and the temperature at the Cu is estimated by using a calibrated Finite Element electro-thermal model. Assuming no contribution of thermo-migration, a maximum likelihood fitting of the lifetimes to the Arrhenius equation provides an activation energy of ~ 0.8 eV which is low compared to the pure activation energy for electromigration, EM-EA, for this technology, suggesting that electromigration is not the only contributing failure mechanism. The contribution of thermo-migration is assessed with a 1D physics-based model which predicts that, under the given conditions, the contribution of electromigration and thermo-migration to void nucleation is ~3x faster than only electromigration. Correcting for this factor gives an activation energy of 0.89 eV which is close to the measured activation energy for electromigration for this technology. This finding allows a more precise estimation of interconnect lifetimes under normal operating conditions with high thermal gradients.