Electromigration of silicon and phosphorus in tantalum polycide interconnections

TaSi2/n⁺-poly-Si interconnections of integrated circuits were investigated after high direct current density stress at elevated temperatures. The current density was increased up to values that caused failures of the interconnections due to electromigration. We found in our experiments that material migrated from the anode to the cathode causing open-circuit failure at the anode region. At the cathode, the material piled up forming hillocks. Material depletion and accumulation are caused by positive or negative mass flow divergences due to temperature gradients near the pads. The accumulated material at the cathode was analyzed by high-resolution Auger and SIMS spectroscopy. The obtained spectra show only silicon and phosphorus peaks with an increased phosphorus concentration at the cathode area. Hence, it can be concluded that in TaSi2/n⁺-poly-Si interconnection mainly material of the polysilicon layer including the dopant phosphorus is moved by electromigration. In contrast to these results of dc experiments, no electromigration occurred during equivalent ac stress.