Electromigration extrusion kinetics of Cu interconnects

Electromigration lifetime and failure mechanism have been investigated for Cu/low-k interconnects at intermediate interconnect levels. It was observed that extrusion fails occurred mostly before resistance shift fails were detected. The activation energy for extrusion fails was determined to be 1.13 eV, comparable to the value of 0.99 eV for the resistance shift fails. This suggests the same failure mechanism for two failure modes: Cu mass transport primarily along the Cu/cap interface. The current exponent was extracted as 1.48 and 1.36 for extrusion fails and resistance shift fails, respectively. Physical failure analysis confirmed Cu extrusion near the anode and void formation at the cathode. Samples with improved pre-clean process before the cap deposition significantly suppressed EM induced extrusions, indicating a mechanically stronger Cu/cap interface. Furthermore, effective atomic sink at the anode end appeared to reduce the compressive stress buildup during EM, as it also significantly mitigated EM induced extrusion.