Electromigration study of Cu/low k dual-damascene interconnects

Electromigration lifetime and failure mechanism have been investigated for Cu/SiLK(TM) interconnects. The activation energies of Cu/SiLK and Cu/oxide were found to be 0.98 eV and 0.81 eV, respectively. The activation energy in the range of 0.8 to 1.0 eV suggests a similar mass transport mechanism that can be attributed to interfacial diffusion. The average lifetime of Cu/SiLK was found to be shorter than that of Cu/oxide at test temperatures. The threshold critical length product of Cu/SiLK structures was determined to be about 1/3 of that of Cu/oxide structures. Failure analysis by FIB revealed a distinct failure mode due to lateral Cu extrusion at the low k/oxide etch stop interface. These results together with the increase observed in Joule heating and thermal resistance show that the thermomechanical properties play an important role in controlling the EM reliability of the low k interconnects. Results of this study suggest that the degradation in thermomechanical properties, in particular interfacial adhesion, reduces the back-flow stress, leading to faster mass transport, shorter EM lifetime and Cu extrusion at the anode in the SiLK structures.