Barrier layer effects on electromigration reliability of Cu/low k interconnects

The effects of barrier thickness scaling and process changes on electromigration (EM) reliability were investigated for Cu/porous low k interconnects. Both EM strong mode lifetime and critical length-current density product (jL)_c were found to be almost independent of the Ta barrier thickness. The results can be accounted for by considering the structural confinement effect based on the effective modulus B. With reducing barrier thickness, early failures emerged in multi-link test structures degrading EM lifetime and the critical (jL)_c product. A non-optimized barrier deposition process can significantly alter the void formation site, leading to a reduction in EM lifetime and (jL)_c product. In this case, failure analyses by FIB and TEM have identified defects related to Cu out-diffusion to induce lifetime degradation and line shorting.