Characteristics of leakage current in the dielectric layer due to Cu migration during bias temperature stress

The dielectric breakdown by Cu ion migration under applied bias is important for the reliability of damascene Cu interconnects as well as Cu through-hole bias in three-dimensional interconnects. The mechanism responsible for the leakage current through the dielectric during bias temperature stress is investigated in Cu/SiO2 and Al/SiO2 metal-insulator-semiconductor samples. When a constant electric field is applied, a gradual increase in leakage currents was observed in the case of Cu/SiO2, but no increase in leakage current before failure was observed for Al/SiO2. The voltage ramping tests conducted on the samples before and during the constant electric field application showed that the increase in leakage current in the Cu/SiO2 sample is by the Poole–Frenkel (PF) conduction mechanism. Cu ions, which migrate into the dielectric under the applied field, act as trap sites for PF conduction. This was confirmed by negative bias test as well as simulation of the leakage current by one-dimensional finite differential methods.