A fundamental AC TDDB study of BEOL ELK in advanced technology

In this study, we thoroughly investigated AC TDDB in BEOL extreme low-k (ELK) dielectric in 10nm technology. We demonstrated that AC TDDB of ELK dielectric has better median-time-to-failure (MTTF) and also much tighter Weibull distribution than constant voltage stress (DC TDDB). In unipolar AC TDDB stress, a very significant recovery process was observed. Through the capacitance recovery analysis, the capture and emission time constants for ELK were found to be ≤ 10 ^-3 and ~10 ^-8 seconds respectively, which implies that there is low probability of charge trapping when the stress period is as fast as 10 ^-3 seconds; meanwhile the charge detrap happens continuously when the stress period is lower than 10 ^-8 seconds. As a result, the unipolar AC TDDB lifetime increased with increasing frequency. In addition, the unipolar AC TDDB improvement shows a power law dependence on the duty ratio due to a very significant charge trap/detrap effect. This is further validated through a physics-based simulation. On the other hand, bipolar AC stress caused the ion diffusion to be accompanied by the backflow Cu ion drift, thus extending the defect growth rate and improving the TDDB performance. However, no significant frequency and duty ratio dependence on the bipolar AC to DC ratio was found because the critical Cu ion concentration was constrained by the ion diffusion mechanism. This study suggests that actual circuit operation in the AC condition should have a much longer back-end TDDB lifetime than the projection by DC stress assessment for BEOL ELK dielectrics.