Dynamic reliability characteristics of ultra-thin HfO2

In this work, we present time-to-breakdown characteristics of HfO₂ under unipolar AC voltage stress on MOS capacitors. It is shown that HfO₂ displays a longer lifetime under AC stress than constant voltage stress. Higher frequency and lower duty cycle in the AC stress resulted in a longer lifetime enhancement. As the thickness decreases, the amount of lifetime enhancement decreases. The enhancement of unipolar t_BD is attributed to less charge trapping during the "on time", t_on and charge detrapping during the off time, t_off. It is proposed that the characteristic time (τ_in) for charge to be trapped in HfO₂ is longer than t_on of unipolar stress. Also, we found a strong correlation between t_on and t_off with regard to how it leads to lifetime enhancement.