Investigation on NBTI-induced dynamic variability in nanoscale CMOS devices: Modeling,experimental evidence,and impact on circuits

With the downscaling of CMOS devices, dynamic variability induced by negative bias temperature instability (NBTI) has become a critical issue. In addition to the time-dependent device-to-device variation (DDV) of NBTI degradation, the cycle-to-cycle variation (CCV) originated from random trap occupation is found non-negligible and should be added into the total dynamic variation. This paper summarizes our recent studies on NBTI-induced dynamic variability, focusing on the CCV effect, with more details on the statistical modeling, circuit reliability simulation methodologies and experimental results. By adding the random trap occupation into consideration, a statistical model for total dynamic variation (DDV + CCV) is proposed. The effective occupancy probability p_eff is introduced as a key parameter for modeling and circuit reliability simulation. With the statistical trap response (STR) method and modified on-the-fly method, the proposed model is validated by the experimental evidence under both DC and AC NBTI. According to the model and experimental results, circuit reliability simulation framework is proposed for both long-term quasi-static and short-term transient performance evaluation with the additional impact of CCV. Two representative digital circuit units, ring oscillator (RO) and SRAM cell, are simulated under different conditions, indicating it necessary to consider the evident influence of the CCV in accurate circuit reliability evaluation. The results are helpful for the reliability/variability-aware circuit design in nanoscale technology.