New observations on AC NBTI induced dynamic variability in scaled high-κ/Metal-gate MOSFETs: Characterization, origin of frequency dependence, and impacts on circuits

In this paper, the frequency dependence of the dynamic variation induced by AC NBTI aging in scaled high-κ/metal-gate devices are experimentally studied for the first time. Challenges in comprehensively characterizing AC NBTI induced variation are addressed by the modified method. The additional variation source in AC NBTI, originating from the variations among each AC clock cycle, is found to be non-negligible and thus should be included in predicting circuit stability. With increasing AC frequency, the mean value (μ) of the V_th shift (ΔV_th) is reduced as expected; however, the variation (σ) of ΔV_th is almost unchanged, which surprisingly disagrees with the conventional model predicting the reduced variation. The origin of this new observation is found due to the competitive impacts of the activated trap number and the trap occupancy probability during device aging. Taken clock-CCV and frequency dependence into account, the impacts of AC NBTI on the SRAM cell stability can be evaluated in terms of both degradation and variation. The results are helpful for the future variability-aware circuit design.