Reproducing the Increased Frequency Dispersion of Ring Oscillators Under Long-Term BTI Aging

抄録

This paper presents a long-term characterization and modeling of Bias Temperature Instability (BTI) aging in ring oscillators (ROs) fabricated in a 65-nm technology. A dedicated measurement platform was used to apply constant stress and to continuously monitor RO frequencies for 300 days under 125 °C ambient temperature and 1.2V supply voltage. Aging degradation measurements on 840 ROs show that the mean degradation follows a power-law dependence throughout the entire observation period, in agreement with trends reported in shorter-duration measurements. In contrast, the RO-to-RO frequency distribution exhibits a pronounced time-dependent broadening. While the mean frequency decreases only marginally (≈0.55% from 1×10³ s to 1×10⁷ s), the standard deviation increases substantially (≈ 43%) and the temporal fluctuation of individual ROs increases by approximately 3×. Since environmental variations primarily induce a global shift of the distribution rather than a change in its shape, the observed variance expansion is attributed to the enhanced temporal variability of individual devices, consistent with the impact of random telegraph noise (RTN). To capture both the average BTI trend and the evolution of dispersion, we further conduct Monte Carlo simulations based on a hole-trapping framework that jointly accounts for BTI and RTN. By incorporating (i) biased clusters of traps with time constants near the onset of enhanced fluctuations, (ii) a threshold-voltage shift contribution proportional to trap time constants, and (iii) initially pre-occupied traps to represent prior stress history, the proposed model qualitatively reproduces the experimentally observed variance expansion under long-term stress.