On Aging-Aware Signoff for Circuits With Adaptive Voltage Scaling

Transistor aging due to bias temperature instability (BTI) is a major reliability concern in sub-32 nm technology. To compensate for aging, designs now typically apply adaptive voltage scaling (AVS) to mitigate performance degradation by elevating supply voltage. Since varying the supply voltage also causes the BTI degradation to vary over lifetime, this presents a new challenge for margin reduction in the context of conventional signoff methodology, which characterizes timing libraries based on transistor models with pre-calculated BTI degradations for a given IC lifetime. In this paper, we study the conditions under which a circuit with AVS requires additional timing margin during signoff. Then, we propose two heuristics for chip designers to characterize an aging-derated standard-cell timing library that accounts for the impact of AVS during signoff. According to our experimental results, this aging-aware signoff approach avoids both overestimation and underestimation of aging-either of which results in power or area penalty-in AVS-enabled systems. Further, we compare circuits implemented with the aging-aware signoff method based on aging-derated libraries versus those based on a flat timing margin. We demonstrate that the flat timing margin method is more pessimistic, and that the pessimism can be mitigated by AVS.