Small embeddable NBTI sensors (SENS) for tracking on-chip performance decay

On-chip circuit aging sources, like negative bias temperature instability (NBTI), hot-carrier injection (HCI), electromigration, and oxide breakdown, are reducing expected chip lifetimes. Being able to track the actual aging process is one way to avoid unnecessarily large design margins. This work proposes a sensing scheme that uses sets of reliability sensors capable of accurately tracking NBTI PMOS current degradations across process, temperature, and varying activity factors. We show that a set of 1000 such small sensors can predict chip lifetime to an uncertainty of 7% to 10%. We also show that, once the total area dedicated to sensing is chosen, the lifetime prediction uncertainty is almost insensitive to the tradeoff between the number of sensors and the area of each individual sensor.