Buffer-ring-based all-digital on-chip monitor for PMOS and NMOS process variability and aging effects

In this paper, we propose an all-digital process variability and aging monitor which utilizes a simple buffer ring with a pulse counter. The proposed circuit monitors the process variability according to a count number of a single pulse which propagates on the buffer ring and a fixed logic level after the pulse vanishes. Using the proposed circuit in combination with a simple ring oscillator which monitors its oscillation period, we can calculate the rise and fall delay values and can monitors the variabilities of PMOS and NMOS devices independently. The experimental results of the circuit simulation on 65 nm CMOS process indicate the feasibility of the proposed monitoring circuit. The proposed monitoring technique is suitable not only for the on-chip process variability monitoring but also for the infield monitoring of aging effects such as negative bias instability (NBTI) and channel hot carrier (CHC).