Total Ionizing Dose Effects on Delay Variability of Integrated Circuits with Metastable Transitions - Selected Delay Stages Embedded into 40~nm bulk CMOS Ring Oscillators

This work investigates variability effects in CMOS circuits under ionizing radiation stress. We studied two types of delay cells embedded in ring oscillators (ROSCs): a basic CMOS inverter and a differential inverter with cross-coupled load, where a short metastable phase occurs in the transition between the binary states. The measurements at several total ionizing dose (TID) steps up to 100 Mrad of three arrays with multiple identical ROSCs enabled the collection of representative statistics. The results include frequency variations with TID with circuit-to-circuit variability and cycle-to-cycle jitter. The examined mean frequency evolves differently for different types of ROSCs. Above all, the frequency variability is significantly decreasing with TID for oscillator arrays based on differential cross-coupled inverters. We demonstrate how this is related to the circuit operation principle. The presented analysis shows that the effect of TID on variability of the circuit parameters may strongly differ from single transistor variability, even in very simple circuits. Especially in circuits with metastable transitions, the balance of parameters can have unexpected effects.