TID Effects on Variability in 40-nm Bulk CMOS

This work investigates the 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 a 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 two arrays with multiple identical ROSCs enabled the collection of representative statistics of frequency evolution with TID and evaluation of circuit-to-circuit variability. We have characterized single MOSFET transistors P-and N-channel under TID stress to support the analysis and interpretation of ROSCs measurements. Further, analyzing the circuits at different power supply levels reveals variability at different overdrive voltages, from strong inversion down to the onset of weak inversion. It is monitored as a function of TID and after high-temperature annealing. The frequency variability of the basic inverter-based oscillator remains relatively constant up to a TID level corresponding to well above the N-channel MOSFET rebound.