A compact pico-second in-situ sensor using programmable ring oscillators for advanced on chip variation characterization in 28nm HKMG

An all-digital on-chip sensor using programmable ring oscillator (PRO) for advanced on chip variation (AOCV) characterization is proposed and verified in 28nm HKMG node. Bypassing technique combined with statistical testing based on array of PROs enables resolution of 1 pico-second. Multiple cells under test (CUTs) are embedded into one single PRO through programmability to get mask area cost effectiveness. Extra variation is eliminated by symmetrical duplicate structure. Test results indicate that there is a flex point around 7-stage on the curves of delay sigma/n vs. stage number. Local variation dominates and decreases significantly with the increase of stage number before 7-stage point. For small dimension, inverter is more sensitive to on-chip-variation than NAND. But no same trend is observed for large dimension. Curves of delay average vs. stage number and delay sigma/n vs. stage number among dies based on the same type of cell indicate a good uniformity.