Within-die and die-to-die variability on 65nm CMOS : oscillators experimental results

This paper presents experimental data and analyses from the oscillation frequency of CMOS ring oscillators (ROs). The measurements are analyzed in order to separate the coefficient of variation (CV = σ/μ) behavior of both within-die (WID) process variations and Die-to-Die (D2D) process variations. Three different RO's sizes distributed over a total of 96 ROs per chip were measured in 32 different chip samples, all from a single MOSIS Multi-Project-wafer in a commercial 65nm CMOS process. All ROs were measured with varying power supply voltages, from nominal VDD (1.2V) down to nearthreshold (0.45V), resulting in a sample space of 18.432 points of interest. Statistical analysis results are shown regarding each RO stage size, the applied power supply and the correlation of both to the WID and D2D variations. The increase on D2D and WID coefficients of variations at the near-threshold supplies is very significant, as explained by increased delay variability at the moderate inversion regime of the FETs. The 96 test ROs occupy a total area of 200 x 480 μm2 in each test chip.