Within-Die Threshold Voltage Variability Estimation Using Reconfigurable Ring Oscillator

Ring oscillators are very popular process monitors due to its easy implementation and high sensitivity to process parameters. This paper presents a technique to accurately estimate the absolute threshold voltage Vth of device under test (DUT) using a reconfigurable ring oscillator (RO). Conventional techniques exploit NMOS-pass transistor based RO to estimate the Vth. However, the RO frequency includes the delay of the NMOS-pass transistor which is the error component in Vth estimation. We propose a test structure which mitigates the delay of the NMOS-pass transistor and accurately estimate the Vth. The DUT is included in each stage of the RO. A large scale of DUTs are incorporated in the reconfigurable RO to estimate the within-die Vth variation. Simulation results in 65nm industrial technology node shows the accuracy improvement of 10% compared to the conventional technique. The Monte Carlo simulations considering within-die variation on the test structure shows a mean error of less than 1ps in the path delay difference due to Vth variation.