Within-Die Threshold Voltage Variability Estimation Using Reconfigurable Ring Oscillator

Abstract

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.