Design of Low-Cost Test Structures for Measuring Within-Die Process Skew Variations

The potential for expanding the use of ring oscillators (ROs) into a new and, as yet, unexplored area was reported in [1]. Simply stated, departures in strengths of PMOS and NMOS transistors from nominal, cause changes in duty-cycles at different nodes of ROs. Given the right RO design, both DC & AC contributions of transistors to process-skew can be determined at the same time.In this paper we address the related engineering problem of creating such RO designs that are specifically tuned for sensing process-skew. Design criteria and methodology are defined for ROs that are sensitive to and amplify the effect of process-skew. Example designs of such ROs are presented along with simulation results to demonstrate their efficacy as process-skew sensors. The simplicity of the proposed designs allows distributing many instances of these sensors on a die, in turn, creating the possibility of characterizing within-die process skew variations.