Adaptive Source Bias for Improved Resistive-Open Defect Coverage during SRAM Testing

SRAM testing is becoming more and more challenging due to issues caused by continuous device scaling. Fabricated SRAMs are submitted to random and systematic process variability, which strongly affect the cell´s behavior and also the ability of test algorithms to detect faults. Traditionally, bias conditions have been used to improve the behavior of the SRAM under process variations by applying body bias to compensate for the effect of variability. Based on the same principle, bias conditions also affect the cell´s behavior when resistive-opens are present, hence affecting test´s defect coverage capability. Both body- and source-bias conditions are analyzed in this paper to find the way to improve defect detect ability in the SRAM cell. Source-biasing has been proven to be the more effective of the two, leading to more than 3X improvement of the defect detected value. Also, by adapting the source-bias conditions to process parameter values, over- and under-testing of the SRAM can be avoided.