WeSPer: A flexible small delay defect quality metric

Testing for small delay defects (SDDs) is important due to their dominance in recent technology nodes. Unfortunately, all the SDD test quality metrics in the literature limit their assessment to the size of the delay defect tested under at-speed or slower clocks, which makes their results misleading under special cases such as faster-than-at-speed testing. Moreover, those metrics are inadequate for assessing the quality of recent SDD test methods that consider the variation of delays in a circuit. In this paper, a novel flexible SDD quality metric that can be adapted according to the available information and the applied test method is proposed. The proposed metric is named Weighted Slack Percentage (WeSPer) as it is defined by a slack ratio weighted by confidence level (CL) multipliers. The flexibility comes from the ability to model test inaccuracies or delay varying effects into the CL multipliers. This paper presents the WeSPer metric, along with a CL multiplier that penalizes overtesting to give a more accurate assessment of the quality of faster-than-at-speed testing. The metric is calculated for several benchmark circuits and compared to other SDD metrics found in the literature. The results show that WeSPer is better than other metrics at representing the quality of SDD tests, especially under faster-than-at-speed testing.