Optimization of Small-Delay Defects Test Quality by Clock Speed Selection and Proper Masking Based on the Weighted Slack Percentage

Classical transition delay fault (TDF) model-based delay tests cannot detect small-delay defects (SDDs) properly in many circuits because they do not set the test slack properly to match the size of the tested delay fault. In this article, we propose a new method of using faster-than-at-speed clocks to enhance a classical TDF-based test pattern set and create an optimized SDD test that can detect effective SDDs. We define effective SDDs as SDDs that can immediately fail a circuit and distinguish them from those that grow over time to fail the circuit (reliability defects). The optimized SDD test developed in this article can either target only effective SDDs or also include reliability defects that are close to fail a circuit. In order to improve the SDD test quality, the proposed optimization method uses the recently proposed weighted slack percentage (WeSPer) as the SDD test quality metric to match each pattern with an appropriate test clock speed and then generates the proper masking vector. The optimization method includes a technique for reducing the final pattern count while keeping WeSPer value high. On a set of benchmark circuits, the proposed technique is able to improve the WeSPer value by up to 32.97% (a relative improvement of 60.98%) compared to classical at-speed testing using the same pattern set. This article also discusses the application of this technique on a self-timed circuit that has previously been at-speed tested. The proposed optimization technique improved the SDD test quality from 60% to 69% for preexisting set of test clocks and patterns. Post-silicon test results for the processor are included in this article.