Test Scores for Improving the Accuracy of Logic Diagnosis for Multiple Defects

In the initial stages of the yield improvement process for a new technology, a faulty unit may contain several defects (a multiple defect). The presence of a multiple defect may cause a state-of-the-art logic diagnosis procedure that is based on simulation and ranking of single modeled faults to produce a large set of candidate faults. Test removal or selection procedures are based on the observation that a large set of candidate faults is obtained when defects interact under certain tests. If the tests are ignored, the accuracy of logic diagnosis is improved. However, when a test is ignored, useful diagnostic information may also be lost. Instead, this brief suggests that it is possible to assign a score to every test based on its contribution to the accuracy of logic diagnosis. The test scores are used for scaling the contributions of the tests to the identification of candidate faults. This results in a reduced set of candidate faults where the faults are identified by tests with higher scores. The procedure requires a single additional logic diagnosis step, with a negligible computational effort. Experimental results are presented for multiple defects in benchmark circuits to demonstrate the effectiveness of the procedure.