Generation of Transparent-Scan Sequences for Diagnosis of Scan Chain Faults

Diagnosis of scan chain faults is important for yield learning and improvement. Procedures that generate tests for diagnosis of scan chain faults produce scan-based tests with one or more functional capture cycles between a scan-in and a scan-out operation. The approach to test generation referred to as transparent-scan has several advantages in this context. (1) It allows functional capture cycles and scan shift cycles to be interleaved arbitrarily. This increases the flexibility to assign to the scan cells values that are needed for diagnosis. (2) Test generation under transparent-scan considers a circuit model where the scan logic is included explicitly. Consequently, the test generation procedure takes into consideration the full effect of a scan chain fault. It thus produces accurate tests. (3) For the same reason, it can also target faults inside the scan logic. (4) Transparent-scan results in compact test sequences. Compaction is important because of the large volumes of fail data that scan chain faults create. The cost of transparent-scan is that it requires simulation procedures for sequential circuits, and that arbitrary sequences would be applicable to the scan select input. Motivated by the advantages of transparent-scan, and the importance of diagnosing scan chain faults, this article describes a procedure for generating transparent-scan sequences for diagnosis of scan chain faults. The procedure is also applied to produce transparent-scan sequences for diagnosis of faults inside the scan logic.