Putting wasted clock cycles to use: Enhancing fortuitous cell-aware fault detection with scan shift capture

Probabilistic approaches to the detection of untargeted defects, such as n-detect and standard LBIST (logic built-in-self-test), generally suffer from the need to apply very long test sets to achieve good coverage. However, more targeted approaches that attempt to explicitly model new types of defects, such as cell-aware faults, so that they can be deterministically detected may also lead to unacceptably long test sets. Generally, when tests are applied to circuits that contain scan chains, test results are only captured once the entire pattern has been shifted in and the desired deterministic pattern has been applied. Intervening shift cycles serve only as overhead. This is done because capturing data in the circuit's scan flip-flops during scan shift would destroy the pattern being shifted in. However, if data is captured in shadow flops in a MISR instead, those shift cycles could be used to obtain additional fault coverage. In this paper, we investigate the ability of the intervening shift cycles to achieve high static cell-aware fault coverage using only the test patterns generated to detect stuck-at faults. We also investigate reducing the number of shadow flops required. Our results show that high cell-aware coverage is achievable even when only a stuck-at test set is applied — in some cases equal to the coverage obtained by a dedicated cell-aware test set.