Detection of Internal Stuck-open Faults in Scan Chains

Nearly half of the transistors in the logic parts of large VLSI designs typically reside inside scan cells. Faults in scan cells may affect functional operation if left undetected. Such undetected faults may also affect the long term reliability of shipped products. Nevertheless, current test generation procedures do not directly target faults internal to the scan cells. Typically it is assumed that scan chain tests, called flush tests, test the scan cells sufficiently. We showed that flush tests applied at slower clock rates, called half-speed flush tests, and tests for scan cell inputs and outputs, detect stuck-at and stuck-on faults internal to scan cells to a similar extent as checking sequence based tests proposed earlier. In this work, we investigate the detection of opens in transistors internal to scan cells. A new flush test and a new method to apply flush tests are proposed to greatly enhance the coverage of opens. We also propose new scan based tests to further increase the coverage of opens. The proposed tests are shown to achieve the maximum possible coverage of opens in transistors internal to scan cells.