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Semiconductor industry goals for the quality of shipped products continue to get higher to satisfy customer requirements. Higher quality of shipped electronic devices can only be obtained by thorough tests of the manufactured components. Scan chains are universally used in large industrial designs in order to cost effectively test manufactured electronic devices. They contain nearly half of the logic transistors in large industrial designs. Yet, faults in the scan cells are not directly targeted by the existing tests. The main objective of this thesis is to investigate the detectability of the faults internal to scan cells. In this thesis, we analyze the detection of line stuck-at, transistor stuck-on, resistive opens and bridging faults in scan cells. Both synchronous and asynchronous scan cells are considered. We define the notion of half-speed flush test and demonstrate that such new tests increase coverage of internal faults in scan cells. A new set of flush tests is proposed and such tests are applied at higher temperatures to detect scan cell internal opens with a wider range of resistances. We also propose new scan based tests to further increase the coverage of those opens. The proposed tests are shown to achieve the maximum possible coverage of opens in transistors internal to scan cells. For an asynchronous scan cell considered, two new flush tests are added to cover the faults that are not detected by the tests for synchronous scan cells. An analysis of detection of a set of scan cell internal bridging faults is described. Both zero-resistance and nonzero-resistance bridging fault models are considered. We show that the detection of some zero-resistance non-feedback bridging faults requires two-pattern tests. We classify the undetectable faults based on the reasons for their undetectability. We also propose an enhanced logic BIST architecture that accomplishes the new flush tests we propose to detect scan cell internal opens. The effectiveness of these new methods to detect scan cell internal faults is demonstrated by experimental results using some standard scan cells from a large industrial design.