Time-driven test methodologies for embedded srams

According to International Technology Roadmap for Semiconductor 2003 (ITRS'03), by 2013 over 90% o f the total System-on-a-Chip (SoC) area w i l l be occupied by memories, e.g., S R A M s . The increasingly dense embedded S R A M s (e-SRAMs) are more prone to manufacturing defects and field reliability problems since they are subject to aggressive design rules. On the one hand, this reduces the memory and SoC yield, thereby increasingly making redundancy necessary; on the other hand, it poses significant test challenges, particularly, the test time required for achieving acceptable test quality. This thesis focuses on reducing test time o f e S R A M s , for both a single and multiple memories. In practice, the test time for a single e S R A M consists of the time for testing Data Retention Faults (DRFs) and the time for testing other faults named as non-DRFs. B y tightly coupling the coupling fault (one o f non-DRFs) test and hard repair techniques, here the non-DRF test time is reduced by up to a factor o f two compared with the one when not coupling those test and repair activities. A n d this reduction is achieved without negatively impacting defect coverage. Based on the memory sizes, its D R F test time is reduced by either using a Design-for-Test (DFT) technique referred to as Pre-Discharge Write Test Mode ( P D W T M ) or by reusing the inherent read or write operation time due to the access to different cells. Furthermore, by considering (i) the trade-off values between the yield gain and the redundancy area overhead, (ii) the delay time for D R F tests, as the two deciding factors, any e S R A M can be categorized into one of four groups. Based on their repair features and D R F test method selections, those four groups are named as N S R D F e S R A M s (with N o or Soft Repair and D F T techniques for retention faults), S R D E (with Soft Repair and the reduced DElay time for retention faults), H R D E (with Hard Repair and the reduced DElay time for retention faults) an H R D F (with Hard Repair and D F T i i techniques for retention faults). Accordingly, four customized March test algorithms, generated from a comparison algorithm, are selected for each group respectively. Wi th the proposed customized algorithms, the test time of any single e S R A M can be reduced by a factor of up to two compared with that required when applying a universal March algorithm. Moreover, the thesis also targets on reducing the test time of multiple distributed small eS R A M s . To the best of our knowledge, the widely used solutions to test/diagnose such multiple e S R A M s are to apply serial memory interfacing techniques, i.e., unidirectional and bidirectional serial interfaces. The approach in this thesis speeds up the referred test solutions by replacing global serial response analyzers with parallel local response analyzers. The serial fault masking and defect-rate dependent diagnosis existing in uni-directional/bi-directional serial interfaces are overcome by designing a pair of serial to parallel and parallel to serial converters. When more eS R A M s are tested in parallel, the accumulate test power might be over the limit and thus a power-constrained scheduling is called for. To further reduce the test time during this scheduling, in this thesis, a "retention-aware" test power model is proposed to replace the original "single-rectangle" model typically used for SoC cores.