PFS - Embedded memory testing: from power measurements to defect encoding

Memory requirements are constantly increasing in System on Chip (SoC) devices. To keep on with this demand, manufacturers rely on embedded Flash memories that are easily scalable and relatively cheap to manufacture. These memories usually take a large percentage of the die area, and their reliability and verification process are thus two of the critical factors for a high yield. The testing efforts ensure that each commercialized device is compliant with the provided specifications but has to deal with a set of faults specific to embedded flash memories. Tests must span from pure analogic power measurements to individual cell-by-cell functionality checks. For the latter, Built In Self Test (BIST) is the most efficient solution given its orders of magnitude reduction in test time (compared to CPU-controlled tests), with a minimal cost due to the increased circuit complexity and die area. Given the regular structure of flash memories, BISTs are easily implemented and replicated multiple times. Such testing solutions allow the verification of all the banks in parallel, further increasing the advantages of the BIST. Carefully organizing both hardware and software components of the BISTs may lead to a significant reduction in test time without affecting the final test quality. Looking at it from another perspective, the BISTs may generate a considerable amount of diagnostic data during the tests. Efficiently collecting this data is crucial for designers and test engineers who can improve the production process or strengthen their design. An intelligent solution for this task is an on-chip method to compact diagnostic information during embedded memory testing. This strategy permits the reconstruction of failure bitmaps without any loss. The proposed method uses a fraction of the memory requested by a coordinatebased bit mapping approach. At the cost of a moderate test time overhead, this strategy permits dramatically increasing the number of devices that can be fully dia