Flash memory testing and built-in self-diagnosis with march-like test algorithms

Flash memories are a type of nonvolatile memory based on floating-gate transistors. The use of commodity and embedded flash memories is growing rapidly as we enter the system-on-chip era. Conventional tests for flash memories are usually ad hoc - the test procedure is developed for a specific design. As there is a large number of possible failure modes for flash memories, long test algorithms on complicated automatic test equipment (ATE) are commonly seen. The long test time results in high test cost. We propose a systematic approach in testing flash memories, including the development of March-like test algorithms, cost-effective fault diagnosis methodology, and built-in self-test (BIST) scheme. The improved March-like test algorithms can detect disturb faults - derived from the IEEE STD 1005 - and conventional faults. As the memory array architecture and/or cell structure varies, the targeted fault set may change. We have developed a flash-memory fault simulator called RAMSES-FT, with which we can easily analyze and verify the coverage of targeted faults under any given test algorithm. In addition, the RAM test algorithm generator - test algorithm generator by simulation - has been enhanced based on RAMSES-FT, so that one can easily generate tests for flash memories, whether they are bit- or word-oriented. The proposed fault diagnosis methodology helps improve the production yield. We also develop a built-in self-diagnosis (BISD) scheme - a BIST design with diagnosis support. The BISD circuit collects useful test information for off-chip diagnostic analysis. It has unique test mode control that reduces test time and diagnostic data shift-out cycles by a parallel shift-out mechanism.