Reliability issues in flash memories: An on-line diagnosis and repair scheme for word line drivers

The aggressive evolution of technologies involves a large amount of problems during and after Flash memory manufacturing. To target automotive or aeronautic applications, Flash memory manufacturers have to consider specific methods and design solutions to improve reliability. Among recent reliability enhancement techniques, ECC are probably the most efficient. However, ECC techniques are limited to correct errors occurring punctually within a word whereas in Flash memory, the presence of high voltage potential can stress peripheral circuits. This stress can lead to an entire faulty bit line or word line (clustering effect). The bit line clustering effect can be corrected by ECC because this effect impacts one or more bits in a word. But ECC remains unable to correct an entire faulty word line caused for instance by a catastrophic defect in a word line (WL) driver. This work proposes an electrical signature analysis of a WL driver output when catastrophic defects related to reliability problems are injected in such circuit. Then, based on these faulty electrical signatures, a solution for on-line diagnosis and repair of defective WL drivers is proposed. Finally, this solution is shown as low cost in term of surface overhead because only 16 high voltage transistors and 38 low voltage transistors are used for each word line. In addition to ECC techniques, the proposed solution could be used by the memory manufacturers to address automotive applications.