Fault Scrambling Techniques for Yield Enhancement of Embedded Memories

Instead of merely using redundant rows/columns to replace faulty cells, error-correcting codes are also considered an effective technique to cure permanent faults for the enhancement of fabrication yield and reliability of memories. However, if the number of faulty bits in a codeword is greater than 1, the protection capability of the widely used SEC-DED (single-error correction and double-error detection) codes will be limited. In order to cure this dilemma, efficient fault scrambling techniques are proposed in this paper. Unlike the fixed constituting memory cells of a codeword in the conventional EDAC schemes, we try to reconstruct the memory cells of code words such that each codeword consists of at most one faulty cell. The corresponding scrambling circuits are also proposed and a simulator is developed to evaluate the repair rates and hardware overhead. According to experimental results, the repair rates can be improved significantly with negligible hardware overhead.