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.

Single isolated fault (SIF) stands for about 60%-70% of the total number of defects and is rather redundancy hungry since a spare row or a column is required for repairing each SIF. Therefore, manufacturing yield will decrease if we do not allocate sufficient spare resources. In this paper, instead of the traditional fault replacement techniques, synergistic techniques that integrate both fault replacement and fault masking techniques are proposed. With our approaches, SIFs are masked instead of the traditional replacement for repairing. For other minor fault types (e.g., faulty rows and faulty columns), the fault replacement technique is used as usual. According to simulation results, repair rates can be improved significantly. The proposed techniques can be integrated with the conventional built-in self-repair with nearly negligible hardware overhead.

NROM is one of the emerging non-volatile-memory technologies, which provides very high data density, low fabrication cost, and better value stability. It is also promising for replacing current floating-gate-based non-volatile memory such as flash memory. In order to raise the fabrication yield and enhance its reliability, a novel test and repair flow is proposed in this paper. Instead of the traditional fault replacement techniques, fault masking techniques are also exploited by considering the logical effects of physical defects when the customer's code is to be programmed. Two techniques are exploited to maximize the possibilities of fault masking-address scrambling and data inversion. Graph models are also proposed for modeling these methods. The proposed methods can be easily incorporated into the ROM BIST architectures. According to experimental results, the fabrication yield can be improved significantly. Moreover, the incurred hardware overhead is almost negligible.