Retention and Reliability Problems in Embedded Flash Memories: Analysis and Test of Defective 2T-FLOTOX Tunnel Window

The evolution of system-on-chip (SoC) designs involves the development of non-volatile memory technologies like flash. Embedded flash (eFlash) memories are based on the floating-gate transistor concept and can be subject to complex hard defects creating functional faults. In this paper, the authors analyze the impact of a defective tunnel window oxide thickness. The authors show that this defective oxide thickness impacts erase or/and write operations as well as the retention and reliability of the eFlash. The proposed solution to detect such a defective oxide consists in using the voltage disturbance due to the coupling phenomenon existing between bit lines. This particular defective mechanism can be modeled as a coupling fault, especially a state coupling fault. Based on this analysis, the authors propose a test solution that uses two checkerboard patterns; checkerboard pattern (CKB) and its complement (CKBI). The authors show that these two patterns have to be applied by using a parallel programming approach, which allows to increase the coupling phenomenon and thus the voltage disturbance