Improving Read Disturb Characteristics by Self-Boosting Read Scheme for Multilevel NAND Flash Memories

A new NAND string and its read operation scheme using self-boosting as a solution for improving read disturb characteristics of NAND flash memories are proposed. By using the proposed self-boosting read scheme, which includes an optimized bias voltage and adjusted threshold voltage (V-th) of dummy cells, the self-boosted channel voltage prevents soft-programming in unselected memory cells during read operation due to reduced electric field across tunnel oxide. Compared to the conventional scheme this leads to a significant improvement in read disturb characteristics. From simulation and measurement results, the worst electric field of the proposed NAND flash memory during read operation is decreased by around 50% and V-th shifts caused by read disturb is lowered by around 40%, compared to conventional NAND. The proposed NAND was fabricated in a 60 nm NAND technology and successfully demonstrated. (C) 2009 The Japan Society of Applied Physic