Adaptive Pulse Programming Scheme for Improving the Vth Distribution and Program Performance in 3D NAND Flash Memory

For triple-level or quad-level 3D NAND flash memory, narrowing the Vth distribution of each state without influencing page program performance is one of the challenges. Considering this challenge, a novel adaptive pulse programming (APP) scheme was proposed. The proposed APP scheme adopted additional verify operations to separate the cells with different programming speed. It enhanced the program effect of slow cells by using increasing programming step voltage, and prevented the fast cells from over programming by using shorter programming pulse width through controlling the voltage of bitline. Compared with general incremental step pulse programming scheme, experimental results on TLC 3D NAND flash showed that, APP scheme could reduce the Vth distribution width of cells by around 15% and at the same time save the program time.