A New Read Scheme for Alleviating Cell-to-Cell Interference in Scaled-Down 3D NAND Flash Memory

Abstract

In this paper, we investigated the cell-to-cell interference in scaled-down 3D NAND flash memory by using a Technology Computer-Aided Design (TCAD) simulation. The fundamental cause of cell-to-cell interference is that the electric field crowding point is changed by the programmed adjacent cell so that the electric field is not sufficiently directed to the channel surface. Therefore, the channel concentration of the selected cell is changed, leading to a V-th shift. Furthermore, this phenomenon occurs more severely when the selected cell is in an erased state rather than in a programmed state. In addition, it was confirmed that the cell-to-cell interference by the programmed WLn+1 is more severe than that of WLn-1 due to the degradation of the effective mobility effect. To solve this fundamental problem, a new read scheme is proposed. Through TCAD simulation, the cell-to-cell interference was alleviated with a bias having a Delta V of 1.5 V from V-read through an optimization process to have appropriate bias conditions in three ways that are suitable for each pattern. As a result, this scheme narrowed the V-th shift of 67.5% for erased cells and narrowed the V-th shift of 70% for programmed cells. The proposed scheme is one way to solve the cell-to-cell interference that may occur as the cell-to-cell distance decreases for a high stacked 3D NAND structure.