Effects of Asymmetric String Structure on Word-Line Interference in the Vertical NAND Flash Memory Devices

Abstract

The vertical NAND flash memory devices with asymmetric string structure were introduced in order to reduce the word-line interference, and their electric characteristics were investigated as functions of the asymmetric factor (AF) by using technology computer-aided design (TCAD) sentaurus simulation tool. The difference in the threshold voltage (Vth) shift of the target cell was decreased with increasing AF; it was 0.435×10⁻³ V at AF of 0 nm, and 0.009×10⁻³ V at AF of 40 nm. This reduction of the word-line interference for vertical NAND flash memory devices with an increased AF was explained by the increased average distance between the target cell and the three closest cells located at the adjacent string.