Channel Thickness and Grain Size Engineering for Improvement of Variability and Performance in 3-D NAND Flash Memory

In this study, to improve the threshold voltage ( $V_{th}$ ) variability and cell performance in three-dimensional (3-D) nand flash memory, we analyzed the electrical characteristics with respect to various channel thickness ( $T_{{ch}}$ ) and average grain size (GS) values. The 3-D random Voronoi grain patterns were applied to a polycrystalline silicon (poly-Si) channel to determine the actual grain shape using technology computer-aided design (TCAD). For statistical analysis, key electrical characteristics such as the threshold voltage ( $V_{{th}}$ ), subthreshold swing (SS), maximum transconductance ( $g_{m}$ ), and on-current ( $I_{{on}}$ ) were extracted from samples with different patterns of grain boundaries (GBs) at specific $T_{ch}$ and GS values. The standard deviation of $V_{th}$ ( $sigma V_{th}$ ) increased with an increase in GS at $T_{ch} $ > 22 nm, and no increase trend was observed for $sigma V_{th}$ at $T_{ch} $ < 22 nm. The mean SS, $g_{m}$ , and $I_{{on}}$ related to the performance improved overall with an increase in GS at the same $T_{{ch}}$ value. Based on a comprehensive analysis of various 3-D grain patterns, optimal structures were proposed in terms of variability and/or performance. Furthermore, based on the results, we suggest suitable $T_{{ch}}$ and GS parameters for the given target of 3-D nand flash devices.