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 ( ${V}_{\text {th}}$ ) variability and cell performance in three-dimensional (3-D) NAND flash memory, we analyzed the electrical characteristics with respect to various channel thickness ( ${T}_{\textsf {ch}}$ ) 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 ( ${V}_{\textsf {th}}$ ), subthreshold swing (SS), maximum transconductance ( ${g}_{m}$ ), and on-current ( $I_{\mathrm{\scriptscriptstyle ON}}$ ) were extracted from samples with different patterns of grain boundaries (GBs) at specific ${T}_{\text {ch}}$ and GS values. The standard deviation of ${V}_{\text {th}}$ ( $\sigma {V}_{\text {th}}$ ) increased with an increase in GS at ${T}_{\text {ch}} >22$ nm, and no increase trend was observed for $\sigma {V}_{\text {th}}$ at ${T}_{\text {ch}} < 22$ nm. The mean SS, ${g}_{m}$ , and $I_{\mathrm{\scriptscriptstyle ON}}$ related to the performance improved overall with an increase in GS at the same ${T}_{\textsf {ch}}$ 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 ${T}_{\textsf {ch}}$ and GS parameters for the given target of 3-D NAND flash devices.