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.