Channel Thickness and Grain Size Engineering for Improvement of Variability and Performance in 3-D NAND Flash Memory
Kihoon Nam, Chanyang Park, Jun-Sik Yoon, Giho Yang, Min Sang Park, Rock‐Hyun Baek
Abstract
In this study, to improve the threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> ) variability and cell performance in three-dimensional (3-D) NAND flash memory, we analyzed the electrical characteristics with respect to various channel thickness ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{\textsf {ch}}$ </tex-math></inline-formula> ) 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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\textsf {th}}$ </tex-math></inline-formula> ), subthreshold swing (SS), maximum transconductance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{m}$ </tex-math></inline-formula> ), and on-current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{\mathrm{\scriptscriptstyle ON}}$ </tex-math></inline-formula> ) were extracted from samples with different patterns of grain boundaries (GBs) at specific <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{\text {ch}}$ </tex-math></inline-formula> and GS values. The standard deviation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma {V}_{\text {th}}$ </tex-math></inline-formula> ) increased with an increase in GS at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{\text {ch}} >22$ </tex-math></inline-formula> nm, and no increase trend was observed for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma {V}_{\text {th}}$ </tex-math></inline-formula> at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{\text {ch}} < 22$ </tex-math></inline-formula> nm. The mean SS, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${g}_{m}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{\mathrm{\scriptscriptstyle ON}}$ </tex-math></inline-formula> related to the performance improved overall with an increase in GS at the same <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{\textsf {ch}}$ </tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${T}_{\textsf {ch}}$ </tex-math></inline-formula> and GS parameters for the given target of 3-D NAND flash devices.