Vertical 3D Memory Technologies

4Vertical NAND Flash

4.1 Overview of 3D Vertical NAND Trends

4.1.1 3D Nonvolatile Memory Overview

There is a strong potential today for 3D nonvolatile memory to replace planar memory for 10 nm nodes and beyond [1]. One candidate for bringing in the 3D era appears to be the NAND flash memory, and its basic operational modes are expected to be similar to those in the planar device. Currently, the basic types of 3D vertical memory are the vertical channel gate-all-around (GAA) NAND flash strings, the vertical gate devices with horizontal thin-film transistor (TFT) double-gate NAND flash strings, and the earlier stacked planar NAND structures discussed in Chapter 2. Both charge-trapping (CT) and floating gate (FG) 3D devices have been investigated. Some characteristics expected with the vertical channel and vertical gate CT NAND flash devices are shown in Table 4.1 [1].

Table 4.1 Comparison of 2D and Two Early 3D NAND Flash Features.

Dimension	2D	3D	3D
Planar	Vertical Channel	Vertical Gate
Gate Structure	Stacked	Gate-all-around	Dual-gate TFT
Unit Cell Size	4F2	6F2	4F2
Scaling Issue	Lithography	Film thickness	Lithography/film thickness
Coupling	Strong	Low	Moderate
Coupling Direction	Word-line + bit-line	Vertical	Bit-line + vertical
Based on J. Choi, K.S. Seol, (Samsung), VLSI Technology Symposium, June 2011 [1].

Various issues are expected in 3D NAND flash configurations. Program disturbs may increase in 3D as a result of the proximity of cells in the ...