TechInsights explores SK Hynix's U-shaped NAND cell architecture.
We at TechInsights have been waiting for SK Hynix 3D NAND products and their cell architecture for many years. We know that SK Hynix has been developing several types of 3D NAND cell structures, including DC-SF (three-dimensional Dual Control-gate with Surrounding Floating-gate, 2010) and SMArT (Stacked Memory Array Transistor with ONO layer, 2012). However, we have been looking forward to seeing the final cell scheme for their products.
TechInsights performed a quick review of their vertical NAND cell architecture. The SK Hynix 3D NAND products, so-called 3D version 2 with H28U74301AMR UFS package markings, were found in the LG Electronics model F800L, also known as the V20 smartphone. Four NAND die are in a 64 GB package, which means each 3D NAND die has 128 Gb, likely using multi-level cell (MLC) rather than triple-level cell (TLC). Currently, other major 3D NAND players — such as Samsung, Toshiba, SanDisk, Micron, and Intel — have their 3D NAND products with TLC.
The die photograph with H27DGS8 die markings consists of four planes, which is different from their previous 2D 1x nm planar NAND die as shown in Figure 1.
Figure 1. 2D planar 1x nm and 36L 3D NAND die photographs from SK Hynix (Source: TechInsights).
Die area is measured at 88.36 mm2, which means that the memory density is 1.45 Gb/mm2 on a die with 67.5% memory array efficiency. The memory density on the die is quite low compared to other players’ 3D NAND TLC products, such as the Micron 32L 3D NAND (2.28 Gb/mm2), Toshiba/SanDisk 48L 3D NAND (2.43 Gb/mm2), and Samsung 48L V-NAND (2.57 Gb/mm2) because the SK Hynix H27DGS8 die used for UFS products might be an MLC die instead of a TLC die.
Referring to memory density on a plane (or a tile for Micron’s 3D NAND die), SK Hynix 3D NAND (ver. 2) die has 2.15 Gb/mm2, which is still lower than Micron’s 32L (2.63 Gb/mm2), Toshiba/SanDisk’s 48L (3.45 Gb/mm2), and Samsung’s 48L (3.56 Gb/mm2).
Table 1 summarizes a comparison of die information and memory density.
Table 1. A comparison of 3D NAND die information from SK Hynix, Micron/Intel, Toshiba/SanDisk, and Samsung (Source: TechInsights).
Figure 2 and Figure 3 show comparisons of memory density and memory array efficiency from major 3D NAND manufacturers. The memory array efficiency (84.9%) of Micron/Intel’s 3D NAND is higher than others because of their array on CMOS circuits in which the CMOS decoders and sense-amps are sitting under the 3D FG-NAND memory array. The memory efficiency of SK Hynix 3D NAND (67.5%) is a little lower than Samsung’s (70.0%) and Toshiba/SanDisk’s (69.9%).
Figure 2. A memory density comparison of 3D NAND products (Source: TechInsights).
Figure 3. A memory array efficiency comparison of 3D NAND products (Source: TechInsights).