Comparing XPoint memory architecture with NAND and DRAM products

By Dr. Jeongdong Choe, Senior Technical Fellow, TechInsights

There has been a great deal of speculation around the composition of Intel’s Optane™ XPoint memory technology: PCM or ReRAM, selector, layouts, patterning technology, technology node, multi-stacked cell structure, die floor plan, interconnection to each electrode (wordlines and bitlines), functional blocks, scalability and process integration.

TechInsights set about to find answers. We have analyzed Optane’s memory cell structure, materials, cell array and memory peripheral array design, layouts, process flow and circuitry. Our Advanced CMOS Essential (ACE) analyses on Intel’s XPoint memory presents our complete findings and market trend predictions. The following paragraphs present some of the highlights.

Intel XPoint memory is based on PCM and selector memory (storage) cell elements. GST-based PCM, Ge-Se-As-Si based Ovonic Threshold Switch (OTS) and two memory cell stacked array architecture are common across Intel’s and Micron’s XPoint technologies.

We examined effective memory cell area efficiency vs. memory array efficiency, and compared it to current DRAM and NAND products. In our previous analysis on XPoint memory die, we found that memory density per die is 0.62 Gb/mm2 and memory efficiency is over 91%. The memory array efficiency, however, may not represent the reality because the memory peripheral and CMOS circuitry cover most of the die area.

We can define the effective cell area efficiency as a ratio of the real area of the cell memory elements (storage) to the total die area. For example, the effective memory cell area efficiency on Toshiba 15 nm 2D planar NAND is 43.9% due to excluding BC, CSL, SSL, GSL dummy wordlines and peripheral area on a die, while memory array efficiency is 72%. Figure 1 shows comparison of the effective memory cell area efficiency for 2D/3D NAND products from Toshiba/SanDisk (Western Digital), Micron/Intel, SK Hynix and Samsung, and 3D XPoint (OptaneTM from Intel).

Figure 1. A comparison of effective memory cell area efficiency on 2D/3D NAND and XPoint memory

Figure 1. A comparison of effective memory cell area efficiency on 2D/3D NAND and XPoint memory

When it comes to the effective unit cell size per 1 bit, NAND flash devices have been scaled down from 2D NAND (320 nm2) to 48L 3D NAND (145.8 nm2) or even to 64L 3D NAND (88.5 nm2) for Toshiba NAND products, while Intel OptaneTM two cell stacked XPoint memory has 800 nm2 (effectively 2F2) (Figure 2).

Figure 2. A comparison of effective unit cell area per bit on 2D/3D NAND and XPoint memory

Figure 2. A comparison of effective unit cell area per bit on 2D/3D NAND and XPoint memory

A comparison of memory density with DRAM products shown in Figure 3 illustrates that XPoint has higher memory density (0.62 Gb/mm2) than Samsung 1x nm (0.19 Gb/mm2), SK Hynix 2y nm (0.15 Gb/mm2) and Micron 20 nm (0.094 Gb/mm2) DRAM dice. Micron announced that the memory density of XPoint would be ten times higher than commercial DRAM products. This is true if we compare it with 30 nm class DRAM products, because most of the 30 nm class DRAM products from major DRAM manufacturers have 0.06 Gb/mm2 memory density. The first commercial XPoint memory die has three times (vs. Samsung 1x DRAM) or six times (vs. Micron 20 nm DRAM) higher memory density than those of current DRAM products.

Figure 3. A comparison of die size and memory density on DRAM (25nm/20nm/18nm) and XPoint memory

Figure 3. A comparison of die size and memory density on DRAM (25nm/20nm/18nm) and XPoint memory

We found that Intel introduced some innovative and compelling technologies on their first XPoint products such as PCM/OTS stack used for memory elements, GST based PCM, Ge-Se-As-Si based OTS and carbon based conductor and 2-bit cell stacked memory array with three electrodes. Intel successfully used a 20nm SADP double patterning technology to build a very uniform GST-based PCM/OTS memory square/island. Complete details on the of TechInsights’ XPoint memory analysis can be found here.

Click here to hear more from Dr. Choe and his TechInsights colleagues on 3D NAND.

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