By Jeongdong Choe, PhD., TechInsights

A few years ago, some of the semiconductor process and device analysts thought 2D planar NAND Flash would soon be coming to an end due to the scaling limits, especially around the 20nm or sub-20nm generation. Do we still think the 2D NAND Flash technologies have hit the scaling wall? According to TechInsights’ deep-dive analysis on current and future NAND Flash technologies, although 3D V-NAND architecture could help with the scaling limit, we believe the 2D MLC and TLC NAND Flash technologies remain strong and cost effective for 14nm, 12nm and even for single-digit nanometer node.

When it comes to 3D NAND technology, Samsung has been developing and mass-producing 32-tier V-NAND architecture (for technical analysis related to the Samsung 3D V-NAND click here) with MLC and TLC for their 850 PRO and 850 EVO since 2014, although, this is not the final goal for Samsung due to a relatively low yield, process complexity and bit-cost viewpoints. More 3D Flash products may appear at the end of this year, or early in 2016, as major NAND players such as Toshiba, SanDisk, Micron, Intel, and SK-Hynix bring out their 3D products with 24-tier, 32-tier or 48-tier FG (floating gate)/CTF (charge-trap-flash) architecture (Figure 1).

Figure 1. 2D and 3D NAND Technology Roadmap (Source: TechInsights Nand Roadmap, download here)

However, the ultimate target for 3D NAND is 128-tier or at least 64-tier structure from the bit-cost viewpoints. In that case, the aspect ratio of Si-channel and common source contacts would be over 80:1, which is a strong burden for process integration engineers. In addition, the uniformity of the 64-tier or 128-tier NAND cell characteristics in a NAND string and their endurance/retention/reliability properties during program/erase operation would be another big challenge for the vertical NAND string architecture.

The scaling limits for 10 nm-class and sub-10 nm 2D planar NAND structures include patterning technology including QPT (Quadruple Patterning Technology), cell-to-cell interference such as cross-talk, poly-Si gap-filling process for control gate (CG), self-aligned STI (SA-STI) for isolation patterning, self-aligned process (SAP) for CG/FG, interconnection methodology including pad layout/design, inter-poly dielectric (IPD) layer engineering, and cell transistor channel/source-drain (S/D) engineering. According to TechInsights’ detailed structural analysis and comparison of 15nm and 16nm NAND flash devices (so called 1Y NAND technology node) such as Samsung 16nm, Toshiba 15nm, Micron 16nm and SK-Hynix 16nm products, we may expect that at least two more next generation 2D planar NAND products having 12nm and less than 12 nm technology would be developed and released from major players near future. As for NAND memory density and die size, Toshiba/SanDisk 15nm TLC products have 1.28 Gb/mm² which is double from other MLC products although Samsung 32-tier 3D V-NAND TLC products have 1.87 Gb/mm² (Figure 2).

Figure 2. Comparison of NAND memory density for each product (Source: TechInsights)

For patterning the three finest lines of the NAND cell structure such as active/STI, gate/wordline (CG/FG) and bitline (usually, metal-2 lines), a quadruple patterning technology (QPT) seems to be very mature for each of the major NAND players. They use their own QPT integration on three critical layers with three or four masks, SOH etching and two-step self-align reverse patterning (SARP) process. Although the critical dimensions have a little skew on every four patterns, they have successfully developed QPT integration with less than 1nm CD (Critical Dimension) and it could be extended into 10nm and even single-digit nanometer NAND products. Fortunately and thanks to state-of-the-art anisotropic plasma etching and ALD/CVD technology, uniformly repeated 8nm patterns would be possible for NAND cell array. Figure 3 shows a comparison of DPT/QPT patterns for each product.

Figure 3. Comparison of DPT and QPT patterns for each product (Source: TechInsights)

Micron uses a 3.3nm thin-FG poly-Si storage node to decrease cell-to-cell interference, while other manufacturers introduce an air-gap process for active, gate wordline (FG/CG) and bitline (metal-2) for thick-FG structure. Especially, the air-gap process has been developed and applied on the channel region of active patterns and FG/CG pillars help decrease the cross-talk.

For an IPD (Inter-Poly Dielectric) or a barrier layer between CG and FG, a multi-layer stacked with thin oxide (O) and nitride (N) layers such as ONO or NONON structure has been used for mid-10 nm class NAND devices, while Micron uses a high-k dielectrics such as HfO/SiO/HfO/Nitrided-SiO which is the same as their 20 nm NAND products. Micron successfully integrated IPD/FG/Tunnel-oxide and decreased FG thickness from 5 nm to 3.3 nm with high-k IPD. It might be further reduced to 10ish nm NAND products by optimizing IPD/FG quantum well structure for their unique thin-FG architecture. A 6 nm tunnel oxide (SiO) is used on Micron, Toshiba/SanDisk and SK-Hynix, while Samsung uses nitrogen-doped oxide in its top and bottom portion.

Triple-row staggered bitline contacts (BC) are used on Toshiba/SanDisk for the first time which is an excellent choice to make things smooth for cell layout and process integration although NAND string overhead is increased from 13% to 19%. Other players still use double-row staggered BC layouts on their 15nm/16nm NAND products (Figure 4).

Figure 4. Double- and triple-row staggered bitline contacts (Source: TechInsights)

Other barriers to extend 2D planar NAND to 10nm such as CG poly fill-ability, anisotropic etching for SA-FG/STI and CG/FG, cell transistor S/D engineering and leaning effect during the process integration are still there. Nevertheless, major players and their equipment vendors will successfully develop and integrate the 10 nm 2D NAND architecture in a few years.

I believe most of the major NAND players have their own matured process integration capability with assistance from ECC and circuit/layout optimization. 2D NAND technology will be further scaled down to 12nm, 10nm, or even 8ish nm which is more cost-effective than 3D V-NAND for near future NAND products.

Jeongdong Choe has more than 20 years of experience on semiconductor process and device integration including NAND Flash, DRAM, logic and advanced memory devices at Samsung and SK-Hynix. He works at TechInsights as a consulting engineer especially focusing on memory and logic process integration.