Yangtze Memory Technologies Co. (YMTC) has reportedly started volume production of 64-layer 3D NAND. The triple-level cell chips with 256GB capacity use the company’s in-house Xtacking architecture for bonding two dies together.
It is the company's first process to go into high volume production. Digitimes’ sources indicate that the Chinese firm has gradually increased its 3D NAND yield and will be ramping its Wuhan factory to 100,000 wafers per month in 2020; although, that could further grow to 150,000 wafers per month. The sources also stated that its state-owned parent company, Tsinghua Unigroup, is constructing a 3D NAND memory fab in Chengdu with a similar wafer capacity, to come online in 2021-2022.
YMTC’s 256GB chips contain 64-layer TLC 3D NAND (which other companies started manufacturing in 2017) and are manufactured with its proprietary Xtacking architecture. The company unveiled the technology last year at the Flash Memory Summit. The chips are manufactured on two wafers. One wafer contains the ‘periphery’ CMOS logic, and the other one has the actual 3D NAND, based on common charge trap technology. The wafers are then bonded together with a process step, with billions of what it calls metal Vertical Interconnect Accesses (VIAs).
The benefit of this technology is a much more efficient use of die area. YMTC claims that die size is reduced by 25% because the area efficiency is over 90% for Xtacking, compared to less than 65% for conventional 3D NAND. It even estimates that at 128 layers, the periphery circuits would take up over 50% of the die area. Xtacking makes YMTC’s 64-layer 3D NAND within 80-90% the density of other’s 96-layer 3D NAND, it claimed. Other companies are also using or working on CMOS under the array technology though, albeit not by bonding two separate wafers.
Additionally, the company says that it allows for a modular approach to technology development, shortening product development by three months and reducing the manufacturing cycling time, the time the product is in the fab, by 20%.
Both wafers are manufactured by YMTC, with the logic wafer produced on a 180nm process. The architecture is also capable of a 3 Gbps I/O speed, which is over two times faster than common today and similar to DDR4. YMTC is working on multiple generations of Xtacking simultaneously; however, it intends to skip the 96-layer generation and move directly to 128 layers.
YMTC is a new, ambitious player in the NAND industry and plays a key role in China’s semiconductor ambitions. The company was founded in 2016 and has over 1,500 R&D engineers. Its CEO had previously worked at Intel and the manufacturing company XMC, now YMTC’s wholly-owned subsidiary.
