Kioxia working on the 3D NANDs of the future

The best SSDs currently on the market use TLC or QLC memories. Kioxia (formerly Toshiba Memory) was the first 3D NAND maker to start talking about 5-bit-per-cell (5bpc) PLC (Penta Level Cell) memory in 2019. Kioxia scientists and engineers certainly didn't want to rest on their laurels. and this year they have shown how 3D NAND 6bpc - Hexa Level Cell or HLC - works and believe that 3D NAND 8bpc - Octa Level Cell or OLD - is also possible. But there are some important aspects to consider.

Credit: Kioxia To store more than one bit per cell, NAND memories must contain multiple distinct voltage levels. For example, MLCs have four voltage levels per cell, TLC uses eight voltage levels, QLC has sixteen voltage levels, and PLC has thirty-two. To store six bits per cell (HLC), 64 voltage levels should therefore be used. To make 3D NAND with such cells, manufacturers have to overcome multiple challenges. You need to find the right materials that can handle the storage of 64 different tension levels and be able to tell them apart. This means that states cannot interfere with each other. Controlling temperatures is also important, and it becomes increasingly difficult to do so as the number of bits per cell increases.

To demonstrate the feasibility of HLC memories, Kioxia scientists took an existing 3D NAND memory chip and they immersed it in liquid nitrogen (77K, -196 ° C) to eliminate the deterioration of the cells caused by the rewrite cycles. Extremely low temperatures also helped reduce the need for insulation films, lower voltage requirements and stabilize materials. Overall, these operations have improved the physical properties and processes that take place in integrated circuits.

Kioxia scientists claimed that not only were they able to write and read six bits of data in a cell and reliably store it for 100 minutes, but they also achieved a lifespan of 1,000 program / erase cycles (P / E). Of course, this is largely due to the temperature of -196 ° C. Under normal conditions, the lifespan of 3D NAND HLC memory would be around 100 P / E cycles, according to estimates. PC Watch colleagues reported that Kioxia presented the experiment results at the 5th IEEE Electron Devices Technology and Manufacturing Conference (EDTM 2021) in April 2021 (submission number: WE2P4-5).

credit : unsplash.com 3D NAND PLC memories have not yet been commercialized and Western Digital (manufacturing partner of Kioxia) believes it will only make sense to do so for some SSDs after 2025. Western Digital also claims that 3D NAND PLCs introduce too many problems for a simple 25% increase in density. In contrast, 3D NAND HLCs increase the density of flash memories by 50% compared to 3D NAND QLCs, so it is more likely to be commercially viable. Furthermore, Kioxia scientists believe that even eight bits per cell with 256 voltage levels are technologically possible. The task of scientists and developers now is to find the right materials, design and controllers to make 3D NAND HLC and OLC operational and commercially viable at room temperature.

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