Kioxia Mulls 7-Bit-Per-Cell 3D NAND For Extreme-Capacity SSDs



Manufacturers of NAND flash memory have always tried to increase the storage density of their memory devices by increasing the number of bits stored per cell. While fundamentally, this is the most challenging way of increasing recording densities, it is also the most rewarding one from a cost standpoint. Companies like Kioxia are constantly experimenting with the number of bits they can store in one cell. This year the company said it had managed to store seven bits per cell (7 bpc), albeit in the lab and under low temperatures. 

To store more than one bit, the NAND cell must hold multiple distinct voltage levels, which is challenging as NAND makers have to find the suitable materials for these cells and then record and read them without errors. Moreover, the number of voltage states increases exponentially with the number of bits. For example, to store four bits, the cell has to hold 16 voltage levels (2^4), but with six bits, that number grows to 64 (2^6). Kioxia’s achievement of storing seven bits per cell requires holding 128 voltage states (2^7). Kioxia presented the paper describing its achievement at the International Memory Workshop 2022 (IMW 2022). 



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