As the core component of SSDs, flash memory is responsible for storing every byte of data. With technological advancements, the increase in the number of voltage levels and stacked layers per flash memory unit has allowed for higher capacity in a single flash memory chip. In the past, it took four flash memory chips to achieve a 1TB SSD capacity, but now a single chip can satisfy a 2TB SSD.
In August 2021, Kioxia utilized flash memory cooled to the boiling point of liquid nitrogen (-196°C) to achieve a density of 6 bits per cell. Recently, Kioxia researchers have made even further progress by successfully developing HLC flash memory (Hepta-level), which stores 7 bits per cell and achieves nearly double the unit density of QLC. It is noteworthy that current 4TB and 8TB SSDs utilize QLC chips, but if Kioxia’s HLC chips are used, SSDs could achieve a capacity of 16TB.
Kioxia has revealed that the reason for achieving such a high density of 7 bits per cell in their HLC flash memory is due to the use of single-crystal silicon as the internal channel material, which enables a significant increase in unit density. Additionally, it is said that the current noise during the operation of HLC Nand has also been significantly reduced. The emergence of HLC means that, even without improvements and developments in stacking technology, larger storage capacity can be achieved simply by increasing the number of bits in storage units.
However, the issues with HLC Nand are also very apparent. The significant increase in density results in improved capacity but sacrifices some bandwidth and speed. Moreover, the lifespan of HLC particles will be noticeably reduced. Based on the current write lifespan of TLC and QLC, the lifespan of a 1TB TLC SSD is usually around 600TBW, while that of a 1TB QLC SSD is less than 400TBW. If HLC is used, the lifespan of a 1TB SSD may be less than 200TBW.
The low lifespan of HLC Nand is not entirely useless, as in some enterprise-level applications such as website file servers, streaming media, data analysis, search engines, etc., data only needs to be written once and then only read operations are required. This is also the main usage scenario for many read-intensive enterprise-level SSDs. In this type of scenario, the advantages of HLC Nand’s large capacity are more prominent, and the issue of their erasable durability can be effectively avoided.
Mr. Liu Maozhi, the Chief Technology Executive of Kioxia, stated at CFMS2023 that storage density is the king, as density equals cost. However, the number of layers in 3D NAND does not equal density. If the layers are too thick, the vertical density will not be high, and if the storage hole spacing is long, the horizontal density will not be high. By widening the options for layer thickness and storage hole spacing, higher layer numbers can be easily achieved, but this is not Kioxia’s choice, because the density is not ideal.
From TLC and QLC to HLC flash memory, all are seeking higher capacity and lower costs, but they also pay the price of speed and durability. The controller chip is the bridge between the Nand Flash and the external hardware. How to use high-density Nand Flash to create larger capacity SSDs, how to reduce data bit errors in high-density flash memory, and how to improve their durability are all significant challenges. Kioxia will continue to accumulate technical expertise and research and development capabilities in the field of flash controller chips and work with partners to explore more advanced storage solutions and gain critical advantages in the era of data explosion.
