Three-level flash gives way to four in bid for greater capacity: Page 4 of 5

April 09, 2019 //By Daniel Zajcev
Three-level flash gives way to four in bid for greater capacity

The future of NAND: 3D QLC with up 512 Layers


Graphical display of voltage levels depending on
NAND Flash technology and the possible stats
per memory cell.

The next logic step is the 3D NAND flash quad level cell (QLC) architecture, which is capable of storing four bits per cell and representing up to 16 different voltage levels. Intel and Micron were the first companies to release a 3D QLC NAND flash product with up to 64-layers. Micron, for example, estimates that using QLC NAND flash instead of legacy hard disk drives would enable datacentres to store 7.7 times more data in the same sized rack. The advanced device architecture of 3D NAND flash will enable vendors to manufacture dies with an even higher number of layers, which in turn further enhances areal density. The fourth-generation devices from Micron and the fifth-generation devices from Samsung, SK Hynix, and Toshiba should allow up to 128 layers. Some companies project that future generations will enable the production of 3D NAND flash with up to 512 layers.

However, the advantages of much greater density do not come without any trade-offs. In terms of performance, in a QLC memory, there are 16 different voltages per cell, which makes writing and reading the data more complex and much slower in comparison to 2D NAND flash. Furthermore, the reliability of the memory decreases relatively rapidly. The validation of individual bits is more demanding and the cells may degrade over several write/erase cycles, making it difficult to determine individual bit values. This may result in data errors. Error correction codes can counter this degradation, but are unable to compensate for the effect entirely. In addition, the program and erase cycles are estimated to be around 500 to 1000, which is significantly lower to 2D and 3D NAND flash architectures.

 

Ready for big data applications

Given the low number of program and erase cycles, storage products that are fitted with 3D QLC NAND flash are best suited for applications that demand high-speed and frequent read operations, but very few write cycles. Suitable application areas include the real-time analysis of big data, data inputs for artificial intelligence, the provision of media for on-demand services, some database applications, and user authentication. For these applications, the total cost of ownership of QLC NAND based data storage is lower than with HDDs, since users need fewer drives to serve the same volume of data, reducing the power bill while also increasing readout rates. The memory architecture’s density may also make it useful in some embedded and mobile applications.

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