How pseudo-SLC mode can make 3D NAND flash more reliable
NAND Flash is an increasingly important storage technology in products such as Solid-State Drives (SSD), SD-Cards and USB drives. In high volume consumer products like smartphones and laptops, cost is one of the most important factors for manufacturers. In other sectors, such as commercial or industrial applications or enterprise servers, secure and reliable data storage is the critical factor.
Pseudo-Single Level Cell (pSLC) is a method of using multi-level cell (MLC), triple-level cell (TLC) and quad-level cell (QLC) NAND flash in a way that reduces the number of bits stored in each cell to one. MLC flash natively stores two bits per cell, TLC stores three bits per cell and QLC stores four bits per cell. Reducing the amount of stored bits in each cell to one increases the reliability and lifetime of the NAND flash memory, while benefiting from the lower cost of the newer technology.
Whatever the application and storage technology used, consumer or industrial, hard disk drives or SSDs, there is continuous pressure to increase storage density due to ever growing volume of data society produces. Increasing the density of NAND flash has been addressed in multiple ways; the reduction in size of the transistors used, the physical layout of the transistors – stacking them in the third dimension in the 3D technology - and the increased number of bits that each cell can store.
The continued scaling of semiconductor manufacturing technology means that as transistor sizes decrease it is possible to fit more memory cells on a chip for the same cost. There are limits to how far this scaling can go, of course, and so flash memory makers have found alternative ways of reducing the cost per bit while also increasing storage density. One of these techniques is to store more data bits in each cell; today this may be two, three, four or even five bits per cell.
However, while this increased storage density reduces the cost per bit, it still has penalty. Storing multiple bits in a single cell increases the demands towards the precision of the write and readout process. This means that MLC, TLC and QLC flash memory operates slower than SLC flash, because more precise control of thresholds and programming voltages are needed. These memories also have much lower write endurance, which is the number of times each cell can be erased and reprogrammed.
This means a divide has arisen been between using SLC with its reliability and high write-endurance or opting for the higher capacity but slower MLC, TLC or QLC flash. The often-overlooked third option, however, is the pSLC mode; a technique that combines the best qualities of both single-level and multi-level technologies.
It exploits the fact that multi-level cell technologies are designed and manufactured using modern processes, unlike SLC memories. As a result they benefit from process scaling, meaning the transistor density – and therefore cell density – is higher. Using pSLC mode takes advantage of the higher density offered by the newer technologies, but modifies the behavior to store just a single bit in each cell instead of multiple bits. By reverting to having just one bit, the flash in pSLC mode can operate faster and more reliably than in its native mode. Storing a single bit per cell also increases the write endurance by up to 20 times.
As a result, although the pSLC mode reduces the capacity of MLC memory by 50%, the capacity of TLC by 66.6% and the capacity of QLC flash by 75%, it still achieves a lower cost per bit than SLC, because of the higher cell density. It improves the performance, reliability and endurance of the newer technologies to something closer to that of SLC. . Implementing the pSLC mode effectively requires a high-quality controller with firmware optimized for the specific flash memory used, in order to provide the highest performance and most reliable data storage. Hyperstone’s flash controllers are designed to support the pSLC mode, giving manufacturers greater design flexibility without sacrificing performance or reliability.