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NAND Flash Architectures - What Comes After 2D SLC NAND Flash?

  Knowledge

2D SLC NAND is currently still the reigning champion when it comes to interchangeable storage media. But the 43nm generation will soon be discontinued, and smaller 24nm structures will suffer the same fate in the medium term, so developers should start looking at possible alternatives.

With 100.000 program/erase cycles, extremely fast access times, low latency, energy efficiency, robustness and availability of low capacities (e.g. microSD with 128MB), as well as several form factors, 2D SLC NAND flash is currently still the reigning champion among interchangeable storage medium technologies for industrial applications. But the 43nm generation will soon be discontinued, and smaller structures will also disappear in the medium term. Because requalification can take months, if not years, developers should already start looking at which storage technology they will be relying on in the future.

Since NAND flash was introduced in 1987, the focus of development has been on cutting costs by shrinking structures. To meet the need for ever increasing amounts of data - with 175ZB of data expected to be generated by 2025 - while also cutting costs, innovation has been looking towards three-dimensional NAND flash memory (3D NAND flash) and also towards increasing the number of bits on a cell with SLC (single-level cell) with one bit per cell, MLC (multi-level cell) with two bits per cell, TLC (triple-level cell) with three, and QLC (quad-level cell) with four bits per cell (Figure 1). The vertical layering of NAND cells increases memory density and thus by extension also memory capacity. But this also goes hand in hand with reduced reliability and fewer program/erase cycles.

Consumer and data centers as primary markets

The flash market is mainly driven by the consumer and data center markets, accounting for around 80% of sales. The fact that the focus is on capacity and not - as with industrial applications - on reliability and service life is reflected in production figures - around 80-90% of NAND flash memory produced today is 3D NAND flash. Supplier roadmaps show that the number of layers increases with each new generation, which in turn allows even greater capacity per NAND chip. Customers benefit from noticeably lower costs, but at the expense of quality. But there are also models that offer the same durability as 2D NAND flash.

3D TLC NAND can enjoy a long service life thanks to Charge Trap Technology combined with smarter controllers, ECC (Error Correcting Code) and LDPC (Low-Density Parity Check Code) methods. However, it is difficult to directly compare the lives of these products against those of 2D SLC NAND flash types. Manufacturers provide entirely different information on endurance, drive waters per day (DWPD) and data retention. The suppliers Intel and Kioxia specify the maximum number of program/erase cycles for their 3D TLC NAND models as 300 to 3.000. By comparison, a 2D MLC NAND flash memory module using a 15nm process technology can manage a maximum of 3.000 program/erase cycles.

Is 3D NAND suitable for industrial use?

But for NAND flash memory to be used in industrial applications, there are other essential criteria. These include long-term availability, PCN handling, a fixed bill of materials (BoM) as well high endurance, data retention, temperature stability and more. Flash memory suppliers can guarantee some of them, including PCN handling and a fixed BoM - and now also functionality in a wide temperature range.

It took a whole five years from the introduction of 3D NAND flash memory until the first modules with industry-grade temperature stability entered mass production. The different cell sizes and the different charge states that this entails in the upper and lower parts of the die create inconsistencies in how individual bit states are read. External temperatures make this process even harder.

At Kioxia, the wide temperature range is currently approved for BiCS3 memory with 64 layers. The new generation from Kioxia, BiCS4 with 96 layers, is not yet qualified for the wide temperature range. This can also mean that not every generation of 3D NAND flash memory will be approved for industrial use.

Smaller capacities under 8GB cannot be created using 3D TLC NAND technology - at least not using current technology - because the 3D TLC memory chips are produced with a capacity of 32GB per die, which means that the minimum capacity available is 32GB. Customers who require capacities of under 4GB continue to enjoy price benefits when purchasing and using 2D SLC NAND products, because the die is essentially financing the smaller base capacity.

But there are now also solutions available for businesses who require smaller capacities in compact form factors, for example for applications involving a graphical interface in conjunction with a display. Using integrated intelligence in the form of features built into the firmware and integrated controller, it becomes possible to create small capacities. For example, Swissbit offers the S-46u-8GB-3D-pSLC with 3D TLC NAND flash and up to 30,000 guaranteed program/erase cycles.

Optimized firmware and controller

Transcend's 452K series is another sign that 3D TLC NAND flash memory combined with increasingly better firmware and controller features may be potentially suitable for industrial applications in the future. The TS128GSSD452K in a 2.5" form factor with a SATA III interface is fitted with BiCS4 3D TLC NAND flash memory. According to supplier specifications and actual testing, the SSD is expected to last for 100.000 program/erase cycles. This is ensured by the operation of the 3D TLC NAND flash in SLC mode along with massive overprovisioning, where part of the memory is not made available for data storage, but instead is set aside for data management.

Conclusion

Even if the final discontinuation of 2D SLC NAND flash memory is still a few years away and Last Time Buy stocking is possible when End of Life is announced, early testing of 3D NAND products are recommended given the protracted nature of the process of incorporating new components into a design.

Given the developments and innovations currently being observed from NAND flash memory and controller suppliers, memory products that look to start living up to the quality of 2D SLC NAND flash memory are to be expected. Whether long-term availability can be guaranteed and 100.000 program/erase cycles will be possible, remains to be seen.

As a partner of Swissbit, Apacer, Intel, Kioxia, Transcend and Wilk, Rutronik is in close contact with the leading suppliers of NAND flash memory. The Rutronik Storage Team provides developers and purchasers with comprehensive support and advice for selecting the right memory technology.

 

Find components at www.rutronik24.com.

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Figure 1: The distribution of charges in SLC, MLC, TLC and QLC NAND flash memory shows why reliability and program/erase counts suffer – unless there are correction mechanisms and better controller and firmware features to counteract this.
Figure 1: The distribution of charges in SLC, MLC, TLC and QLC NAND flash memory shows why reliability and program/erase counts suffer – unless there are correction mechanisms and better controller and firmware features to counteract this.