Flash memory degrades with normal usage. How quickly this occurs depends upon a range of different factors. If a memory card is to reliably perform its task over a period of years without needing to be replaced - as with an industrial embedded memory solution, or perhaps in an outside location such as a telecommunications system - then it must be of a higher quality than a standard product.
(Photo courtesy: Swissbit AG)
In this case, the use of electronic components with an extended range of temperature tolerance and robust workmanship are typical requirements. A more important aspect is that the storage media are able to preserve data reliably for a long time. If the issue is durability, then the active assistance of the controller is required: the internal operation of a flash memory is crucial to how long it will remain writable and not lose any data. In order to understand this, one needs to know how flash memory devices age.
The cells of a NAND flash can only withstand a limited number of programming and erase cycles. Why? The programming voltage generates a tunnel effect by which electrons are pushed onto the floating gate or charge trap layer in which they are stored. The problem is this: over a large number of programming cycles, high-energy electrons also accumulate in the oxide layer. As a result, over time the threshold voltage changes to the point where ultimately the cell is no longer readable.
An aging cell: Electrons accumulate in the tunnel oxide layer causing the threshold voltage to gradually change. Cracks in the tunnel oxide layer induce leakage current paths permitting the charge to flow off. Read errors increase to the point where the block becomes a “bad block” that needs to be retired. (Photo courtesy: Swissbit AG)
There is also a further aging effect: conductive paths form in the oxide layer, causing the cell to gradually lose its amount of charge – and with it the stored bit. This effect is exacerbated by high temperatures. Experiments on a 25 nm Multi-Level Cell (MLC) NAND have shown that after 50% of the allowed erase cycles have been consumed the retention falls to about 25% of the initial values at 55 °C. At 85 °C it falls to below 10%.
The effect also increases with time the closer the cell approaches its maximum program/erase cycles (P/E cycles). The effect on retention is huge: hence, while both new single-level and multi-level cell NANDs reliably provide 10 years of retention, this figure declines to only one year by the end of their lifetimes. But with MLC NANDs, this point is reached after 3,000 P/E cycles and with SLCs after 100,000. That is another reason why SLC is so popular for especially challenging applications.