Automotive
evaluate NAND Flash-based storage devices with confidence that they are robust and reliable enough for use in vehicles.
NAND Flash cell operation at high temperature
To see how technology can counter the effect of high temperatures on NAND Flash cells, it is important to understand the operation of these cells. In flash memory, data bits are represented by stored charge (electrons) in cells. When NAND Flash technology first came on to the market, memory arrays were made of Single-Level Cell (SLC) elements. In SLC NAND, the cell stores one bit of data – a 1 or 0. As technology advanced, NAND Flash chip manufacturers responded to demand for higher memory density by developing Multi-Level Cell (MLC) technology, which stores two bits per cell, and then Triple- Level Cell (TLC) technology, with three bits per cell (see Figure 1). This means that the cell volume-per-bit has declined with each new generation of NAND Flash. NAND cell size also shrinks as semiconductor fabrication processes advance from older process nodes to the latest sub-10nm nodes. The high memory density of today’s TLC NAND Flash devices means that a storage device such as Silicon Motion’s FerriSSD can provide up to 480GB of data capacity in a surface-mount BGA package with a tiny footprint of just 20mm x 16mm. But the small size of TLC cells means that they wear out at a faster rate than SLC cells, a factor which the flash controller embedded in a storage device has to take into account. Every Program/ Erase (P/E) cycle slightly degrades the oxide layer in the cells on which a P/E operation is performed. Smaller TLC cells have a thinner oxide layer than the larger SLC cells, so they degrade faster, and
www.cieonline.co.uk.
can on average withstand fewer P/E cycles. Proprietary NANDXtend technology solves this problem in Ferri series storage devices. NAND Flash cells also experience electron leakage over time. If too much charge leaks from a cell, its data can no longer be read out. ‘Data retention’ – the length of time for which data can be stored in a cell – declines as more P/E cycles are performed. And heat accelerates electron leakage, so data retention also declines faster as temperature rises, as Figure 2 shows.
So, this is the problem for NAND Flash- based storage devices for automotive applications such as the infotainment system: in a car’s centre console, an infotainment ECU might be required to operate at temperatures as high as
85°C. But data loss is not acceptable in infotainment applications such as mapping and navigation. And the AEC-Q100 standard mandates a defect rate of zero when tested at temperatures up to 85°C (for Grade 3 qualification).
It is a problem that a Silicon Motion storage controller, and the firmware which it runs, can solve.
Central role of storage controller A NAND Flash-based storage system consists of two basic elements: n A NAND Flash array n A NAND Flash controller IC The basic role of the controller is the bridge between the NAND Flash cells and the host processor which writes to and reads
from the memory. The controller manages the mapping of bits to cell addresses. Silicon Motion has more than 20 years’ experience of developing the specialized controller ICs which manage NAND components. Its deep understanding of NAND characteristics enables it to design both highly optimized ICs and related firmware controller platforms. In fact, more NAND Flash components, including 3D Flash products from Intel, Kioxia, Micron, Samsung, SK Hynix, Western Digital and YMTC, are supported by Silicon Motion controllers than by those of any other company.
Silicon Motion’s understanding of the behaviour of NAND Flash at high temperatures underlies the operation of
Components in Electronics
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