As the semiconductor market continues its march forward, creating smaller trace-width silicon solutions on the latest fab equipment, designers should carefully analyze the characteristics of the end product. In the case of NAND flash memory, the finer trace widths deliver two opposing characteristics.
First, the trace widths employed on the latest memory reduce the cost per gigabyte, lower power use, and increase performance. These are three very valuable qualities for many markets. But—you knew the “but” was coming—the finer the trace width of NAND flash, the lower the endurance, the higher the inter-cell noise coupling, and the higher the error rates.
For industrial-grade flash storage devices, the most reliable NAND flash is a highly desired commodity. SLC (single-level-cell) NAND flash, with only one bit per cell, is exclusively used. However, which SLC NAND generation is also an important consideration.
Several generations of SLC NAND flash from the handful of major suppliers being fabricated are available today. They range from 43 nm at the high end to less than 15 nm at the low end. Based on actual and published test data performed by CactusTechnologies, endurance of the 43-nm SLC NAND is 90,000 cycles/physical block, 32 nm is 70,000, and 24 nm is 60,000. If endurance is the only criteria used to determine the best SLC NAND, the 43 nm is surely ahead, but not an order of magnitude or more.
Additional analysis of SLC NAND error rates by trace width shows a different picture. 43-nm SLC NAND has an order of magnitude lower error rate than 32-nm SLC. As 24-nm and lower SLC NAND components make their way to the market, error rate will become more of an issue.
Generally, more powerful ECC engines inside the flash card/SSD controller can correct these errors, but it’s a clear indicator of the reliability of the NAND flash itself. At the point the controller’s ECC engine can no longer correct these errors, a cell must be retired.
For the most reliable SLC NAND flash on the market today, 43-nm SLC NAND is the hands down winner. Embedded OEM system designers would be prudent to consider flash storage devices built with 43-nm SLC for applications where reliability is essential.
Steve Larrivee is VP Sales & Marketing for Cactus Technologies and has over 30 years’ experience in the data storage market, including 10 years with SanDisk and 5 years at Seagate Technology.