There has been quite a buzz in the industry lately about NAND flash products that are capable of storing more than two bits per cell, so I wanted to just take a minute and provide our perspective.
Simply put--what ultimately matters is having the lowest cost-per-bit solution
in volume production at a given moment in time, not how many bits per cell are stored. Now, that said, there’s no question that being able to store more bits per cell results in lower cost. However, there are some serious trade-offs that we think make this option not viable at this time. Most notably--performance and reliability suffer. In fact, we estimate that the performance (measured in write-cycle throughput) for going from two to three bits per cell using the same NAND architecture and process technology could be as much as halved. And reliability, (measured in write-cycle endurance) could be up to an order of magnitude worse (yes—up to 10x worse). Scary stuff. Because having a product that may have a lower bit cost, but doesn’t meet today’s level of performance and reliability limits the value of that product. It’s also worth noting the greater burden on the system implementation of going beyond today’s well-understood MLC technology, such as making sure the controller provides adequate ECC coverage.
Perhaps the most critical factor is the time frame of when the new product is introduced relative to the next-generation process technology. So, for example--most NAND manufactures introduce MLC products first on their latest-and-greatest process node, and then introduce other products as the process matures. But, if you introduce a new product with more than two bits per cell and
your next-generation process technology is production-ready shortly thereafter, it is likely that your new product will not be successful, since the next generation process will provide lower cost, higher performance, and higher reliability with the good-old standard MLC.
Now, all that said, at our core we are innovators--and I’ll tell you that we are
currently developing products that go beyond two bits per cell. But we also feel that the technology as a whole hasn’t come into its own just yet. It will, but what’s clear now is that lithographic scaling is the key to achieving the lowest costs while delivering the performance and reliability that the industry needs.
About Our Blogger
Kevin is Micron Technology's Director of NAND Marketing.