NAND 201: the continued evolution of NAND Flash

A lot has changed with NAND Flash memory since my original NAND 101 article was published in 2006. From the evolutionary changes of a continually shrinking NAND cell, to the performance-enhancing innovations that support increasingly advanced designs, this follow-on article will chronicle the developments in NAND technology from 2006 through early 2011.

Market Changes
In 2006, single-level cell (SLC) NAND Flash devices were mainstream products that accounted for more than 80% of the devices on the market. At that time, many NAND Flash vendors were struggling with two bits per cell, known as multilevel cell (MLC), and SLC device densities were in the range of just a few gigabits. Today, for example, Micron offers a range of NAND products with densities up to 512Gb in a single device.

Figure 1 shows the past, present, and projected output mix for the major NAND cell technologies. The high runners are all MLC devices, which have replaced SLC devices in holding approximately 80% of the total market. While 16-level-cell technology grew to a few percent in 2010, it is expected to drop to 0% in 2011 due to the continued difficulty of reliably placing 16 discrete thresholds in a single cell. 8LC devices, which pack three bits per cell, are expected to grow from less than 10% in early 2009 to almost 30% by the end of 2011. These devices are used primarily in value-minded consumer products that can operate with lower NAND performance and fewer PROGRAM/ERASE cycles (also known as endurance). Traditional MLC devices, which group two bits (four levels) per cell, are ideal for applications that demand higher performance and endurance; thus, MLC drives the majority of NAND output. Lastly, SLC NAND is the technology of choice for high-performance, high-endurance, and high-reliability applications. Later, we will discuss some separate, specialized NAND devices that have been born out of necessity.



Figure 2 shows the expected application adoption of 3-bit-per-cell (8LC) NAND devices. In addition to early drivers such as Flash cards and USB thumb drives, several other consumer application designs are expected to be based on this technology.





NAND Basic Operations
While the NAND cell itself has remained essentially the same over the last several years (albeit much smaller), almost everything else about NAND has changed (see Table 1; (Click graphic for hi-res PDF)).