The following article was contributed by Dev Paul, memory manager at Semiconductor Insights. Based in Kanata, Ontario Canada, Semiconductor Insights provides technical and patent analyses of integrated circuits and structures.
Explosive growth in wireless and consumer applications is attracting DRAM players into flash (Hynix, ST, Infineon, and potentially Micron) and driving all vendors to increase density and drive down cost per bit. Samsung is a proponent of NAND flash and leads this market with Toshiba/Sandisk, a distant second. As well, Samsung has displaced Intel, a proponent of NOR, as the leader in the overall flash market.
There are two key NAND architectures -- Single Bit per Cell (SBC) and Multi Level Cell (MLC). MLC's more complex architecture means it usually lags SBC technology by one or more process generations. Vendors can maximize density by using an SBC architecture in a more advanced process (Samsung), or by deploying MLC architectures (Toshiba).
Samsung established its flash leadership position with SBC technology. SI recently analyzed Samsung's K9F2G08U0M NAND flash and confirmed the company's claim that it is indeed the first to 90-nm NAND flash production. SI's analysis revealed a Samsung die size of 144mm2, neutralizing its closest NAND competitor from Toshiba -- a 130-nm 2Gb MLC flash with a 149mm2 die size. Die size comparisons of other devices analyzed by SI are included in Table 2.
While Samsung went with SBC, Toshiba has pursued an MLC architecture. One of the major advantages of MLC is the ability to deliver leading storage densities in relatively mature processes, offering a cost and yield advantage with the potential to offset and/or overtake the die shrink advantage offered by a more advanced process. Toshiba recently released that it will double the density in its 90-nm process with a 4 Gb NAND flash. If successful, Toshiba will have a strong cost advantage, threatening Samsung's market dominance.
Regardless of density, leading NAND flash chips are typically introduced with a die size in the 135mm2 to 150mm2 range (see Table 1). Referring back to 512 Mb devices, manufacturers have not departed substantially from this die size range. What has changed is the Mb achieved per square mm, with an approximate doubling for each generation. Flash vendors typically try to keep the die size of their leading density die sizes below 150 mm2, since yields (and thus profitability) drop substantially beyond this point.
Table 2 provides a more comprehensive breakdown of the characteristics of selected products. Most of the products display fairly similar characteristic at each density node, regardless of implementation. Aside from Hynix's HY27US08121MTIB product, all other products share the same range when it comes to Mb/mm2.
The fact that Hynix's 512 Mb HY27US08121MTIB product, which at 6.12 Mb/mm2 is similar to a 1 Gb device, suggests that the 1 Gb version will be available shortly. Being late to the NAND Flash market, it is a wise strategy for Hynix to enter with a lower risk 512 Mb product than going straight to 1 Gb. Regardless, Hynix currently has no share in the NAND flash market and will have to put a major push to achieve a substantial position in this market. Given its technology and its manufacturing prowess, it is a dark horse that could erode the market share of some of the existing incumbents.
As illustrated in Table 2, the playing field is fairly level when it comes to Mb/mm2 at a given device density. Vendors will need to act aggressively to improve cost structure at each density node. Samsung has a competitive edge today by being early to market with an SBC architecture in a leading 90-nm process. Longer term, MLC based vendors may have the advantage, if they can implement the complex sensing mechanisms required to differentiate four voltage levels in ever decreasing voltage tolerances in 90- and 65-nm.
Regardless of implementation, strong market drivers and increased competition will force all flash vendors to continually drive down cost per bit using a combination of process, packaging and circuit innovations.
Further technical analyses on the Samsung 90-nm NAND flash and other NAND flash products presented in Table 2 are currently available at the Semiconductor Insights Web site. Analysis of Toshiba's 4 Mb TC58NVG2D4BFT00, MLC NAND Flash in 90-nm is expected to be available soon.