NAND flash approaches square off

Two memory makers, Samsung and Toshiba, have long dominated the rapidly growing NAND flash market. Samsung, the largest player, is a dominant force in the memory market in general and continues to push into advanced process geometries to maintain its competitive advantage. This teardown will examine both companies' latest flash iterations, placing the developments in context relative to the efforts of such other players as Hynix and the IM Flash venture of Micron and Intel.

Historically, Samsung has concentrated its development efforts on single-level-cell (SLC) flash, in which each flash cell stores only a single bit of information. Toshiba has also moved aggressively with advanced process technologies, but its competitive advantage lies with its ex- perience and prowess in multilevel-cell (MLC) NAND flash design. MLC flash stores 2 bits of information at each memory cell site, allowing Toshiba to pack more bits into a given area of silicon and lowering the company's manufacturing cost for a given memory size. Thus Toshiba can lag Samsung in process technology and still be the die density leader.

Toshiba has manufactured several generations of MLC flash, including the recent release of an 8-Gbit version on a 70-nanometer process. Last year, Toshiba used 90-nm technology to go head-to-head with Samsung's 73-nm devices. Toshiba's 90-nm MLC flash had a bit density of 29 Mbits/ mm², which exceeded Samsung's 73-nm bit density of 25.8 Mbits/mm².

Toshiba's devices even have smaller die sizes than Samsung's for a given memory density. Toshiba's 4-Gbit 90-nm NAND, for example, has a die size of 138 mm², compared with 156 mm² for Samsung's 4-Gbit 73-nm NAND. This makes Toshiba more competitive on cost. And since NAND flash for file storage is certainly a game of pennies, we hear a lot of buzz around iPod design wins for the cost leader.

But while MLC is a great equalizer in some respects, the incredibly hot demand for flash chips these days blurs the view. Storage card manufacturers need cheap chips, but they also need stability in their supply. For that reason, Kingston Technology was in talks with Samsung to purchase SLC chips as a second source (as reported in DigiTimes). Kingston took this step despite estimates that MLC solutions were about 30 percent cheaper than SLC designs last year.

While Samsung has long advocated SLC NAND over MLC, the first signs of a shift in its thinking were offered in ISSCC papers on MLC technology that the company presented in 2004 and 2005. Samsung still does not have any marketing material on MLC flash on its Web site, but it does have a 4-Gbit MLC NAND flash. Although we have analyzed samples of this device, it is still difficult to locate samples in the market. The die size of the device is 156 mm², which is still 18 mm² larger than Toshiba's 90-nm MLC NAND flash. Samsung will need to improve its MLC technology for future NAND generations if it is to be competitive with Toshiba.