Flash storage technology will soon see a three-fold improvement in data density thanks to a joint development at Intel and Micron that will allow the production of 3.5 TB flash sticks and 10 TB standard-sized SSDs. Meanwhile, a new 48-layer cell technology development by Toshiba could pave the way for higher write speeds, more reliability and lower costs in solid state drives.
The flash memory inside our phones, tablets and SSDs is getting faster, cheaper and more capable every year, but there is a limit to how much data we can pack inside a given area of silicon, and current technology is already pushing that limit. To keep the trend going, manufacturers are needing to move two-dimensional memory cells into the third dimension.
It’s all about electrons. Each memory cell stores bits by retaining a set amount of charge, which relates to the number of electrons that are "trapped" inside that cell. As cells get smaller, fewer and fewer electrons are stored in each cell, and this makes cells much more prone to errors.
Similarly, we have gone from storing one bit over two memory cells in the earliest designs to storing one bit per cell ("single-level cells," or SLC), two bits per cell ("multi-level cells," MLC) and even, more recently, three bits per cell ("triple level cell," or TLC). Packing more bits in a single cell makes flash memory cheaper and much more data-dense, but also less reliable because fewer electrons correspond to each bit.
The solution has been to go to 3D. By stacking tens of memory cells on top of each, many more can fit on the surface of a single die. At the same time, the cells can still stay relatively large and therefore store more electrons. This achieves the perfect combination of high data density and reliability.
Intel and Micron’s new 3D NAND technology stacks flash cells vertically in 32 layers to store 32 GB in a MLC die and 48 GB in TLC die. Squeezing 16 dies in a single package means that a flash thumb drive (which normally fits five packages) could now store as much as 3.5 TB, and standard 2.5-inch SSDs could now fit as much as 10 TB of data – about a threefold improvement over the current technology.
Traditionally, single-level cells have had the advantage of faster write speeds, lower power consumption and higher endurance, which made them better suited to industrial applications like data center storage. But now Intel and Micron claim their two- and three-bits per cell designs are reliable enough for such applications, which is good news especially given that their cost per megabyte is bound to be lower. Power consumption is also reduced thanks to a new sleep mode that cuts power to inactive dies.
One critical decision was the choice to use a floating gate cell design. This is an architecture where the cells' transistors feature a second, insulated gate that traps the precious electrons until a strong external voltage is applied. It’s a common design for flash cells in two dimensions, but this marks the first use of floating gate cells in a 3D NAND flash memory.
Meanwhile, Toshiba has announced the world’s first 48-layered 3D NAND flash memory (outdoing Intel’s 32). Its design doesn’t rely on floating gate cells but rather a two-bit-per-cell system that uses a so-called "charge trap" to prevent electrons from escaping. This technology is still in its infancy and is much less data-dense than Intel’s latest development, but it could be an interesting one for the future as it’s said to increase reliability and boost write speeds, both desirable qualities in solid state drives.
According to Micron and Intel, the MLC and the higher-performance TLC 3D NAND will both begin volume production by the end of the year. If everything goes according to plan, it's possible that we'll see the higher-capacity SSDs and thumb drives available for sale as early as next year.
