3D Nand SSDs

The SSD Capacity Problem

There's every reason to consider solid-state drives (SSDs) as the primary storage medium for PCs and laptops. The benefit of SSDs over traditional spinning drives is immense. No moving parts enables smaller form factors to be created, while the intrinsic speed of SSDs far outstrip that of even the fastest mechanical spinners.

And there's more. The lack of moving parts is also a boon when copying or writing small files, as SSDs can be hundreds of times faster than traditional drives at these tasks. Want to speed up your computer? One of the cheapest ways to do this is to invest in an SSD.

However, not all SSDs are created equal. Composed of a controller and Nand flash memory, capacities increase in two ways: firstly, the manufacturer adds more chips to a PCB, so a drive with eight chips with 64GB of capacity offers twice the amount of storage as a drive with four chips of the same size/density. There's a limit to how many chips can be placed on to a PCB, especially as form factors such as M.2 shrink the real estate further. A second method is to increase the density and therefore capacity of each chip.

This second aspect can be achieved by two methods, too. One can add more bits per cell - the building blocks of Nand - and therefore increase capacity for the same die size. For example, single-level cell (SLC) memory contains one bit per cell, multi-level cell (MLC) generally carries two while triple-level cell (TLC) has, as the name implies, three bits, which means 3x the capacity of SLC. The downside of adding more bits per cell is reduced performance (because there are multiple levels of charge that need to be sent to each cell) and reduced endurance, and these two problems represent a trade-off between cost, speed and endurance.

SLC memory is fast, durable and expensive, MLC strikes a decent balance, while TLC has been ushered in to provide the plentiful storage for the most cost-effective SSDs. Each of these technologies is based on having bits and cells arranged on a single layer of memory, and you can still only fit so much when physical space is at a premium on popular form factors such as M.2. Think of the Nand residing on traditional SSDs as a bungalow, with each cell representing a room and each bit denoting a person in a room. Having multiple people in a room (MLC or TLC) is handy for increasing capacity, but we're now at the point where chips cannot be made any denser - more rooms or more people in a room - without running into stability problems. Enter 3D Nand.

A New Way To Increase Capacity - 3D Nand

Wouldn't it be great if one could build Nand chips more like apartment blocks rather than bungalows? You wouldn't increase the Nand footprint but you would have many more floors, or layers, with which to increase capacity. This is exactly what newer SSD memory, known as 3D Nand, purports to do.

3D Nand represents cell layer-stacking technology where multiple layers are built up on a single Nand chip. Stacking 32 layers clearly increases the amount of space available to cells and bits, meaning that much higher capacities can be built without changing manufacturing processes. It's worth knowing that the cells are stacked into multiple layers, not the chips themselves, so they will look the same to the untrained eye.

This type of stacking doesn't just make implicit sense; there are other benefits too. As the manufacturing process becomes ever smaller - we're already at 10nm now - having lots of cells arranged near each other in a single layer brings unwanted, harmful interference into play. Stacking multiple layers provides far more room for cells and bits, mitigating the negative effects of interference. It's a win-win situation from a design point of view.

The potential for significantly more capacity in the same die space, promise of all-but eliminating interference, and no real increased cost in manufacturing due to 3D Nand being able to use the same tool-set as traditional planar Nand combine to ensure that all major manufacturers have variants of 3D Nand in their respective catalogues.

The No-Compromise Solution

Intel/Micron, SK hynix and Samsung dominate the 3D Nand landscape, and each has their unique method of implementation. Intel and Micron use the far greater area made possible by vertical layer stacking to increase the size of each Nand cell. There's no problem doing so because, even though the companies' 3D Nand is built on a larger, cheaper manufacturing process, there's ample space, as discussed above. This would not have been at all possible with 2D (planar) Nand - such a product would have been hugely uneconomic to produce in volume.

Larger cells improve both the performance and endurance to such an extent that it is better than planar, flat Nand. Micron says that it is able to produce 256Gbit MLC and 384Gbit TLC dies without compromising speed or reliability. Think about that for a second; there's up to 384 billion storage bits in a piece of Nand smaller than a fingertip.

What this really means to you, the consumer, is the possibility of consumer SSDs in excess of 2TB and enterprise-grade drives hitting 10GB - all made possible by 3D Nand. Micron recently demonstrated a 3.5TB SSD, composed of 3D MLC Nand, no bigger than a stick of gum. At this moment in time, the major players use 3D Nand composed of 32 layers, with 48 layers on the horizon.

The Future

3D Nand will make 2D Nand obsolete sooner rather than later. Intel has a number of 3D Nand-based drives in its arsenal, while Samsung and SK hynix's larger-capacity models are being transitioned to 3D Nand.

The introduction of 3D Nand represents a milestone in increasing the capacity of SSDs whilst not increasing the cost or footprint, thus paving the way for multi-TB consumer drives coming next year.