Storage tech to look out for

Last month we saw some major developments in two storage technologies - Vertical NAND (V-NAND) and Resistive RAM (RRAM) - which have the potential to change the way we view storage. We currently have two choices - spinning mechanical hard drives and flash based solid state drives (SSD). The spinning hard drives still offer the best cost per GB as compared to flash-based SSDs, but each of these technologies come with their own set of issues. Hard drives lack the speedy access times and low-power goodness offered by an SSD. The only way to currently get the best of both worlds - lower cost per GB and fast access times - is to invest in a hybrid drive. But as far as SSDs are concerned, while the costs may have come down, they are still grappling with their own set of issues. NAND cell reaching its limits?The basic building block of an SSD or any flash based storage device is the NAND cell, more specifically a planar NAND cell. With each successive generation, the NAND cell shrinks in size. The advantages that you get with it are more economical SSDs, lower power consumption, higher densities which allows you to have higher capacity drives and so on. So while 25nm NAND cells were common last year, this year there are 19nm NAND cells seen in some SSDs and that number will reduce. But only to a limit. There are three types of NAND - Single-level cell (SLC) NAND which has only one bit per NAND cell, Multi-level cell (MLC) NAND having two bits per NAND cell and Triple-level cell (TLC) NAND having three bits per NAND cell. Higher bits per cell, reduce manufacturing and material costs, so a TLC-based SSD will be cheaper than an MLC based one whereas the SLC based ones will cost magnitudes over their MLC counterparts for the same capacity. While you can read from an SSD unlimited times, there is only a limited number of times you can write to the NAND cell before it goes bad. Once each NAND cell in an SSD goes bad, it is said to have reached its write endurance. A cell can only take a certain amount of Program/Erase cycles before it reaches its endurance limit. With higher bits per cell and shrinking cell size, the endurance will also take a hit. So while an older 34nm multi-level cell (MLC) NAND had 5000 P/E cycles, the 19nm MLC NAND has around 3000 P/E cycles. As the process node shrinks further, the number of P/E cycles will come down as well till you reach a point where it will not make practical sense to shrink the cell size further. 3D NAND / V-NANDSamsung has come up with a solution to address the issue discussed above with its three-dimensional NAND cell or V-NAND. According to its website, “Samsung’s three-dimensional V-NAND (Vertical NAND) flash memory is fabricated using an innovative vertical interconnect process technology to link the 24-layer 3D cell array based on Samsung's 3D Charge Trap Flash (CTF) structure.” 

3D V-NAND evolution (Image courtesty: Samsung)

Traditional NAND cells use a floating gate architecture where the charge is stored in the floating gate which is the conducting material. Samsung uses its technique called the charge trap flash, which uses an insulator in place of a conducting floating gate and stores charge in the insulating material temporarily. For the V-NAND, Samsung has converted the planar CTF design to a non-planar one where the silicon substrate is in the centre surrounded by the insulating material which is then surrounded by the control gate. Samsung then stacked these cells on top of each other to create a 24-layer 3D cell array. According to Samsung this addresses the scalability issue as well as reduces interference between the cells thereby increasing endurance. Last month Samsung announced its first generation 128 Gb MLC V-NAND solution which it claims delivers twice the density in the same physical space as a 19 or 20nm solution. Currently Samsung is only looking at the enterprise segment with its 480GB and 960GB capacities. You can read more here. Samsung claims that its V-NAND SSDs will offer a 20 percent performance improvement and 40 per cent less power consumption. While the current product will have 128Gb per NAND cell, Samsung claimed that it would be looking at taking that number to 1Tb in future generations. Expect this technology to trickle down to consumer SSDs in the next couple of years. Resistive RAM (RRAM)Most mobile devices these days max out at 64GB of flash storage. While that is a lot of storage, often times, you have to make a compromise as to what data you would want to store on your mobile device. The 64GB capacity is the maximum limit as far as microSD cards go and most phones/tablets will sport 8GB or 16GB with or without an external card. But what if we told you that somewhere in the near future you will be able to carry terabytes of data on your mobile devices? A start-up in California called Crossbar Inc has come out with Resistive RAM or RRAM (also called ReRAM) module which is a non-volatile memory technology which promises high density flash storage volumes. It must be noted that Crossbar Inc. isn’t the only company operating in the resistive RAM space, but they are the first to actually come out with a working RRAM prototype manufactured in an unknown third party fab. The chip was manufactured on the 25nm process node with 8GB of storage. HP is another name in the resistive RAM space but its ‘memristor’ is yet to see the light of day. 

Crossbar RRAM comprises three main layers, two electrodes separated by a switching material (Image courtesy: Crossbar Inc)

As the name implies, Resistive RAM or RRAM works by creating a resistance between the electrodes rather than storing the charge. The RRAM electrical structure comprises a non-metallic bottom electrode, an amorphous silicon switching medium and a top metallic electrode. When a voltage is passed across the electrodes, it creates a filament across the electrodes - around 10nm in size. This creates a pathway for the electrons to pass through and when it’s destroyed it stops the flow of electrons. So in effect, the resistive filament acts as an on/off switch. 

Crossbar RRAM occupies a smaller footprint as compared with similar capacity NAND solution (Image courtesy: Crossbar Inc)

According to Crossbar, its non-volatile RRAM will be capable of storing 1TB of data on a single 200 mm^2 chip. This capacity can be increased by 3D stacking. Crossbar also claims that their RRAM modules occupy lesser space than current similar-capacity NAND modules, boast of 20 times better write performance, 20 times lesser power consumption and are 10 times more long lasting. These claims need not be the actual real-life performance benchmarks, but just an indication of where the RRAM is positioned. Also since it will have CMOS compatibility, you can technically integrate storage on the system-on-chip itself. So think of an SoC with multiple stacked RRAMs taking capacities well beyond 1TB. George Minassian, CEO of Crossbar has stated that the RRAM technology will be seen in products such as microcontrollers by 2015. The futureSSDs will certainly be good for another decade, well at least in India where a majority of the population is still on spinning hard drives. The innovations discussed above have the potential to be truly disruptive, if they meet the right pricing which should still take a while after they are commercially available. But just imagine a future where you no longer have to rely on external storage for your mobile devices. Will we even bother with storing in the cloud when we have terabytes available on our mobile devices? Who knows?