Fujitsu announced that it has licensed Nantero's carbon nanotube-based NRAM (Non-volatile RAM) and will participate in a joint development effort to bring a 256Mb 55nm product to market in 2018. Carbon nanotubes are a promising technology projected to make an appearance in numerous applications, largely due to their incredible characteristics, which include unmatchable performance, durability and extreme temperature tolerance. Most view carbon nanotubes as a technology far off on the horizon, but Nantero has had working prototypes for several years.
The battle for the future of non-volatile memory is heating up, as numerous products, such as 3D XPoint, work their way to market. Carbon nanotubes feature their own unique blend of attributes that may just outstrip them all if the price is right. I have to admit, the thought of a carbon nanotube SSD is enticing. Let's take a closer look.
The Particulars
NRAM is a flexible technology that vendors can use to produce memory or storage products. The real attraction of carbon nanotubes begins with their DRAM speeds, but unlike DRAM, the material retains stored data after the power source is removed. The material itself switches between states in picoseconds (a trillionth of a second), but the DRAM interface limits performance to the nanosecond range (>5ns).
The "persistent" capability mirrors that of other emerging technologies, such as 3D XPoint, ReRAM and PCM, but NRAM purportedly features higher performance than the competition. Other products also suffer limited endurance thresholds, whereas Nantero's NRAM has been tested up to 10^12 (1 trillion) cycles. The company stopped testing endurance at that point, so the upper bounds remain undefined. It is safe to assume that endurance is essentially unlimited for most practical, and even impractical, applications.
Heat tolerance factors into any endurance equation, but NRAM can withstand temperatures up to 800 degrees C, though the company has only tested the current version at 200 degrees C during an active state (fulfilling data requests). The spec noted that NRAM retains stored data for more than 1,000 years at 85 degrees C and more than 10 years at 300 degrees C.
How It Works
The NRAM carbon nanotubes are 2nm in diameter. Much like NAND, fabs arrange the material into separate cells. NAND employs electrons to denote the binary value held in each cell (1 or 0), and the smallest lithographies hold roughly a dozen electrons per cell. NRAM employs several hundred carbon nanotubes per cell, and the tubes either attract or repel each other with the application of an electrical current, which signifies an "on" or "off" state. NRAM erases (resets) the cells with a phonon-driven technique that forces the nanotubes to vibrate and separate from each other. NRAM triggers the reset process by reversing the current, and it is reportedly more power efficient than competing memories (particularly at idle, where it requires no power at all).
The technology currently stores one bit per cell. However, varying the voltage causes some groups of nanotubes (within each cell) to touch each other, while others will not come into contact. This presents the capability to support multiple bits per cell (similar to MLC and TLC NAND), which increases density. Nantero also sees a path forward with vertical 3D arrangements. Much like NAND, NRAM can scale density through lithography shrinks (it projects sub-5nm) or 3D multi-layered architectures. Fabs can also stack NRAM die with wire bonding and TSV techniques to create high-density packages.
Spinning The Silicon
The technology is amazingly simple to produce. It's merely a thin layer of carbon nanotubes that are spin-coated onto a normal wafer and then sandwiched between two layers of interconnects. The interconnects use a crosspoint implementation. Unlike NAND, which has to erase cells in large blocks, NRAM is bit-addressable. The ability to program and erase each bit separately provides tremendous performance and allows it to operate as either memory or storage. The products communicate through the DDR4 interface, which actually constrains the performance of the underlying media, but it could theoretically communicate through other protocols, such as NVMe, as well.
The manufacturing process employs normal lithography and etching techniques on standard CMOS wafers. In fact, Nantero claims to be the only company with a patented (175+ and counting) CMOS-compatible process. The easy CMOS manufacturing technique doesn't require expensive re-tooling, which leaps the economic hurdles that prevent many promising technologies from making it to market.
The Path To Market
Fujitsu is just the first company to announce that it's producing the technology. Nantero claimed that it is working with six other (unannounced) fabs, and some are already developing 28nm 3D designs that enable "multi-GB" products that are denser than DRAM. Nantero indicated that its partners will announce these products soon. The company also has 12 other (unspecified) customers in place, but because Nantero merely licenses the technology to others, it cannot provide details.
The extreme endurance and heat tolerance makes the new technology very promising for embedded caches on SoCs and processors. Numerous other applications require extreme temperature tolerances, such as components used in oil and gas drill heads. Unsurprisingly, Schlumberger, one of the largest oil and gas exploration companies, has invested heavily in the technology.
Nantero, which began its work in 2001, has produced several working prototypes and rocketed the technology into space on a 2009 space shuttle Atlantis mission. An independent third-party lab has already tested its products, which lends credence to Nantero's claims.
Keys To Success
Pricing is always the key ingredient to success, and Nantero indicated that the leading-edge products will ship with half the cost of DRAM, and prices will fall as production ramps and the vendors shrink lithographies and stack layers. Increased density will also allow the technology to penetrate a broader range of applications.
Intel's and Micron's (IMFT) competing 3D XPoint technology is much closer to market, but pricing remains hazy, and it will require expensive new production techniques and tools, which doesn't help. Also, 3D XPoint is a "captive" technology that will be unique to the IMFT alliance, whereas Nantero is more than happy to license its technology to a range of fabs. It's doubtful that NRAM would be a serious competitor to 3D XPoint in the short term, but considering that 3D XPoint isn't even on the market yet and also faces significant pricing challenges, it could be a threat over the coming years.
The important takeaway is that carbon nanotube products have busted out of the research lab and into production fabs. The early products will focus on highly specialized applications, such as embedded caches and storage-class memory applications. If the price is right, the technology will trickle down into more familiar devices, such as SSDs, in the future.
