It’s been a while since we checked in on Crossbar, the next-generation memory company working on a
NAND flash replacement. The company’s resistive RAM (RRAM or ReRAM) technology stores data by creating resistance in a circuit rather than trapping electrons within a cell. Now, the company has announced that it’s moving towards commercializing its designs. That means it’s proven that it can build hardware at existing foundries and can seek vendors to bring solutions to market.
There are several intrinsic advantages to ReRAM as compared to NAND flash. NAND has limited endurance, its lifespan degrades as process nodes shrink and cells become smaller, and the amount of error-correction required at each new node is steadily increasing. Performance gains have slowed since clocking NAND faster also tends to cause it to degrade, and the bulk of improvements are now delivered by improving either the NAND controller or the system interface — not the underlying performance of the NAND itself.
ReRAM solves many of these problems. Unlike NAND, it doesn’t need to be erased before its programmed, and it’s much faster than NAND by multiple metrics. It also consumes less power — Crossbar claims that NAND requires 1360 picojoules per cell to program, while RRAM cuts this to just 64 picojoules per cell. Programming power is just one aspect to overall SSD power consumption, but the company also claims that its technology supports storing two bits of data per cell (analogous to MLC NAND) and can be stacked into 3D layers.
The company also claims that its technology can be used to reduce the complexity of the microcontroller itself — a significant potential advantage as this is one area where complexity and cost have been increasing as the task of flash management becomes more complicated.
NAND controller design vs. RRAM controllers
Commercialization of consumer hardware, however, remains some time away. Crossbar has demonstrated that its designs can scale up into the terascale, but that doesn’t mean it’s ready to bring products at that density to market.
The firm is now licensing to ASIC, FPGA, and SoC developers, with samples arriving in 2015. Early expected target applications are the embedded and low-level applications shown in the chart above, where very little storage is required and low-power operation is essential.
One thing that hasn’t changed is the long-term roadmap for actual NAND flash replacement. Here’s where the realities of economic scaling come home to roost. Samsung, Intel, Micron — these companies have invested tens of billions of dollars into NAND production, and they aren’t going to shift to a new standard on a dime. For all that the tech industry likes to pride itself on rapidly adopting the latest and greatest technology, the truth is far different — the most successful products in computing are those that extend previous work in a cost-effective manner. Come-from-behind overtake maneuvers are actually quite rare, which is one reason why storage mediums tend to live for decades even when faster solutions are available in the market.
Right now,
3D NAND flash (Samsung calls it
V-NAND) will drive the market from at least 2015 to 2018. That doesn’t mean we won’t see RRAM in consumer or enterprise applications — the market has snapped up more-expensive NAND flash solutions that leverage standards like PCI Express or the upcoming
NVMe, particularly when these products can enable faster response times for high-frequency stock trading or other latency-critical applications. RRAM may not “feel” much faster than NAND, but it has the potential to provide better response times at latencies that matter to computers, and that’s enough of a reason for certain segments to adopt the equipment.
Earlier this year, we covered advances in other NAND flash replacements, such as
phase change memory (PCM). These designs have demonstrated substantially improved performance compared to NAND, but also face significant scaling challenges and cost concerns. RRAM uses conventional CMOS hardware and can operate at scales down to 5nm. NAND flash, in contrast, isn’t expected to scale below 10nm on even the most optimistic roadmaps, and it’s not certain it will even get that low.
3D NAND will extend this further by allowing companies to step back up to higher nodes (Samsung’s current V-NAND is built on 40nm process technology). It’s not necessarily fair to call that a stopgap when it could slap 5-10 years on NAND scaling, but it’s still a long-term functional limitation. We’re going to need to replace flash with some alternate form of memory in the long term if we want to continue to improve power consumption and scale compute capability upwards, and right now RRAM looks like the most practical near-term option.
