08 August 2013

ReRAM, the memory tech that will eventually replace NAND flash, finally comes to market

A new memory technology company, Crossbar, has broken cover with a new ReRAM design it claims will allow for commercialization of the technology. The company’s claims aren’t strictly theoretical; today’s announcement reveals that the design firm has successfully implemented the architecture in silicon. While that’s not the same as initiating mass production, it’s an important step in the search for a NAND flash replacement.

ReRAM (also known as RRAM) works by creating resistance rather than directly storing charge. An electric current is applied to a material, changing the resistance of that material. The resistance state can then be measured and a “1″ or “0″ is read as the result. Much of the work done on ReRAM to date has focused on finding appropriate materials and measuring the resistance state of the cells. ReRAM designs are low voltage, endurance is far superior to flash memory, and the cells are much smaller — at least in theory.
Crossbar memory characteristics. (Click to zoom in)

Crossbar has been working to turn theoretical advantages into practical ones. The company’s design is ready for mass production but will target low-density applications for now — think embedded microcontrollers. Demonstrating the capabilities of the part now is important to grabbing investor attention. Crossbar might be a small player, but it’s a small player in a field that’s attracting a lot of prominent attention from major companies; SK Hynix, Panasonic, and HP are all working on ReRAM designs. Long-term, the same principles that make ReRAM function might allow its use as a DRAM replacement, though mass storage ReRAM and ReRAM-DRAM might use different architectures, with one emphasizing long-term storage and the other accelerating random access.

Flash in the pan

ReRAM is the most likely candidate for replacing NAND flash and make no mistake — we need a NAND flash replacement. Sub-20nm NAND roadmaps are peppered with references to 1X and 1Y technology as a means of implying node scaling when lower nodes aren’t actually on the table. The broad plan is to rely on 3D die stacking as a means of improving cost-per-GB as opposed to transitioning to smaller 2D process geometries. Flash will still scale to 14nm within the next few years, but every smaller process node sharply increases the amount of ECC RAM required, degrades longevity, and requires greater over-provisioning and more intelligent recovery schemes at the controller level. This, in turn, slows down performance and increases die sizes. SLC (single-layer cell NAND) doesn’t really suffer from these issues, but it’s inordinately expensive.

We don’t know where, exactly, the limit it, but the ITRS predicts that NAND below 7nm, in 2D or 3D form, isn’t going to happen, period. That’s more or less when CMOS itself runs out of steam, and even getting down to 7nm is currently dubious given the troubles with EUV lithography and the advent of double/quad patterning. The endurance issue will eventually bite into enterprise and database use, or force those industries to adopt SLC NAND. The bottom line is that regardless of when it happens, NAND scaling isn’t going to continue indefinitely.

The current hope is that ReRAM will be ready for widescale adoption by the 2017-2018 timeframe. The first 3D NAND devices are currently expected in 2015, which means commercial ReRAM deployment would begin well before NAND hits its absolute scaling limit. Given the difficulty of ramping an entirely new technology, it wouldn’t surprise us if NAND’s last generations focus primarily on low-end consumer applications, while ReRAM comes in at the top for the enterprise market, where endurance and write requirements are difficult to meet with smaller NAND geometries.

Put in context, then, the work Crossbar is doing to bring ReRAM to market is essential early work towards building the practical standard of the future. Not that ReRAM is guaranteed — there’s always the possibility of a problem or another technology might suddenly have a breakthrough moment. But as things stand today, ReRAM appears to be the memory technology with the fewest obstacles standing between it and commercialization as a long-term replacement for NAND.


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