Crossbar founder finds metals move – in a solid

by Robin Harris on Friday, 27 June, 2014

NVRAM maker Crossbar’s co-founder and professor at U Michigan Wei Lu has published a paper describing a never-before-seen phenomena: metal nanoparticles moving in a solid. Crossbar is pushing RRAM – Resistance RAM – but there is a problem with most RRAM implementations: no one knows how they work.

RRAMs been made to work, seen to work, and they have great properties that promise to supersede NAND flash in enterprise applications, but not knowing their mechanics in detail is a problem. For example, it’s hard to optimize the production process if you don’t know what the underlying physics are.

According to the press release (paper is behind a pay wall):

Lu, who led the project, and colleagues at U-M and the Electronic Research Centre Jülich in Germany used transmission electron microscopes to watch and record what happens to the atoms in the metal layer of their memristor when they exposed it to an electric field. The metal layer was encased in the dielectric material silicon dioxide, which is commonly used in the semiconductor industry to help route electricity. They observed the metal atoms becoming charged ions, clustering with up to thousands of others into metal nanoparticles, and then migrating and forming a bridge between the electrodes at the opposite ends of the dielectric material.

They demonstrated this process with several metals, including silver and platinum. And depending on the materials involved and the electric current, the bridge formed in different ways.

The bridge, also called a conducting filament, stays put after the electrical power is turned off in the device. So when researchers turn the power back on, the bridge is there as a smooth pathway for current to travel along. Further, the electric field can be used to change the shape and size of the filament, or break the filament altogether, which in turn regulates the resistance of the device, or how easy current can flow through it.

The StorageMojo take
With the many compromises required to use NAND flash for enterprise storage, its declining durability as feature sizes shrink, and the relatively small cost of media in high-performance storage, it seems likely NAND flash will not be the medium of choice in 10 years time. Consumer and mobile apps will continue to use it, but a more robust medium is helpful for mass storage reliability and economics.

But RRAM has a steep hill to climb. Billions have been spent on flash factories and hundreds of millions on making it robust enough for high-performance use.

That said, Crossbar is taking the right approach to RRAM: their process is compatible with today’s CMOS foundries; they can produce 3D chips more simply than 3D NAND; and, as this paper demonstrates, they probably know more about this technology than anyone else. That’s a good start.

Courteous comments welcome, of course. Other RRAM teams are welcome to comment.

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