RRAM storage – Silverton Consulting

Read an article the other day about the “Atomristor: non-volatile resistance switching in atomic sheets of transition metal dichalcogenides” (TMDs), an ACS publication. The article shows research that discovered an atomic sheet level version of a memristor. The device is an atomic sheet of TMD that is sandwiched between two (gold, silver or graphene) electrodes.

They refer to the device switching non-volatile resistance (NVR) from low to high or vice versa but from our perspective it could just as easily be considered a non-volatile device usable for memory, storage, or electronic circuitry.

Prior to this research, it was believed that such resistance switching could not be accomplished with single atomic, sub-nanometre (0.7nm) sized, sheet of material.

NVR atomristor technological properties

The researchers discovered that NVR switching can occur at different device temperatures, sheet areas, compliance current, voltage sweep rate, and layer thickness. In all five degrees of freedom were tested to show that TMD atomristors had wide applicability and allowed for significant environmental and electronic variability.

Not only was the effect extremely versatile, the researchers identified multiple materials which could be used for the atomic sheet. In fact, TMD are a class of materials and they showed 4 different TMD materials that had the NVR effect.

Surprisingly, some TMD materials exhibited the NVR effect using unipolar voltages and some using bipolar voltages, and some could use both.

The researchers went a long way to showing that the NVR was due to the atomic sheet. In one instance they specifically used non-lithographic measures to fabricate the devices. This process utilized graphene manufacturing like methods to produce an atomic sheet ontop of gold foil and depositing another gold layer ontop of that.

But they also used standard fabrication techniques to build the atomristor devices as well. Using these different fabrication methods, they were able to show the NVR effect using different electrodes types, testing gold, silver, and graphene, all of which worked similarly.

The paper talked of using atomristors in a software defined radio, as a electronic circuit/cross bar switch, or as a memory/storage device. But they also indicated that it could easily be used in a neuromorphic computer as well, effectively simulating neuron like computations.

There’s much more information in the ACS article.

How does it compare to flash?

As compared to flash, atomristors NVR devices should be able to provide higher levels (bits per mm) of density. And due to the lower current (~1v) required for (bipolar) NVR setting, reading and resetting, there’s a lower probability of leakage of stored charges as they’re scaled down to nm sizes.

And of course it comes in 2d sheets, so it’s just as amenable to 3D arrays as NAND and 3DX is today. That means that as fabs start scaling 3D NAND up in layers, atomristor NVR devices should be able to follow their technology roadmap to be scaled up just as high.

Atomristor computers, storage or switch devices

Going from the “lab” to an IT shop is a multifaceted endeavour that takes a lot of time. There are many steps to needed to get to commercialization and many lab breakthroughs never make it that far because of complexity, economics, and other factors.

For instance, memristors were first proposed in 1971 and HP(E) researchers first discovered material that could produce the memristor effect in 2008. In March 2012, HRL fabricated the first memristor chip on CMOS. In Dec. 2017, >9 years later, at their Discover Conference, HPE showed off “The Machine”, a prototype of a memristor based computer to the public. But we are still waiting to see one on the market for sale.

That being said, memristor technologies didn’t exist before 2008, so the use of these devices in a computer took sometime to be understood. The fact that atomristors are “just” an extremely, thinner version of memristors should help it be get to market faster that original memristor technologies. But how much faster than 9-12 years is anyone’s guess.

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Comments?

Picture Credit(s): All graphics and pictures are from the article in ACS

Chip componentry

The chip’s sensors are based on carbon nanotubes. Aside from a layer of silicon at the bottom, all the rest of transistors used in the chip are also based off of carbon nanotube FET (field effect transistors).

The RRAM storage class memory is a based on a dielectric material which uses electrical resistance to store non-volatile data.

The bottom layer is a silicon based CPU. On top of the silicon is a carbon nanotube layer. Next comes the RRAM and the top layer is more carbon nanotubes making up the sensor array.

Architectural benefits

One obvious benefit is having data storage directly accessible to the CPU is that there’s no longer a need to go off chip to access data. The 2nd major advantage to the chip architecture is that the sensor array can write directly to RRAM storage, so there’s no off chip delay to provide sensor readout and storage.

Another advantage to using carbon nanotube FET’s is that they can be an order of magnitude more energy efficient than silicon transistors. Moreover, RRAM has the potential to be much denser than DRAM.

Finally, another major advantage is that this can all be built in one 3D chip because carbon nanotube and RRAM fabrication can be done at relatively cooler temperatures (~200C) vs. silicon fabrication which requires relatively high temperatures (1000C). Silicon cannot be readily fabricated in multiple layers because of the high temperatures required which will harm lower layers. But you could fabricate the lowest layer in silicon and then the rest as either carbon nanotube FETs or RRAM without harming the silicon layer.

Transistor/RRAM counts

The chip as fabricated has a million RRAM cells (bits?) and 2 million nanotube FETs. In contrast, in 2014, Intel’s 15-core Xeon Ivy Bridge EX had 4.3B transistors and current DRAM chips offer 64Gb. So there’s a ways to go before carbon nanotube and RRAM densities can get to a level available from silicon today.

However, as they have a bottom layer of silicon they can have all the CPU complexity of an Intel processor and still build RRAM and carbon nanotubes FETs on top of that. Which makes this chip architecture compatible with current CMOS fabrication techniques and a very interesting addition to current CPU architectures.

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Unclear to me why they stopped at 4 layers (1-silicon FET, 1 carbon nanotubes FET, 1 RRAM and 1 carbon nanotubes FET [sensor array]). If they can do 4 why not do 5 or more. That way they could pack in even more RRAM storage and perhaps more sensor layers.

Also, not sure what the bottom most layer of carbon nanotubes is doing. If I had to hazard a guess, it’s being used for RRAM control logic. But I could be wrong.

I could see how these chips could be used for very specialized sensor applications, with a limited need for data storage. The researchers claim many types of sensors can be created using carbon nanotubes. If that’s the case, maybe we might see these sorts of chips showing up all over the place.