#CHIPS: "Memristive ReRAM Employs Silicon Oxide"

With a tip of their hat to James Tour's pioneering demonstration of a memristor-like resistive memory technology at Rice University in 2010 (see Rice's Silicon Memristor Aims to Beat HP) the University College London announced what its claims is the first room temperature resistive random access memory (ReRAM) technology. ReRAMs store information on nonvolatile dynamic resistors--called memristors by their inventor Leon Chua. Mosr ReRAMS are based on a memristive layer between crossbars and will be available commercially from Hewlett Packard with Hynix, Sharp with Elpida, and separately from Samsung and Panasonic by next year, with IBM and IMEC to follow. The memristive layer between crossbars uses current flowing one way of the other to migrate vacancies that change its resistance--usually in titianium, hafnium or tantalum--thereby creating a dynamic nonvolatile resistance between crossbar switches. The big advantage of ReRAMs is that they are denser than flash, but faster than DRAM. The uniqueness of Rice's and now UCL's material, is that their vacancy migration occurs in silicon oxides instead of metal oxides, potentially lowering the cost and increasing the yields of future silicon ReRAMs: R. Colin Johnson


A photo of the UCL ReRAM device. Credit: UCL/Adnan Mehonic

Here is what UCL says about their silicon-based RRAM: The first purely silicon oxide-based ‘Resistive RAM’ memory chip that can operate in ambient conditions – opening up the possibility of new super-fast memory - has been developed by researchers at UCL.

Resistive RAM (or ‘ReRAM’) memory chips are based on materials, most often oxides of metals, whose electrical resistance changes when a voltage is applied – and they “remember” this change even when the power is turned off.

ReRAM chips promise significantly greater memory storage than current technology, such as the Flash memory used on USB sticks, and require much less energy and space.

The UCL team have developed a novel structure composed of silicon oxide, described in a recent paper in the Journal of Applied Physics, which performs the switch in resistance much more efficiently than has been previously achieved. In their material, the arrangement of the silicon atoms changes to form filaments of silicon within the solid silicon oxide, which are less resistive. The presence or absence of these filaments represents a ‘switch’ from one state to another.

Unlike other silicon oxide chips currently in development, the UCL chip does not require a vacuum to work, and is therefore potentially cheaper and more durable. The design also raises the possibility of transparent memory chips for use in touch screens and mobile devices.

Our ReRAM memory chips need just a thousandth of the energy and are around a hundred times faster than standard Flash memory chips. The fact that the device can operate in ambient conditions and has a continuously variable resistance opens up a huge range of potential applications.

The team have been backed by UCLB, UCL’s technology transfer company, and have recently filed a patent on their device. Discussions are ongoing with a number of leading semiconductor companies.

For added flexibility, the UCL devices can also be designed to have a continuously variable resistance that depends on the last voltage that was applied. This is an important property that allows the device to mimic how neurons in the brain function. Devices that operate in this way are sometimes known as ‘memristors’.

This technology is currently of enormous interest, with the first practical memristor, based on titanium dioxide, demonstrated in just 2008. The development of a silicon oxide memristor is a huge step forward because of the potential for its incorporation into silicon chips.

The team’s new ReRAM technology was discovered by accident whilst engineers at UCL were working on using the silicon oxide material to produce silicon-based LEDs. During the course of the project, researchers noticed that their devices appeared to be unstable.

UCL PhD student, Adnan Mehonic, was asked to look specifically at the material’s electrical properties. He discovered that the material wasn’t unstable at all, but flipped between various conducting and non-conducting states very predictably.

The technology has promising applications beyond memory storage. The team are also exploring using the resistance properties of their material not just for use in memory but also as a computer processor.

The work was funded by the Engineering and Physical Sciences Research Council.
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