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Scientific Method / Science & Exploration

Logic circuits that program themselves: memristors in action

Integrated circuits incorporating memristors are able to successfully perform …

Since 1972, scientists have known there are four basic circuit components, but if you've spent any time in an electrical engineering classroom, you probably only have experience with three: capacitor, inductor, and resistor. The fourth basic component, the memristor, had remained stuck in the domain of theory--a nice idea that even the theorists thought had few practical uses. Last year, scientists at Hewlett-Packard (HP) demonstrated the first functional solid-state memristor, made from thin films of TiO2, and discovered it had an abundance of unique and highly promising properties.

A study released Monday by The Proceedings of the National Academy of Sciences shows that these same TiO2 memristors can be fabricated into functional and reprogrammable integrated circuits. Scientists at HP combined a crossbar architecture of memristors with field effect transistors (FETs) to produce a convincing proof-of-concept device that includes circuits that can dynamically reprogram themselves, acting a bit like a solid-state nerve cell-like operation--a holy grail of electrical engineering.

The theoretical understanding of a memristor predicted that the current moving through a memristor would be proportional to the flux of magnetic field that had flowed through the material. This caused scientists to look for memristor behavior in magnetic materials, but it turned out they were looking in all the wrong places. HP found resistance in TiO2 thin-film multilayers could be controlled by varying an applied voltage and, critically, that the resistance change was non-volatile--the hallmark of a memristor.

The key to this behavior is creating alternating layers of high-quality (high resistance) and defect-rich (low resitance) TiO2 thin films. Charge transport in TiO2 is dominated by O2- conduction, instead of the electrons of most semiconductor devices. Applying a voltage between the layers causes the O2- defects to diffuse into the low defect region, reducing the device's overall resistance. By applying a reverse bias, the vacancies diffuse back into the defective layer and the resistance returns to its original state. 

A functional memristristor must also behave electrically as a single component, so the TiO2 layer thickness must be restricted below two nanometers, which prevents separate conduction through the individual layers.

To make the new device, n-type FETs were patterned onto a silicon wafer using normal CMOS processing techniques and covered with a protective oxide layer. A UV nanoimprint process was used to create a two-dimensional grid pattern memristor wires in and their fanout connections to the FETs. Details of this process are scarce, most likely to protect HP's intellectual property. Finally, electrical connections between the fanouts and FETs were made by photolithography (to spatially locate the vias) and aluminum metal deposition. 

The yield was 20% for functional and addressable memristor devices; this was limited by broken nanowires and leads between the memristors and FETs. However, more than 90% of the addressable devices were successfully tested, and the devices showed stable operation between -3 and 4 volts applied bias. And those devices worked as promised: the high-voltage conductivity was over 10,000 times greater than the zero-bias conductivity.

To demonstrate that they worked in a programmable logic array, the memristor grid was programmed to perform a Boolean sum-of-product operation. The resistance of each memristor junction was mapped using a probe station and a logic circuit was designed based on these measurements. ON memristors were programmed by applying a 4.5 V bias using the contacting nanowires, while OFF memristors were programmed with a 2.2 V bias, and logic operations were performed between 0 and 1 V to prevent accidental de-programming of the circuits. The circuits successfully performed the sum-of-product operation at 2.8 kHz.

The full potential of memristors was demonstrated when the devices were made to actively re-program themselves. Dynamic reprogramming was achieved by linking the output of the sum-of-products circuit described above to another memristor inside the device. Instead of simply returning a value of 1 or 0 (voltages of 0.3V or 1.45V, respectively) the voltage was applied to another memristor in the system. This voltage reprogrammed the target memristor to the ON position. In this way, calculations carried out in the device can reprogram circuits in other areas, effectively allowing the device to reprogram itself and adapt to different situations. While the memristor logic circuit may not have become self aware or searched out Sarah Connor, the result was extremely impressive, and it opens doors to exciting new systems.

Despite the wow factor, you can certainly expect to wait some time before buying your own personal memristor device. Obviously, the yield is far too low for commercialization, and operating at 2.8 khz will not exactly set the word on fire--it's not at all clear whether the O2- conduction mechanism is scalable to faster operation. Still, the possibilities for operational logic circuits that can reprogram themselves are unending. These possibilities will most likely drive substantial innovation, particularly because most of the problems lie in the fabrication and aren't the result of an inherent physical limitations. 

PNAS DOI:10.1073/pnas.0806642106

Listing image by NASA

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