Find a penny, pick it up, and all the day you’ll have good luck. That saying has been around since pennies were actually made mostly of copper, not just copper-coated like they are today. Copper can also be found in our bloodstream. It has been used to carry water and transfer heat. Most recently, it has been used for interconnects in semiconductors. But what about putting the metal to work as part of a nonvolatile memory technology?
Researchers from Arizona State University have achieved exactly that through nanoionics, a method popularized by nanoionic supercapacitors, lithium batteries, and other fuel cells that have nanostructured electrodes. With nanoionics, metallic ions can be moved around to form a pathway between electrodes. While traditional memories move electrons among ions, nanoionics moves the ions themselves.
Each memory cell is constructed using a solid-state ionic conductor known as a solid electrolyte sandwiched between two metal electrodes (a cathode and anode). The solid electrolyte is made out of a glass-like material containing metal ions.
In the normal non-biased state, the glass resists the flow of current from anode to cathode. Apply a few hundred millivolts to the electrodes, and electrons will find themselves binding to the metal ions. The metal will then group together, forming a bridge between the electrodes (see the figure).
This virus-sized bridge provides a conduction path between electrodes, lowering the resistance between nodes and providing a path for current flow. This would typically be recognized as the “on” state representing a logic one. Reversing the voltage between electrodes easily breaks the bridge. Once the bridge is broken, the cell returns to a high-resistance state, which represents a logic zero. The bridge stays put until the voltage is reversed, regardless of the power state of the semiconductor, making it nonvolatile.
Put it all together and you have programmable metallization cell memory, or PMC. According to ASU, bit-for-bit, the ion-based technology is one-tenth the cost of flash and 1000 times more energy-efficient. PMC is also more future-friendly than its charge-based counterparts like flash, which are becoming more difficult to scale because they lose reliable charge capacity as they shrink. When a gate is downscaled, squishing the electronics closer together results in greater heat and power dissipation.
And if you’re concerned PMC may not be compatible with existing semiconductor fabrication equipment, think again. The latest manifestation of PMC uses copper ions in silicon dioxide to form the solid electrolyte. “Copper and silicon dioxide have been used in the semiconductor industry for many years, and PMC technology is compatible with existing back-end processes and equipment sets,” said Michael Kozicki, founder of ASU spinoff Axon Technologies and co-inventor of the technology.
The implications of this compatibility include a low barrier to entry, making the decision to use the technology less risky. Since the market for nonvolatile memory has exploded over the past few years, and with the ever-present scaling problem with other memory technologies, looking for a near drop-in alternative just makes sense. And, Axon Technologies recently received its 27th patent on PMC-based technology.
“We have developed strong technical and business relations with several companies in the memory and storage industry,” Kozicki explained, “and we have also licensed our PMC technology to two of the world’s largest memory companies.”
AXON TECHNOLOGIES CORP.