Researchers have produced a novel memory device set to rival transistor-switched silicon-based memory.

Conventional memory chips in electronic devices are made up of transistors, resistors and capacitors built in layers on a silicon wafer through a photolithographic process, during which precise patterns are etched on the silicon to form the chip. Today’s technology allows several million transistors to be built on a piece of silicon the size of a pinhead, but many researchers believe this form of memory has been pushed to its limits.

Researchers have been trying to create electromechanically driven switches small enough to rival transistor-switched silicon-based memory. Unlike transistors, electromechanically driven switches contain moving parts. Not only do electromechanical devices have excellent ON-OFF rations and fast switching characteristics, but the physical separation between the switch and capacitor in such devices means the data leakage problem is significantly reduced. However, until now, the technology has not been a viable alternative to silicon-based arrangements because it involved larger cells and more complex fabrication processes.

Professor Gehan Amaratunga and a team of international researchers have remedied these drawbacks by creating a novel nanoelectromechanical (NEM) switched capacitor based in vertically aligned multi-walled carbon nanotubes (CNTs).

Rather than creating chips through a photolithographic process, nanotubes are grown in place on a silicon wafer by allowing a carbon-carrying gas to absorb onto a hot nickel surface, which acts as a catalyst for the nanotube growth. The length of time for which the nanotube is grown determines its length, which in turn determines its mechanical properties such as stiffness and resonant frequency. The resonant frequency of the nanotube structure determines the maximum switching speed of the NEM switch and its stiffness determines how much charge is needed to deflect it into contact with the other element of the cell.

One nanotube which stores an electric charge bends towards a static nanotube. When the two touch, an electrical contact is created and charge can flow to a capacitor structure formed around the static nanotube. This charge is used to represent a bit of information; a charged capacitor represents 1/ON and an uncharged capacitor represents 0/OFF.

The vertical nature of the NEM capacitor structure allows for high integration densities, reducing both process costs and size requirements. There is a sharp transition between the ON and the OFF state of the device, reducing the amount of power required for its operation.

While nanoelectromechanical devices based on carbon nanotubes have been reported previously, this is the first time researchers have been able to control the number and spatial location of nanotubes over large areas with the precision needed for the production of integrated circuits.