Tiny Etch-a-Sketch

A simple technique can draw--and erase--wires to create denser computer memory.

It may be the world's tiniest Etch-a-Sketch. Researchers have demonstrated a new technique that could be used to create rewritable logic circuits and denser computer memory. Using an atomic force microscope (AFM), the researchers were able to draw nano-sized wires and dots that could be repeatedly erased and written.

Led by Jeremy Levy of the University of Pittsburgh, the researchers used an AFM tip like a pencil, drawing electrically conductive paths--which act like metallic wires--on a special material. The lines were as thin as three nanometers, making them considerably narrower than the lines that can be drawn using electron beam lithography--one of the most precise techniques for etching devices out of silicon.

The researchers used a two-layer material developed by Jochen Mannhart's group at the University of Ausberg, in Germany. The base is made of a strontium titanate crystal, with a thin layer of lanthanum aluminate on top. The interface between the two materials can be switched from insulating to conducting by applying a voltage across the interface.

Levy and Mannhart's groups collaborated on a project to draw fine conductive lines at the interface by probing the surface of the material with an AFM, which has a nanoscale tip that can apply a voltage across a small area. The lines the groups drew were both fine and long; their length was limited only by how far the AFM tip would scan.

Levy and his colleagues showed that reversing the voltage and dragging the AFM tip across a wire severed it, breaking the conduction. By measuring how far they had to drag the tip to sever the wire, they were able to estimate the wire's breadth. Exposing the material to blue light also erased all the wires drawn in the material.

"The fact that it is rewritable is very important," says Harold Hwang of the University of Tokyo, in Japan, who was not involved in the new study. "In a conventional semiconductor device, once you fabricate the device, that's it."

Being able to draw these conductive patterns could allow researchers to create circuits that can be reconfigured on the fly, Levy says. The researchers also showed that the wires might be able to work as transistors. Although it's hard to imagine them competing head-on with the well-developed techniques for silicon chips, Levy says, the technique could be used for high-density memory.

By sending a voltage pulse through the AFM tip, "we could write isolated islands at very small scales, on the order of a couple nanometers," Levy says. "It's about 100 times higher density than what you can do with magnetic materials," the basis for today's data storage.

Levy finds it "exciting" that the material can form conductive wires and transistors, and potentially store information. "Usually, these things are done with different materials, completely different platforms. But here, it's all in the same material." Also, researchers have had some success growing strontium titanate on top of silicon, Levy says, so it could be possible to integrate the new material with existing silicon chips.

The study, which was recently published in Nature Materials, found that the wires and dots stayed in their state for at least 24 hours. Levy thinks that they will last much longer and is currently testing this theory.

"The sort of things they're doing with the scanning probes in this paper are relatively straightforward," says Stephen Streiffer of Argonne National Laboratory, in Illinois. He adds that researchers should be able to use arrays of AFM tips on these materials to draw multiple wires and dots at once.