Multilayered silicon could bea breakthrough for electronic technology

PASADENA—Researchers at the California Institute of Technology have found a way to stack silicon layers on chips in a way that could lead to significant new advances in silicon-based electronic devices.

In the October issue of the Journal of Vacuum Science and Technology B, Caltech's Fletcher Jones Professor of Applied Physics Thomas McGill and his colleagues report on their work growing a novel silicon structure through a process known as molecular beam epitaxy.

The process begins with an existing silicon wafer, onto which an insulating layer of cerium dioxide just a few atoms thick is grown. Finally, a single crystal of silicon can be grown back onto the cerium dioxide.

The end result is a three-dimensional device using cerium dioxide as an insulator with crystalline silicon on top. Beginning with this top layer of silicon, the wafer is then ready to begin the process again. In this manner, layer upon layer of devices may be grown one after another on the same chip.

"The implications are very significant," says McGill. "For years there have been predictions that progress will eventually stop in silicon electronics because the devices will have been shrunk as much as they can.

"But this new technology could allow you to get the functionality increase by stacking instead of shrinking," he says.

McGill says the group has stacked only a single extra layer of silicon so far. However, the key is the demonstration that the cerium oxide is indeed acting as an insulator, and that the silicon on top is single crystalline and suitable for further growth.

"In principle, you can stack forever," McGill says.

According to Caltech grad student Joel Jones, another member of the team, this technique is especially interesting because it also allows the fabrication of a new group of novel silicon devices.

"We've already fabricated a primitive tunnel switched diode from the multilayered chips," Jones explains. "This is a single device that exhibits memory. At a given voltage, you can have two different stable currents depending on how you've switched the device."

This phenomenon is called negative differential resistance, allowing two current states of different amperage to exist at the same voltage.

Similar effects can be found in other devices enabled by this new technique, including resonant tunneling diodes. These devices can be exploited for novel memory storage, as well as used to enhance the performance of numerous other microelectronic circuits.

"The silicon industry is a $100 billion industry," says McGill. "This could be a major contributor in 10 to 15 years."

In addition to McGill and Jones, the authors of the paper are Edward Timothy Croke, Carol M. Garland, and Ogden Marsh.