Faster Plastic Circuits

A new way to create high-quality silicon components on plastic could make flexible computing practical.

Electronic displays can now be made on flexible materials, and they're appearing in limited applications. But the high-speed processing power to run them still requires expensive -- and rigid -- silicon wafers. If all the components could be built onto the same flexible surface, though, it could save money, improve reliability, and perhaps allow for radical new designs.

Researchers have built working circuits now on plastic that are fast enough to make this integration possible. At Sarnoff Corporation in Princeton, NJ, and Columbia University, researchers have succeeded in operating circuits at 100 megahertz -- as much as a hundred times faster than previous ones on plastic.

"To my knowledge, 100 megahertz is the fastest anyone has ever had any circuit working from transistors made directly on plastic," says Michael Kane, a solid-state devices researcher at Sarnoff, who reported the results last week at the Institute of Electrical and Electronics Engineers meeting in Washington, DC.

"It is an impressive piece of work," says Sigurd Wagner, electrical engineering professor at Princeton University, who also does research with flexible electronics, a field that's focused in part on creating inexpensive ways to build large electronic devices -- "much bigger than you could ever do on wafer-based silicon."

The Sarnoff/Columbia advance could lead to displays measuring three meters or more diagonally that can also be rolled up and easily transported. One possible market: the Pentagon, which is interested in such a device for use in field command centers.

Fast transistors on plastic could also lead to portable phased-array antennas. Such antennas direct a transmission at a precise target, which saves power and makes communications harder to intercept. Today's phased-array antennas cost $100,000 and take up at least one square meter, meaning they have to be mounted to a vehicle, according to Kane.

An antenna using Sarnoff's new technology could cost a few thousand dollars and fit inside a backpack, to be unrolled on the ground when needed. What's more, a soldier carrying such an antenna "could travel more lightly, because he or she won't have to take as much power with him," says Kane.

Currently, large-area displays and some flexible displays depend on a disorderly form of silicon, "amorphous" silicon, that can be fabricated at temperatures low enough to work with plastic. The researchers at Sarnoff and Columbia built their prototype by finding a way to crystallize amorphous silicon that is already deposited on plastic.

The core of the technology is a new laser-based process, developed by James Im, a materials science professor at Columbia University, that heats one narrow band of amorphous silicon at a time. This process makes well-aligned crystals that let electrons move quickly, allowing for the higher processing speeds. Researchers at Sarnoff helped adapt this process for use with a plastic substrate. For example, they introduced special barriers that spread out the heat from the laser, preventing the plastic from deforming.