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Electronic Design

The Transparent IC—Now You See It, Now You Don't!

John Wager, a professor of electrical engineering at Oregon State University, has to squint hard when he looks at his research team's latest invention, a transparent integrated-circuit. "You can put it in a window and not even know it's there," he says.

The experimental five-stage circuit, a ring oscillator, promises to open the door to a new generation of see-through electronics. One possible application would be transparent backplane displays that appear to hang invisibly in space (see the figure).

Besides fancy desktop and group presentation screens, the technology also could lead to enhanced "heads-up" information systems based on displayintegrated "smart windshields" rather than expensive and finicky projection systems in automobiles and other vehicles.

Transparent chips additionally could serve as the foundation for more efficient solar cells and enhanced LCDs, Wager says. The technology promises to help electronics manufacturers cram more circuitry into small spaces as well. "You can save a lot of space, particularly in portable devices, when the circuit board is also the screen," says Wager.

The chip is made from indium gallium oxide, a compound that offers high electron mobility, chemical stability, and physical stability. All of these characteristics, plus the ability to manufacture the chips at low temperatures, ultimately should help make the technology relatively inexpensive to produce in large quantities.

Wager believes his technology will give organic and polymer devices a run for their money in applications like large-area, flexible, disposable, and printed electronics. "After just a couple years of work, we're actually seeing better performance than organics and polymers," he says. "We're seeing mobilities, for example, that are higher than the theoretical limit of organic and polymer materials."

Although the prototype IC was constructed on a glass substrate, Wager is confident that he can build a version of the chip on a flexible plastic substrate that will still outperform its organic and polymer counterparts. Wager also foresees no problems in scaling the current prototype into larger, more complex circuits.

"A ring oscillator provides the proof that we can do a circuit," he says. "It also gives us a means of predicting what operating speeds we can achieve."

The IC can be produced with conventional photolithography techniques. Wager claims that his technology, produced by sputtering, is more manufacturable than organic or polymer devices, which are typically made by either vacuum evaporation or spin-coating processing. "Most industries would prefer to go into high-volume manufacturing using sputtering," he says.

Although Wager is hesitant to predict when transparent chips may be a commercial mainstay, he's encouraged by how quickly his research has progressed. It took less than three years to move from a discrete transparent transistor to the fivestage oscillator. "I don't see any technological or scientific show stoppers in the way," he says. "A lot of it is going to depend on whether a killer app appears that can make someone a lot of money."

Oregon State University

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