Flexible Transistors on a Roll
What if creating transistors were as easy as printing the New York Times? In that world, abundant, cheap and flexible electronics could be embedded in countless every-day objects. Manufacturers would use fewer pollutants, gadgets would contain more recyclable materials, and computers would cost less. That’s exactly what Silicon Valley startup Rolltronics Corporation and its partner, Iowa Thin Film Technologies, envision. The two companies recently demonstrated the world’s first working silicon transistors made using a radical new “roll-to-roll” manufacturing technique. In this process, a continuous sheet of flexible polymer is unrolled from one spool, covered with circuit-board-like patterns of silicon, and collected on another spool.
The advance, its proponents say, could speed implementation of devices such as cheaper desktop displays and flexible electronic paper. “We’re working toward something that could be very beneficial to people,” says Frank Jeffrey, president of Ames, IA-based Iowa Thin Film. The company, which receives funds from Rolltronics to pursue the development of its roll-to-roll processing technology, has manufactured Rolltronics’ first working transistor. “I consider it a great accomplishment,” Jeffrey says.
Rolltronics, based in Menlo Park, CA, is planning new company divisions to incorporate flexible, roll-to-roll electronics into radio-frequency ID tags, digital X-ray detector panels, biometric sensors and backplanes, the layers of electronics that control the pixels in active-matrix displays. “Each of these could be a multi-billion-dollar market,” says Rolltronics CEO Michael Sauvante. “Right now, we are an R&D company, but very shortly we could be a full-fledged production company.”
The Role of Roll-to-Roll
Rolltronics’ technology exploits an economic opening left by today’s chip fabrication plants. In these facilities, integrated circuits and memory chips are burned onto crystalline silicon one wafer at a time.
Chipmakers can squeeze tens of billions of transistors onto each wafer. But the economies of scale that come from such batch processing disappear with low-density applications such as the drive electronics for liquid-crystal displays, where only one transistor is needed for each widely-spaced pixel. “It costs the same amount to make this one transistor as it would to make thousands,” explains James Sheats, Rolltronics’ chief technical advisor.
The alternative? Think newspaper. In the roll-to-roll system, amorphous (non-crystalline) silicon transistors are deposited in precise patterns onto a continuously unwinding ribbon of plastic as it rolls through a vacuum chamber. The final cost of four to eight cents per square centimeter is “vastly lower” than that of crystalline silicon wafers, says Sheats, making it affordable to space transistors as far apart as the pixels in a display.
So far, the smallest structure produced using this method is about 10 micrometers across. That’s 50 times fatter than the finest features produced using conventional photolithography. But that’s okay, since these electronics are not designed for devices that require high density, like memory chips or microprocessors.
“There are a lot of applications, like your cell phone or PDA, that need transistors that can be spread over a large surface area, and that is what we can make cost-effectively,” says Sauvante.
He proudly unpacks the company’s demo unit: a plastic square bearing one of Rolltronics’s transistors, attached via alligator clips to a small, gray box. The box sends a periodic charge to the transistor’s “gate” layer, allowing current to flow from one of the transistor’s terminals to the other. When this current returns to the box, it’s amplified and used to turn a red light-emitting diode on and off.
As a display of computational power, it isn’t exactly stunning, but it does prove that Rolltronics’ transistor can-like all transistors-act as a switch, clearing the way for the manufacture of more complicated silicon-on-plastic structures. “Inside of two years from now we’ll be shipping products based on this technology,” Sauvante predicts. The first of these will likely include backplanes for organic light-emitting displays and electronic paper systems such as those under development at Cambridge, MA-based E Ink and Palo Alto, CA-based Gyricon Media.
Rolltronics isn’t the only company trying to bring flexible transistors to market (see “Flexible Transistors”). Bell Labs, for example, has developed a system for printing multi-layered plastic transistors on small sheets of flexible polymer using a finely patterned rubber stamp. And Lucent has fused Bell Labs’ printed transistors with sheets of electronic ink from E Ink to demonstrate a working electronic paper prototype.
But Sauvante contends that because Bell Labs’ transistors are themselves made of plastic, “they’ll never be capable of reaching the switching speeds of silicon.” Rolltronics’ silicon-on-plastic circuits, he says, are “the only viable approach” for making integrated e-paper devices where the display, the backplane, and high-speed drive electronics-such as analog-to-digital converters and row- and column-drivers-are all flexible.
Sauvante’s enthusiasm about Rolltronics’ technology has begun to spread. “What they are doing is very, very promising, and it looks to me like it has a very high probability of success,” says Nicholas Sheridon, director of research at Gyricon Media. “And if it does succeed, it could have a very big effect on making transistor arrays more available to people, for things like displays.”
People, Planet, Profits
At the same time as Rolltronics refines its technology, of course, the company will have to attend to its bottom line. That may be tricky, given the still experimental nature of the company’s technology, investors’ wariness toward new high-tech ventures, and the recent slump in demand for electronics that’s cutting into revenues at Intel, AMD, Hitachi and other major electronics manufacturers.
Still, new flexible transistor technologies could bring payoffs for the whole industry. “Putting transistors on organic substrates [like plastic] is another step in the electronics revolution that puts more functions into the daily life of the consumer,” says Jerry Marcyk, director of Intel’s Components Research Lab. Another key advantage: the roll-to-roll process is more environmentally friendly than conventional semiconductor production technology. It uses fewer toxic solvents and cleaners and can be carried out with many recyclable materials.
For now, Rolltronics can work without much fear of competition from semiconductor industry leaders, most of whom still focus their R&D primarily on making faster, denser microchips on silicon. Sauvante believes these chip-making giants “are essentially jostling around a pie that’s getting smaller and smaller, instead of going out to find the game-changing new paradigm. That’s what Rolltronics represents.”
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