Flexible displays: What's the holdup?

The other approach, advocated by Plastic Logic, a developer of flexible display substrates, involves depositing transistors on a flexible plastic film. The company gets around the heat issue by using a cold deposition process, substituting organic semiconductors for silicon ones.

Plastic Logic creates semiconductors with organic materials: carbon-based molecules or polymer chains. It uses solvents to change the organic materials into a solution, and uses that solution to create organic semiconductors that can be deposited onto the substrate at room temperature.

"There's no inherent reason why you can't drive OLEDs with organic transistors, rather than silicon transistors put onto flexible plastic," says Seamus Burns, director of display engineering at Plastic Logic.

Discrete vs. roll-to-roll manufacturing

Another choice facing flexible display manufacturer researchers is whether it's more cost effective to stick with the traditional, discrete manufacturing techniques used in building today's LCDs, called mask alignment, or to use an emerging printed, continuous, roll-to-roll process.

The first method, pursued by ITRI and the FDC, involves bonding flexible plastic substrates to a glass backing and then peeling off the plastic layer after the TFT array has been applied to it. By using this method, manufacturers can modify the processes they already use to manufacture conventional, flat-panel LCDs. Their investment in LCD panel fabrication facilities is largely preserved and they can go to market faster.

But the approach is difficult to scale up to very large substrates, such as those used in televisions, and manufacturing costs will likely be higher than those for conventional glass-based displays, explains Carl Taussig, director of HP's Information Surfaces Lab.

The alternative, exemplified by HP's self-aligned imprint lithography (SAIL) technology, is a process in which the transistor arrays that make up the TFT layer of the display are printed on continuous sheets of plastic film using what the industry calls a roll-to-roll process. "HP's SAIL technology offers the possibility of dramatically lower costs [than today's discrete manufacturing processes], and the ability to scale to very large areas," says Taussig.

Today's active-matrix display factories are "fabulously expensive" to build, Taussig continues. "It may cost upwards of $5 billion to make such a factory. If we can make [displays] using print-like technologies [such as SAIL], the capital equipment costs will go down dramatically, the throughput will be much higher and the materials costs a lot lower. Displays could cost one tenth of what they cost today."

The FDC's Colaneri has his doubts about the concept. "There a lot of cavalier statements about cost advantages being made by roll-to-roll advocates, but I've yet to see a serious analysis that indicates why that could be true," he says. (Story continues on next page)