Electronic Design

Roll-To-Roll Processing Becomes A Priority

Whenever flexible-display manufacturing is discussed, the term "roll-to-roll" processing often is invoked. Much like newspaper production, this process cost-effectively creates electronic devices on a roll of a large flexible substrate (see the figure). Circuits made with thin-film transistors (TFTs) and other devices can be easily patterned onto such substrates, which can be up to thousands of feet long and 10 to 20 feet wide. Some electronic devices can be patterned directly on the substrate, much like an inkjet printer deposits ink. For most semiconductors, though, the devices must be patterned using photolithography techniques.

Proponents of roll-to-roll processing argue that the large-scale manufacture of flexible displays may not be practical using batch processing methods of photolithography and vapor deposition. Yet when it comes to the feasibility of roll-to-roll processing, an equal number of skeptics instead favors modifying existing processing equipment, like those used for LCDs, to make cost-effective flexible displays.

"It's very reasonable to assume that as the LCD industry goes to generation-six processes and higher, they will retire some of their older lines. And those older lines, which will be fully depreciated, will be working toolsets, and they will be modified appropriately and adapted to a flexible display application," says Greg Raupp, director of the Flexible Display Center at Arizona State University, where government and industry funding is being funneled toward developing key manufacturing processes that will make flexible displays commercially viable. Raupp argues that we "might" potentially get lower-cost flexible displays using roll-to-roll processing, but it won't be the necessary higher-quality display possible using existing photolithographic and vapor-deposition techniques.

The final word on roll-to-roll processing has yet to be rendered, since it's a technology still in development with lots of activity going on. Novaled, for example, is working to make the manufacture of organic LEDs (OLEDs) practical. The company is developing what it calls a Rollex process for putting OLEDs on flexible substrates. Funded by the German government's Federal Ministry of Education and Research, the program is aimed at developing highly efficient, low-priced OLEDs.

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