By Tom Cheyney
Small Times Contributing Editor
Feb 26, 2007—For the emerging flexible thin-film, organic, and printed electronics markets to flourish, most industry professionals agree that roll-to-roll (R2R) processing must be implemented on the factory floor. Indeed, thin-film photovoltaics, OLEDs, and RFIDs are already in pilot or volume production using R2R techniques.
But for the manufacturing of large-area and conformable displays, paperlike e-books, and high-performance solid-state lighting to take place, it remains an open question whether inkjet, thermal laser imaging, or other printing technologies; optical, imprint, or digital lithography; adapted semiconductor and LCD processing methods; inorganic or organic material; or a combination of the various methodologies and chemistries will be leveraged into successful, scaleable R2R approaches.
Why R2R? The main reason is cost, which must be cut by at least 50% compared with batch-processed components, according to Display Search's Barry Young during his presentation at the recent Flexible Display & Microelectronics Conference. Applied Materials' Hans Maidhof echoed the cost-cutting sentiment, saying that "flex will only succeed if it's cheaper."
"Thin films reduce the cost of semiconductor materials, continuous fabrication increases utilization and reduces production costs, and application of industrial processes simplifies production while providing high manufacturing rates¿. R2R [also] leverages form factor to lower overall customer costs," noted Maidhof.
The challenges facing those trying to commercialize continuous processing are cultural as well as technical. "R2R and web converting folks are not used to semiconductor requirements and vice versa," Hewlett-Packard Labs' Carl Taussig told Small Times. "We pretty much build everything, and building [your own] tools slows development."
Included among the equipment that Taussig and his team have built is the self-aligned imprint lithography (SAIL) system. This high-resolution tool, along with stamping, mastering, and other proprietary technologies, has allowed HP (with its partner PowerFilm Solar) to build what he claims is "the first flexible TFTs and [active-matrix] backplanes made fully with R2R processes," adding that the first integrated display "will come out any day now."
Eran Elizur of Kodak's graphic communications group described a seventh-generation dry-process, maskless production tool for printing thermal color filters, which resides "at a customer site in Asia for evaluation." The system handles 2250 x 2250 mm substrates, employs five 5-micron-resolution laser heads, has 3-micron imaging accuracy with an imaging speed of up to 2 meters per second, and supposedly decreases manufacturing costs per panel by 30%.
For R2R processes to be consistent, efficient, and high yielding, there must be reduced contamination and defectivity levels, precise endpoint control, very high uniformity, subnanometer-level surface roughness, and assured reliability.
Applied's Maidhof admitted that, although his company has a cleanroom-compatible vacuum web coater system, the "particle issue is most important" and "not totally solved in our tools."
"With an endless process, where's the endpoint?" quipped Taussig of HP Labs. For their amorphous-silicon TFT process, his group uses interferometry to assess the endpoint of back-channel etching and fluorescence techniques to monitor and control the thickness of the polymer mask etch.
"Surface roughness is still an issue with flex and is not good enough for making transistors," explained PARC's Bob Street. He told Small Times that the roughness average needs to be a few angstroms, but that plastic substrates remain "5 to 10 times rougher than glass." Pointing out the susceptibility of flexible substrates to scratching, Street said that the plastics people "need to learn how to improve quality."
Dan Gamota of Motorola, which has produced "more than 60 miles of printed electronics" and is "close to getting dielectric layers 1 to 5 microns thick," believes that the key challenge facing printed electronics is "how to [perform] quality control and characterization [tests] on rolls many hundreds or thousands of feet long. How do you check individual transistor device mobility on 2000 feet of film?"
Display Search's Young zeroed in on the limitations of current R2R technology. "Do you have a process where a particular display is going to stay constant from roll to roll? There's not a lot of flexibility in roll-to-roll manufacturing."