For decades, the $70 billion flat-panel display (FPD) industry has hitched itself to wafer fab technologies, leveraging semiconductor processes and related tool sets to build bigger and better liquid-crystal displays and other FPDs. In the next 10 years, however, new technologies promise to change significantly, creating a divergence between FPD production lines and IC wafer fabs.
For example, developments continue to gain momentum to move today's glass-based FPDs to lighter low-cost flexible substrates. This transition will require lower-temperature processes than IC processes and - most likely - traditional batch fabrication steps will give way to roll-to-roll (R2R) manufacturing systems.
"Today's manufacturing paradigm in flat-panel displays is based on the history of semiconductor processing - in wafer or sheet form using batch or cluster-type tools," noted M. Robert Pinnel, CTO of the US Display Consortium. "If we want to get to really large areas [of displays] at low cost, the belief is that we must change the manufacturing paradigm to something that is a high-throughput process, which comes more from the experiences of the printing or photographic film industries," he said, referring to growing R&D efforts to shift FPD production to R2R formats and away from increasingly large plates of glass.
To accelerate development of R2R, the US Display Consortium has become the first industry investor in the new Center for Advanced Microelectronics Manufacturing (CAMM) at Binghamton U. in New York, a partnership between Binghamton U., Endicott Interconnect Technologies, and Cornell U. Initial development of R2R tool concepts will target the toughest transitions from traditional wafer-based processing to low-temperature roll-based production.
"The biggest areas of focus must be the combination of coating and lithography techniques," Pinnel told WaferNews. Traditional spin coating and other techniques for applying thin films and photoresist to wafers and glass substrates will not be applicable in R2R manufacturing. "You have to go toward the printing industry and learn how to apply materials through slot coating, screen printing, or some other methods more amenable to high-throughput-rate roll-to-roll processing," he said.
The second major issue is how to pattern circuits. "You cannot use classical steppers to do lithography," Pinnel said. "That's why the industry is looking at screen printing, inkjet printing, and laser transfer processes - any number of options by which lithography can become additive in a roll-to-roll format." The US Display Consortium is attacking new FPD technologies on two fronts: R2R manufacturing challenges inside the CAMM center, and materials processing at the new Flexible Display Center in Tempe, AZ.
To get R2R lithography rolling, the US Display Consortium has contracted Azores Corp., Wilmington, MA, to deliver a modified step-and-repeat exposure system for the initial R2R R&D pilot line at the CAMM center. The modified stepper - a G-line stepper adapted to accommodate a web flow of flexible material in the R2R line - and an R2R vacuum thin-film deposition system from CHA Industries, Fremont, CA, are expected to be up and running by early next year.
"We have some modest objectives at the start," said Bahgat Sammakia, professor of mechanical engineering and director of Binghamton U.'s Integrated Electronics Engineering Center and Small Scale Systems Packaging Center. The CAMM's initial target for R2R production speeds is about two feet/min, but "we will certainly start much slower than that," he told WaferNews.
In addition to displays, the CAMM center also is pursuing other applications, including plastic ICs, radio-frequency ID tags, and even possibly large-area flexible foldable radar. In about three years, test-bed devices are expected to be rolling off the R2R line. Pinnel believes other less complex device applications will be ahead of flexible displays in reaching viable processes for R2R manufacturing, but he hopes that will enable R2R for displays.
While the initial prototype production tools planned for CAMM's R&D line are expected to be a "stepping stone" toward high-speed roll-to-roll processes, Azores CEO Elvino da Silveira is optimistic that photolithography steppers could prove to be a viable production tool for R2R lines. In initial tests, Azores has developed a new lens control system, which dynamically corrects for distortions in flexible substrate materials during process steps. The company plans to integrate an outsourced R2R handling system into the stage of its experimental stepper.
"We certainly feel the stepper approach works, and we have a good way of dealing with the scale-related effects, due to compaction and expansion [of the flexible materials]," explained da Silveira. "We also have a proven approach for layer-to-layer alignment and believe we can get the throughput needed by making optics bigger, but that would be for a production-oriented machine." The initial prototype G-line stepper will target 3-micron resolution with a small lens for 80mm exposure fields.
Imprint lithography and inkjet concepts for R2R production may still require additional developments to deal with overlay issues as well as compensation for shrinking and expansion of flexible materials during process runs, suggested da Silveira. While optimistic about the prospects of R2R, the Azores CEO and others admit it's unclear when R2R will be ready for production.
Regardless, industry analysts believe FPD manufacturing will continue to diverge from wafer fab technologies, as panel makers are squeezed by cost pressures in a consumer product-driven market. "Lithography, etch, and a few processes are similar, but beyond that FPD manufacturing is becoming highly specialized [compared to chip manufacturing]," said equipment analyst Risto Puhakka, president of VLSI Research Inc., who estimates that up to 15% of chip equipment supplier revenues now come from display manufacturing.