BOSTON Working prototypes of "electronic paper," which unites organic transistors with a unique spray-on electronic ink, will be shown this week at the fall meeting of the Materials Research Society. Developed in a joint technology program between E Ink Corp. (Cambridge, Mass.) and Lucent Technologies Bell Laboratories (Murray Hill , NJ.), the display technology attempts to imitate the advantages of conventional paper and ink: flexibility, low cost and the ability to be read using ambient light.
With thin sheets of plastic as a substrate, the flexible displays contain a layer of drive electronics built with organic, rather than silicon, materials and a top layer of E Ink's microcapsule-based display material, which responds to an electric field by switching color. A colored organic material inside the microcapsules rotates in the field using an effect similar to electrophoresis. The basic physics of the process was devised at the Massachusetts Institute of Technology's Media Laboratory.
This flexible active-matrix backplane comprises an array of several hundred interconnected plastic transistors with feature sizes of 10 microns.
Building electronic paper with the novel ink turned into a quest for a low-cost, printable process for defining the drive electronics. Each electronic ink pixel needs a transistor behind it in order to switch. "We've succeeded in moving away from lithography no high-temperature processes or vacuum chambers, for example. We have kept the temperature of the process below 100C so that we can be compatible with organic and plastic materials," said Pierre Wiltzius, director of semiconductor research at Bell Labs. "Instead, we will have a low-temperature, reel-to-reel plastic film-based technology."
The reel-to-reel process also depends on the ability to print the organic electronics using a rubber-stamp method. "These are close to conventional rubber stamps, except that they have very fine features, on the order of 10 microns," he said.
The really novel aspects of the display technology the electronic ink and organic transistors have turned out to be the most cooperative components. "People tend to focus on the organic transistor technology as the really interesting aspect of this, but we now know of a variety of materials that work well," Wiltzius said. "It was other processes, such as building a reliable thin-film dielectric, that turned out to be difficult."
The prototype also has some temporary components, such as evaporated gold wires for interconnect and a special plastic membrane between the electronics and the ink, that will eventually be eliminated from the manufacturing process.
Eventually, the process will use organic, printed conductors, but the researchers wanted to eliminate the added complication of that process in building the current prototype. Wiltzius also does not see any fundamental problems of compatibility between the electronics and the organic ink.
"The microcapsules can break and possibly could create some problem with the electronics, but we don't foresee any difficult problems here," he said. It was a simple matter to insert the plastic membrane between the two systems, since the entire display is assembled using conventional plastic lamination equipment. "We just wanted to be sure that we had eliminated any possible problems with the electronic ink in the prototypes," he said.
Several prototype displays have been built, each consisting of a 25-square-inch display area made up of several hundred pixels. The electronic ink maintains its state when power is off, so that the displays consume very little power. The performance of the organic transistors is comparable to amorphous silicon thin-film transistors, which are being used in other low-cost display programs.
E Ink researchers have also made some improvements in the performance of the electronic ink, said Wiltzius. The medium creates a reflective surface with a contrast ratio of 10:1, which compared favorably with newsprint's 8:1 contrast ratio.
The flexible electronic paper will probably be appearing in commercial products in five years, according to a statement from the two companies.