17 Jun 2002
Alternative lithography methods are set to change the face of displays as we know them today.
Researchers from Bell Labs and E Ink, US, are the first to successfully use microprinting to produce highly functional microelectronic structures. By building a thin film of organic, semiconducting transistors on a plastic sheet and integrating microencapsulated electrophoretic "ink", John Rogers and colleagues made a paper-thin, flexible display that could replace today's costly, rigid, power-intensive electronic displays. (see Proceedings of the National Academy of Sciences 98, 4835-4840, 2001).
"The transistors have characteristics that are comparable to or better than rigid silicon-based devices that are fabricated using conventional photolithographic methods," claimed the researchers.
To microprint the displays, the team first fabricated a rubber stamp that carried an imprint of the required circuit pattern, and coated it with a solution of self-assembling molecules. The researchers selected a poly(ethylene terephthalate) sheet for the display substrate and a nanometer-thick, transparent layer of indium tin oxide for the front surface. Both substrates were coated with gold to form electrical contacts, before stamping on the circuit pattern. By etching away any exposed gold and removing the solution, the team was able to construct a gold circuit pattern. They aligned and joined each patterned surface, a step that often fails in conventional photolithographic patterning, to form the transistor array.
"This technique, combined with the circuit's good performance, validates microprinting as a means for constructing realistic systems," the team said.
The display's optical characteristics rely on E Ink's "electronic ink", which consists of microcapsules filled with white pigment particles suspended in a black fluid. The ink sits on the transistor array, and the electric field generated by each transistor forces the white particles within local microcapsules to rise towards the display's transparent surface. According to the team, the optical properties of the display match those of low-resolution signs that use similar electronic inks, and they want to build on this.
"Our prototype displays have 256 pixels, which is not enough for most consumer applications," said the team. "However many of the processing approaches can be extended to systems with more pixels and higher resolutions."