Soft Lithography Used To Fabricate Transistors On Curved Substrates

"Conventional photolithography works great for many applications -- such as cramming a lot of information into small amounts of silicon real estate," said Ralph Nuzzo, a UI professor of chemistry and of materials science and engineering. "The desire for new patterning processes is being driven by the need to fabricate components over large formats and to use unconventional materials."

Because soft lithography is compliant, the patterning process can conform to small surface irregularities, flexible substrates and three-dimensionally curved surfaces. Possessing the advantages of block printing, the patterning technique can be used in applications where photolithography can not.

"Thin-film transistor arrays deposited on spherically curved substrates could be used in optical detectors to take pictures over a very wide field of view," said John Abelson, a UI professor of materials science and engineering. "The human eye, for comparison, focuses images on a nearly spherically curved retina that neatly accommodates the relatively simple optics of the eye's lens."

Soft lithographic patterning techniques -- such as micron-scale polymer molding -- on curved substrates do present special challenges, Nuzzo said. "For example, the mold must be flexible enough to conform to the curvature of the substrate, yet stiff enough to preserve the integrity of the pattern."

To test the general effectiveness of polymer molding, Nuzzo, Abelson and graduate students Martin Erhardt and Hyun-Chul Jin fabricated thin-film transistors on both planar and curved substrates using two different transistor architectures.

"One design was a common gate, common channel architecture for single-level patterning on a spherically curved glass substrate." Abelson said. "The other was an isolated channel, inverted staggered architecture with multilevel pattern registration on a planar glass substrate."

To fabricate the microstructures on a spherically curved substrate, the researchers first deposited thin films of aluminum, silicon and silicon nitride. A patterned silicone mold was then placed in contact with the substrate, and a polyurethane precursor flowed into the mold by capillary action. Following curing, the mold was peeled away, leaving a polyurethane pattern on the substrate, roughly 30 microns high. Typical etching and metallization steps completed the fabrication process.

"While many engineering issues, from throughput to electrical performance, must still be solved," Nuzzo said, "this work demonstrates the versatility of the soft lithographic patterning technique."

A paper describing the fabrication process is scheduled to appear in the journal Chemistry of Materials. Funding was provided by the Defense Advanced Research Projects Agency.