Soft Lithography Prints TFTs on Curved Substrates

Researchers at the University of Illinois (Urbana-Champaign) are using a soft lithography technique known as micron-scale polymer molding to fabricate circuits on non-planar substrates. Though they have used only modest design rules, they have shown the capability of the method to print high-quality patterns.

There are several existing examples of soft lithography, the main idea being that replication is done using molding or printing without involving photochemistry. The compliance of the process enables it to conform to surfaces despite irregularities or even deep curves.

Described in a recent issue of Chemistry of Materials (Nov. 20, 2000), the lithography technique — referred to as MIMIC (micromolding in capillaries) — uses capillary action to fill a patterned elastomeric mold. The patterned mold is placed against the substrate, then the capillary action flows a polyurethane precursor into the mold. Removing the mold after curing leaves a polyurethane pattern ~30 µm thick. Typical etching and metallization proceeds from there.

To avoid concerns with multilevel registration on a curved surface (the process tools have yet to be invented to achieve this), the TFT architecture chosen required only one mask level. And, although MIMIC can print features as small as 1 µm, the researchers chose to limit the test to a 20 µm resolution. "What we're really trying to do is develop chemistries that leverage these types of processes," said Professor Ralph G. Nuzzo. "You can do that just as well at 20 µm as you can at 1 µm."

The group's intention is not to develop a commercial process for such printing techniques, but to demonstrate the technique's competency in a class of materials and heterostructure growth. Nuzzo, a professor in both chemistry and materials science and engineering, is leading the cross-disciplinary team into explorations of chemical and physical properties.

"There's a very rich interplay between materials science and chemistry. The two leverage each other very well," Nuzzo said. In some cases of patterning work, he noted, the resist is only one molecule thick instead of the typical microns of resist. In fabricating a direct sol-gel via liftoff, for example, microns of resist would generate a catastrophic topology. "How is it that one molecule can be an effective liftoff method? That's not a trivial bit of chemistry," he said.

Regardless of what commercial development will take place down the road, MIMIC is not intended as a high-resolution lithography technique. Commercialization will likely take place at design rules somewhere between the abilities of screen printing and state-of-the-art lithography methods, Nuzzo noted.

The method will become commercial, he predicts, because it will enable the curved arrays needed for improved imaging, for instance. The project that the researchers are currently working on is funded by the Defense Advanced Research Projects Agency (DARPA). One relevant application for defense is non-planar array detectors that are modeled on the optics of the human eye.

"This gives you a wide depth of view with very simple optics," Nuzzo said. "But there are all kinds of problems. It's not just an off-the-shelf kind of thing."

Nuzzo is convinced that those production problems will be solved within the next several years. Although those first solutions are bound to be high-end, the processing methods will undoubtedly trickle down to consumer applications, he noted.

For additional information on lithography, go to www.semiconductor.net/lithography