A new technique leverages the properties of common table sugar to transfer lithographic patterns onto the most complex of surfaces.
As impressive as the evolution of (semiconductor) lithography has been over the past decades, there are still some things it can’t do, and probably never will: patterning anything else than rigid and flat substrates. Patterning soft or curved surfaces requires printing the design onto a planar surface first and then transferring it onto the nonplanar or flexible target. Carriers such as flexible tape can only bend so much, however, and not all materials can withstand the contact pressure required for pattern transfer.
Liquid transfer techniques, where the transfer material is floated on the surface of water and the target surface is pushed through it, are much gentler. But they’re tricky too; with a freely flowing liquid, it can be hard to place the print precisely where you want it on a new surface.
The work of Gary Zabow combines the best of both worlds. The researcher at the National Institute of Standards and Technology (NIST) stumbled upon a micro-transfer technique that can handle surface topographies of any complexity and with radii down to the nanoscale. The key ingredient: hard candy.
Washed away
Zabow had been experimenting with sugar-based mixtures to envelop arrays of microscopic magnetic dots. This way, his colleagues in a biomedical lab could easily dissolve the carrier away in water, without leaving any chemicals behind that might interfere with their experiments.
One day, Zabow found a beaker he’d left out, its bottom coated with a gooey mess – the sugary mixture had melted. Figuring he’d clean up a bit, he dissolved the sugar, expecting to collect his micromagnets in the water. But they weren’t there. Instead, he found them on the bottom of the beaker, casting a rainbow reflection. This colorful show meant that the original pattern in which the dots had been fixed inside the sugar had been preserved.
This led to the birth of the Reflow-driven Flexible Xfer (Reflex) process: using a simple mixture of sugar and corn syrup, micropatterns can be easily transferred. Adding corn syrup is an old trick in the food processing industry to prevent the sugar from crystallizing. After all, growing crystals might disturb the pattern that’s being transferred.
Dissolved in a little bit of water, the sugar mixture can be poured over the pattern on a flat surface. Once the water evaporates, the candy hardens and can be lifted away with the pattern embedded. The candy with the print is then placed over the new surface and melted. The sugar/corn syrup combination maintains a high viscosity as it melts, letting the pattern maintain its arrangement as it flows over curves and edges. Then, using water, the sugar can be washed away, leaving just the pattern behind.
Human hair
Reflex proved compatible with metal, plastic, paper, glass, polystyrene, semiconductor, elastomer, hydrogel and multiple biological surfaces, including ones of complex geometry. Among the more impressive feats was printing onto the sharp point of a pin and writing “NIST” in gold lettering on a human hair.
Zabow thinks his method will expand the microprinting toolkit and extend the reach of precision planar microlithography to highly nonplanar substrates and microstructures. “The semiconductor industry has spent billions of dollars perfecting the printing techniques to create chips we rely on,” he says. “Wouldn’t it be nice if we could leverage some of those technologies, expanding the reach of those prints with something as simple and inexpensive as a piece of candy?”