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Nano Printing Goes Large

A rolling nanoimprint lithography stamp could be used to print components for displays and solar cells.

A printing technique that could stamp out features just tens of nanometers across at industrial scale is finally moving out of the lab. The new roll-to-roll nanoimprint lithography system could be used to cheaply and efficiently churn out nano-patterned optical films to improve the performance of displays and solar cells.

Nano press: This 10-by-30-centimeter plastic sheet (top) has been patterned with a series of nanoscale polymer lines using roll-to-roll nanoimprint lithography (bottom). The film is iridescent because of the way its nanoscale features scatter light.

Nanoimprint lithography uses mechanical force to press out a nanoscale pattern and can make much smaller features than optical lithography, which is reaching its physical limits. The technique was developed as a tool for miniaturizing integrated circuits, and a handful of companies, including Molecular Imprints of Austin, TX, are still developing it for this application.

So far, however, it’s been difficult to scale up nanoimprint lithography reliably. To achieve the resolution needed to print transistors, for example, it’s necessary to use a flat stamp that’s a few centimeters square and must be repeatedly moved over a surface. This isn’t practical when printing large-area films for many other applications. “Displays and solar cells require printing over a much larger area and then cutting it up into sheets,” says Jay Guo, associate professor of electrical engineering and computer science at the University of Michigan. “You have to do it in a continuous fashion.”

To solve this problem, Guo developed a stamp that can be used for roll-to-roll nanoimprinting over large areas. His setup uses a polymer mold wrapped around a rolling cylinder to press a pattern into a material called a resist that sits on top of either a rigid glass backing or a polymer one. To make the finished component, the pattern is then fixed by a flash of ultraviolet light. The process, described in the journal ACS Nano,can be done continuously at a rate of a meter per minute, and Guo says he’s used it to print features as small as 50 nanometers over an area six inches wide. That resolution isn’t good enough to make integrated circuits, but it is adequate for printing optical devices such as light concentrators and gratings.

This isn’t the first time that roll-to-roll printing has been explored for nanoimprint lithography. But Yong Chen, professor of materials science and engineering at the University of California, Los Angeles, says the Michigan group “has made this process more reliable with lower defect density.”

At first glance the new roll-to-roll printer resembles a newspaper printing press, but it’s much more complex. The quality of the final nano product depends on achieving the right balance of properties in the printing materials. Silicon and other rigid materials used to make normal nanoimprint lithography stamps can’t be wrapped around a cylinder. So Guo selected a polymer that’s stiff enough to work as a reliable stamp, but also pliable enough to wrap around the printer’s rolls. The finished resist also should stick to the substrate without being too viscous, and it must cure rapidly without shrinking.

“This work is an important industrial advance, which should [enable] a wider application of nanoimprinting,” says Stephen Chou, professor of electrical engineering at Princeton University and a pioneer of nanoimprint lithography since the late 1990s.

The process developed by Guo’s group could be used to make nanophotonic devices on a large scale and high-performance printed electronics, adds Ali Javey, assistant professor of electrical engineering and computer sciences at the University of California, Berkeley. However, Javey, who is developing roller-printing methods for electronic materials such as silicon nanowires, cautions that the longevity of the molds must be resolved before the technique is likely to be widely adopted by the industry. “It would be quite attractive if the mold does not have to be replaced often, in order to make the process as continuous as possible,” Javey says.

The Michigan researchers will work on shrinking the resolution achieved by the technique and developing it for manufacturing. Guo says his group is working with companies that are interested in using the printing process for their products. “This is a baseline technique that can be used to make many things,” he says.

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