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A Shortcut to Designer Nanostructures

A new nanolithography technique works rapidly over large areas.

A new nanolithography method could bring down the costs of making experimental computer chips for electronics research and arrays of biomolecules for cell biology. The method makes it possible to deposit fine patterns of materials, or carve them away, using large arrays of silicon pens sitting on springs; it combines the ability to pattern arbitrary designs that have nanoscale features with the ability to work quickly and over relatively large areas.

Nanodot design: The dots making up this image, which shows the pyramid from a U.S. dollar bill, are 150 nanometers apart. An array of these pyramid images, measuring 30 by 33 micrometers each, was made on a gold film using a new nanolithography method.

The most common methods for making custom nanostructures are dip-pen lithography, which involves depositing molecules using the tip of an atomic-force microscope, and electron-beam lithography, which entails carving them away with electron beams. Both methods let researchers realize new designs with nanoscopic features, but they are incredibly time-consuming and expensive.

For the past decade, Chad Mirkin, professor of chemistry at Northwestern University, has been working on ways to reduce the cost and time needed for nanoscale manufacturing. Mirkin invented dip-pen lithography in 1999; in 2008, he developed a more practical approach using polymer pens instead of microscope tips. The pens are cheaper than the microscope tips, easier to work with, and work over larger areas. These pen arrays can be sprayed with different molecular inks on their tips, and then attached to the moving arm of a scanning-probe microscope to trace out designs. Polymer-pen arrays aren’t very good at patterning nanoscale features, though, because the tip of the pen is soft. “You can only go so small,” Mirkin says.

Now Mirkin has developed an array that works in a similar way but can create much smaller features. When pushed over a surface using a scanning-probe microscope, the new arrays—made of hard silicon tips attached to a springy polymer backing—can either deposit molecules to make nanostructures, or act like tiny electric chisels, carving material away. It’s this combination of the hard, fine silicon tip with the give allowed by the underlying polymer layer that enables higher resolution. Mirkin calls the method “hard-tip, soft-spring lithography.”

This week in the journal Nature, Mirkin reports using this method to create patterns with features smaller than 50 nanometers. In one demonstration, the researchers used the arrays to carve out 30-by-30-micrometer replicas of the pyramid on the U.S. dollar bill on gold films. Printing a centimeter-square area of these pyramids took about 200 minutes. They also printed patterns using biomolecules and electrical materials.

“This advance has a good chance of transitioning scanning-probe lithography from academic [use] to being an important production and prototyping tool broadly used across the semiconductor and biotechnology industries,” says Joseph DeSimone, professor of chemistry at the University of North Carolina at Chapel Hill.

One likely application for the lithography technique is the production of small numbers of specialty computer chips, says DeSimone. There is increasing demand for small batches of chips for testing new circuit designs, as well as for specialized chips for niche applications, particularly in the military. Making a new chip requires making a new mask that’s the equivalent of a photographic negative used to pattern the circuits onto a wafer. “There is a huge unmet need to make chips using maskless approaches,” says DeSimone.

In the short run, Mirkin says, cell biologists will likely find applications for the technique in their labs. The technique could help them understand the way nanoscale cellular interactions control stem-cell differentiation and the spread of cancer throughout the body, he says. Using the approach, large arrays could be covered with hundreds of thousands of cells to get statistically significant information about how they react to these spatially patterned chemical cues.

Milan Mrksich, professor of chemistry at the University of Chicago, says Mirkin’s new lithography technique could enable completely new areas of research. It might enable new studies of cell adhesion, for instance. Biologists know that a cell’s attachment to a surface is determined by tiny nanostructures called focal adhesions that vary in size. These are important because when cell adhesion breaks down, a cancer cell might break free from a tumor and spread throughout the body. Mrksich says patterned arrays made using Mirkin’s technique could show cell biologists how the size of the focal adhesions regulates cell behavior.

“This method should open up desktop fabrication capabilities to many more researchers,” says Mirkin. A company called Nano Ink has commercialized previous lithography methods from his lab. He says the university is likely to license the nanolithography method to a company, not necessarily Nano Ink. Mrksich is also on the scientific advisory board of that company.

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