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.
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.