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Drawing Circuits with Nano Pens

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The demand for ever faster, cheaper electronics is pushing the lithography-based manufacturing techniques standard in the semiconductor industry to their limits. Now researchers report a cheap, fast lithography technique that uses arrays of flexible polymer nano pens to precisely pattern millions of complex structures in parallel. The technique, which the researchers have used to create an integrated circuit (and lilliputian versions of the Olympics logo), can be employed to make lines whose sizes range from a few nanometers to millimeters thick.

The technique, developed by Chad Mirkin, a chemist at Northwestern University and director of the International Institute for Nanotechnology, uses arrays of pyramid-shaped polymer pens whose tips are dipped in solutions of chemicals that may feature almost any molecule, including proteins and acids; the pens are then traced over a surface by a mechanical arm to create millions of structures in parallel. The width of the lines drawn by each pen can be carefully controlled by varying the force exerted on the flexible pen tips. Because Mirkin's pens trace out designs programmed by computer software, they can quickly switch between complicated designs, making possible the creation of complex patterns whose features are very close together.

Mirkin has used the pens to pattern acid on a silicon wafer coated with gold; he then etched, based on the pattern, a gold integrated circuit. Polymer-pen lithography also shows promise for patterning biological molecules. Indeed, says Mirkin, the technique could work with almost any molecular "ink," including proteins for capturing and studying cells. The arrays of polymer pens cost less than a dollar each to make.

Polymer-pen lithography is an improvement over dip-pen lithography, a technique that Mirkin has been developing since 1999. Dip-pen lithography uses arrays of sharp, stiff cantilevered probes--the same ones used for atomic force microscopy. Mirkin created a company, NanoInk, to commercialize the technology. But, he acknowledges, "its ultimate utility has been limited by problems with throughput, cost, and complexity." The size of its molecular strokes has been restricted to a relatively narrow range, the cantilevers are prone to breaking, and the number of structures that can be made in parallel is limited.

"If this works," says Grant Willson, an engineer at the University of Texas at Austin, "it will speed the process" of patterning structures with nano pens. The new version of dip-pen lithography could make the technology much more commercially practical. But Mirkin's technique will be competing in a crowded field, notes Willson. Researchers aiming to pack circuits with ever smaller features for ever faster chips are taking many different nanofabrication approaches. Some, for example, are creating optical antennas to focus light into very small beams to extend the capabilities of photolithography. Others have turned to beams of electrons or ions, or use heat deformation to form patterns.

Harald Fuchs, director of the Interface Physics Group at the University of Münster, in Germany, says that the major advantage of Mirkin's technique over other nanofabrication methods is precision and flexibility. The pens could be used to write a pattern in one molecular ink, get dipped in another, and then write another layer. To make even more complex patterns, says Fuchs, each pen tip could be dipped in a different ink.

Mirkin says that Northwestern is talking to companies, including his own NanoInk, about commercializing polymer-pen lithography. The technique, he says, will make the dip-pen technology "accessible to a large number of people."



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