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A Cheap, Fast Way to Write Nanoscale Patterns

6 August 2010 11:55 am
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Mirkin Group/International Institute for Nanotechnology, Northwestern University

Hybrid. A new nanopatterning technique combines the advantages of near-field microscopy with photolithography.

Today's microchips, communications gear, and medical diagnostics are typically made by writing nanoscale patterns over large areas of silicon wafers and other high-tech materials. The process is either extremely expensive or painfully slow, however. Now scientists have come up with a hybrid approach that could offer researchers a way to craft prototype nanoscale devices quickly and cheaply, speeding up the already blistering pace of developments in the field.

The standard computer chip–patterning technique, called photolithography, works by shining light through a prepatterned stencil onto a light-sensitive polymer that sits atop a wafer of silicon or another electronic material. Chemicals then etch away at the polymer and the silicon, creating a pattern that matches the original stencil. Intel and other chipmakers already use photolithography to pattern features on chips as small as 32 nanometers. But the high cost of the technique—it requires multibillion-dollar clean-room facilities—keeps it out of the hands of researchers looking to prototype novel devices.

An alternative technique called near-field scanning optical microscopy can also etch nanoscale features. Making use of atomic microscopy, it places tiny light emitters extremely close to a surface to illuminate small patches of a light-sensitive polymer. But because it doesn't use stencils to pattern an entire surface at once, it is far slower than photolithography.

Now a team led by Chad Mirkin, a chemist at Northwestern University in Evanston, Illinois, has combined near-field techniques with conventional photolithography to pattern large areas of silicon and other materials without an expensive fabrication facility. Mirkin's team previously pioneered a technique called polymer pen lithography, creating tiny plastic tips shaped like inverted pyramids, which use ink to write features onto a surface. Mirkin's new technique, called beam-pen lithography, uses similar tips made from a transparent polymer. The researchers coat all but the tips of their pyramids with a thin layer of gold. When they then shine light on the base of an array of pyramids, it passes through the polymer and out the tips onto a photosensitive layer atop a silicon surface.

In their study, published online this week in Nature Nanotechnology, Mirkin and colleagues report using an array of 15,000 tips to pattern 15,000 replicas of the Chicago skyline, each consisting of 182 dots, each dot about 450 nanometers across. Mirkin notes that the technique can also be combined with photolithographic stencils to produce virtually any pattern. He adds that since the paper was submitted, he and his colleagues have made arrays containing 11 million tips, and he expects to be able to make arrays containing billions of tips, drastically boosting patterning speeds.

"It's a brilliant concept," says Joseph DeSimone, a chemist and nanofabrication expert at the University of North Carolina, Chapel Hill. He notes that the technique can't yet match the resolution of conventional photolithography and that it needs improvements to keep the patterns aligned if the array is used repeatedly on different parts of a surface. But DeSimone says those problems should be manageable. That would make the tool a powerful way for nanoscientists to rapidly and cheaply prototype novel devices, such as new micromechanical diagnostics and unique microelectronic designs.

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