NEWS OF THE WEEK
NANOTECHNOLOGY
February 12,
 2001
Volume 79, Number 7
CENEAR 79 7 pp.9
ISSN 0009-2347

WRITING BETWEEN THE LINES
New procedure decorates small structures with nanoscale features

MITCH JACOBY

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FILL IN THE BLANK Penn State researchers fabricate metal structures in gaps of larger structures. Their method can be used to paint a 30-nm-wide gold dot in the center of a hole in a gold square (above). Similarly, a 110-nm gap between gold pads (below) is gradually shrunk with organic layers that guide a 25-nm-thick gold line into the tight space.
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Going from small to smaller, researchers at Pennsylvania State University have demonstrated a technique for scaling down structures produced via lithography and other common fabrication procedures. The method provides a new way to create nanoscale components of miniature devices with fine control over size and proximity to other tiny structures.

Device manufacturers currently have at their disposal a growing collection of tools for making pieces of electronic circuits and micromachines with dimensions on the nanometer scale. But each method has limitations. For example, electron beam lithography--useful for preparing ultrasmall wires and transistors--is often difficult to apply to products with a target size below 100 nm.

Using the output of today's conventional fabrication procedures as a starting point, postdoctoral researcher Anat Hatzor and associate chemistry professor Paul S. Weiss have come up with a technique to grow structures measuring just a few nanometers in width and simultaneously position the products adjacent to larger parent structures with roughly 1-nm accuracy.

Based on chemically selective material deposition and removal steps, the method uses individual molecular layers as spacers to place newly created structures at set distances from existing ones [Science, 291, 1019 (2001)].

"We're trying to bridge the gap between the top-down approach of nanofabrication and the bottom-up approach of molecular science," Weiss says.

Near-term applications of the procedure are likely to come in the area of traditional semiconductor devices, Weiss says. With the Penn State advance, well-developed semiconductor technologies can be pushed to produce devices with even smaller, more closely spaced components.

"But the real motivation for the study comes from molecular electronics," Weiss notes. "If we want to be able to hook up functional molecules between electrodes, we need to be able to make electrodes with spacings that precisely match the length of those molecules."

Describing the work as "a big step forward," Christopher B. Murray, manager of nanoscale materials and devices at IBM's T. J. Watson Research Center, explains that "the procedure brings together lithography and molecular-scale assembly in a complementary way by allowing each fabrication method to work at the length scale at which it works best."

In one demonstration, the group applies mercaptoalkanoic acid layers each 2 nm thick to gold pads separated by about 100 nm. The organic layers adsorb selectively on gold but do not coat the underlying oxidized silicon substrate.

Next, the researchers evaporate metal into the gap. Finally, the team uses solvents to remove the organic layers, leaving behind a metal trail, or trace, between the gold pads. The position and thickness of the trace is determined by the number of organic layers applied. The method has been used to create traces of several shapes and can be applied to a variety of chemical substances, Weiss points out.



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