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STED Effect Enables Chips with Finer Features

Photonics.com
Dec 2011
CAMBRIDGE, Mass., Dec. 22, 2011 — A new way to break through wavelength-related limits to feature size in state-of-the-art silicon chips could enable further leaps in computational power.

The microchip revolution has seen a steady shrinking of features on silicon chips, packing in more transistors and wires to boost their speed and data capacity. But in recent years, the technologies behind these chips have begun to bump up against fundamental limits, such as the wavelengths of light used for critical steps in their manufacture.

Trisha Andrew of MIT’s Research Laboratory of Electronics said that the new technique — which she and her colleagues described in a recent edition of Physical Review Letters — allows the production of complex shapes rather than just lines and can be carried out using less expensive light sources and conventional chip-manufacturing equipment.

“The whole optical setup is on a par with what’s out there” in chip-making plants, she said. “We’ve demonstrated a way to make everything cheaper.”


Trisha Andrew and her colleagues have developed a new way to create reduced feature sizes on silicon chips. (Photo: M. Scott Brauer)

In 2009, Andrew’s team described a way of creating finer lines on chips, dubbed absorbance modulation (See: Narrower Chip Patterns Made). As in the earlier work, the new system relies on a combination of approaches: namely, interference patterns between two light sources and a material that changes color when illuminated.

But, Andrew said, a new step is the addition of a photoresist, which produces a pattern on a chip via a chemical change following light exposure. The pattern transferred to the chip can then be etched away with a chemical developer, leaving a mask that can control where light passes through that layer.

Although traditional photolithography is limited to producing chip features larger than the wavelength of the light used, the method devised by Andrew and her colleagues has produced features one-eighth that size. Others have achieved similar sizes before, she said, but only with equipment whose complexity is incompatible with quick, inexpensive manufacturing processes.

The new system uses “a materials approach, combined with sophisticated optics, to get large-scale patterning,” she said, adding that the technique should make it possible to reduce the size of the lines even further.

The key to beating the limits usually imposed by the wavelength of light and the size of the optical system is an effect called stimulated emission depletion imaging, or STED, which uses fluorescent materials that emit light when illuminated by a laser beam. If the power of the laser falls below a certain level, the fluorescence stops, leaving a dark patch. It turns out that, by carefully controlling the laser’s power, it is possible to leave a dark patch much smaller than the wavelength of the laser light itself. And by using the dark areas as a mask and sweeping the beam across the chip surface to create a pattern, these smaller sizes can be “locked in” to the surface.

That process has been used to improve the resolution of optical microscopes, but researchers had thought it inapplicable to photolithographic chip making. The innovation by Andrew and her colleagues was to combine STED with the earlier absorbance modulation technique, replacing the fluorescent materials with a polymer whose molecules change shape in response to specific wavelengths.

Besides enabling the manufacture of chips with finer features, the technique also could be used in other advanced technologies, such as in the production of photonic devices, which use patterns to control the flow of light rather than the flow of electricity. “It can be used for any process that uses optical lithography,” Andrew said.

The creator of STED, Stefan Hell of Max Planck Institute for Biophysical Chemistry in Göttingen, Germany, calls this work “strikingly simple and elegant” and “a most impressive demonstration of the idea of using photochromic molecules to create features that are both finer and closer together than half the wavelength of the light.”

“The work shows a concrete pathway to creating tiny and dense features at the nanoscale,” Hell added. “Because of its future potential, it needs to be actively pursued.”

The paper’s authors also include Rajesh Menon, formerly a research engineer at MIT and now an assistant professor of electrical engineering and computer science at the University of Utah, and Nicole Brimhall and Rajakumar Varma Manthena, both also of Utah.

For more information, visit: www.mit.edu  


GLOSSARY
fluorescence
The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
photolithography
A lithographic technique using an image produced by photography for printing on a print-nonprint, sectioned surface.
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