July 30, 2009 -- A new technique to produce ultra-smooth metal surfaces could secure a future of sensitive medical detectors, more efficient solar cells and faster computers, according to new research published today in the journal Science.
The key to this new technique is a five-minute epoxy that could actually be found at a local hardware store.
Scientists from the University of Minnesota created nano-scale structures on silicon templates that can be used over and over again, which should lower costs of manufacturing across a variety of technologies.
"Up to this point each device had to be hand-crafted one at a time, and because each device is different, they behave differently," said David Norris, one of the co-authors of the Science paper. "Our process allows you to turn out many identical devices that all act the same."
The scientists started with a single block of silicon. They then placed a mask over the block and treated the exposed area with potassium hydroxide to chemically etch various structures into the silicon. This process created a pattern in the silicon, which served as a template.
Other methods of etching patterns, such as electron beams, are available as well.
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The Minnesota scientists created bulls-eyes, pyramids and a handful of other patterns, although virtually any shape can be made.
With the mask removed, the scientists then deposited gold, silver or copper over the silicon template, which filled in the various pores.
So far, this is fairly standard procedure; the semiconductor industry and other sectors have been doing this for 15 years to create ultra-smooth flat surfaces to use with the technologies they manufacture. However, the technique used to remove the silicon on ultra-smooth, patterned surfaces is new.
Traditionally, companies and researchers dissolved the silicon template away. It's effective, but not efficient; for each device, a new silicon template has to be created.
The Minnesota scientists have figured out how a way to reuse these templates. Instead of destroying the silicon, they simply peel the metal off, revealing the ultra-smooth patterned surface underneath.
There are two ways to peel off these tiny structures. The first is to create a thin metal layer, 100-200 nanometers thick for silver, and then pour five minute epoxy over the metal. Once the epoxy hardens, the researchers simply peel the epoxy, and the metal underneath, off the silicon.
The other way to save the silicon is by adding more metal. When the metal layer becomes about one tenth of a millimeter thick it's strong enough to peel off.
Once the metal is removed, the silicon template can be reused up to 30 times, enabling the creation of identical devices. Because the silicon template can be molded to virtually any shape, this technique could have a wide range of applications.
For example, tiny pyramids created using this technique could focus beams of optical light onto a single point much smaller than any magnifying glass could manage. For scientists to take a picture of something tiny, they have to use a wavelength of light that is smaller than the object they are trying to image.
Focusing light onto such a small area could enable scientists to see, for example, even a single cancer marker in a blood sample. In general, the earlier cancer is detected the greater the chance that a person will survive.
Another possible application for this technique is in the production solar cells. Many solar panels spread relatively inefficient, but cheap solar cells over a large area. Gathering light and focusing it at one particular point filled with more expensive, but more efficient solar cells could help raise the amount of power gathered from the sun.
Patterning tiny optical structures could help lead to optical, instead of electrical, computing as well. Using light waves instead of electrons means computer chips would be smaller, faster and hold more data than electron-based chips, says Teri Odom of Northwestern University.
All of these are potential applications will have to try this new manufacturing technique individually to ensure that it works, but scientists are excited by the myriad possibilities.
The initial costs of etching the silicon will remain high, says Odom. However, since the templates can be used over and over again, they should lower costs for all of these fields.
Henri Lezec, a researcher at the National Institute of Standards and Technology who is familiar with the work, agrees with Odom.
"This technique will lead to devices of higher quality that are easier to fabricate and can enable large area fabrication, said Lezec. "This is a very practical piece of work that could benefit a large number of research communities."
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