9 June 2008—A team of Korean researchers may have come up with an economical way to construct laptop-sized light sources for making the next-generation of computer chips. The team, led by Seung-Woo Kim, of the Korean Advanced Institute of Science and Technology, in Daejon, has demonstrated a method to generate extreme ultraviolet (EUV) light using bow-tie shaped nanostructures and a very fast femtosecond laser. The entire apparatus is small enough to fit on a tabletop, according to their report in the 5 June issue of Nature .
The South Korean method could help to overcome a big problem that threatens to derail the industry’s decades-long march of miniaturization. Transistors today have become so small that a billion of them can be packed onto a computer chip the size of a fingernail. (Today’s state-of-the-art chips have features that are 65 or 45 nanometers in size). These features are etched onto silicon wafers using very-short-wavelength light, but to make faster, more compact circuits, transistors must shrink further. For that to happen, a new, shorter wavelength light source is needed to define the patterns on the chip.
Current plans predict that chip makers will begin to need to switch to EUV for state-of-the-art chips by 2011 or 2012, when the features that make up transistors must be just 22 nm. Finding a suitable light source for EUV lithography machines has proved much more difficult than expected. The South Korean technique is attractive for its small size and optical quality.
In an accompanying commentary in Nature , Mark I. Stockman, a physicist at Georgia State University, in Atlanta, says the South Korean method ”is a deft new way to produce EUV radiation—one that could be considerably more economical than previous approaches.”
Industrial firms working on EUV sources typically blast droplets of tin with high-powered lasers to produce bursts of EUV light. But researchers have been exploring another approach in the lab, which involves starting with pulses of light from a femtosecond laser and stepping up the laser’s frequency. So far, though, the stepping-up process has proved to be cumbersome, costly, and unwieldy. The laser pulses are used to ionize atoms of a gas such as argon, and then the electrons released by ionization are accelerated by an intense electric field. Subsequently, the electrons recombine with the ions, shedding the excess energy as light that is made up of harmonics of the original laser pulses—multiples of the laser’s initial frequency. The highest order harmonic typically falls in the EUV range, and the rest are filtered out.
Kim and colleagues simplified the stepping-up process by generating the electric field needed to produce the high harmonics using a bow-tie shaped nanostructure made up of two cones separated by a small gap. They used a standard titanium-sapphire femtosecond laser that emitted a train of 10 femtosecond laser pulses with a wavelength of 800 nm. The output beam had a modest peak power of 100 kilowatts, and each pulse had energy of 1.3 nanojoules, which gave a modest pulse intensity that needed to be amplified by a factor of at least 100 to generate EUV light. They fed a jet of argon gas over the bow-tie nanostructures, and then they fired the focused laser beam into the gap between the gold nanostructures. The intense electric field produced in the gap by the interaction of the laser light with the metallic points stripped electrons from the argon. When the electrons recombined with their atoms, they produced light of higher order harmonics—up to the 17th harmonic, which was 47 nm, right in the EUV range.
While an EUV light beam was produced, it is still not quite of a narrow enough wavelength or powerful enough for commercial applications.