Laser-focused deposition produces nanowiresA team of researchers at the National Institute of Standards and Technology (NIST; Gaithersburg, MD), led by Jabe¥McClelland, has fabricated thousands of nanowires simultaneously with widths as small as 66 nm--about one-thousandth the size of a human hair. The grou¥believes that its results could have implications for the production of microelectronic and data storage devices because future microfabrication techniques will have to be able to make an array of components including transistors and wires in the sub-100-nm regime. Optical lithography is a candidate for making these sub-100-nm components, but the technique may be limited by optical diffraction. Electron-beam lithog raphy is another possibility but is currently hampered by the need for extended exposure times and proximity effects.
The NIST process uses laser light to focus neutral atoms as they deposit on a silicon surface--in this case the "atom optics" process focuses chromium atoms with a laser beam to form rows with spacing and thickness that correspond exactly to the wavelength of the laser light. For the process to work, the atomic resonance of the chromium atom must be near the frequency of the laser light.
The research team used a single-frequency, stabilized ring dye laser operating with stilbene-3 and pumped by an argon-ion laser to provide a standing-wave output at 425 nm with a single-beam power of about 30 mW at the substrate surface. The laser output was frequency-shifted to 500 MH¥above the atomic resonance of chromium with an acousto-optic modulator. The beam was focused to a 130-µm-diameter spot and grazed across the silicon substrate in a vacuum chamber. A stream of chromium atoms was released to flow through the laser beam toward the surface. The atoms tumbled down the peaks in the laser wave, sticking onto the silicon surface in rows whose thickness varied according to the laser output wavelength.
The substrate containing the rows of deposited atoms was then plasma etched. The thinner rows of chromium were etched through to the underlying silicon substrate, while the remaining (thicker) rows of chromium formed fine wires between the trenches on the silicon surface. "The whole process relies on the strong interaction between the chromium and the laser beam," McClelland explained. "Chromium has a strong absorption with the laser beam."
"The laser focusing technique is isotope selective," McClelland said. It resulted in the wires each being composed of predominantly chromium-52. Other isotopes, of which natural chromium contains 16%, formed a background deposition. Scientists at Colorado State University (Fort Collins, CO), meanwhile, have used aluminum in an attempt to make similar wires, and at Lucent Technologies (Murray Hill, NJ), researcher Gregory Tim¥has experimented with making sodium wires.
McClelland characterized the NIST nanowires as being well defined. They ranged u¥to about 150 µm in length (see figure). The NIST officials believe that the biggest advantage of the technique is that the combination of laser focusing and plasma etching can cover a relatively large surface with nanowires in a short time.
One key to the success of the research was use of a reactive-ion etch plasma that allowed the researchers to sputter-remove chromium uniformly without significantly increasing the roughness. This allows background material to be removed while the essential features remain intact. The result is a large, coherent array of clearly separated metallic nanowires.
The research team believes there are a variety of avenues for future development. Emphasis could be placed on covering a large area with an array of chromium wires. In fact, a single exposure area of several square millimeters is possible. To achieve uniform wires over this area, however, precise control of the etching will be necessary and the role of contamination and oxides will have to be explored, the researchers believe. They also suggest using other substrates such as gallium arsenide or sapphire.
The research team included McClelland, Rajeev Gupta, Robert Celotta, and George Porkolab. McClelland, Gupta, and Celotta are members of the Electron Physics Grou¥at NIST. Porkolab was at the Laboratory for Physical Sciences at the time of the research.
C. David Chaffee