Jan 15, 2008
Particles cut the cost of making nanoholes
Scientists from Northwestern University, US, have come up with an affordable, high-throughput process for creating uniform arrays of nanoholes and nanopillars. The group uses its technique as a way of making very efficient lasers and infrared detectors, but the maskless method has a broad range of applications from filtration through to solar cells.
At the heart of the low-cost process is a self-assembled network of silica and polystyrene spheres, which behaves as an array of patterning lenses. The nanohole diameter is adjusted by simply varying the UV exposure energy, and the lattice period (the distance between the holes) can be controlled by changing the sphere diameter.
"We can produce very uniform sub-100 nm features using a UV source (200 nm) that costs around $10,000," Hooman Mohseni of the university's bio-inspired sensors and optoelectronic laboratory (BISOL) told nanotechweb.org. "Dimensions of this size are currently only accessible with relatively expensive methods such as X-ray lithography and nanoimprinting, or by using low-throughput techniques such as e-beam lithography and AFM-based lithography."
To pattern their samples, the researchers cover a photoresist-coated substrate with an aqueous suspension of either silica or polystyrene spheres that self-assemble into a uniform two-dimensional array. Next, samples are UV exposed using a conventional photolithography tool for a fraction of a second. During this short period of time, only material at the base of the spheres is exposed due to the focusing effect of the particles.
Calculations show that the beam waist is a very weak function of the sphere diameter, which allows the team to produce a regular pattern of holes even using particles of slightly different sizes.
Once the exposure step is complete, the silica spheres are eliminated by dipping the sample into hydrofluoric acid for 30 seconds. Polystyrene particles are removed using sonication. Samples are then developed in AZ-300 MIF for 30 seconds, rinsed in deionized water and dried in nitrogen.
The researchers presented their results in Nanotechnology.
About the author
James Tyrrell is editor of nanotechweb.org.