Sep 23, 2011
Photonics goes flexible
Researchers at Boston University in the US have managed to nanopattern on a wide range of flexible substrates using nanostencil lithography. The high-throughput fabrication technique, which involves just a single step, could be used to make novel photonics devices that can be wrapped around curved objects. Medical imaging and environmental monitoring are two potential application areas.
The last decade has seen considerable progress in flexible electronics and a host of novel applications, such as electronic paper-like displays, artificial skin, electronics for in vivo brain monitoring and electronic eye-like imagers, have already seen the light of day. However, photonics is still lagging behind in this respect, particularly at nanoscale dimensions. This is because we cannot employ traditional nanofabrication techniques (like electron beam lithography or focused ion beam lithography) to make nanophotonic structures on flexible or curved surfaces. So-called soft lithography techniques, which work well for flexible electronics, are also limited in resolution and are not precise enough at the dimensions needed to make nanophotonics devices.
A team led by Hatice Altug of Boston has been busy helping to advance the nanophotonics and plasmonics fields. Plasmonics is a relatively new branch of photonics in which devices exploit both light and electrons. The researchers recently showed that they could nanopattern device components on rigid surfaces such as glass, silicon and calcium fluoride using a technique called nanostencil lithography. And, happily, the devices made were as good in terms of optical response as those made by conventional e-beam lithography.
Now, the scientists say that they can nanopattern on a wide range of flexible substrates using the same technique. The team has already made plasmonic antenna arrays and metamaterials on unconventional substrates including PDMS (a widely-used polymer in micro- and nanofluidics), parylene-C (a biocompatible polymer) and low-density polyethylene film, better known as "cling-film". All three of these polymers are technologically important because they can withstand high strain without causing any damage to devices built on them.
Altug and co-workers succeeded in overcoming the resolution-limiting factors of nanostencil lithography and managed to fabricate sharp-edged nanostructures (similar to those achieved by e-beam lithography) on PDMS with an accuracy of less than 10 nm. The researchers also showed that they could produce large numbers of "bow-tie" antenna structures with sub-100 nm side lengths using this technique. What is more, the nanostructures made were mechanically tunable – that is the resonances of the plasmonic structures could be actively tuned by applying mechanical strain to the stretchable substrates.
"We also showed that nanoscale lithography could be applied for nanopatterning curved objects," Altug told nanotechweb.org. "Normally, directly nanopatterning on non-flat substrates is very challenging with existing nanofabrication technologies as they are inherently 2D. We overcame this problem by first generating nanostructures on planar, thin flexible polymer films and using this film as a 'carrier' to transfer the nanostructures directly to a curved surface."
If needed, the carrier polymer can then be removed – by selective etching – leaving behind only the desired nanostructures atop the curved surfaces, adds team member Serap Aksu. The team proved that its technique worked by transferring plasmonic nanoparticles onto optical fibres. Such "photonic probes" could be used to monitor changes in the body in places that are difficult to access – for example the bloodstream and deep-lying tissues. They might also be employed in hostile environments in the field – for instance, areas contaminated with chemicals or biological agents.
Everyone is familiar with stencilling in arts and crafts. Nanostencil lithography is basically the same but on a much smaller scale. Altug and colleagues created their stencil, which had a pre-defined aperture design, on mechanically robust SiN membranes. The researchers then placed the stencil on a polymer substrate and deposited a metal layer, such as gold. The surface of the substrate encourages bonding of stencil to substrate and so ensures that there is no gap between the two. "When we remove the stencil we get nearly perfect transfer of the nanoparticle pattern with geometries complementing the apertures on the stencils," explained team members Min Huang and Alp Artar. "After deposition, the stencil is easily cleaned and ready to use again."
And, unlike other soft lithography methods that involve multiple pattern transfers, nanostencil lithography allows for easy fabrication of nanostructures in a single step, which saves time and effort. What more could you ask for?
The team now plans to optimize its fabrication technique.
The work is reported in Advanced Materials.
About the author
Belle Dumé is contributing editor at nanotechweb.org