Making lithographic masks from azobenzene-containing polymers (azopolymers) rather than conventional photoresist provides a convenient and fast means of performing optical interference lithography. That's the view of researchers from Aalto University in Finland and Tokyo Institute of Technology in Japan, who have now used this technique to fabricate large areas of silicon nanostructures (Adv. Mater. 23, 4174–4177; 2011).

Azopolymer films are attractive because the efficient polarization reversibility of azobenzene makes it easy to create photo-induced surface patterns. Unlike traditional photoresist, which is sensitive to the intensity of short-wavelength visible light, mask formation in azopolymers is driven by polarization modulation within the incident interference pattern, and is therefore insensitive to stray light. Moreover, because azopolymers are sensitive to a wider wavelength band of light and exhibit better tolerance to overexposure than traditional photoresist, complex surface patterns can be created by applying multiple interference patterns.

Credit: © 2011 WILEY

The fabrication procedure developed by Andriy Shevchenko and co-workers involves spin-coating a thin film of azopolymer onto the top of a silicon substrate. Illuminating the surface with an optical interference pattern generates a surface-relief grating in the film. The next step is to partially etch the polymer by reactive-ion etching and then dry-etch the silicon substrate in regions that do not contain the polymer mask. The final step is to strip away the remaining mask, leaving only the desired silicon pattern. Unfortunately, because the azopolymer mask is soft, etches in the silicon are slightly angled rather than perfectly rectangular, which limits the achievable etching depth of the technique.

To overcome this problem, the researchers adopted an advanced fabrication approach that adds a 20 nm layer of amorphous silicon and a 5-nm-thick alumina layer beneath the azopolymer film. Alumina is more resistant than azopolymer to reactive-ion etching and thus functions as a hard mask with steep side walls. The amorphous silicon provides good adhesion to the azopolymer film. The amorphous silicon layer is etched through the soft mask using reactive-ion etching followed by wet etching of the alumina layer, after which the silicon is dry-etched and the mask is stripped off.

Using this advanced approach, the researchers were able to fabricate high-quality one-dimensional periodic surface structures in silicon with feature sizes of the order of 100 nm over an area of around 1 cm2. Two-dimensional silicon patterns can be obtained by exposing the same interference pattern twice and rotating the sample by 90° between exposures. Changing the exposure and etching parameters provides control over the geometry of the etched structures.

The use of azopolymers relaxes the environmental requirements of optical lithography, allowing it to function at longer wavelengths and under room lighting. The nanostructures fabricated in this work could be useful in a variety of fields, including surface-enhanced fluorescence, Raman scattering spectroscopy and photonic-integrated circuits.