Nanolithographic Fabrication on Curved Substrates

Today, many applications require nanofabrication on curved surfaces. Examples include but are not limited to nanostructured antireflective coatings, diffraction gratings, and other functional nanopatterns on lenses or optical fibers. However, traditional nanolithographic patterning, which originates from the production of electronic devices and circuits, issuited only for flat substrates. Nowadays, novel nanopatterning approaches based on self-assembly from the bottom-up are extensively explored as a simple and cost effective alternative for beam-based lithography. Self-assembly in principle is not sensitive to the substrate curvature, and hence is applicable to all kinds of surfaces. Yet, self-assembly based lithography on curved surfaces, such as lenses, has been barely explored so far.

Here, we demonstrate a novel approach for nanofabrication of periodic nanopatterns on lenses. To produce the nanopattern on lens, we coated its surface with a closely packed monolayer of polystyrene nanobeads using Langmuir-Blodgett through. Then, the nanobead pattern is transferred by metal evaporation and liftoff. To ensure that the metal is deposited uniformly over the lens surface, we devised and programmed a sample holder that allows continuously changed tilt and rotation during the evaporation. We characterized the nanopattern formed after the liftoff by AFM, and found that it is uniform all over the lens. This approach opens a pathway to endless applications that require nanofabrication on unconventional substrates, which have been impossible up today.

To produce the actual nano-structures on flat glass surface, deep reactive etching was needed. Before the etching, needed optical depth was calculated in order to promise the correct reflectance measurement in accordance with its transmission wavelengths. In order to do that, refractive relative index was calculated based on periodicity of the pattern and the air/substance area ratio. The obtained antireflective nano-structures reduced the reflectance by 20-55% with dependence on the optical depth. The next step will be to develop DRIE process to be applied for curved surface in order to produce antireflective nano-structures.