Laser interference exposure lithography for fabricating super-hydrophobic polymer microstructures

In nature, a variety of insects, plants, or other living species utilizes micro-scale fibers/hairs to possess hydrophobic surface on their bodies. Hydrophobic surface is important especially for small living species to keep their fragile bodies against surface tension of water droplets and to keep their body surface clean. By being inspired by such natural microstructures, we artificially fabricated such periodic pillar arrays made of polymer. For the fabrication of periodic pillar structures, we developed laser interference exposure lithography system. We used a third-harmonic light of Nd:YVO4 ultraviolet laser, with its wavelength of 355 nm, as the light source. The laser beam was expanded by a beam expander, and was divided into four beams by a diffraction type beam splitter. The four laser beams were gathered by a pair of single lenses into one point, so that interference fringe exhibiting square-lattice light hot spots was formed. We put a photoresist film at the position where the laser beams were gathered, and exposed the film under the laser light. The photoresist was cured according to the interference pattern. After developing procedure, we obtained spuare-lattice polymeric pillar structures. The distance and tilting angle of the pillars can be tuned by the incident angles of four beams. The thickness and the height of the pillars can be can be tuned by the exposure dose of laser beams, and the diameter of our prepared resist film, respectively. By a single shot laser exposure for 0.3 seconds, we simultaneously fabricated an array of ∼60,000 pillars with their diameter and the height of 4.5 µm, and 3.5 µm, respectively, in a dimension of 4mm². We will show hydrophobicity of the pillar structures investigated by the measurement of contact angle of water. We will also show the effect of the aspect ratio and tilting angle of micro-pillars on the hydrophobicity.