## 3 Results and Discussion  
### Laser Interference Lithography Fabrication  
Laser interference lithography was used to fabricate hexagonal arranged hole-like patterns in SU-8-2. These structures were achieved by irradiating a spin-coated layer of non-polymerized SU-8. To obtain the hole-like patters, two exposure steps were applied with a rotation of 60\({}^{\circ}\) between them. On the positions corresponding to the interference maxima, the SU-8 material starts to polymerize forming the periodic structure. By varying the laser fluence between 1-8 mJ/cm\({}^{2}\), the amount of polymerized material can be controlled and thus the diameter of the produced holes. In the case of small spatial periods, lower fluences are required to avoid the full polymerization of the SU-8 layer. This first study permitted to determine the most adequate fluences for the fabrication of the biomimetic hierarchical surface structures. In this case, fluences of 7.5 mJ/cm\({}^{2}\) and 4 mJ/cm\({}^{2}\) for the 5 \(\upmu\)m and 0.5 \(\upmu\)m patterns were used, respectively. Scanning electron microscope images of resulting two-level hierarchical patterns fabricated using three different strategies are shown in Figure 1.  
Figure 1a,b shows a hexagonal hole-like structure with a 0.5 \(\upmu\)m spatial period covered with a 5.0 \(\upmu\)m hole-like structure. This strategy for fabricating hierarchical structures has been developed before as reported in [27]. First, a single-scale pattern with 0.5 \(\upmu\)m spatial period was applied on a silicon wafer as described in the experimental section. After the hard baking step, the patterned wafer was spin coated again and the exposure setup was changed to fabricate a 5 \(\upmu\)m hole-like pattern. Finally, the wafer was baked, developed, rinsed and hard baked again to finish the second layer. It is visible in Figure 1a that the 5.0 \(\upmu\)m hole-like pattern dominates the surface topography. Only within the holes of the large-scale structure, the small-scale structure is visible. In addition, since two layers of the resist were applied, the 0.5 \(\upmu\)m geometry can be observed below the large pattern. In Figure 1c, hierarchical patterns fabricated with a similar strategy are shown. In this case, the largest periodic pattern was fabricated first. Then, the pre-patterned substrate with the 5.0 \(\upmu\)m structure was covered with a second layer of SU-8 which was later exposed to the interference patterns with the 0.5 \(\upmu\)m spatial period geometry. As it can be seen in the image, the small-scale structure dominates the surface, since it covers the whole surface of the 5 \(\upmu\)m pattern. The difference to a single-scale pattern is that the 0.5 \(\upmu\)m pattern topography is now modulated with the 5 \(\upmu\)m topography lying underneath. Consequently, the small-scale pattern is on different height levels at different lateral positions. Finally, a second strategy for the production of multi-scale patterns was used. Here, a single layer of SU-8 was exposed first with the 5 \(\upmu\)m spatial period setup and later with the 0.5 \(\upmu\)m spatial period setup. Between the irradiation steps any other treatment was performed. As it can be seen in Figure 1d, the small-scale structure of 0.5 \(\upmu\)m spatial period is visible mainly at the regions corresponding to the interference minima positions of the larger 5.0 \(\upmu\)m pattern. It is also evident from the image that  
Figure 1: SEM images of (**a**) 5 \(\upmu\)m hexagonal hole-like structure on top of a 0.5 \(\upmu\)m hole-like structure, (**b**) the same with a larger magnification. (**c**) 5 \(\upmu\)m hole-like structure covered with 0.5 \(\upmu\)m of spatial period pattern and (**d**) 5 \(\upmu\)m and 0.5 \(\upmu\)m exposure consecutively in the same layer. In all cases, 7.5 mJ/cm\({}^{2}\) and 4 mJ/cm\({}^{2}\) were used for the 5 and 0.5 \(\upmu\)m pattern, respectively. both geometries are at the same level, since only one layer of the resist was applied.