# Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
## 2 Preparation of 3D Polymer Nanonetworks
While DLW has great advantages in manufacturing arbitrary structures with a high resolution, it suffers from fatal drawbacks such as a very slow production speed and the development of two-photon absorbing materials that are not diversified. In this respect, MBIL, also called holographic lithography, is a precise manufacturing technique for periodic 3D nanonetwork structures, which complements the low productivity of DLW (Figure 2d) [49; 50; 51; 52]. The principle of MBIL is to transfer the 3D periodic intensity distribution resulting from the interference between four coherent beams separated by a single laser, to photosensitive polymer. Various types of periodic structures can be created by adjusting the optical parameters (e.g., incident angle or polarization) of the four interfering beams. Because this method is a parallel process, unlike DLW, which is a serial process, it has advantages in terms of the large-scale fabrication of periodic 3D nanonetworks; however, the degree of structural freedom is not high. Moreover, because the four interfering beams must be temporally and spatially coherent to generate interference, the structural reproducibility is very sensitive to vibration, and inevitably requires a complex optical setup. To evolve into more industrial processes, [PERSON] et al. developed PMIL, also known as proximity-field nanopatterning (PnP), in which all complex interference optics are integrated into a single conformal phase-mask (Figure 2e) [53; 54; 55; 23; 56; 57; 58]. The principle of this technique is to imprint the Talbot interference, caused by the interference of diffractions arising from the phase mask with the surface relief structure, onto the photopolymer. The used phase-mask is generally made of a poly(dimethylsiloxane) (PDMS)-based elastomer, which can adhere to the substrate without being sensitive to surface  
Figure 2: Schematic representation of the fabrication methods of 3D nanonetwork-structured polymer templates: (**a**) electrospinningning (reprinted with permission from [37]. Copyright 2004 John Wiley and Sons); (**b**) block-copolymer (BCP) lithography (reprinted with permission from [45]. Copyright 2013 The Royal Society of Chemistry); (**c**) direct laser writing (DLW) (reprinted with permission from [46]. Copyright 2011 The Optical Society (OSA)); (**d**) multibeam interference lithography (MBIL) (reprinted with permission from [47]. Copyright 2011 The Optical Society (OSA)); (**e**) proximity-field nanopatterning (PnP) (reprinted with permission from [48]. Copyright 2006 The Optical Society (OSA)). roughness, thereby enabling high-resolution contact-mode lithography with high-vibration tolerance. The generated Talbot interference and 3D periodic nanonetworks can be varied by adjusting the incident wavelength of the laser and phase-mask geometry, and can be predicted and designed by optical simulations such as finite-difference time domain (FDTD), finite element modeling (FEM), or rigorous coupled-wave analysis (RCWA) [59]. Above all, PML has the advantage of expanding the production area of the 3D periodic nanonetwork to the wafer scale by increasing the size of the phase mask and exposure source. However, the existing technical issue is to improve the degree of structural freedom through an inverse algorithm design [60; 61]. More information on the recent progress of the 3D nanofabrication techniques mentioned above can be found in the literature [13; 30; 51; 62; 63; 64; 65; 66; 67; 68]. Figure 3 shows the representative examples of polymer templates with a regular or irregular 3D nanonetwork structure fabricated by the above 3D nanofabrication techniques, including electrospinning, BCL, DLW, MBIL, and PnP.