# Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
## 3 Conformal Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks  
The above-mentioned 3D nanofabrication techniques mainly produce polymer-based nanonetwork structures. However, to apply such nanostructured materials to various fields, it is necessary to  
Figure 3: SEM images of polymer templates with irregular or regular 3D nanonetwork structures fabricated through 3D nanofabrication techniques: (**a**) electrospinning (reprinted with permission from [69]. Copyright 2014 Elsevier); (**b**) BCP lithography (reprinted with permission from [10]. Copyright 2014 The Royal Society of Chemistry); (**c**) DLW (reprinted with permission from [44]. Copyright 2004 Nature Materials); (**d**) MBIL (reprinted with permission from [18]. Copyright 2009 The Royal Society of Chemistry); (**e**) one-photon PnP; (**f**) two-photon PnP (reprinted with permission from [48]. Copyright 2006 The Optical Society (OSA)); (**g**) one-photon PnP with a binary phase-mask (reprinted with permission from [23]. Copyright 2015 John Wiley and Sons). compensate for the relatively poor thermal or mechanical resistance of the polymer, or to impart additional functionality (e.g., conductivity, reactivity, etc.). To achieve this, many studies have been performed to fill the interconnected pores of 3D nanonetworks with functional inorganic materials, including metals and ceramics [31]. Typical filling methods include electrochemical deposition, sol-gel reaction, and CVD. However, these bottom-up filling techniques eliminate the porous structure of 3D nanonetworks in a composite state. The removal of polymer templates by a high-temperature (>450 \({}^{\circ}\)C) heat treatment or an O\({}_{2}\) plasma etching process allows an inverse porous structure; however, it is generally difficult to obtain high porosity (>90%). In contrast, ALD has a unique feature of the conformal coating of the surfaces of porous structures based on surface adsorption and reaction. In this case, we can selectively change surface properties or reduce pore size while maintaining a porous structure. Unlike bottom-up filling, ALD generates a hollow structure after removing the polymer template, thereby creating an ultra-lightweight and highly porous material similar to aerogel [29; 70]. A conventional thermal ALD process begins when a precursor is fed into a chamber and adsorbed onto a polymer template. At this time, the substrate temperature should be maintained below the dissociation temperature of the precursor to prevent undesirable thin film formation. The dissociation temperature of the precursor ranges from about 100 to 700 \({}^{\circ}\)C, depending on the type of precursor. In polymer template-assisted ALD, a temperature of about 200 \({}^{\circ}\)C or less is preferred to prevent reflow of the polymer, even if the film quality deteriorates. The precursor is generally delivered to the polymer template with a carrier gas and adsorbed on the substrate by chemical attraction. To improve the uniformity of the thickness of the deposited film, a sufficiently long supply time is required to cover the nanoporous structure completely with the precursor at this stage [23; 70; 72]. In the next step, it is necessary to supply the purge gas to wash away the remaining precursor. Ar, an inert gas that does not react with the precursor, is generally used as the purge gas. Once the precursor supply is stopped and the purge gas is supplied, the remaining gaseous precursor in the chamber and the precursor adsorbed weakly on the template are rinsed away by the purge gas, leaving only one layer of the precursor that has been in strong chemical bonding with the template. A third step in the ALD process is the supply of reactants to the precursor-coated polymer template. The reactant chemically reacts with the precursor adsorbed on the template to form a thin film. Except for some precursors that require thermal activation, most precursors react spontaneously with the reactants even at room temperature.