# Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
## 3 Conformal Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
Heating is sometimes required to improve the properties and crystallinity of the deposited materials. When a metal oxide-based dielectric film is deposited, water is usually used as the reactant. However, when water is adsorbed on the inner wall of the chamber, water may easily condense, and the condensation may not be removed, which may cause contamination problems. For this reason, alcohol, oxygen, ammonia, and ozone, which are less reactive than water but prone to purging, can be used as alternatives. The ALD process is completed by supplying purge gas to purge the remaining reactants. If the purging time is very short, the reactants may remain in the chamber and cause unwanted chemical reactions with the precursors supplied in the next cycle. From the beginning to this stage is one cycle of the ALD process. One of the key parameters in ALD is growth per cycle (GPC), which means the thickness of the deposited film in one cycle. The GPC depends on the process temperature, the precursor, the substrate, and so on. The GPC value of ALD is usually less than a few angstroms, which is relatively low compared to CVD because the physically adsorbed molecules are supposed to fly off during the purge step. In polymer template-assisted ALD, it is very important to understand the nucleation and growth behavior of the material deposited on the polymer [73; 74; 75; 76]. In fact, the formation of an inorganic thin film by ALD is highly dependent on the density of the polymer and the functional groups of the polymer surface. Occasionally, polymers act as inhibitors to the nucleation of inorganic materials during the ALD process. Based on this phenomenon, some studies have achieved area-selective ALD (AS-ALD) by patterning polymer resists or self-assembled monolayers (SAMs) [77; 78; 79; 80; 81]. In the case of a soft polymer with low crosslinking density, the gaseous precursor can diffuse into the polymer and react with the polymer to form an organic-inorganic hybrid material. This is called vapor phase infiltration (VPI) [82; 83; 84]. In general, various inorganic materials such as oxides, nitrides, and sulfides can be deposited through thermal ALD. Figure 4 shows various examples of the successful conformal deposition of inorganic thin films on 3D polymer nanonetworks using the thermal ALD process described above. Figure 4: Various inorganic thin films conformally deposited on 3D nanonetwork polymer templates through thermal ALD: (**a**) SEM image of a ZnO thin film deposited on a polymer template formed by electrospinning; (**b**) TEM image of (a) (reprinted with permission from [71]. Copyright 2012 American Chemical Society); (**c**) TEM image of Al\({}_{2}\)O\({}_{3}\) and ZnO thin films deposited on a polymer template formed by BCP lithography (reprinted with permission from [9]. Copyright 2015 The Royal Society of Chemistry); (**d**) cross-sectional SEM image of an Al-doped ZnO thin film on a polymer template formed by DLW (reprinted with permission from [14]. Copyright 2011 The Optical Society (OSA)); (**e**) cross-sectional SEM image of an Al\({}_{2}\)O\({}_{3}\) thin film deposited on a polymer template formed by MBIL (reprinted with permission from [18]. Copyright 2009 The Royal Society of Chemistry); (**f**) cross-sectional SEM image of an Al\({}_{2}\)O\({}_{3}\) thin film deposited on a polymer template formed by PnP (reprinted with permission from [26]. Copyright 2018 American Chemical Society); (**g**) cross-sectional SEM and EDS images of a TiO\({}_{2}\) thin film deposited on a polymer template formed by PnP (reprinted with permission from [21]. Copyright 2013 The Royal Society of Chemistry). The device configuration of thermal ALD is very simple; however, the deposition rate is rather slow, because the precursor and gaseous reactant react only by thermal energy to form a thin film, and the film quality is poor when the deposition is performed at low temperatures. Moreover, the known precursors and reactions for thin film formation at low temperatures are very limited.