# Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
## 3 Conformal Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
In general, the deposition of metal is very difficult to achieve via thermal ALD. PEALD was developed to overcome these issues with thermal ALD. In the PEALD process, the precursor supply and purge steps are the same as they are for thermal ALD; however, plasma is generated in the third reactant supply step to improve the reactivity. In general, O\({}_{2}\), H\({}_{2}\), NH\({}_{3}\), or ozone plasma is used to generate highly reactive ions or radicals. Owing to the high reactivity of reactants, in PEALD, the fourth stage purge time can be significantly reduced compared with thermal ALD. As a result, PEALD can achieve higher productivity, lower process temperature, and better film quality (e.g., purity and density) compared to conventional thermal ALD. In particular, Pt, one of the noble metals widely used as a catalyst, can be deposited through PEALD at room temperature. The deposition of Pt via thermal ALD was performed using MeCpPtMe\({}_{3}\) and O\({}_{2}\) in a temperature window of 200-300 \({}^{\circ}\)C [88]. However, for certain polymer templates, this temperature range is still too high for application. [PERSON] et al. successfully reported a three-step PEALD process that yielded a high-quality Pt film with a resistivity of 18-24 \(\mu\Omega\) at room temperature by sequentially supplying MeCpPtMe\({}_{3}\), O\({}_{2}\) plasma, and H\({}_{2}\) gas or H\({}_{2}\) plasma [85]. The H\({}_{2}\) pulse serves to reduce the PtO\({}_{x}\) that can be deposited at low substrate temperatures. They also demonstrated that Pt can be deposited on polymer, textile, and paper substrates through this room temperature process (Figure 5a-c). In addition, PEALD has succeeded in depositing various dielectrics and semiconductors such as Al\({}_{2}\)O\({}_{3}\), AlN, and GaN on natural or electrospun polymer networks at a process temperature below 200 \({}^{\circ}\)C (Figure 5d-f) [86; 87; 89; 90]. However, PEALD has not yet been applied to regular 3D nanonetworks. Figure 5: Various inorganic thin films conformally deposited on 3D nanonetwork polymer templates through PEALD: **(a)** digital image of a Pt thin film deposited on a woven cotton fabric; **(b)** SEM image of (a); **(c)** elemental composition of (a) (reprinted with permission from [85]. Copyright 2013 American Chemical Society); **(d)** SEM image of an AlN thin film deposited on an electrospun 3D nanonetwork (reprinted with permission from [86]. Copyright 2012 John Wiley and Sons); **(e)** TEM image of a GaN thin film deposited on an electrospun 3D nanonetwork; **(f)** elemental composition of (e) (reprinted with permission from [87]. Copyright 2015 The Royal Society of Chemistry). So far, using nanostructured polymer templates for PEALD has not been actively studied compared with thermal ALD. Despite the advantages of PEALD, such as fast deposition rates and low process temperatures, some of its drawbacks may have hindered the use of nanostructured polymer templates [91]. For example, PEALD is generally confronted with issues such as low conformality, due to the directional flux of plasma-activated species and unwanted plasma damage to the polymer template. Also, remote plasma sources (e.g., inductively coupled plasma) for PEALD are typically only a few inches, thus limiting large-scale processing. To further expand the convergence of PEALD and 3D nanonetwork templates, much effort and attention is needed to solve these issues. Table 1 summarizes the representative examples of the deposition of inorganic materials via thermal ALD or PEALD on polymer templates with 3D nanonetwork structures and the process conditions used therein. More information on ALD materials and processes can be found in an online database ([[https://www.atomiclimits.com/alddatabase/](https://www.atomiclimits.com/alddatabase/)]([https://www.atomiclimits.com/alddatabase/](https://www.atomiclimits.com/alddatabase/))).