# Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
## 1 Introduction  
Atomic layer deposition (ALD), a type of chemical vapor deposition (CVD), is a unique thin film deposition technique that can control the thickness of thin films at the angstrom level based on sequential self-limiting, gas-solid surface reactions [1; 2; 3]. One of the key features of ALD is the ability to form a conformal thin film insensitive to surface topology, because the principle of ALD depends on surface adsorption. This results in excellent step coverage even for trench structures with extremely high aspect ratios, which is useful for semiconductor devices [4; 5]. Another feature of ALD is that it is capable of low-temperature processing compared to other vapor-phase deposition techniques [6]. If precursors and systems are designed well-enough to allow the process temperature to be below 200 \({}^{\circ}\)C, then ALD can be applied to polymer substrates as well. In recent years, the potential of ALD based on these benefits has expanded to nanotechnology [7]. As a representative example, ALD has been combined with three-dimensional (3D) polymer nanostructuring methods such as electrospinning, block-copolymer (BCP) lithography [8; 9; 10; 11; 12], direct laser writing (DLW) [13; 14; 15; 16; 17], multibeam interference lithography (MBIL) [18; 19; 20], and phase-mask interference lithography (PMIL) [21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. These unconventional nanofabrication techniques specialize in the production of 3D polymer nanonetworks that can serve as templates for subsequent materialconversion processes [31]. Polymer templates have the advantage of being easily removed by thermal treatment, chemical dissolution, or plasma etching, unlike metal or ceramic templates, which require the use of hazardous acid solutions [32]. In polymer template-based strategies, ALD can be used for various purposes, such as (a) modifying the surface of the polymer template from hydrophobic to hydrophilic [23], (b) reducing the porosity of the polymer template [18], (c) adjusting the optical or mechanical properties of the polymer template [14; 26], and (d) producing hollow inorganic materials converted from the polymer template [15; 16; 21; 22; 27; 33]. An important technical requirement for depositing inorganic thin films on 3D polymer nanonetworks through ALD, which serve as templates, is to maintain the least-possible process temperature below the melting point of the polymer template to protect the 3D nanostructures during the process. For example, an epoxy-based negative-tone photoresist called an SU-8 (Microchem Co.) is widely used to prepare 3D nanostructured polymer templates through lithographic techniques because of advantages including excellent mechanical stability (\(E\)\(\sim\)2 GPa), high photosensitivity and resolution, and a thick film processing ability above several hundred microns [34]. However, because the glass transition temperature (\(T_{g}\)) of SU-8 after cross-linking is only \(\sim\)200 \({}^{\circ}\)C, subsequent processes with SU-8 templates should be performed below \(T_{g}\) to prevent a reflow phenomenon. Most nanostructured polymer networks produced by other techniques also have poor heat resistance [35; 36]. Therefore, to realize various materials and properties through 3D polymer template-based ALD, it is very important to secure a variety of methods for depositing inorganic thin films at temperatures below 200 \({}^{\circ}\)C, especially at room temperature. This short review focuses on recent cases of successful formation of conformal inorganic thin films on 3D nanostructured polymer templates, using thermal or plasma-enhanced ALD (PEALD) at process temperatures below 200 \({}^{\circ}\)C (Figure 1a-c). We also discuss the material properties, applications, and prospects of 3D nanostructured inorganic or composite materials through this strategy.