## 2 Fabrication Strategy for Functional Materials with 3D Periodic Nanostructures
### Key Considerations for 3D Nanostructured Template-Assisted Electroplating
For example, a study to increase the contact area between the substrate and the 3D nanostructure through control of the surface roughness of the substrate, or to optimize the interfacial energy between the substrate and the 3D nanostructure through component control of the photoreactive polymer, may be considered as a follow-up study. Next, the complete filling of 3D nanostructured templates with inorganic materials without defects such as voids or seams is a very important issue as well in the electroplating step in relation to the quality and functionality of the resultant materials. When depositing a material by electroplating on a template with a nanoconfined structure rather than an open structure, this is more likely to result in non-uniform deposition that can cause voids or seams. One of the main reasons for non-uniformdeposition is the lack of interaction between the hydrophilic components of the electrolyte solution and the hydrophobic polymer template. There are several ways to improve the uniformity of the filling while reducing defects, one of which is by adding surfactants to the solution (Figure 5a) [87; 88; 89; 90]. For example, [PERSON] et al. reported complete filling of Cu in a deep etched Si hole pattern by adding poly(ethylene glycol) (PEG), bis-(3-sodiumsulofopropyl disulfide) (SPS), and Janus Green B (JGB) [51]. PEG, SPS, and JGB act as a suppressor, an accelerator, and a leveler, respectively, during electroplating. PEG and JGB contributed to reducing the plating rate at the edges and surfaces of the hole pattern, while SPS contributed to increasing the plating rate inside the hole. Through the optimization of the concentration of each surfactant, void-free dense filling was achieved by compensating for the plating rate that varies locally across the hole pattern. The current density is also an important variable that must be optimized for the defect-free filling of inorganics in the nanostructured templates. Essentially, as the current density increases, the plating rate or productivity increases. However, if the current density is too high, dendritic or tree-like structure growth is preferred, resulting in imperfect filling of the nanostructured templates. In addition, the current density tends to be maximized at the interface between the conductive part (seed layer) and the insulating part, so a thicker layer is generally formed at the edge of the substrate. This phenomenon is called as the edge effect. In the case of 3D nanostructured templates, this non-uniformity of the current density becomes more problematic since localized electrical interfaces are distributed over the entire area of the substrate and are continuously generated in volume as plating proceeds. To ensure the complete filling of the electroplated materials in the nanostructured templates, it is desirable to lower the current density to an optimum level for precisely controlling the plating rate. Such a strategy can be easily found in the case of the 3D packaging process, in which a metal should be completely filled in through silicon vias (TSVs) through electroplating for electrical interconnection [50]. When the high current density is applied, the outside of the hole is easily blocked with an electroplated film before even filling the inside of the hole. This is a highly fatal defect in the electrical interconnection that reduces the reliability of the device. On the other hand, if the current density is too low, the plating rate or productivity significantly decreases. The impurity level of the electroplated film may increase as well. Therefore, the current density should be maintained at an optimal level to ensure super-conformal deposition, which is closely related to the defect density of the nanostructured templates (Figure 5b). Figure 4: Schematic illustration of “window” process for electroplating inorganic materials into 3D porous templates. The waveform of the current applied to the electrochemical cell is also an important variable in determining the quality of the filling in template-assisted electroplating.