## 3 Results and Discussion  
### Effect of Parameters on Pore-Filling Percentage  
The experiment was carried out under different current densities (5 A/dm\({}^{2}\), 10 A/dm\({}^{2}\), 15 A/dm\({}^{2}\)) at 40 \({}^{\circ}\)C. Figure 3a shows the pore-filling results under 5 A/dm\({}^{2}\), nearly all pores had been filled and the pore-filling percentage had reached 97.5%. When current density was 10 A/dm\({}^{2}\), the pore-filling percentage was decreased to 61.8%, as shown in Figure 3b. When the current density was 15 A/dm\({}^{2}\), only a few pores were filled, and the pore-filling percentage decreased to 35.8% (Figure 3c). The curve of pore-filling percentage versus the current densities is displayed in Figure 3d. These results revealed that lower current density is beneficial for higher pore-filling percentages. High current density will bring a higher driving force, which increases the movement speed of copper ions and enlarge the uneven movement of copper ions, resulting in different growing rates in different pores [17; 18; 19]. A uniform growing rate of CNPs in different pores is required for high pore-filling percentages. The mechanism is shown in Figure 4a, some nano-pillars with high growing rates firstly reached the AAO film surface and then spread out onto the surrounding area. After the continuous copper bulks formed on the top surface, the pillar filling process in these pores under the copper bulks stopped, as indicated in Figure 4b. The blocking of pores and impeding of pillars were caused by the small copper bulks. The percentage of blocked pores (PBP) at high current density was counted by Equation (1) using Figure 4c:  
\[\mathrm{PBP=N_{b}/N_{a}=(S_{b}\times p)/(S_{a}\times p)=S_{b}/S_{a}} \tag{1}\]  
In which \(\mathrm{N_{b}}\) and \(\mathrm{N_{a}}\) are the number of blocked pores and the number of all pores, respectively. \(\mathrm{S_{b}}\) and \(\mathrm{S_{a}}\) are the area of AAO film covered by copper bulk and the whole area of AAO film, respectively. \(\mathrm{p}\) is the area percentage of pores. The area was counted by pixels in the Photoshop software. The percentage of blocked pores was 41.2% for the samples fabricated under 40 \({}^{\circ}\)C and the current density of 15 A/cm\({}^{2}\) at 4 h. With a lower current density, a uniform growth happened and the nano-pillar in each pore grew sufficiently, resulting in a high pore-filling percentage, as shown in Figure 4e-f.  
Figure 3: Top-view of copper nano-pillars (CNPs) filled in AAO interposer with different electric current densities with 8 h: (**a**) 5 A/dm\({}^{2}\); (**b**)10 A/dm\({}^{2}\); (**e**)15 A/dm\({}^{2}\) and (**d**) the relationship of the pore-filling percentage versus current density. To investigate the effect of temperature on the nano-pillar growth behavior, the current density is fixed at 10 A/dm\({}^{2}\). Figure 5 shows the AAO film surfaces after electrodeposition process under different deposition temperatures (20 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C). Figure 5a shows that a low pore-filling percentage of 40.2% was obtained at 20 \({}^{\circ}\)C. When the electrodeposition temperature increased up to 40 \({}^{\circ}\)C, more pores were filled, and the pore-filling reached 61.8%. Lastly, the pore-filling percentage reached up to 89.8% under 60 \({}^{\circ}\)C. The relationship between the pore-filling percentage and temperature is shown in Figure 5d. The pore-filling percentage increased with the electrodeposition temperature. Figure 4: **(a)** The schematic depiction of nano-pillars electrodeposition at high current density; **(b)** SEM images of side-view at high current density; **(c)** top-view of SEM images at high current density; **(d)** the schematic of nano-pillars electrodeposition with low current density; **(e)** side-view of SEM images at low current density; **(f)** top-view of SEM images top-view at low current density.