# A Novel Top-Down Fabrication Process for Vertically-Stacked Silicon-Nanowire Array
## 3 Fabrication Results  
For the fabrication of vertically-stacked silicon-nanowire arrays, it is necessary to define a structure with protrusion and support, as shown in Figure 3. Indeed, the difference between the protrusion and support widths determines the wet-oxidation process time, which, in turn, determines the dimensions of the silicon nanowires fabricated in the protrusion. In addition, if the width of the support is too narrow, or if the support does not form, then the thermal oxide layer will not grow uniformly during the wet-oxidation process. Therefore, the most critical step of the proposed fabrication method is the DRIE process, especially the isotropic-silicon etching step, which determines the scallop size. The experimental results of the DRIE process with the tuning of the isotropic-silicon etching time are shown in Figure 4 and summarized in Table 2. By adjusting the duration of the silicon etching, the size and depth of the scallop can be adjusted. A deep and wide scallop size can be obtained by a long silicon etching time. To find the optimal process conditions, the duration of the silicon-etching step is divided into five conditions: 5, 7, 10, 15, and 18 s. After the DRIE process, cross-sectional SEM imaging is performed to measure the protrusion width (\(W_{P}\)), vertical gap between protrusions (\(H_{P}\)) and the support width (\(W_{S}\)) for the isotropic silicon etching results. As a result, the etch rates for \(W_{P}\), \(H_{P}\) and \(W_{S}\) are measured to be 0.04, 0.12, and 0.07 \(\upmu\)m/s, respectively. Among the five process conditions, a suitable condition for the difference in width between the protrusion and the support is determined through experimentation. Indeed, a silicon-etching time of 15 s produces an adequate scallop size for the fabrication of the vertically-stacked silicon nanowires. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Coil** & **Platen** & **Pressure** & \multicolumn{3}{c}{**Gas Flow (sccm)**} & \multirow{2}{*}{**Time (s)**} \\  & **Power (W)** & **Power (W)** & **(mTorr)** & & **C\({}_{4}\)F\({}_{8}\)** & **SF\({}_{6}\)** & **Ar** & \\ \hline Polymer & \multirow{2}{*}{825} & \multirow{2}{*}{1} & \multirow{2}{*}{22} & \multirow{2}{*}{100} & \multirow{2}{*}{0.5} & \multirow{2}{*}{30} & \multirow{2}{*}{5} \\ deposition & & & & & & & \\ Polymer etch & 825 & 13 & 23 & 0.5 & 50 & 30 & 3 \\ Silicon etch & 825 & 13 & 23 & 0.5 & 100 & 30 & 5, 7, 10, 15, 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: DRIE process steps for process development. Figure 3: Fabrication result after DRIE, showing silicon protrusions and supports. In Figure 5, the fabrication results of the vertically-stacked silicon-nanowire arrays are presented. As is well known, for oxidation with lower oxide thickness, the rate of oxide growth is much faster than that predicted by the Deal-Grove model [24]. However, in the case of oxidation of a certain thickness, there is a region with almost linear characteristics. In this paper, the conditions that can \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Etching Time (s)** & **5** & **7** & **10** & **15** & **18** \\ \hline \hline \multicolumn{1}{c}{Protrusion width (\(W_{P}\)) (\(\mu\)m)} & 1.74 & 1.59 & 1.50 & 1.37 & 1.19 \\ \multicolumn{1}{c}{Vertical gap between} & \multicolumn{1}{c}{0.75} & 1.02 & 1.33 & 1.75 & 2.46 \\ \multicolumn{1}{c}{protrusions (\(H_{P}\)) (\(\mu\)m)} & \multicolumn{1}{c}{1.19} & 0.95 & 0.72 & 0.43 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measurement results after various time duration during the DRIE process. Figure 4: Experimental results of the DRIE process with various isotropic-silicon etching times. (**a**) Dimension parameters; (**b**) 5 s (typical Bosch process case); (**c**) 7 s; (**d**) 10 s; (**e**) 15 s; (**f**) 18 s.  
grow 500 nm oxide for the fabrication of silicon nanowires having a width of approximately 300 nm are conducted with a process time between 60 and 90 min.