## 3 Principle of DEE Process  
DEE is a process that repeats the electrodeposition, deposited layer patterning, and electrochemical etching steps in one cycle. Figure 2 is a diagram showing the DEE process. In the first step, electrodeposition, a deposited layer is created on the surface of a metal workpiece, as shown in Figure 2a. In this case, the deposited layer must be formed to have a uniform thickness while serving as a protective layer to enable uniform patterning of the deposited layer in the laser patterning step. In the second step, the deposited layer is patterned in the desired patterns using a laser beam, as shown in Figure 2b. At this time, the damage to the base metal surface by laser irradiation should be minimized. The third step is the electrochemical etching step in which the deposited layer and metal base material are simultaneously dissolved by the anodic dissolution, as shown in Figure 2c. During the electrochemical etching step, the anodic dissolution does not occur on the surface of the base material in the area covered by the deposited layer, and the anodic dissolution of the base material occurs only in the area where there is no deposited layer. The electrochemical etching step can be performed until the deposited layer is entirely dissolved. When the electrochemical etching step is finished, the difference in dissolved depth between the protected area of the workpiece by the deposited layer and the unprotected area of the  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Characteristics** & **Value** \\ \hline Laser source & ND:YAG \\ Mode of operation & Pulsed \\ Pulse duration & 100 ns \\ Average power & \(<\)20 W \\ Pulse repetition rate & \(<\)80 kHz \\ Moving speed of spot & 600 mm/s \\ Central emission wavelength & 1064 nm \\ Spot size & 50 \(\upmu\)m \\ Line spacing of beam spot & 10 \(\upmu\)m \\ \hline \hline \end{tabular}
\end{table}
Table 1: Optical characteristics of ytterbium pulsed fiber laser beam.  
Figure 1: Experimental setup for deep electrochemical etching (DEE).  
workpiece is within a few micrometers. When these three steps are performed, one cycle of the DEE process is completed.  
After the first cycle of the DEE process is completed, the cross-sectional shape of the metal specimen has a hemispherical profile. This is because isotropic etching proceeds in most metals. Therefore, if the electrodeposition step is performed in the second cycle of the DEE process, the deposited layer is formed along the hemispherical profile. In the laser patterning step, since the bottom surface of the deposited layer formed along the hemispherical shape is removed and the side deposited layer is left, the etching in the depth direction can be performed while minimizing the side etching in the electrochemical etching step.  
Even in the second cycle, if the deposited layer is removed, the electrochemical etching must be stopped because side etch can no longer be prevented. Therefore, if the cycle consisting of electrodeposition, deposited layer patterning, and electrochemical etching is repeatedly performed, deep etching can be performed while minimizing side etch. Similar to the DRIE process, the DEE process can adjust the processing precision of vertical walls by adjusting the etching depth per cycle. In addition, the DEE process can also implement through-shaped patterns with a high aspect ratio.