4.1. Shape Evolution of DEE Process Conditions
A copper layer was deposited on the stainless steel surface through an electrodeposition process. A copper-deposited layer was formed under the conditions of 0.5 M of sulfuric acid, 0.5 M of copper sulfate mixed solution, an applied voltage of 2 V, and an electrodeposition time of 60 s. Shin et al. [
13] suggested laser beam irradiation conditions to minimize damage to the surface of stainless steel, the base material. In this study, the copper layer was removed under conditions that minimized the surface damage of stainless steel, which is the base material [
13]. The laser patterning conditions of the deposited layer in the DEE process are shown in
Table 2. In order to minimize damage to the base material, the copper layer was removed by performing laser patterning three times under the laser beam conditions in
Table 2.
Figure 3 shows the result of the actual patterning of the copper layer under the conditions of
Table 2 as an optical image. The dark areas are the copper layer, and the light areas are the surface of the stainless steel from which the copper layer has been removed.
The electrochemical etching of metals is basically isotropic etching. Therefore, in the DEE process, the etching shape changes depending on the etching step conditions.
Figure 4 shows the shape change of the cross-section in isotropic etching. As shown in
Figure 4, in electrochemical etching, the etch factor is determined by the side etch and etching depth. Since the electrochemical etching of the DEE process is based on the isotropic etching of the metal, in order to increase the overall etching depth in the DEE process, side etching should be minimized. In the electrochemical etching of metal, the limit of the physical etch factor is about 1.5 [
1,
2,
3]. If the etching time is excessive in the electrochemical etching step of the DEE process, it means that the etching proceeded even after the deposited layer protecting the sidewall was removed. If the side wall is not protected, the surface profile of the cross-section proceeds in a hemispherical shape, so the taper angle of the vertical wall in the etched final shape increases. The big taper angle of the vertical wall per cycle of the DEE process means that the precision of the overall machining shape is lowered after the DEE process is finished. Therefore, it is necessary to select an electrochemical etching depth per cycle of the DEE process that can minimize the taper angle. In the DEE process, the etch factor can be calculated by measuring the width before electrochemical etching as L
1, the width at the repeated cycles as L
2, and the total depth in the cycle as d.
Figure 5 shows the result of a square pattern performed by repeating the DEE process 10 times at an excessive etching time of 10 s.
Figure 5b is an enlarged picture of the white square dotted line in
Figure 5a. As shown in
Figure 5b, it can be seen that the taper angle is large in the vertical wall formed from the top surface to the bottom surface. This means that the more the DEE process was repeated under excessive etching conditions, the greater the taper angle was generated. In
Figure 6, it can be confirmed that a big taper angle occurred as a result of measuring the surface profile of the cross-section of line AA’ in
Figure 5a. This is because the side etches proceeded even after the copper layer was removed due to an excessive etching time in the etching step. If the DEE process is repeated under the condition of having an excessive etching time, a very big taper angle is generated, and the shape of the vertical wall becomes irregular. Therefore, in order to minimize irregular shape of the side wall in the DEE process, an appropriate etching time condition per cycle of the DEE process is required. Shin et al. confirmed that an excessive taper angle occurs when the etching time exceeds 10 s in selective electrochemical etching using a copper layer formed at 2 V with a 60 s condition [
13]. In the DEE process, in order to minimize the taper in one cycle, an etching time of fewer than 10 s should be selected.
Figure 7a is a picture of one cycle of the DEE process completed by performing electrochemical etching after laser patterning of the deposited layer.
Figure 7b–e is the results of performing the DEE process five times, 10 times, 15 times, and 20 times, respectively. Electrodeposition conditions are 2 V and 60 s, and laser patterning conditions are shown in
Table 2.
Figure 7c is an optical image when the number of repeated cycles in the DEE process is 10 times, and the etching depth at this time is about 37 μm.
Figure 7e is an optical image when the number of repeated cycles in the DEE process is 20 times, and the etching depth at this time is about 81.6 μm. As shown in
Figure 8, as the number of repeated cycles increases in the DEE process, the etching depth increases. As shown in
Figure 8, as the number of repeated cycles increases up to five, it can be seen that the average depth of 20 μm per cycle has been processed. This is for the experiment repeated 20 times and can be said to show reproducibility with an average etching depth of 4.08 μm per cycle.
4.2. Deep Etching by the DEE process
Figure 9a is a graph showing etching depth according to repeated cycles of the DEE process. As the number of repeated cycles in the DEE process increases, it can be seen that the etching depth increases almost linearly. The etching depth per cycle in the DEE process is about 4 μm.
Figure 9b is a graph showing the width L
2 according to repeated cycles of the DEE process. The width L
2 increases significantly until the number of repeated cycles in the DEE process cycle is five, but the increase in width L
2 is slightly slowed in the section from five to 10 repeated cycles in DEE. In addition, it can be seen that the increase in width L
2 is remarkably reduced from the section where the number of repeated cycles in the DEE is 10 times or more. This is because the etching depth is not sufficient to create vertical walls when the number of repeated cycles in the DEE is less than 10 times, so the undercut phenomenon appears larger than in other repeated cycle sections. However, after 10 or more DEE process cycles, the increase in L
2 reduces because the copper layer protects the generated vertical wall. This is because the copper deposited layer serves as a protective layer to minimize side etch. When the number of repeated cycles in the DEE process cycle is more than 10, the side etch decreases, and the etching depth continuously increases. Due to these factors, the etch factor increases as the number of repeated cycles in the DEE process increases. As a result, the increase of the etch factor in the DEE process means that deep etching is possible as in the DRIE process. Based on the data in
Figure 9, the average etching depth and standard deviation per cycle were calculated. The average etching depth per cycle from one to five cycles was 3.6 μm, the average etching depth per cycle from five to 10 was 3.8 μm, the average etching depth per cycle from 11 to 15 was 4 μm, and the average etching depth per cycle from 16 to 20 cycles was 4.92 μm. At this time, it is confirmed that the standard deviation of the average etching depth per cycle in each of the four sections was 0.5, showing high reproducibility.
Figure 10 shows the microsquare groove structure fabricated through the DEE process. This microsquare groove structure was obtained by repeating the DEE process cycle 56 times. As a result of measuring the surface profile, it was confirmed that a structural pattern with an etching depth of 220 μm and pitch spacing of 200 μm was successfully fabricated. In addition, the average etching depth per cycle was measured as 3.92 μm. By performing the DEE process on a one-hundred-μm-thick stainless steel specimen, a through-rectangular pattern was successfully fabricated, as shown in
Figure 11. Perforated areas from some bottom surfaces were first observed when the number of repeated cycles of the DEE process exceeded 20. As a result of repeating the DEE process cycle up to 27 times in order to reduce the remaining nonpenetrating portion, it can be seen that the remaining nonpenetrated area was reduced, as shown in
Figure 11. These results mean not only the blind shape but also the through shape pattern could be implemented through the DEE process.