J. Eur. Opt. Society-Rapid Publ. 22, 29( 2026) 295
Fig. 6. Results of EBSD analysis of the machined electroplated Cu surface.( a) Grain map and( b) grain size distribution histogram.
are similarly low. Thus, the electroplated Cu and electroless-plated NiP surfaces exhibited comparable smoothness and were smoother than the OFC surface. Furthermore, the electroplated Cu and electroless-plated NiP surfaces exhibited similar spatial frequency characteristics over the entire frequency range.
In the previous section, we described that the electroplated Cu and electroless-plated NiP surfaces exhibited hills at intervals of approximately 90 lm. Although the peaks corresponding to these hills exist, the peaks do not appear in the PSD curves because they must be located in the low-frequency region outside the plotted range. However, it is possible that the increase at the left end of the PSD curve of the electroless-plated NiP surface is related to the presence of these hills. Moreover, Figure 4 shows that each PSD curve exhibits a similar sharp peak at a spatial frequency of approximately 5.0 10 3 mm �1. This corresponds to a tool-mark interval of about 2 lm attributed to turning.
Figure 5 shows the PSD curves calculated from the surface profiles measured with the AFM. Similarly, the onedimensional( 1D) PSD curves obtained from the AFM measurements were calculated using in-house software. The software calculates the PSD for all cross-sectional profiles across the measured surface, and the analysis was performed over the full evaluation length of each profile. The PSD curves shown in the figure represent the average of these profiles and correspond to the surface data of the measured area shown in Figure 2. The PSD of the OFC surface was the highest among all materials at spatial frequencies below 1 10 4 mm �1, likely attributable to the presence of the tiny holes on the surface. For the OFC surface, the high PSD in the low-frequency region below 1 10 4 mm �1 is considered to result in the largest RMS value among the three materials. On the other hand, the PSD of the electroplated Cu surface was the highest among all materials at spatial frequencies above 2 10 4 mm �1, which is considered to result from the sand-like texture appearing on the surface.
Overall, from the PSD analysis results shown in Figures 4 and 5, it can be concluded that the PSD of the electroplated Cu surface is lower than that of the OFC surface at spatial frequencies below 2 10 4 mm �1. Furthermore, at spatial frequencies below 1 10 3 mm �1, thePSD of the electroplated Cu surface is as low as that of the electroless-plated NiP surface, indicating comparable surface quality.
3.3 Material microstructural analysis
The microstructure of the machined electroplated Cu was analyzed by EBSD. This analysis was carried out on the machined surfaces without post-treatments such as polishing and etching, because the machined electroplated Cu was smooth enough for analysis. Figure 6 shows the results of the EBSD analysis, in which( a) the grain map over an area of 6 lm 6 lm and( b) its grain size distribution histogram are shown. EBSD data were analyzed by defining misorientations exceeding 5 ° as grain boundaries. The measurements were conducted by scanning an electron beam at a pitch of 20 nm, and the lower detection limit of the EBSD instrument was 40 nm. Thus, fine grains with sizes ranging from 40 to 280 nm were detected on the electroplated Cu. The black areas in Figure 6a are considered to correspond to either micro-grains smaller than 40 nm or amorphous regions.
Next, the microstructure of the machined electroplated Cu surface was analyzed by XRD. The electroless-plated NiP surface was also analyzed as a reference for comparison; in general, the electroless-plated NiP with a phosphorus concentration of 11 % is amorphous [ 32, 33 ]. The XRD analysis was also carried out for the machined surfaces without post-treatments. Figures 7a and 7b show the XRD profiles of the electroless-plated NiP and electroplated Cu surfaces, respectively. In Figure 7a, the XRD profile of the electroless-plated NiP surface is broad, which indicates that the electroless-plated NiP is amorphous, as generally known. In contrast, the XRD profile of the electroplated Cu surface in Figure 7b isnotbroad. Instead, multiple peaks attributed to Cu were detected. This indicates that there was no amorphous region on the electroplated Cu surface, and the electroplated Cu is crystallized and exists in a polycrystalline state. Table 1 shows the peak fitting results for the major Cu-related reflections shown in