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J. Eur. Opt. Society-Rapid Publ. 22, 29( 2026)
tool wear and chipping during diamond machining. Furthermore, when a copper electroplating layer is applied to the surface of OFC, it may enable the fabrication of optical components that combine extremely smooth optical surfaces with high thermal conductivity.
In our previous studies [ 23, 24 ], we demonstrated the successful fabricating of mirror arrays with high shape accuracy using an ultraprecision milling machine with a singlecrystal diamond tool. In the machining, a mirror surface was formed on a 1-mm-thick electroplated Cu layer on a block, allowing the mirror array to be machined into the thick copper layer. The reason for the use of electroplated Cu was that it is considered to cause less tool wear and chipping than electroless-plated NiP. Moreover, we expected that the grain size of electroplated Cu would likely be smaller than that of OFC, providing smooth surfaces without irregularities. In our previous studies, we focused on the shape accuracy of the electroplated Cu surface in ultraprecision milling, and we have not studied the roughness of the machined surface in detail.
For optical components, the surface roughness after machining is a critical factor because it directly affects optical performance. To the best of our knowledge, the surface characteristics of electroplated Cu after ultraprecision diamond machining have not yet been fully clarified [ 23 – 31 ]. Understanding these surface characteristics is important for the design and fabrication of high-performance optical components. Therefore, the objective of this study is to clarify the surface roughness characteristics of electroplated Cu after ultraprecision machining and to compare them with those of OFC and electroless-plated NiP.
To achieve this objective, the surfaces of electroplated Cu, OFC, and electroless-plated NiP were machined by ultraprecision turning with a single-crystal diamond tool, and the resulting surface roughnesses in areas of 140 lm 105 lm and1lm 1 lm were evaluated. Moreover, the microstructure of the machined surfaces was analyzed by electron backscatter diffraction( EBSD) and X-ray diffraction( XRD) to investigate the relationship between the microstructure and the surface topography. This study provides insight into the feasibility of electroplated Cu as an alternative material for ultraprecision optical fabrication.
2 Experimental methods
The electroplated Cu specimens and the OFC specimens were cylinders with a diameter of 20 mm and a height of 10 mm. The NiP-electroless-plated specimens were cylinders with a diameter of 20 mm and a height of 7 mm. Cu plating and NiP plating were carried out by Kyowa Sangyo Corporation, Japan. The thicknesses of the electroplated Cu and electroless-plated NiP were approximately 0.2 and 0.1 mm, respectively. The substrates for the electroplated Cu and the electroless-plated NiP were OFC and stainless-steel cylinders, respectively. Cu electroplating was carried out using a plating solution in which Cu sulfate served as the main constituent. Electroless-plated NiP used in this study had a phosphorus concentration of approximately 11 %.
The face of the specimens was turned with an ultraprecision turning machine with a single-crystal diamond tool having a nose radius of 0.5 mm. First, all specimens were roughly turned. After replacing the rough-turning tool with a new one, finish turning was performed using the same tool for all materials. The finishing process was performed sequentially on the OFC, electroplated Cu, and electroless-plated NiP so that electroless-plated NiP, which was considered most likely to cause tool damage, was machined last. The depth of cut was set to 5 and 2 lm in the rough and finishing processes, respectively. The tool feed rate was 2 mm / min, and the spindle speed was 1000 rpm for each process. At these tool feed rate and spindle speed, the distance between tool marks is calculated to be 2 lm.
After turning, the surface topographies of the machined surfaces were measured using a white-light interferometer( WLI)( Zygo, NewView 7300) and an atomic force microscope( AFM)( Shimazu, SPM-9700HT). In the measurement using the WLI, an area of 140 lm 105 lm was measured with a horizontal resolution of 220 nm. In the measurement using the AFM, an area of 1 lm 1 lm was measured with a horizontal resolution of 4 nm. Both measurements were performed at approximately the midpoint in the radial direction of the workpiece, 5 mm from the center. Moreover, the microstructure of the machined surfaces was analyzed by XRD( Rigaku, SmartLab) and EBSD( Oxford Instruments, Symmetry). XRD analysis was performed using the in-plane method. This method enables the investigation of the surface structure by limiting X-ray penetration through grazing incidence.
3 Results and discussion
3.1 Machined surface topography
Figure 1 shows the topographies of the machined surfaces measured using the WLI. Figures 1a – 1c correspond to the topographies of the OFC, electroplated Cu, and electroless-plated NiP surfaces, respectively. As shown in Figure 1a, the OFC surface shows steps, which probably result from crystal grains. In Figure 1a, the height of the step through the center of the measured area is approximately 5 nm, and measurements performed at several other locations also revealed step structures showing comparable overall morphology with minor variations in contour and height. The root-mean-square( RMS) roughness of the OFC surface was 1.78 nm. As shown in Figure 1b, the electroplated Cu surface shows no steps. The RMS roughness of the electroplated Cu surface was 0.87 nm, showing a smoother surface than the OFC surface. As shown in Figure 1c, the electroless-plated NiP surface also shows no step structures, but two hills can be observed with an amplitude of approximately 3 nm and an interval of approximately 90 lm. The RMS roughness of the electroless-plated NiP surface was 1.21 nm. Since similar hills were observed in measurements at several locations, it is considered that concentric waves with a pitch of approximately 90 lm were generated on the surface. Referring again to the electroplated Cu surface shown in Figure 1b, high regions can be observed at approximately 90 lm intervals,