Evaluation of Surface Topography and Microstructure of Electroplated Copper Ultraprecision-machined with a Single-crystal Diamond Tool

We investigated in detail the surface characteristics of ultraprecision-machined electroplated copper (Cu). Electroplated Cu, as well as electroless-plated nickel phosphorus (NiP) and oxygen-free copper (OFC) for comparison, was machined using an ultraprecision turning machine with a single-crystal diamond tool. The resulting surfaces were measured over an area of 140 µm × 105 µm by white-light interferometry, revealing that the electroplated Cu surface exhibited the lowest surface roughness of 0.83 nm root-mean-square (RMS) among the three materials. For a smaller area of 1 µm × 1 µm measured by atomic force microscopy, the electroplated Cu surface exhibited a sand-like texture with a surface roughness of 1.74 nm RMS. Power spectral density (PSD) analysis revealed that the electroplated Cu surface had a lower PSD than the OFC surface below a spatial frequency of 2 × 10⁴ mm⁻¹ and was comparable to the electroless-plated NiP surface below a spatial frequency of 1 × 10³ mm⁻¹. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) analyses indicated that the electroplated Cu was composed of fine crystalline microstructures, which was probably the reason for the sand-like texture. Overall, the electroplated Cu surface exhibited excellent smoothness and machinability, superior to the OFC surface and close to electroless-plated NiP surface, demonstrating its suitability for ultraprecision optical components.