J. Eur. Opt. Society-Rapid Publ. 2026, 22, 29 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2026030 Available online at: https:// jeos. edpsciences. org
EOSAM 2025 Guest editors: Omar El Gawhary, Stefan Witte, Ignacio Moreno
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
Evaluation of surface topography and microstructure of electroplated copper ultraprecision-machined with a single-crystal diamond tool
Hideo Takino 1, *
, Hidenori Aizawa 2, and Masahiko Kanaoka 3
1 Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan 2 Sendai Nikon Corporation, Natori, Miyagi 981-1221, Japan 3 JTEC Corporation, Ibaraki, Osaka 567-0086, Japan
Received 3 December 2025 / Accepted 14 March 2026
Abstract. 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 lm 105 lm 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 lm 1 lm 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 10 4 mm �1 and was comparable to the electroless-plated NiP surface below a spatial frequency of 1 10 3 mm �1. Electron backscatter diffraction and X-ray diffraction 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.
Keywords: copper.
Surface, Machining, Diamond turning, Electroplated Cu, Electroless-plated NiP, Oxygen-free
1 Introduction
In optical fabrication, an ultraprecision machine tool with a single-crystal diamond tool is effectively used to obtain smooth and highly accurate metal surfaces for optical components, such as those of mirrors and injection molds [ 1, 2 ]. Electroless-plated nickel phosphorus( NiP) is a widely used material for single-crystal diamond machining to achieve a smooth surface [ 3 – 14 ]. However, the major issues in machining electroless-plated NiP are tool wear and chipping [ 4, 5, 7 – 10, 13, 14 ].
To overcome this problem, alternative materials that can achieve smooth optical surfaces with lower tool wear are desirable. Because copper is softer than electroless-plated NiP, machining copper may help reduce the wear and chipping of the diamond tool. Oxygen-free copper( OFC)
* Corresponding author: takino. hideo @ it-chiba. ac. jp, takino. hideo @ gmail. com is one candidate material because it can be machined with a single-crystal diamond tool [ 15 – 22 ]. However, the machined surface of OFC often exhibits irregularities owing to crystal grains, resulting in an increase in surface roughness [ 15, 16, 18, 19, 21 ]. Although OFC has high thermal conductivity and is suitable for optical components subjected to high thermal loads when adequate cooling is applied, these surface irregularities can affect the surface quality when extremely smooth optical surfaces are required. Therefore, it is important to investigate alternative copper-based materials that can maintain the advantages of OFC while enabling the formation of smoother optical surfaces by ultraprecision diamond machining.
Electroplated copper( Cu) is therefore expected to be a promising alternative material for optical components. Because electroplated Cu generally has finer grains than OFC, it may enable the formation of smoother optical surfaces. In addition, electroplated Cu is also softer than electroless-plated NiP and is therefore expected to reduce
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