High efficiency polishing of micro-structured NiP alloy using isotropic electrochemical etching for achieving sub-nanometer roughness

Electroless plated nickel phosphorus (NiP) alloy is an essential engineered material used in optical applications, particularly in the field of extreme ultraviolet (EUV) technology, where high reflectivity of short-wavelength light is required. However, it is still challenging to achieve highly efficient sub-nanometer polishing of NiP plating, especially for micro-structured NiP surface. This study introduces isotropic etching polishing (IEP) as a novel ultra-precision processing technique for NiP plating, which is a damage-free and quick metal polishing technology through the amalgamation of contiguous etching pits. IEP was performed for 4 mins, leading to the attainment of a sub-nanometer surface exhibiting a Sa roughness of 0.065 nm, which verified efficiency and feasibility of the method. The IEP of NiP plating under various applied voltages is categorized into three stages: the etching stage, the limited current plateau stage, and the gas evolution stage. Experimental results bear witness to the direct correlation between the material removal rate (MRR) and surface roughness of NiP plating, specifically in relation to the sulfuric acid content within various electrolyte ratios. The most efficacious electrolyte composition was found to be 5:100 (H2SO4:CH3OH). Furthermore, the technology achieved ultra-smooth and shape-preserving polishing with a surface roughness below 0.1 nm, as confirmed by the comparisons of both the grating microstructure and Fresnel lens before and after IEP. The findings presented in this study are highly valuable for comprehending the process development and viability of IEP for NiP plating.