Conformal polishing of NiP grating microstructures using SiO2/Al2O3 mixed abrasive slurry

Nickel-phosphorus (NiP) coatings are widely used for surface modification of metal mirrors due to their excellent physical and chemical properties, such as in the collector mirrors of extreme ultraviolet (EUV) lithography machines. Grating microstructures can be fabricated on NiP coatings using Single-Point Diamond Turning (SPDT) to enhance the optical performance of mirrors. However, the SPDT process often introduces periodic marks on the surface, increasing surface roughness, causing optical flux loss, and reducing reflectance, which negatively impact the performance of mirrors. In this study, SiO₂/Al₂O₃ mixed abrasive is employed for Chemical Mechanical Polishing (CMP) to investigate conformal polishing processes for NiP grating microstructures. The mixed abrasive leverages the advantages of both abrasives: Al₂O₃ abrasive effectively removes periodic marks and reduces the spatial frequency errors in the 10¹∼10² mm⁻¹ range, while SiO₂ abrasive further smooths the surface and reduces the spatial frequency errors in the 10²∼10³ mm⁻¹ range. Through experiments, factors such as Al₂O₃ particle size, Al₂O₃ abrasive concentration, pH, and H₂O₂ concentration are comprehensively considered to precisely control the synergistic effects of the mechanical removal and chemical reactions of CMP, balancing the optimization of surface quality and conformality of microstructures. By optimizing the polishing slurry and utilizing a combination of soft and hard polishing pads, a uniform RMS roughness of 0.8 nm (scanned area: 218 × 218 μm²) was achieved across a 100 mm diameter NiP grating microstructures sample. Importantly, the grating microstructures retained their shape, with only an 84 nm reduction in the grating height and a 12.98 μm increase in the transition area width after polishing. These results highlight the excellent conformal polishing capability of the process. This study provides a promising strategy for the conformal polishing of large-area and high-precision NiP grating microstructures on EUV collector mirrors, while also offering an important reference for the industrial-scale production of other optical microstructures.