Novel MRF fluid for ultra-low roughness optical surfaces

Abstract

Over the past few years there have been an increasing number of applications calling for ultra-low roughness (ULR) surfaces. A critical demand has been driven by EUV optics, EUV photomasks, X-Ray, and high energy laser applications. Achieving ULR results on complex shapes like aspheres and X-Ray mirrors is extremely challenging with conventional polishing techniques. To achieve both tight figure and roughness specifications, substrates typically undergo iterative global and local polishing processes. Typically the local polishing process corrects the figure or flatness but cannot achieve the required surface roughness, whereas the global polishing process produces the required roughness but degrades the figure. Magnetorheological Finishing (MRF) is a local polishing technique based on a magnetically-sensitive fluid that removes material through a shearing mechanism with minimal normal load, thus removing sub-surface damage. The lowest surface roughness produced by current MRF is close to 3 A RMS. A new ULR MR fluid uses a nano-based cerium as the abrasive in a proprietary aqueous solution, the combination of which reliably produces under 1.5A RMS roughness on Fused Silica as measured by atomic force microscopy. In addition to the highly convergent figure correction achieved with MRF, we show results of our novel MR fluid achieving <1.5A RMS roughness on fused silica and other materials.