Super-smooth polishing on aspherical surfaces (II): achievement of a super-smooth polishing

This paper discusses super-smooth polishing technology for excimer lasers, soft X-rays, and other short-wavelength light applications. Short-wavelength light elements require surface quality and contour accuracy superior to traditional specifications, as well as free-form contours. For this reason, our target for free-form contours of a 500 mm diameter was set to 0.08 mm PV for contour accuracy, and 0.2 nm RMS for surface roughness. To improve surface quality we employed local pitch polishing, utilizing a flexible tool laminated with an elastic sheet, which adapts well to various contours. For greater contour accuracy, we developed the CSSP (Canon Super-Smooth Polisher), which polishes 500 mm diameter optical elements. The CSSP polishing process achieves a contour accuracy of 0.078 mm PV and a surface roughness of 0.13 nm RMS on a 500 mm diameter fused silica toroidal mirror. We also fabricated both CaF2 and CVD-SiC, materials widely used in short-wavelength light elements.