Super-smooth polishing on aspherical surfaces

This paper discusses super-smooth polishing technology researched and developed for excimer lasers, soft X-rays, and other short-wavelength light applications. Short-wavelength light requires surface quality and contour accuracy superior to traditional specifications, as well as free-form contour elements. For this reason, the final target for free-form contours of OE500 mm or more was set to 0.08 mu m 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 layer, which adapts well to various contours. For greater contour accuracy, we developed the CSSP (Canon Super-Smooth Polisher), which polishes OE500 mm optical elements, and has a unique structure incorporating an on-machine contour measurement device built onto a single base plate. The CSSP polishing process achieves a contour accuracy of 0.078 mu m PV and a surface roughness of 0.13 nm RMS on a OE500 mm fused silica toroidal mirror. We also achieved the targets in the case of both CaF 2 and CVD-SiC, materials widely used in short-wavelength light applications.