Ion beam milling fabrication of a small off-axis ellipsoidal mirror, diffraction limited to 1 μm resolution at 14 nm

Abstract

One interest of extreme ultraviolet wavelengths is their potentially high optical resolution. Such optics require high precision, low roughness surfaces onto which will be deposed adequate multilayers giving near-normal incidence reflection. For a spatially resolved interferometer, we have manufactured and tested with a reasonable degree of confidence an 8×8 mm off-axis ellipsoidal mirror, below the diffraction limit for a 14 nm wavelength. The ion beam milling technique employed for the fabrication allows to preserve the low roughness of the initial spherical substrate, and although we only achieved a 0.4 nm root-mean-square roughness, better could easily be done. At these precisions, testing is as important and as difficult as the figuring itself. The resulting mirror combines high theoretical resolution (1 μm) over a large object field (∼1 mm in diameter).