Ion beam etching of a flat silicon mirror surface: A study of the shape error evolution

An ion beam etching process has been developed to improve the flatness of an X-ray mirror surface. The basic principle consists of moving a mirror at variable and controlled speed under an ion beam whose erosion capability was thoroughly characterized beforehand. At every step of the polishing procedure, several metrology measurements are applied to assess both mirror figure and finish. Two flat-shaped mirrors were produced to demonstrate the capability of the approach. The residual slope error was measured to be of 70 nrad over a mirror length of 70 mm while the root mean roughness remained statistically unaffected.