Homogenized ion milling over the whole area of EUV spherical multilayer mirrors for reflection phase error correction

For accurate reflection phase manipulation of spherical multilayer mirrors used in extreme ultraviolet (EUV) imaging, a wide-area ion beam with a homogenized radial distribution was produced for period-by-period ion milling. Measured variations of the milling depth with incident angle showed that Si and Mo have the same angular dependence within the effective aperture used in our imaging optics. By using the designed homogenizer mask, the ion-milling depth was successfully homogenized to within an error of ±1.9% over a 50-mm-wide concave mirror. Furthermore, a versatile homogenizer mask with adjustable opening angle plates was developed. With this mask, an ion-milling depth-profile homogeneity of ±1.7% was realized. Although a slight decrease in the peak EUV reflectance was measured as the incident angle decreased, the effectiveness and practicality of our correction method has been demonstrated.