A method of reducing surface roughness of DUV reflectance coatings for a DUV mirror to improve the reflectance of the DUV mirror includes: forming the reflectance coating on a substrate, the reflectance coating including a film stack comprising multiple dielectric layers, including an uppermost layer. The method also includes adding to the uppermost layer a cap layer comprising SiO2 and having an upper surface with an initial RMS amount of surface roughness. The method further includes adding a sacrificial layer to the upper surface of the cap layer, wherein the sacrificial layer comprises SiO2. The method also includes etching the sacrificial layer down to the cap layer so that the upper surface of the cap layer has a final RMS amount of surface roughness that is less than the initial amount of surface roughness.