Effect of Techniques for Polishing Molybdenum Mirrors on their Optical Stability under Cleaning D2–N2 Plasma

The effect of cleaning with D 2 –(2–4) mol % N 2 glow discharge plasma on the reflectivity of Mo(111) single-crystal mirrors is studied. The surface of each mirror is prepared by mechanical polishing with diamond pastes or diamond powder. Polishing of the mirrors is accompanied by the formation of a layer of material up to 3 microns thick, different in structure from the bulk metal. The peculiarity of this layer is a gradual improvement in its structure further away from the surface of the mirror and transition from an amorphous layer to a layer with nanoscale crystallites, and then a gradual transformation into the structure of an unbroken single crystal. When polishing, the diamond abrasive is pressed into the surface layer of the mirror. During the plasma-cleaning process, carbon particles pressed into the surface layer of the mirror initiate the development of negative roughness (pits or depressions). At the same time, a 5–10-nm layer of molybdenum nitride is formed. In disordered layers of the mirrors, the probability of the origination and growth of blisters is reduced. Aluminum is chosen as the metal that changes the total reflection coefficient of Mo. The use of a D 2 –N 2 mixture to increase the rate of Al sputtering in the last stage of cleaning is replaced by plasma exposure in deuterium with ions, which initiates the dissociation of molybdenum nitride and the removal of nitrogen from the surface layer of the mirror. As a result of prolonged plasma exposure, the reflectivity of the mirrors polished with diamond paste increases to a steady-state value close to the reflection of the reference mirror.