Ion Polishing of Metal Surfaces

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Abstract
Various combinations of ion polishing and vacuum annealing were used in an attempt to reduce the optical absorptance and to smooth the surfaces of polycrystalline metal mirrors for high power lasers. The samples included micromachined copper and conventionally polished copper and molybdenum mirrors. It was found that ion polishing and vacuum annealing resulted in a large decrease, 40 to 50%, in the absorptance at 10.6 pm for conventionally polished copper samples and a small decrease, 5 to 10%, in the absorptance of micromachined copper and conventionally polished molybdenum mirrors. After ion polishing and annealing, the absorptance of the conventionally polished copper mirrors was as low as that of the micromachined mirrors. Ion polishing proved beneficial in removing surface defects, such as machining marks, scratches, and embedded polishing grit, and oxide layers; however, for large amounts of material removed, differential sputtering of the various crystallites led to grain boundary delineation and surface roughening. Thus, there exists an optimum amount of material to be removed by ion polishing which results in both maximum removal of surface defects and minimum roughening of the surface. The optimum amounts were found to be: 2500 to 5000 A for micromachined copper, 5000 to 7000 A for conventionally polished copper, and less than 1500 A for molybdenum.

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