Scattering and Surface Finish of Unconventional Mirror Materials

In many systems applications, mirror materials are needed that have, in addition to a good optical surface, high thermal conductivity, light weight, a low thermal expansion coefficient, rigidity, toughness, or other combinations of properties not available in conventional glass or fused quartz optics. Metals, ceramics, semiconductors, and other materials have been considered for mirror substrates. The requirement of an extremely smooth, low scatter surface has spurred a large effort to learn how to polish several unconventional materials, and good results have been achieved in some cases. We will describe in detail the polishing of two of these materials, silicon and molybdenum, and will also mention others. Differential interference contrast (DIC) micrographs and surface profiles of selected materials will be shown, along with rms roughness values obtained from total integrated scattering (TIS) measurements and roughnesses measured directly with a surface profiling instrument.