One of the most important considerations in the design and
manufacture of multilayer mirrors is material compatibility. Ideally, the materials selected should not chemically react or interdiffuse, thereby avoiding the formation of second phases, thus enhancing the smoothness of the layer interfaces and the reflectivity of the optic. The transparency of the spacing layers can therefore be augmented by materials of low absorption. Initial findings with B4C layers indicate deposits that show smoother layering than those containing carbon, with the advantages of greater stability, lower absorption, and therefore greater reflectivity. The manufacture, microstructure, and reflectivity of W/B4C multilayers are discussed.