Ion Etching of Thin W-Layers - Enhanced Reflectivity of W-C Multilayer Coatings

Thin layers of tungsten were deposited on carbon films by electron beam evaporation and subsequently ion etched by bombardment with Ar+ ions of 200 eV. We used an in-situ soft X-ray reflection monitor as a diagnostic tool and observed a consistently higher reflectivity for a given thickness of the W layer after ion etching than as-deposited. We ascribe the difference to a considerable decrease of the surface roughness induced by the ion bombardment process. This smoothening effect occurred mainly during etching of the first 1.5 nm. Since the interface roughness is an important parameter influencing the efficiency of X-ray reflecting optics, we applied ion etching to each tungsten layer in a W(1.3 nm)-C(1.5 nm) multilayer coating of 10 periods: We deposited tungsten layers having a thickness of 2.8 nm and subsequently removed about 1.5 nm using ion etching. For comparison we deposited an identical coating without ion etched tungsten layers. We report a relative increase of the reflectivity at the N Kalpha emission line by a factor 3 for the multilayer coating having ion etched tungsten layers.