Limits to Ion Beam Etching of Mo/Si Multilayer Coatings,

Recently ion beam etching of electron beam deposited metal layers on C has resulted in higher x-ray reflection coefficients. In spite of the widely recognized importance of MoSi multilayer coatings for projection lithography, little work has been done to use the etching technique on multilayer mirrors made of this combination of materials. As MoSi multilayer mirrors of d-values greater than 12nm. have already been made with reflectivities very close to the theoretical maximum 2, our main interest is in multilayers with smaller d-values. We have investigated the effect of ion beam etching on the morphology and surface smoothness of Mo on top of Si layers and of Si on top of Mo layers. In this study we varied the species of the etching ions Ar and Kr and the ion energy from 200 up to 1500 eV. Increased x-ray reflectivities could be observed for every combination of ion species and energy.