Diffuse-x-ray-scattering measurements of roughness on ion-etched multilayer interfaces

The effect of ion etching on the interface morphologies of Mo/Si multilayers is investigated via measurements of small-angle diffuse scattering. A comparison is made between two electron-beam-deposited multilayers composed of ten Mo/Si double layers, one with each surface ion etched after deposition. It is observed that ion etching preferentially reduces the short-length-scale interfacial roughness. The non-ion-etched sample exhibits a nearly constant diffusely scattered intensity as a function of in-plane momentum transfer (${\mathit{q}}_{\mathit{x}}$) over the measurable ${\mathit{q}}_{\mathit{x}}$ range. Thus, roughness contributions from very small length scales are dominant. In contrast, the ion-etched sample produces an exponential decay in the diffuse intensity as a function of ${\mathit{q}}_{\mathit{x}}$, which is quantitatively inconsistent with predicted line shapes of existing models describing the effect of ion etching on surface morphology. The data is fit by a model that includes an algebraic decay of the real-space height-height correlations. Possible causes for the discrepancy between data and theory are discussed.