Soft x-ray multilayers produced by sputtering and molecular beam epitaxy (mbe): Substrate and interfacial roughness

Interfacial and substrate roughness can greatly reduce the performance of multilayer optical coatings for soft x-rays. Due to the short wavelength of soft x-ray light, multilayers with roughness on the order of a few Angstroms are required to achieve adequate performance. In the present work, roughness measurements have been made on uncoated silicon wafers and float glass using a WYKO TOPO-3D phase shifting interferometer. The silicon wafers are found to be slightly smoother than the float glass. The effects of different cleaning methods and of the deposition of silicon "buffer layers", on substrate roughness also have been examined. An acid cleaning method is presented that gives more consistent results than detergent cleaning. Healing of the roughness due to sputtered silicon buffer layers was not observed on the length scale probed by the WYKO. Sputtered multilayers have been characterized with both the WYKO interferometer and low-angle x-ray diffraction to yield information about the roughness of the top surface and of the interfaces of the multilayer. Simulations based on these measurements show the need for extremely smooth layers for certain mirrors. Preliminary results on film growth in our recently acquired Molecular Beam Epitaxy (MBE) system are also presented. Although a MBE acid cleaning method greatly increases the roughness of the substrates, Reflection High Energy Electron Diffraction (RHEED) shows this cleaning method is not necessary to achieve epitaxial growth.