Substrate surface quality effects on multilayer performance in synchrotron radiation premonochromator applications

Abstract

Because multilayer structures are relatively wideband devices and can be fabricated on various substrates, they have been proposed for use as premonochromators in synchrotron radiation beamlines as a means to mitigate the extremely high power densities found there. A question therefore arises regarding the optical quality that can be achieved with these devices. Since earlier work showed that multilayers deposited on Si(111) surfaces seriously degraded the energy resolution in an EXAFS beamline, we sought to distinguish between effects related to substrate quality and effects inherent in multilayer fabrication. In this work we equipped a special small-angle diffractometer with an exit beam monochromator so that we could directly model an EXAFS beamline. By comparing EXAFS spectra taken using a multilayer premonochromator to spectra taken using an undiffracted beam, we were able to sense degradations in energy resolution as small as 0.5 eV. We then compared the performance of multilayers simultaneously deposited on a variety of substrates to isolate substrate- and deposition-dependent effects. We thereby showed that for tungsten/carbon multilayers the substrate quality is the primary factor in optical quality, with Si(111) samples degrading resolution up to 40 eV, whereas Cervit samples flat to λ 10 with 1.5 Å surface roughness introduced no measurable distortion whatsoever.