Speckle in coherent x-ray reflectivity from Si(111) wafers

Abstract

We report the observation of x-ray speckle in the reflected beam from Si(111) wafers illuminated at grazing incidence. An intense coherent 8-keV x-ray beam was prepared using a wiggler source and multilayer monochromator optics. We demonstrate that the speckle patterns are specific to the region of the sample that is illuminated. From the trade-off between surface sensitivity and signal as a function of perpendicular momentum transfer, we infer that the speckle is due to the surface morphology on a micrometer length scale. We document and explain the evolution of the speckle patterns from nearly specular at low ${q}_{z}$ to highly structured at larger ${q}_{z}.$