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

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 qz to highly structured at larger qz. copyright 1997 The American Physical Society