Characterization of Ultrathin Multilayers for Applications in X-Ray Optics

Abstract

Ultrathin multilayers have unique physical properties which are of great practical importance. Our interest here is in layer systems suitable for X-ray optical devices, in particular for X-ray mirrors. Optimization of device performance depends upon such factors as the layer uniformity (thickness and composition), interface roughness, and structural irregularities. Techniques such as Rutherford backscattering, X-ray diffraction and Auger profile analysis provide valuable structural information but averaged over comparatively large specimen regions. High-resolution electron microscopy has been used in the present study to gain complementary insight into the local microstructure of various layer systems, based on combinations of Rh,W,C,B4C, grown by diode rf-sputtering. Samples suitable for electron microscopy were prepared in cross-section using standard grinding, dimpling and argon ion-milling techniques. Most observations were made with a JEM-4000EX HREM operated at 400kV, with selected area electron diffraction (SAED) to determine the average bilayer thickness by reference to the diffraction pattern of the Si (110) substrate. Optical diffractograms (ODMs) and digital processing were used for accurate measurement of the lattice spacings visible in the multilayers.