Beryllium-Based Multilayer Structures
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D. G. STEARNS*, K. M. SKULINA*, M. WALL*, C. S. ALFORD, R. M. BIONTA , D. M. MAKOWIECKI*, E. M. GULLIKSON**, R. SOUFLI**, J. B. KORTRIGHT** AND J. H. UNDERWOOD" Lawrence Livermore National Laboratory, P.O. Box 808, Livermore CA 94551 **Center of X-ray Optics, Lawrence Berkeley Laboratory, Berkeley CA 94720
ABSTRACT Multilayer (ML) structures composed of Mo-Be, Ru-Be and Rh-Be with bilayer periods of 6 nm have been grown using dc magnetron sputter deposition. The ML microstructure has been characterized using x-ray diffraction and high-resolution transmission electron microscopy, and the normal incidence reflectivity has been measured at soft x-ray wavelengths. -
INTRODUCTION Multilayer (ML) coatings important in x-ray 3•optical applications • •1 are becoming increasingly •2 • such as extreme ultraviolet lithography', x-ray microscopy and astronomy3 . Normal incidence reflectivity as high as 66% at 13.4 nm has been achieved using Mo-Si ML coatings4 , and this material combination is generally considered to be optimal for x-ray wavelengths above the Si L-absorption edge at 12.4 nm. There are strong motivations for developing ML x-ray optics for operation at wavelengths shorter than 12.4 nm, where use of Mo-Si is no longer feasible.
For example, imaging at shorter wavelengths can provide an improvement in resolution. In addition, the x-ray penetration depth is larger at shorter wavelengths, which can allow the use of single-layer resists in lithography and thicker specimens in microscopy. In this paper, we present the first results on the structural characterization of berylliumbased ML coatings composed of Mo-Be, Ru-Be and Rh-Be, which have theoretical normal incidence reflectivities of near 80% at wavelengths above the Be K-edge at 11.2 nm. We find significant variation in the microstructure of ML films of the different material systems grown by sputter deposition. The best layering is obtained for Mo-Be, where the Mo layers are crystalline, the Be layers are amorphous and interdiffusion is limited to a ~ 1-nm-wide interlayer region. The Ru-Be and Rh-Be ML exhibit significant interdiffusion and compound formation. As reported previously5 , a near-normal incidence reflectivity of 68% at 11.4 nm is measured for the Mo-Be ML coating. GROWTH AND CHARACTERIZATION The ML coatings were deposited using dc magnetron sputtering in an Ar plasma discharge. The deposition system 5 consisted of two 2-inch-diameter circular targets mounted vertically for sputtering in a sideways orientation, and positioned a distance of 9 cm from the substrate surface. The substrates were mounted on a fixture that provided computer-controlled rotation 329 Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
between the two sources in a pendulum-like motion. Hence each layer of material was produced by passing the substrate twice in front of a source. In addition, the substrates were spun at a rate of 2 Hz throughout the deposition. The coatings were deposited on polished, single-crystal < 110 > Si wafers having a ~2 nm overcoa
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