Glancing Angle Exafs Studies of Tungsten-Carbon Multilayers
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GLANCING ANGLE EXAFS STUDIES OF TUNGSTEN-CARBON MULTILAYERS G. M. LAMBLE, S. M. HEALD*, D. E. SAYERS,
AND E. ZIEGLER+
North Carolina State University, Raleigh, NC 27607 *Brookhaven National Laboratory, Department of Applied Science, Upton, NY 11973 +Materials Science and Technology Division, Argonne National Laboratory, Argonne, IL 60439.
ABSTRACT Results are presented from glancing angle EXAFS studies of three tungsten-carbon multilayer systems, of different W/C thickness ratios, by monitoring the fine structure above the tungsten L3-edge. The purpose of the investigations was to determine the structural changes occurring in the
multilayer
as a
result
of moderate
annealing.
Surprisingly,
dramatic
changes are observed in the structural environment of the W atoms as a 0 result of heating to temperatures as low as 350 C for 3 hours. It is found that, when the W layer is sufficiently thick, and the W/C layer thickness favorable, W2 C is preferentially formed and crystallization is extensive. The experiment demonstrates the suitablility of the EXAFS technique for the study of multilayer systems since, 1) contribution to the signal from the interface is significant and 2) the phenomenon is not dependent on the
existence of long range order. INTRODUCTION The fabrication of multilayered structures from pairs of materials with a large difference in refractive index has provided an extension of the low energy limit for monochromatized X-radiation beyond that attainable with natural crystals. The creation of these multilayers with a "tailored" d-spacing of sufficient quality for them to function as optical elements requires very stringent experimental procedure, there are additional problems associated with the behavior of the multilayer itself. A better understanding of these chemical and physical properties is required in order to optimize its performance. There is understandable concern about the thermal stability of W/C multilayers which undergo dramatic structural changes after heat treatments. In an x-ray diffraction experiment Takagi et al. [11 demonstrated that structural changes occurring, after annealing various multilayer sam0 ples to 730 C for several hours, depend largely on the thickness ratio of
In particular, when the tungsten layer is very thin (10-20 X) an anomalously large expansion of the d-spacing is observed at 730 0 C, and analysis of the resulting samples shows the components to be crystallized in some form or other. Some controversy exists as to the identification of the annealed products [1-3] and also as to whether the expansion is a result [4] or cause [21 of agglomeration of the tungsten atoms. Since while the multi-
W/C.
layer structure is destroyed at this temperature, changes in the d-spacing are observed long before this occurs. We present the results from glancing angle EXAFS studies of various W/C multilayer samples; monitoring the fine structure above the tungsten L3 edge. The experimental technique is a particularly useful probe of the interface region where substantial che
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