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ABSTRACT Tungsten-carbon (W/C) multilayer structures are used as X-ray mirrors and other optical elements. The optical properties of such elements are highly sensitive to changes in strain due to thermal processing. Sensitive curvature measurements were performed on 40A period W/C multilayer structures on Si substrates using a two beam laser reflection technique. A compressive stress of approximately 1530 MPa was measured in these sputtered multilayer films. Thermal annealing to 5000C in air and under vacuum resulted in very little strain relaxation in the multilayers but X-ray diffraction data show a slight increase of the multilayer period. Significant strain relaxation, though, was observed when a 400A W buffer layer was included. Thermal annealing of these samples to 400-500'C resulted in large strain relaxation due to the formation of ct-W crystals in the buffer layer. Moderate oxide formation on air annealed samples as measured by SIMS was shown not to be a dominant mechanism of strain relaxation. INTRODUCTION Thin multilayer films of alternating high and low optical density materials are being used as artificial Bragg diffractors in optical elements such as X-ray mirrors. W/C multilayers on polished Si substrates are commonly used for such X-ray mirrors. Macroscopic bending of these structures due to stress in the thin films may have a considerable effect on their optical properties. Stability of these mirrors under common thermal processing is therefore of considerable interest. Stress in sputtered films has been found to depend on sputtering conditions [1]. Sputtered tungsten films in particular have been observed with 500 MPa tensile to 1600 MPa compressive stress depending on substrate temperature [2]. In this study, we report measurements of the stress in sputtered W/C multilayers by a wafer curvature measurement technique. The relaxation of the as-grown strain upon thermal annealing to 400-500*C is also studied both with and without oxide formation. The effects of oxidation, crystal growth, and compound formation in multilayers and in various thick buffer layers are considered to explain the mechanism of strain relaxation.

EXPERIMENTAL Multilayer films were deposited by dc magnetron sputtering with 5 mTorr Ar sputtering gas. The substrates were not intentionally heated. The base pressure in the vacuum system was

3x10" 7 Torr. In most samples, a 400A buffer layer of W or C was first deposited on a 20 mil

thick 3 inch diameter (100) Si substrate. A multilayer consisting of 20 layers of 40A nominal period (20k C and 20A W) was then deposited on the buffer layer. The high quality of these multilayers has previously been exhibited by their high reflectivity and low interfacial roughness 215

Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society

[3, 4]. Annealing studies were carried out on samples that were cleaved from the same wafer. Curvature measurements were made by a two beam laser reflection technique [5] referred to as SSIOD (surface-stress-induced optical deflection.) The appar