Thermally induced structural modification of Mo-Si multilayers

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Abstract
The effect of elevated temperature on the structural stability and performance of Mo-Si multilayer mirrors is investigated. Mo-Si multilayers deposited by magnetron sputtering are annealed at temperatures ranging from 200 to 800 °C. A detailed and consistent picture of the thermally induced changes in the microstructure is obtained using an array of complementary measurement techniques including small- and large-angle x-ray scattering and high-resolution electron microscopy. The first significant structural changes are observed at 400 °C, characterized by an increase in the width of the amorphous interlayer regions, as well as the nucleation of microcrystallites of silicide in these regions. At higher temperature the Mo layers transform completely into polycrystalline mixtures of Mo5 Si3 and MoSi2 in both the hexagonal and tetragonal phase. The layers of silicide remain intact but exhibit a structural instability, resulting in severely warped layers surrounded by pockets of amorphous Si and voids. By 800 °C the layered structure is completely destroyed and the composition is predominately tetragonal MoSi2 . The performance of the multilayers as normal-incidence x-ray mirrors is measured and correlated with the observed structural modifications. Finally, our results are compared and contrasted with other annealing studies of the Mo-Si system.