Abstract
Wavelength, reflectance, and stress stability of multilayers were studied as a function of postdeposition annealing up to . These multilayers are of interest as normal incidence coatings for wavelengths above the boron K-absorption edge. multilayers deposited at low sputtering pressure have high compressive stress. Zero stress can be achieved at , but annealing at is sufficient to reduce stress by . This stress relaxation is accompanied with a multilayer period expansion of and a decrease in normal incidence reflectivity. The multilayer period remains stable up to , while intrinsic stress changes from compressive to tensile. A four-layer model with amorphous molybdenum and boron carbide layers separated by amorphous layers of molybdenum borides () is presented. These interlayers are present already in the as-deposited state and continue to grow with increasing temperature. Their presence lowers the optical contrast and the achievable reflectivity. However, they also increase multilayer thermal stability. At temperatures , a noticeable decrease in reflectivity associated with the phase transition from amorphous to crystalline molybdenum boride is observed. This is accompanied with an increase in interface and surface roughness and a change in stress as a function of temperature.
© 2011 Optical Society of America
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