Wavelength, reflectance, and stress stability of $\mathrm{Mo}/{\mathrm{B}}_4\mathrm{C}$ multilayers were studied as a function of postdeposition annealing up to $900°\mathrm{C}$ . These multilayers are of interest as normal incidence coatings for wavelengths above the boron K-absorption edge. $\mathrm{Mo}/{\mathrm{B}}_4\mathrm{C}$ multilayers deposited at low sputtering pressure have high compressive stress. Zero stress can be achieved at $360°\mathrm{C}–370°\mathrm{C}$ , but annealing at $<200°\mathrm{C}$ is sufficient to reduce stress by $\sim 40\%$ . This stress relaxation is accompanied with a multilayer period expansion of $\sim 0.02\mathrm{nm}$ and a $<0.5\%$ decrease in normal incidence reflectivity. The multilayer period remains stable up to $\sim 600°\mathrm{C}$ , 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 ( ${\mathrm{Mo}}_x{\mathrm{B}}_y$ ) 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 $>600°\mathrm{C}$ , 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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