Thermal and stress studies of normal incidence Mo/B4C multilayers for a 6.7 nm wavelength

Wavelength, reflectance, and stress stability of Mo/B(4)C multilayers were studied as a function of postdeposition annealing up to 900 °C. These multilayers are of interest as normal incidence coatings for wavelengths above the boron K-absorption edge. Mo/B(4)C multilayers deposited at low sputtering pressure have high compressive stress. Zero stress can be achieved at 360 °C-370 °C, but annealing at <200 °C is sufficient to reduce stress by ∼40%. This stress relaxation is accompanied with a multilayer period expansion of ∼0.02 nm and a <0.5% decrease in normal incidence reflectivity. The multilayer period remains stable up to ∼600 °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 (Mo(x)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 °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.