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

Wavelength, reflectance, and stress stability of Mo/B4C multilayers were studied as a function of post-deposition annealing up to 900 degrees C. These multilayers are of interest as normal incidence coatings for wavelengths above the boron K-absorption edge. Mo/B4C multilayers deposited at low sputtering pressure have high compressive stress. Zero stress can be achieved at 360 degrees C-370 degrees C, but annealing at < 200 degrees C is sufficient to reduce stress by similar to 40%. This stress relaxation is accompanied with a multilayer period expansion of similar to 0.02nm and a < 0.5% decrease in normal incidence reflectivity. The multilayer period remains stable up to similar to 600 degrees 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 (MoxBy) 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 degrees 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. (C) 2011 Optical Society of America