Evolution of interfacial structure of Co-based periodic multilayers upon annealing

The evolution upon annealing from room temperature to 600 degrees C of the microstructure of Co/Mo2C/Y and Co/Y/Mo2C multilayer stacks has been studied by transmission electron microscopy combining HAADF-STEM, EDX and HRTEM. Both kinds of samples show significant amount of intermixing even in the as-deposited state. The mixing further increases upon annealing. Most of the mixing occurs between the Co and the Mo elements, the Y layers staying comparatively preserved. After annealing at 600 degrees C a surprising sharpening of the interfaces is observed. It is ascribed to the formation of Co-Mo compounds. Combining these analyses with the results we have already published, we can explain in a consistent way the evolution of the x-ray reflectivity measurements of the samples upon annealing. Understanding the evolution the optical properties of such stacks designed to work in the soft x-ray range is essential for improving the optical performance of multilayer structures.