Structure and stress studies of low temperature annealed W/Si multilayers for the X-ray telescope

Low stress W/Si multilayer mirrors are demanded in the hard X-ray telescopes to achieve the high angular resolution. To reduce the stress of the as-deposited multilayer and maintain a high reflectivity, two groups of low-temperature annealing experiments were performed on the periodic multilayers with a d-spacing of similar to 3.8 nm. The temperature-dependent experiments show that the 150 degrees C annealing can slightly increase the reflectivity while the stress reduced only by 24%. Higher temperature annealing induced a larger reduction of the stress and the multilayer reached an almost zero stress state at 250 degrees C. The stress relaxation was accompanied by a small drop of reflectivity of <= 5% and a period compaction of <0.02 nm. The time-dependent experiments indicate that most of the stress changes occurred within the first 10 minutes while a prolonged annealing is not useful. The X-ray scattering and transmission electron microscopy were further used to study the microstructure changes of the multilayers. It is found that the W/Si multilayer exhibits an amorphous structure before and after annealing, while an enhanced diffusion and intermixing is the main reason for the stress relaxation and structure changes.