Reflective multilayer optical elements may be subject to elevated temperatures in instrumentation based on intense synchrotron light, free-electron laser, solar space missions, or laser plasma sources. Moreover, at-wavelength characterization, usually performed with synchrotron radiation, may impose a temperature rise of the sample by hundreds of degrees. Thus, if the optical element is temperature sensitive, the reflective performance may change during use or during characterization, influencing the measurement being carried out.
We have investigated the thermal stability of amorphous Cr/Sc multilayer X-ray mirrors has been investigated. The interdiffusion process has been studied quantitatively by monitoring the evolution of low-angle Bragg peak intensities in hard X-ray reflectivity and X-ray diffuse scattering measurements.
Cr/Sc multilayers with periods ranging from 0.8-6.4 nm, layer thickness ratios between 0.2-0.8 and number of periods from N=50 to N=300 have been deposited by unbalanced magnetron sputtering. The substrate was maintained at an ambient growth temperature (<50 ©C), and ion assistance was applied to produce smooth and sharp interfaces for maximum reflectivity performance.
Isothermal annealing was performed in the range 25-600 °C in situ a diffractometer. The temperature was increased in steps of 50 °C and kept for ~12 hours. At each temperature the intensity decreased exponentially with annealing time, in accordance with linear diffusion theory. Within the studied temperature range apparent activation energies of 0.55±0.1 eV for interdiffusion was obtained, and the calculated effective interdiffusion coefficient was as low as 10-22 m2/s. A non-linear diffusion behaviour was also observed in the beginning of the annealing experiments. This observation has impact for the description of interdiffusion at nearly atomically abrupt interfaces.
Thermal stability up to 350 °C was found, where the intensity of the first Bragg peak had decreased to approximately 75% of the initial value. After further annealing up to 600 °C the multilayer structure was completely vanished. The microstructural changes during the annealing process, as investigated using transmission electron microscopy and electron diffraction, is also presented. |