Stress and structure properties of X-ray W/Si multilayer under low temperature annealing

Abstract

The X-ray timing and polarization telescope proposed in China is for imaging spectroscopy in an energy range of 1-30 keV. To obtain the high energy spectrum response with a large effective area, W/Si multilayer mirrors each with a mirror thickness of only 0.3 mm are used. This makes the figure accuracy of the mirror and the distortion caused by the multilayer stress an important issue during the telescope development. W/Si multilayer mirror is an important component of X-ray telescope for astronomical observation. To reduce the effect of the multilayer stress and maintain a high reflectivity at the same time, the W/Si multilayers prepared by magnetron sputtering deposition are annealed at low temperatures of 150 ℃, 175 ℃ and 200 ℃, respectively, for 3 h. The stress of the multilayer is determined based on the surface figure measurements of each sample before and after annealing. The X-ray reflectance and layer structure of the multilayer are characterized by the grazing incidence X-ray reflectometry (GIXR) and the reflectance fitting curves. The first Bragg peak reflectivity of the as-deposited sample is 67% at 8.04 keV and the multilayer stress is around -260 MPa. After annealing at 150 ℃ for 3 h, the first Bragg peak reflectivity and the layer structure are almost the same as before annealing, while the stress reduces 27%. The fitting results display almost the same interface widths of the multilayer before and after annealing. As the temperature increases to 175 ℃, the first Bragg peak reflectivity drops by about 2%. The multilayer structure begins to deteriorate and the W/Si interface widths increase from 0.346 nm/0.351 nm to 0.356 nm/0.389 nm, according to the fitting results, while the stress reduces about 50%. After annealing at 200 ℃ for 3 h, the stress reduces 60% and the stress decreases down to about -86 MPa. However, the first Bragg peak reflectivity drops by 17%, and the layer structure undergoes significant change after annealing. The W/Si interface widths increase from 0.352 nm/0.364 nm to 0.364 nm/0.405 nm. The GIXR results also show that the d-spacing between the multilayers decreases after annealing, and a higher annealing temperature causes a larger decrease. The stress reduction should be mainly caused by the enhanced atomic diffusions at the interface and inside the layer structure during the annealing. The enlarged interface and the possible compound formation contribute to the decrease of X-ray reflectance and the layer compactness. These results provide important guidance for developing low-stress X-ray multilayer mirrors.