Effects of sputtering parameters and separator plates on the structure and stress of W/Si multilayers in x-ray telescope applications

Abstract

To develop an x-ray focusing telescope for the Chinese x-ray timing and polarization mission, W/Si multilayers were fabricated using a dedicated cylindrical deposition facility with direct current magnetron sputtering technique. To reduce the interface width of the multilayers, separator plates were mounted at both sides of the substrates to collimate the sputtering particles. The average interface width was reduced from 1.03 nm (without separator plates) to 0.29 nm (with separator plates) for a W/Si multilayer with a 3.8-nm d-spacing. To achieve a high reflectivity multilayer mirror with low stress, the effects of different background and working gas pressures on periodic multilayers during the deposition process were studied. The stress of the W/Si multilayer (d=3.8 nm) slightly increased from −294 to −358 MPa as the background pressure increased from 4×10−5 Pa to 8×10−4 Pa and the first-order Bragg peak reflectivity at 8.04 keV (λ=0.154 nm) decreased from 68% to 54%. As the working pressure increased from 0.4 to 0.8 Pa, the stress of the W/Si multilayer significantly changed from −306 to 100 MPa. However, the first-order reflectivity also decreased dramatically from 67% to 8%, and the average interface width increased from 0.29 to 1.2 nm.