Structural characterization of Au/Cr bilayer thin films using combined X-ray reflectivity and grazing incidence X-ray fluorescence measurements

Gold thin-film mirrors are widely used as X-ray reflective optics to manipulate X-rays especially in synchrotron beamlines. Application of a thin underlying buffer layer of chromium enhances the adhesion of gold film to the substrate material and provides long term structural stability. In the present work, Au/Cr bilayers deposited using the DC magnetron sputtering technique with different thickness values on a Si substrate are studied to better understand the correlation between micro-structure properties and residual stress. In order to obtain precise structural properties of Cr binding layer and gold layer, as well as in-between nanoscaled interfacial structure, nondestructive probing is carried out by simultaneously utilizing synchrotron-based combined X-ray reflectivity (XRR)-grazing incidence X-ray fluorescence (GIXRF) techniques. It has been clearly observed that when thickness of Cr layer is relatively small, a composite interlayer structure is formed at the Au/Cr boundary and at the Cr/Si interface boundary due to the interdiffusion of two layer mediums. However, when the thickness of the Cr layer is relatively larger (similar to 90 angstrom), emergence of such an interlayer medium does not take place. The grazing incidence X-ray diffraction (GIXRD) measurements have also been performed to investigate crystalline structure and residual stress of the gold layer medium. Our results show that relatively large residual stress exists in the Au layer if thickness of Cr layer is small. The surface roughness morphology of the top gold layer obtained from the atomic force microscopy measurements showed the partial agreement with the results obtained from the XRR-GIXRF technique.