Aperiodic W/B4C multilayer systems for X-ray optics: Quantitative determination of layer thickness by HAADF-STEM and X-ray reflectivity

X-ray microanalysis of materials by synchrotron radiation has generated an increasing demand for advanced multilayer based X-ray optical elements that can be used in beam-lines as monochromators of small or large spectral bandwidth, or in components for shaping high-intensity X-ray beams with highest reflectivities. Modern thin film technologies allow the fabrication of appropriate multilayer systems with reproducible control of layer thicknesses and resulting excellent reflectivity properties that are well adjusted to the requirements. Aperiodic multilayer systems combine the advantage of large bandwidths and decent reflectivity values. Their development necessitates quantitative control of the processing and of the resulting microstructures on the nanometer scale. We have investigated aperiodic multilayer systems consisting of W/B4C bi-layers on silicon, with single layer thicknesses ranging from about 1 nm to 2 nm. The microstructure and the bi-layer and single layer thickness are characterized quantitatively on cross-sectional samples by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The thickness of bi-layers is determined by geometric phase analysis and that of single layers by evaluation of STEM intensity line profiles. Comparisons with nominal thickness values and with those derived from X-ray reflectivity scans show very good agreement.