STRUCTURAL CHARACTERIZATION OF MULTILAYERED NANOSTRUCTURES BY X-RAY DIFFRACTION AND SCANNING PROBE MICROSCOPY
K. Temst, M.J. Van Bael, V.V. Moshchalkov, C. Van Haesendonck, Y. Bruynseraede
Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium
Advances in thin film deposition techniques have opened the way to the production of artificial multilayer structures, tailored to the needs of the experimenter. Multilayers and superlattices have provided a wealth of new physical phenomena, mostly due to the fact that the artificially introduced multilayer periodicity can be matched to a particular relevant physical length scale, e.g. the magnetic interaction length or the superconducting coherence length. New optical, structural, electric, magnetic, and superconducting properties have been discovered and gradually multilayers have been applied in micro-electronics and neutron and x-ray optics. These properties are strongly influenced by the degree of structural perfection of the multilayers. Their quality is degraded by layer thickness fluctuations, interface roughness, and interdiffusion [1]. Reduction of interface roughness, for instance, has been proven indispensable for the imaging propertiesof multilayer x-ray mirrors. Interface roughness is not always an adversary, however, as it has been shown to enhance the magnitude of the giant magnetoresistance effect in magnetic mutilayers.
Interface roughness in multilayers has most often been studied by X-ray reflectivity and diffraction (XRD). Roughness is always linked to a certain lateral sampling length measured in the interface or surface plane. A problem associated with XRD experiments is the fact that this length scale is not known a priori. Whereas XRD yields an average roughness over all multilayer interfaces, atomic force microscopy (AFM) will provide information on the outer surface of the multilayer. AFM also offers the advantage of producing a direct mapping of the roughness as a function of the lateral length scale by changing the scan size in the experiment.
The aim of this work is to correlate the structural roughnesses as determined by XRD and AFM in order to study in detail the multilayer quality and to establish the lateral length scale involved in x-ray measurements. This will be illustrated by experimental XRD and AFM results obtained on Nb/Cu superlattices [2], Pb/Ge superlattices in which a regular array of submicrometer holes ('antidots') has been defined [3], and submicron Co dots. The x-ray diffraction patterns were analysed using the SUPREX model calculation [1]. This one-dimensional kinematical model of the multilayer structure takes into account the roughness due to variations in the number of monolayers within a layer, fluctuations of the interface distance, and disorder within the layers. The roughness values obtained from the analysis of the x-ray diffraction patterns will be compared to the rms roughness values measured directly with the AFM. It will also be shown how x-ray diffraction provides information on the lateral periodicity of a lattice of antidots in Pb/Ge multilayers, as well as on the regularity of arrays of submicron Co dots. In the latter case the periodicity and geometry of these lateral structures are revealed in reciprocal space mappings.
Work supported by the Fund for Scientific Research
Flanders (FWO), the Belgian IUAP and the Flemish GOA programmes.
K.T., M.J.V.B., and C.V.H. are Post-Doctoral Research Fellow,
Research Fellow, and Scientific Director, respectively, of the
FWO.