Manufacture and characterization of soft X-ray mirrors by laser ablation

Abstract

The preparation of layered synthetic microstructures (LSM) of a nanometer thickness scale is one particular application of laser pulse vapour deposition (LPVD). Ultimate efforts have to be undertaken for providing a very high degree of reproductivity of individual layer parameters and vanishing roughness of interfaces throughout the entire layer stack when these heterostructures are intended for soft X-ray optical applications. The preparation technique involving materials ablation by CO 2-TEA-laser irradiation and subsequent particle condensation on high quality silicon substrates is described. The process proceeds in a rather clean vacuum environment (base pressures ≤ 5 × 10 -8 mbar) providing means for target motion and contamination-free exchange as well as for substrate motion. The growing layer thickness can be monitored in-situ by a quartz oscillator device. A complex program for the analytical characterization of layer stacks involving up to 80 individual layers of 1–5 nm thickness is explained by presenting results obtained form Ni/C specimens showing a distinguished X-ray optical quality. The morphological and microstructural data has been obtained by TEM cross section investigations, X-ray diffraction measurements and ellipsometry. Information about chemical composition of the LSM has been deduced from depth profiling measurements carried out by Auger electron spectroscopy (AES) and secondary neutral mass spectrocopy (SNMS). An ultimate depth resolution of the order of 1 nm has been achieved. X-ray diffraction characteristics computer simulation has been applied for a quantitative explanation of the measured results and thus for modelling the initial state of the layer stacks.