Characterisation of Mo/Si multilayers with small angle reflectivity measurements.

We developed an optical system for soft x-ray projection lithography [1] with a laser plasma x-ray source. In the optical system Mo/Si multilayer mirrors are used for near normal incidence reflectivity. High reflectivity of each multilayer is mandatory because in the optical system four reflecting surfaces are used. A reliable tool for characterisation of multilayers is needed in the process of determining the optimum parameters for the production of high reflectivity multilayer mirrors. Therefore we developed software to analyse the hard x-ray reflectivity measurements. With this software, that is based on a fast Fourier transform (FFT) of the Bragg peaks, a distinction between interface-roughness and layer-thickness errors can be made. These two parameters have considerable influence on the near normal incidence reflectivity. Furthermore, reflectivity measurements are done at various wavelengths to find the material densities. These are also important parameters for the near normal incidence reflectivity. This article will restrict itself to the analysis Mo/Si multilayers deposited at 36°C and 215°C with or without ion etching. These multilayers are made for a wavelength of 13.5 nm at normal incidence.