Diffractive X-ray optical elements made by thin film coating techniques such as multilayer Laue lenses (MLL) and multilayer zone plates (MZP) are promising approaches to achieve resolutions in hard X-ray microscopy applications of less than 10 nm. The challenge is to make a lens with a large numerical aperture on the one hand and a decent working distance on the other hand. One of the limiting factors with the coated structures is the internal stress in the films, which can lead to significant bending of the substrate and various types of unwanted diffraction effects. Several approaches have been discussed to overcome this challenge. One of these is a three-material combination such as Mo/MoSi2/Si, where four single layers per period are deposited. Mo and Si represent the absorber and spacer in this case while MoSi2 forms a diffusion barrier; in addition the thicknesses of absorber and spacer are chosen to minimize residual stress of the overall coating. Here the diffraction efficiency as well as the profile of the beam in the focal plane are discussed in order to find a tradeoff between lowest residual stress and best diffraction properties.

The application of thin film coating processes for the fabrication of diffractive X-ray optical elements like sputteredsliced zone plates or multilayer Laue lenses (MLL) is a very promising approach for X-ray focusing down to spot sizes of < 10 nm. However, for practical useful focal length in the order of several millimeters or a few centimeters, multilayer thicknesses of several 10 μm up to a few 100 μm are necessary in order to have large enough numerical apertures of the lenses. Currently one of the main challenges is to coat low-stress multilayers with large total thicknesses in the order of 100 μm. Usually sputter deposition results in thin films with significant compressive stress. With new material combinations such as Mo/MoSi2/Si/MoSi2 and W/WSi2/Si/WSi2 the overall stress can be reduced to almost zero if the individual thicknesses are properly adapted. In the case of these four-layer-systems only the period thickness dp follows the zone plate law. In case of Mo/MoSi2/Si/MoSi2, stress-free multilayers are obtained with dMo = 0.5*dp, dMoSi2 = 0.16*dp and dSi = 0.34*dp.

Most of the currently used reflective coatings for EUV and X-ray mirrors are periodic nanometer multilayers. Depending on the number of periods and the absorption in the multilayer stack a certain band width of the incoming radiation can be reflected. In order to increase the integral reflectance or to accept larger ranges of incidence angles, non-periodic multilayers are needed. With the transition from periodic to non-periodic multilayers new challenges arise for the deposition process. Since the reflectance spectra are sensitive to every single layer thickness a precise coating control and an exact knowledge of the interface reactions are required. Furthermore substrate roughness influences the reflectance spectra. With an advanced coating process using additional ion bombardment during thin film growth the integrated reflectance of broadband mirrors can be conserved even for an initial substrate roughness of about 0.7 nm rms.

The topic of this paper is the fabrication and characterization of EUV reflective coatings based on molybdenum/silicon (Mo/Si) multilayers. For the fabrication of such nanometer structures, the technologies of magnetron sputter deposition (MSD) and ion beam sputter deposition (IBSD) are used in IWS Dresden. The main challenges for extreme ultraviolet (EUV) optics are high reflectance, precise thickness profiles, low internal stress and long-term stability. Reflectances > 70 %, uniformities > 99.9 % and overall internal stresses < 20 NTa have been reached. In addition to sophisticated deposition technologies, precise metrology tools are mandatory for the characterization of the coatings. Together with a number of partners, IWS Dresden has developed a stand-alone EUV reflectometer that makes it possible to measure EUV reflectance R and peak position lambda on substrates with diameters of up to 500 mm. Current improvements of the reflectometer resulted in differences compared to calibrated measurements at PTB/BESSY of Delta R -0.1-0.6 % and Delta lambda(50) = +4...+9 pm.

This paper deals with multilayer monochromators for synchrotron beamlines that are produced by magnetron and ion beam sputter deposition (MSD and IBSD). Different material combinations (W/B4C, W/Si, Mo/B4C, MO/Si) with period thicknesses between I rim and 10 nm have been fabricated and measured at the synchrotron BESSY II (optics and KMC1 beamlines). The main challenge for the deposition of nanometer multilayers is to find growth conditions where the interfaces between adjacent layers are abrupt (no interdiffiasion sigma(d)) and smooth (no roughness sigma(r)). The interface width sigma (sigma(2) = sigma(2)(d) + sigma(2)(r)) becomes increasingly important for smaller period thicknesses. One decisive point for the interface formation is the kinetic energy distribution of the particles arriving on the substrate surface. In MSD, the sputter gas pressure is the main parameter for influencing the kinetic energy of the particles. In IBSD, an assist ion beam source can be used to bombard the growing film with inert gas atoms of a specific energy. Using this option, the best compromise between low interdiffusion and low roughness can be found for every material combination. Investigations of the reflection of W/B4C multilayers with period thicknesses of 1.2 nm and number of periods N = 50, 300 and 600 show that no roughness increase occurs with increasing N. Typical values for the interface widths are sigma = 0.27...0.28 nm.

In this paper we will give an overview of the preparation, characterization and typical results of nm-multilayer coatings prepared by pulsed laser deposition (PLD) and magnetron sputter deposition (MSD). We have achieved particularly outstanding results for the Ni/C and Mo/Si material systems, e.g. extreme ultraviolet (EUV) reflectances of >71% with Mo/Si. The thickness deviations of the multilayer period on flat and curved substrates with macroscopic dimensions (150mm diameter) are reduced down to <0.05%. The standard deviation of the thicknesses from run-to-run is <0.2%.