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Influence of sputtering pressure on the nanostructure and the X-ray reflectivity of iridium coatings
(2017)
Reflective mirror coatings made of iridium are used in X-ray telescopes of the Chandra X-ray Observatory (CXO) launched in 1999 by the National Aeronautics and Space Administration (NASA) to investigate astronomical sources at photon energies below 10. keV. These coatings were produced in a DC magnetron sputtering process and have so far proven their suitability for space-based applications. We are considering in the present paper the processing of thin iridium films for lightweight telescopes using the radio frequency magnetron sputtering technique with an oblique angle deposition. The coating development presented here is focused on the influence of total sputtering pressure on film properties as well as on its impact on the mirror's performance. Characterisation methods such as X-ray diffractometry, X-ray reflectometry, atomic force microscopy and transmission electron microscopy have been used. Correlations between morphology, density, surface micro-roughness, crystal structure of the iridium layer and the expected reflectivity of the X-ray mirror are described and discussed.
Future investigations of astronomical X-ray sources require light weight telescope systems with large collecting areas and good angular resolution. The Wolter I type telescope design offers a suitable possibility for obtaining performant X-ray mirrors with high collecting areas. The technology based on replicated slumped glass optics using thin glasses thereby provides the opportunity to fulfil the light weight and mass production requirements. In NASA's telescope NuSTAR this technology has been proven as advantageous compared to previous systems. Coating thin glasses with iridium, gold or platinum enhances the reflectivity of X-ray mirrors.
Future space-based X-ray observatories need to be very lightweight for launcher mass constraints. Therefore they will
use a reduced mirror thickness, which results in the additional requirement of low coating stress to avoid deformation of
the initial precisely shaped mirror substrates. Due to their excellent reflection properties iridium coatings are sometimes
applied for grazing incidence mirrors in astronomical X-ray telescopes. At Aschaffenburg University of Applied
Sciences the coating of thin iridium films by an RF-magnetron sputtering technique is under development. The work is
embedded in collaborations with the Max-Planck-Institute for Extraterrestrial Physics in Germany, the Czech Technical
University in Prague, the Osservatorio Astronomico di Brera in Italy, the German Leibniz Institute for Solid State and
Materials Research in Dresden, and the French Institute Fresnel. Sputtering with different parameters leads to iridium
films with different properties. The current work is focused on the microstructure of the iridium coatings to study the
influence of the substrate and of the argon gas pressure on the thin film growing process. Correlations between coating
density, surface micro-roughness, the crystalline structure of the iridium layers, and the expected reflectivity of the X-ray
mirror as well as coating stress effects are presented and discussed. The final goal of the project is to integrate the
produced prototype mirrors into an X-ray telescope module. On a longer timescale measurements of the mirror modules
optical performance are planned at the X-ray test facility PANTER.
In the field of astronomical X-ray telescopes, different types of optics based on grazing incidence mirrors can be used. This contribution describes the special design of a lobster-eye optics in Schmidt's arrangement, which uses dual reflection to increase the collecting area. The individual mirrors of this wide-field telescope are made of at silicon wafers coated with reflecting iridium layers. This iridium coatings have some advantages compared to more common gold layers as is shown in corresponding simulations. The iridium coating process for the X-ray mirrors was developed within a cooperation of the Aschaffenburg University of Applied Sciences and the Czech Technical University in Prague. Different mirror parameters essential for a proper function of the X-ray optics, like the surface microroughness and the problematic of a good adhesion quality of the coatings were studied. After integration of the individual mirrors into the final lobster-eye optics and the corresponding space qualification testing it is planned to fly the telescope in a recently proposed NASA rocket experiment.