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Influence of Sputtering Pressure on the Microstructure and Layer Properties of Iridium Thin Films
(2018)
Iridium layers with low stress, high density, and low surface roughness find widespread use in different high-technology applications. This paper presents a study of the influence of the sputtering pressure on the properties of iridium thin films and of its effect on the substrate surface microstructure. We analysed the dependence of the microstructure, crystalline structure, electrical resistivity, and deposition rate on the sputtering pressure and surface defects of the substrate. For the latter, plasma etching of the substrate was performed for different processing times and its effect on the surface roughness of substrates and, subsequently, on the grown iridium films, was examined. The sputtering pressure and the substrate plasma etching time both had a strong influence on the microstructure and surface roughness. These microstructural changes are in good agreement with the tendency described in the Thornton Structure-Zone Model for different sputtering pressures and the microstructure phase map of Alvarez. The electrical resistivity, deposition rate, and crystalline structure were highly dependent on the sputtering pressure.
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.
X-ray astronomy uses space-based telescopes to overcome the disturbing absorption of the Earth´s atmosphere. The telescope mirrors are operating at grazing incidence angles and are coated with thin metal films of high-Z materials to get sufficient reflectivity for the high-energy radiation to be observed. In addition the optical payload needs to be light-weighted for launcher mass constrains. Within the project JEUMICO, an acronym for “Joint European Mirror Competence”, the Aschaffenburg University of Applied Sciences and the Czech Technical University in Prague started a collaboration to develop mirrors for X-ray telescopes. The X-ray telescopes currently developed within this Bavarian- Czech project are of Lobster eye type optical design. Corresponding mirror segments use substrates of flat silicon wafers which are coated with thin iridium films, as this material is promising high reflectivity in the X-ray range of interest. The deposition of the iridium films is based on a magnetron sputtering process. Sputtering with different parameters, especially by variation of the argon gas pressure, leads to iridium films with different properties. In addition to investigations of the uncoated mirror substrates the achieved surface roughness has been studied. Occasional delamination of the iridium films due to high stress levels is prevented by chromium sublayers. Thereby the sputtering parameters are optimized in the context of the expected reflectivity of the coated X-ray mirrors. In near future measurements of the assembled mirror modules optical performances are planned at an X-ray test facility.
Iridium-based coatings for mirrors of x-ray telescopes are studied. In particular, stress-induced deformation is characterized and shown to be compressive and equal to −1786 MPa. Two methods for stress compensation are then studied. One relies on the deposition of silica on the back surface of the substrate and a second one relies on the deposition of a chromium sublayer. Advantages and drawbacks of each of these techniques are presented.
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.