Investigation of boron carbide and iridium thin films, an enabling technology for future x-ray telescopes

Abstract

We present an experimental examination of iridium and boron carbide thin film coatings for the purpose of fabricating X-ray optics. We use a combination of X-ray reflectometry and X-ray photoelectron spectroscopy to model the structure, composition, density, thickness and micro-roughness of the thin films. We demonstrate in our analyses how the two characterization techniques are complementary and from this we derive that an overlayer originating from atmospheric contamination with a thickness between 1.0-1.6 nm is present on the surface. The magnetron sputtered iridium films are measured to have a density of 22.4 g/cm 3 . The boron carbide film exhibits a change in chemical composition in the top ∼2 nm of the film surface when exposed to the ambient atmosphere. The chemical reaction occurring on the surface is due to an incorporation of oxygen and hydrogen present in the ambient atmosphere. Lastly, we present a correlation between the absorption edges and the emission lines exhibited by the thin films in an energy range from 50-800 eV and the impact on the reflectivity performance due to contamination in thin films.

Compatibility of iridium thin films with the silicon pore optics technology for Athena

“…The XRR measurements were fitted in IMD 24 using a model with a silicon dioxide substrate and an iridium thin film layer. The material densities were assumed as the nominal values tabulated for the CXRO/LLNL determination of optical constants, as previous studies suggest that the iridium is deposited with the same density within < 0.1% 13 . For the 1.487 keV data, a hydrocarbon overlayer, modeled as a simple COH compound with a fixed density of 1.00 g/cm 3 , was included to account for a surface contamination layer.…”

“…As the optics development is scaling up for the industrialized mirror plate production and assembly [2][3][4] , the development of X-ray mirror coatings for Athena is progressing alongside, [5][6][7][8][9][10][11][12][13][14] with several aspects of material stability [14][15][16] and optimization of performance 17 being investigated. The thin film designs under study for the Athena X-ray mirrors at present include iridium single-layers, and bilayers of iridium with a low-Z overcoat of either boron carbide, silicon carbide, or carbon.…”

Design, performance, and utilization of the Low-Energy X-ray Reflectometer at DTU space

“…To compare with 1.487 keV measurements from another beamline, LEXR measurements were carried out on two samples previously measured at the Optics Beamline at BESSY II synchrotron 24 and reported in Ref. 25. Figure 21 shows the data from BESSY, overplotted with LEXR data, and the best-fit parameters are listed in Table 3.…”

The effect of deposition process parameters on thin film coatings for the Athena X-ray optics

“…The thin film coating development for the Athena mission is progressing rapidly. Materials, such as, iridium, chromium, boron carbide, silicon carbide and carbon are comprehensively investigated [4][5][6] . The thin film coating development strategy, we developed for the Athena mission, emphasises a de-risking of the thin film chemical stability, thin film reproducibility and thin film compatibility with the SPO process technology, while achieving an excellent throughput performance.…”