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

Growth behavior of Ir metal formed by atomic layer deposition in the nanopores of anodic aluminum oxide

The conformal coating or surface modification in high aspect ratio nanostructures is a tough challenge using traditional physical/chemical vapor deposition, especially for metal deposition. In this work, the growth behavior of iridium (Ir) metal formed by atomic layer deposition (ALD) in anodic aluminum oxide (AAO) templates was explored deeply. It is found that the surface hydrophilicity is crucial for the nucleation of ALD Ir. An in situ ALD Al₂O₃ layer with an ultra-hydrophilic surface can greatly promote the nucleation of ALD Ir in AAO nanopores. The effect of the Ir precursor pulse time, diameter, and length of AAO nanopores on the infiltration depth of ALD Ir was investigated systematically. The results show that the infiltration depth of ALD Ir in AAO nanopores is in proportion to the pore diameter and the square root of the Ir precursor pulse time, which follows a diffusion-limited model. Furthermore, the Ir precursor pulse time to obtain conformal Ir coating throughout all the AAO channels is in proportion to the square of the aspect ratio of AAO templates. In addition, the conformal Ir deposition in AAO nanopores is also related to the Ir precursor purge time and the O₂ partial pressure. Insufficient Ir purge time could cause a CVD-like reaction, leading to the reduction of the infiltration depth in AAO. Higher O₂ partial pressure can facilitate Ir nucleation with more Ir precursor consumption at the entrance of nanopores, decreasing the infiltration depth in AAO nanopores, so appropriate O₂ partial pressure should be chosen for ALD Ir in high aspect ratio materials. Above all, our research is valuable for surface modification or coating of metal by ALD in high aspect ratio nanostructures for 3D microelectronics, nano-fabrication, catalysis and energy fields.

Iridium thin-film coatings for the BabyIAXO hybrid X-ray optic

Reflective coatings are an essential feature of X-ray telescopes. Their overall performance relies heavily on substrate compatibility and how well they conform to the optics assembly processes. We use X-ray reflectometry (XRR) to demonstrate the compatibility of shaping flat substrates coated with iridium, and show that specular and nonspecular reflectance before and after shaping is on par with traditional hot-slumped coated substrates. From 1.487 and 8.048keV measurements, we find that the substrates have rms roughness of 0.38nm and magnetron sputtered iridium deposits with rms surface roughness of 0.27-0.35nm. A hydrocarbon overlayer from atmospheric contamination is present with a thickness of 1.4-1.6nm and a density of 1.2-1.6g/cm³. Both the traditional hot slumped and the flat substrates undergoing post-coating shaping have a similar characteristic surface morphology and are equally well-suited for use with X-ray optics. Finally, we demonstrate by simulation the improved effective area achieved by using a low-Z overlayer, and illustrate the performance of a hybrid optic coated with optimized bilayers for a Primakoff axion spectrum emitted by the sun.

Influence of Substrate Materials on Nucleation and Properties of Iridium Thin Films Grown by ALD

Ultra-thin metallic films are widely applied in optics and microelectronics. However, their properties differ significantly from the bulk material and depend on the substrate material. The nucleation, film growth, and layer properties of atomic layer deposited (ALD) iridium thin films are evaluated on silicon wafers, BK7, fused silica, SiO2, TiO2, Ta2O5, Al2O3, HfO2, Ru, Cr, Mo, and graphite to understand the influence of various substrate materials. This comprehensive study was carried out using scanning electron and atomic force microscopy, X-ray reflectivity and diffraction, four-point probe resistivity and contact angle measurements, tape tests, and Auger electron spectroscopy. Within few ALD cycles, iridium islands occur on all substrates. Nevertheless, their size, shape, and distribution depend on the substrate. Ultra-thin (almost) closed Ir layers grow on a Ta2O5 seed layer after 100 cycles corresponding to about 5 nm film thickness. In contrast, the growth on Al2O3 and HfO2 is strongly inhibited. The iridium growth on silicon wafers is overall linear. On BK7, fused silica, SiO2, TiO2, Ta2O5, Ru, Cr, and graphite, three different growth regimes are distinguishable. The surface free energy of the substrates correlates with their iridium nucleation delay. Our work, therefore, demonstrates that substrates can significantly tailor the properties of ultra-thin films.