**2. Experimental**

Ru thin film deposition was performed with an Oxford Instruments OpAL open load reactor tool. In this study, Ru(EtCp)2 (bis(ethylcyclopentadienyl)ruthenium, Strem Chemicals, Kehl, Germany) and O2 were used as metalorganic precursor and co-reactant, respectively. Ru(EtCp)2 as a liquid precursor has a relatively high vapor pressure of 0.24 mbar at 80 ◦C [26]. This temperature was applied to bubble the precursor with 150 sccm argon (Ar) gas flow into the ALD reactor. Under a working pressure of approximately 0.1 mbar, the coatings were fabricated in a thermal ALD process at substrate temperatures between 200 ◦C and 300 ◦C. Major reaction products during this ALD process are carbon dioxide (CO2) and water (H2O). The overall chemical reaction can be suggested as:

$$\text{Ru(EtCp)}\_2\text{ (g)} + \text{}^{37}/\_2\text{ O}\_2\text{ (g)} \rightarrow \text{Ru(s)} + 9\text{ H}\_2\text{O (g)} + 14\text{ CO}\_2\text{ (g)}\tag{1}$$

Growth rate experiments of the Ru films were performed at a deposition temperature of 250 ◦C. The ALD cycle consists of four repeated steps: Ru(EtCp)2 precursor pulse, precursor purge with Ar (150 sccm), co-reactant pulse with 50 sccm O2, and a final purge with 150 sccm Ar flow. The optimized time for each step was constant at 2 s, 4 s, 3 s, and 4 s, respectively. Furthermore, a plasma enhanced ALD (PEALD) process was tested. Therefore, O2 plasma was ignited with 100 W RF power at 100 sccm flow rate. The substrate was exposed for three seconds to the O2 plasma instead of the thermally activated O2 gas flow. Super-polished silicon (Si) wafers with a crystal orientation of (100) and amorphous fused silica (SiO2) were used as conventional substrates for optical coatings.

For comparison, Ru coatings were fabricated on the DC-magnetron sputtering system NESSY 3 [27]. In this industrial system, coatings were deposited in an Ar atmosphere on Si substrates with a working pressure of 10−<sup>3</sup> mbar and a source power of 500 W.

All samples produced were measured by grazing incidence X-ray reflectometry (XRR) with Cu-Kα radiation (λ = 0.154 nm) to characterize the coating properties. The XRR data were fitted with a simple single layer model (Ru on substrate, whereby the roughness of the coating is also considered) using the Leptos 7 software package (Bruker Corporation) [28]. The extracted simulation results provide information on the coating thickness, coating density, and surface roughness. The same measurement setup was used for X-ray diffraction experiments (XRD). The crystal sizes were estimated according to the Scherrer equation [29].

Furthermore, the surface was investigated with a Carl Zeiss Σigma scanning electron microscope (SEM, Carl Zeiss, Oberkochen, Germany) with a constant acceleration voltage of 10 kV and energy dispersive X-ray analysis (EDX) for chemical characterization. Surface roughness analysis was additionally carried out through atomic force microscopy (AFM, Dimension 3100 with Nanoscope IV controller, Digital Instruments, Santa Barbara, CA, USA) measurements.

The XUV reflectometry (XUVR) was carried out by the Physikalisch Technische Bundesanstalt (PTB, Bessy II, Beamline PTB-EUV, Berlin, Germany) [30] at a fixed grazing incidence angle of 10◦ varying the wavelength between 2 nm and 25 nm. The reflectivity curves were simulated with the IMD-software [31] using the optical constants of Henke et al. [32]. Similar to the XRR simulations, a single-layer model has initially been applied, but had to be extended by a thin RuO2 and C layer.

The surface composition was studied with X-ray photoelectron spectroscopy (XPS, XR 50 M X-ray source with FOCUS 500 monochromator, SPECS Surface Nano Analysis GmbH, Berlin, Germany) using an ultrahigh vacuum (UHV) surface analysis system. The photoelectrons were excited by monochromatic Al-Kα radiation (*E* = 1486.71 eV) under 55◦ angle of incidence and detected with a PHOIBOS 150 hemispherical electron analyzer (SPECS Surface Nano Analysis GmbH, Berlin, Germany).

Additionally, an Auger electron spectroscopy (AES, Varian Vacuum Division, Palo Alto, CA, USA) depth profile was performed with an Auger cylindrical mirror spectrometer. A focused 5 keV electron beam under an angle of incidence of 30◦ and a cylindrical mirror analyzer (CMA) were used. Sputtering was carried out with krypton (Kr) at an energy of 2 keV and a current of 10 μA.
