**5. Conclusions**

Ruthenium thin films grown by ALD have been evaluated for optical applications. High-density Ru films have successfully been deposited on Si and fused silica substrates. Temperatures above 230 ◦C are required for a film formation using Ru(EtCp)2 and O2. Higher deposition temperatures above 275 ◦C have led to blisters that increased the surface roughness significantly and reduced the adhesion properties. With an optimized deposition temperature of 250 ◦C, we have explored the film growth and the resulting optical properties. The polycrystalline growth and the corresponding evolution of the surface roughness have led to major reflection losses at short wavelengths. Sputtered Ru coatings show similar density but are much smoother than ALD coatings. The ALD samples show a thin (<2 nm) RuO2 surface layer and 0.8 at % residual O in bulk material. Carbon impurities were not measurable. Further experimental development is required to leverage on the major benefit of ALD to realize conformal coatings on complex-shaped substrates towards coatings with high reflectance and stability for optical applications.

**Author Contributions:** Conceptualization, R.M., L.G., and A.S.; ALD Depositions, L.G.; SEM Characterization, S.W.; AFM Measurements, V.B., XRR and XUV Analysis, R.M.; XPS and AES Analysis, F.O. and T.F.; Writing–Original Draft Preparation, R.M. and P.S.; Writing–Review and Editing, P.S., L.G., T.F, and A.S.; Supervision, A.S.; Project Administration, A.S., N.K. and A.T.; Funding Acquisition, A.S., N.K. and A.T.

**Funding:** This research was funded by the Fraunhofer Society Attract Project (No. 066-601020), the Fraunhofer IOF, Center of Excellence in Photonics and the Deutsche Forschungsgemeinschaft (DFG) Emmy-Noether-Project (No. SZ235/1-1).

**Acknowledgments:** The authors thank David Kästner for the technical support and Philipp Naujok for helpful discussions. We also acknowledge constructive comments of reviewers to improve the article.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
