**Growth of Atomic Layer Deposited Ruthenium and Its Optical Properties at Short Wavelengths Using Ru(EtCp)2 and Oxygen**

**Robert Müller 1,2, Lilit Ghazaryan 1,2, Paul Schenk 1,2, Sabrina Wolleb 2, Vivek Beladiya 1, Felix Otto 3, Norbert Kaiser 2, Andreas Tünnermann 1,2, Torsten Fritz <sup>3</sup> and Adriana Szeghalmi 1,2,\***


Received: 18 September 2018; Accepted: 10 November 2018; Published: 20 November 2018 -

**Abstract:** High-density ruthenium (Ru) thin films were deposited using Ru(EtCp)2 (bis(ethylcyclopentadienyl)ruthenium) and oxygen by thermal atomic layer deposition (ALD) and compared to magnetron sputtered (MS) Ru coatings. The ALD Ru film growth and surface roughness show a significant temperature dependence. At temperatures below 200 ◦C, no deposition was observed on silicon and fused silica substrates. With increasing deposition temperature, the nucleation of Ru starts and leads eventually to fully closed, polycrystalline coatings. The formation of blisters starts at temperatures above 275 ◦C because of poor adhesion properties, which results in a high surface roughness. The optimum deposition temperature is 250 ◦C in our tool and leads to rather smooth film surfaces, with roughness values of approximately 3 nm. The ALD Ru thin films have similar morphology compared with MS coatings, e.g., hexagonal polycrystalline structure and high density. Discrepancies of the optical properties can be explained by the higher roughness of ALD films compared to MS coatings. To use ALD Ru for optical applications at short wavelengths (λ = 2–50 nm), further improvement of their film quality is required.

**Keywords:** atomic layer deposition; sputtering; ruthenium; thin film; optical properties; structural properties; soft X-ray; XUV
