**1. Introduction**

Ultrathin metal films are essential for numerous applications, especially in microelectronics [1], heterogeneous catalysis [2], soft X-ray optics, and sensing. Ruthenium, as a relatively low-cost noble metal, is an attractive material when high oxidation resistance is needed [3]. Smooth and high-density Ru thin films are a preferred solution, for example, as electrodes for dynamic random access memories (DRAM) [4–7], metal-oxide-semiconductor field-effect transistors (MOSFET) [8], metal-insulator-metal capacitors [9,10], and grazing-incidence soft X-ray mirrors [11]. The atomic layer deposition (ALD) technology enables pinhole-free and conformal films with sub-nanometer thickness control. Since conventional physical vapor deposition (PVD) technologies cannot realize conformal coatings on complex shaped substrates, ALD is being considered as a promising technology for optical coatings. High-efficiency metal wire polarizers for UV spectral range have already been realized based on frequency doubling technique with iridium (Ir) coatings by ALD [12,13]. Iridium ALD coating has been also applied for Fresnel zone plates for X-ray microscopes at 1 keV synchrotron radiation [14]. Although ALD of Ru processes has been widely investigated for their electronic properties [2,6–9,15–19], their optical properties have not been analyzed yet. The large interest for Ru ALD arises since it is considered as a favorable capacitor electrode in DRAM [3,4,6,18], as gates in metal oxide semiconductor transistors [5,8], or is applied as nucleation seed layers for copper interconnect formation [19]. As an optical coating, Ru is currently realized for capping layers [3,20,21] and grazing incidence mirrors [22–25] by magnetron sputtering (MS).

This article presents the optical and structural properties of Ru ALD in the soft X-ray and XUV (extreme UV) spectral range. Furthermore, the coating properties are compared with conventional sputtered Ru films and the potential of the ALD technology for optical applications is discussed.
