**3. Results**

Atomic layer deposition of Ru has been reported using several metalorganic precursors. Hämäläinen et al. presented a review of the reported Ru ALD processes with various precursors [33]. They summarized growth rates, deposition temperatures and the evaporation temperature of different precursors relating to the corresponding co-reactants. Besides Ru(Cp)2 [6,34] and Ru(Thd)3 [33,35], Ru(EtCp)2 [5,10,34,36] is the most commonly used precursor. Its flexible deposition properties have led to the choice of Ru(EtCp)2 to start here the optical coating development. Besides thermal and plasma enhanced ALD, the precursor enables the deposition with a wide selection of co-reactants, e.g., air [6], O2 [26,34], ozone (O3) [10], ammonia (NH3) [19,34], and hydrogen (H2) [5]. The main by-products of the chosen process (Ru(EtCp)2 and O2) are water and carbon dioxide [37] (see Equation (1)) and thus it fulfills safety requirements. Furthermore, the liquid precursor Ru(EtCp)2 is readily available in adequate ALD bubblers for precursor delivery.
