Abstract
An electroless Ru plating bath is prepared by mixing Ru precursor (K2RuCl5·xH2O), oxidizer (NaClO), stabilizer (NaOH), and reducing agent (NaNO2) simultaneously in deionized water at a molar ratio of 1:1:20:10. Instead of conventional direct reduction route, the RuCl52− experiences an oxidative-reductive sequence to form metallic Ru on an activated Si substrate. Spectra from ultraviolet-visible and X-ray absorption spectroscopy indicate that the RuCl52− is oxidized to form RuO4 initially, followed by a slight reduction becoming RuO42−. The RuO42− solution is relatively stable and is able to undergo further reduction to render metallic Ru via heterogeneous nucleation and growth. Images from scanning electron microscope demonstrate a solid film of 100 nm with scattered protrusions and cavities after 120 min plating time. Analysis from atomic force microscope determines its surface roughness of 7.8 nm. From X-ray diffraction patterns, the as-deposited film reveals an amorphous structure but turns crystalline after Ar annealing at 400°C for 2 h. Curve-fitting of Ru 3p3/2 signal from X-ray photoelectron spectroscopy suggests a film composition of 92.49 atom % Ru and 7.51 atom % RuO2. The electroless Ru plating bath exhibits impressive life time (137 h) and negligible homogeneous precipitation without involving surfactants and unnecessary chemical additives.
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