Surface and sub-surface thermal oxidation of ruthenium thin films

For next generation Extreme UV photolithography, multilayer coatings may require protective capping layers against surface contamination. Ruthenium, as a low-oxidation metal, is often used as a reference material. The oxidation behaviour of Ru thin films has been studied using X-ray reflectometry (XRR) and Atomic force microscopy (AFM). XRR is sensitive to the in-depth electron density distribution and as such to the formation of ruthenium oxides. In-situ XRR during atmospheric oxidation reveals formation of a low density RuOX layer near the surface, followed by sub-surface formation of a high density RuOX layer. Surprisingly, the ratio between RuOX formation and Ru consumption is only 1.2, in contrast to the value of 2.3 reported in literature. AFM studies show that the “missing” RuOX is found in surface agglomerates that grow with increasing oxidation temperature and duration. These agglomerates appear around 200˚C, consistent with RuO2 nucleation and growth reported in literature.