Chemical etch resistance of sputter-deposited metal oxide thin film coatings in atomic hydrogen at 700 °C

We report a systematic experimental study of the resistance of thin film metal oxides (MO_x) to chemical reduction by atomic hydrogen (H*) at 700 °C. Thin films of Y₂O₃, HfO₂, ZrO₂, TiO₂, Nb₂O₅ and Al₂O₃ are selected based on their relevance to various coating applications in, for instance, integrated circuits and extreme ultraviolet lithography (EUVL) scanners. In the study 15–20 nm thick MO_x thin films were thermally annealed at 900 °C and thus stabilized before exposure to H* at 700 °C. Comprehensive characterization using X-ray photoelectron spectroscopy, X-ray reflectivity, X-ray diffraction, atomic force microscopy, and in-situ ellipsometry revealed two distinct categories of behavior, which are combined with equilibrium thermodynamics assessments. Upon exposure to H*, Y₂O₃, HfO₂, ZrO₂, and Al₂O₃ exhibited high resistance to reduction, with minor to no morphological, structural or compositional changes, consistent with thermodynamic predictions. In contrast, TiO₂ and Nb₂O₅ underwent phase transformation and reduction to lower oxidation states. At present systematic reports in the literature of MO_x interaction and their reducibility by H* are scarce. We here provide insight into general trends of MO_x stability that is relevant for the need of chemically stable coatings in reactive environments like H*, but also low-ion energy H plasmas that reside in EUVL scanners.