The interfacial reactions in the structures of Si/SiO2(500 nm)/Ru (65 nm) (up to 950 °C) and Si/SiO2(500 nm)/Ru (72 nm)/Al‐0.8 wt % Si (630 nm) (up to 700 °C), were studied by Rutherford backscattering spectrometry, Auger electron spectroscopy, x‐ray diffractometry, selected‐area electron diffraction, energy‐dispersive x‐ray spectroscopy, scanning electron microscopy, and transmission electron microscopy. A thin film of Ru was stable on a SiO2substrate up to 30 min in a vacuum anneal at 950 °C. In vacuum, a fast interfacial reaction between the Ru thin film and the Al‐Si overlayer started between 550 and 575 °C, resulting in the formation of a Si/SiO2/ RuAl2/(Ru4Al13)Al‐Si layer sequence. Similar to the reactions between Pt group metals and Al, the Ru/Al reaction is nonuniform. It is controlled by a nucleation mechanism and is accompanied by void formation. ©1995 American Institute of Physics.