X-ray photoelectron spectroscopy investigation of MgAl₂O₄thin films for humidity sensors

MgAl₂O₄thin films, to be studied as active elements for humidity sensors, were deposited on Si/SiO₂substrates by radio-frequency sputtering. This paper discusses the x-ray photoelectron spectroscopy (XPS) investigation of these films. XPS demonstrated that the thin films had a stoichiometry close to that of MgAl₂O₄. The evaluation of the modified Auger parameter α' for Al gave structural information about the order of the crystalline structure of the thin films. The combination of Ar⁺ion etching and XPS analysis showed the simultaneous presence of Mg, Al, and Si at the film-substrate interface. The thicknesses of the interfaces were calculated between 7 and 10 nm. The analysis of the binding energy (b.e.) values of the XPS peaks at different etching depths showed that O 1s and Si 2pb.e. values were characteristic of a silicate at the interface, whereas in the substrate they were typical of silica. This suggests a chemical interaction took place between film and substrate with the formation of a silicate layer at the interface, which may be the cause of the good adhesion of MgAl₂O₄films to silica, as observed by peel tests with Scotch tape.