X-ray photoelectron spectroscopy investigation of MgAl2O4 thin films for humidity sensors

MgAl2O4 thin films, to be studied as active elements for humidity sensors, were deposited on Si/SiO2 substrates 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 ?' 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 2p b.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 MgAl2O4 films to silica, as observed by peel tests with Scotch tape.