Microstructure and Electrical Properties of MgAl₂O₄ Thin Films for Humidity Sensing

Active elements for humidity sensors based upon MgAl₂O₄ thin films or sintered pellets were investigated. Thin films were deposited on Si/SiO₂ substrates by radiofrequency (rf) sputtering. Sintered MgAl₂O₄ pellets were prepared by traditional ceramic processing. Scanning electron microscopy (SEM) analysis showed that the thin films were rather dense and homogeneous, made up of clustered particles of about 20–30 nm, while the pellets showed a wide pore‐size distribution. X‐ray photoelectron spectroscopy (XPS) demonstrated that the thin films have a stoichiometry close to that of MgAl₂O₄. Sintered MgAl₂O₄ is crystalline, while it is disordered in thin‐film form. The presence of two different components of the Al 2p peaks was correlated with the structural difference between pellets and thin films. The relationship between good film–substrate adhesive properties and the chemical composition at the interface was studied. The electrical properties of the sensing elements were studied at 40°C in environments at different relative humidity (RH) values between 2% and 95%, using ac impedance spectroscopy. MgAl₂O₄ thin films showed interesting characteristics in terms of their use in humidity‐measurement devices. Resistance versus RH sensitivity values showed variations as high as 4 orders of magnitude in the RH range tested for thin films, and 5 orders of magnitude for pellets. The differences in the electrical behavior of MgAl₂O₄ pellets and thin films were correlated with their different microstructures.