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/Si O2 substrates by radiofrequency (rf) sputtering. Sintered MgAl2O4 pell ets were prepared by traditional ceramic processing. Scanning electron microscopy (SEM) analysis showed that the thin films were rather dens e and homogeneous, made up of clustered particles of about 20-30 nm, w hile the pellets showed a wide pore-size distribution. X-ray photoelec tron spectroscopy (XPS) demonstrated that the thin films have a stoich iometry close to that of MgAl2O4. Sintered MgAl2O4 is crystalline, whi le it is disordered in thin-film form. The presence of two different c omponents of the Al 2p peaks was correlated with the structural differ ence between pellets and thin films. The relationship between good fil m-substrate adhesive properties and the chemical composition at the in terface was studied. The electrical properties of the sensing elements were studied at 40-degrees-C in environments at different relative hu midity (RH) values between 2% and 95%, using ac impedance spectroscopy . MgAl2O4 thin films showed interesting characteristics in terms of th eir use in humidity-measurement devices. Resistance versus RH sensitiv ity values showed variations as high as 4 orders of magnitude in the R H 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.