Fast, selective, and stable high temperature humidity sensors enabled by microfabricated yttrium-doped barium zirconate thin films

This work tested the viability of microfabricated humidity sensors based on Y-doped BaZrO3 by developing thin film deposition processes, fabricating devices, and characterizing the device response. At high temperatures, this material becomes conductive depending on temperature, water vapor, and other gas concentrations. Such devices should help increase efficiency and decrease emissions through improved combustion process control. Using microfabrication may lead to reduced size and faster sensor response. Two hundred and twelve variations of thin film layers were deposited and characterized out of which 112 were used in sensors.;BaZrO3:Y thin films (200--750 nm) were sputtered onto oxidized n-type silicon substrates at room temperature from a ceramic target in an Ar sputtering ambient. Various deposition pressures and powers were used to correlate process parameters with film properties. Films were annealed at 800 and 1000°C (3 hours, air) and characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) to determine microstructure, surface morphology, and film composition. For comparison, 30--500 nm thick films were deposited onto identical substrates using PLD (various substrate temperatures) and characterized as deposited. Selected samples were annealed at 1000°C (3 hours in air) and characterized again. Transmission electron microscopy (TEM) of 30 and 50 nm PLD films confirmed the particle sizes found by AFM of 19--25 nm. Although all layers show Barium deficiencies, stable process windows were established for sputtering and PLD close to stoichiometric compositions.;Sensors with the sensitive material deposited on top of the interdigitated test structure (IDE) showed no response to changes in humidity. Sensors with the IDE placed on top of the sensing film showed sensitive response, suggesting a strong surface dominated sensing effect. Ti/Pt as contact metal yielded an unrepeatable humidity response. Cr/Au gave sensitive, selective, and long term stable humidity response.;All films were exposed to varying partial pressures of water vapor, (400 to 650°C) with and without exhaust gas mixtures. Sensitivities of 0.2 to 62 atm-1 were demonstrated with tenfold selectivity towards other gases and sensor life time in excess of a year. Response times are 4--20 times faster than reports in literature.