Pulsed laser deposited Y-doped BaZrO_3 thin films for high temperature humidity sensors

摘要

Pulsed laser deposited (PLD) Y-doped BaZrO_3 thin films (BaZr_(1-x)Y_xO_(3-y/2), x = 0.2,y>0), were investigated as to their viability for reliable humidity microsensors with long-term stability at high operating temperatures (T>500℃) as required for in situ point of source emissions control as used in power plant combustion processes. Defect chemistry based models and initial experimental results in recent humidity sensor literature [1,2]. indicate that bulk Y-doped BaZrO_3 could be suitable for use in highly selective, high temperature compatible humidity sensors. In order to accomplish faster response and leverage low cost batch microfabrication technologies we have developed thin film deposition processes, characterized layer properties, fabricated and tested high temperature humidity micro sensors using these thin films. Previously published results on sputtering Y-doped BaZrCh thin films have confirmed the principle validity of our approach [3]. However, the difficulty in controlling the stoichiometry of the films and their electrical properties as well as mud flat cracking of the films occurring either at films thicker than 400 nm or at annealing temperature above 800 ℃ have rendered sputtering a difficult process for the fabrication of reproducible and reliable thin film high temperature humidity microsensors, leading to the evaluation of PLD as alternative deposition method for these films.rnX-ray Photoelectron Spectroscopy (XPS) data was collected from as deposited samples at the sample surface as well as after 4 min of Ar~+ etching. PLD samples were close to the desired stoichiometry. X-ray diffraction (XRD) spectra from all as deposited BaZrO_3:Y films show that the material is polycrystalline when deposited at substrate temperatures of 800 ℃. AFM results revealed that PLD samples have a particle size between 32 nm and 72 nm and root mean square (RMS) roughness between 0.2 nm and 1.2 nm. The film conductivity increases as a function of temperature (from 200 ℃ to 650 ℃) and upon exposure to a humid atmosphere, supporting our hypothesis of a proton conduction based conduction and sensing mechanism. Humidity measurements are presented for 200-500 nm thick films from 500℃ to 650℃ at vapor pressures of between 0.05 and 0.5 atm, with 0.03-2% error in repeatability and 1.2-15.7% error in hysteresis during cycling for over 2h. Sensitivities of up to 7.5 atm~(-1) for 200 nm thick PLD samples at 0.058 atm partial pressure of water were measured.