Comparison of Y-doped Bazro_3 thin films for high temperature humidity sensors by RF sputtering and pulsed laser deposition

摘要

This paper presents a comprehensive comparison of the fabrication processing parameters and the electrical, mechanical, chemical, and sensing properties of sputtered and pulsed laser deposited (PLD) Y-doped BaZrO_3 thin films for the use in high temperature (400-650℃) water vapor partial pressure measurements.rnRF sputtered thin films with thicknesses of between 200 and 750 nm, at pressures between 0.45 and 50 mTorr and deposition powers between 50 and 300 W were deposited and annealed at 800℃ or 1000℃ for 3 h in air. Correlations between mechanical film stress, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM) stoichiometry data, crystallographic, surface morphology and process and film parameters are presented. Furthermore, the baseline resistivity was characterized as function of film thickness and temperature.rnPLD samples with thicknesses between 200 and 500 nm were deposited at room temperature to 800℃. All samples were characterized as deposited. 200 and 500 nm thick samples were also annealed at 1000℃ for 3 h in air and characterized post-anneal. The Ba concentration vs. substrate temperature remained stable within 12-16%, correlating with those Ba concentrations that yield good sensitivity (0.25-8 atm~(-1)). The low dependence of the Ba concentration on the deposition parameters indicates that the PLD deposition process allows larger process windows for the deposition of good sensitivity thin film material compared to sputtering.rnWater vapor partial pressure sensitivity indicated a decrease in sensitivity with decrease in Ba concentration in the films. Partial transition of the Y-doped BaZrO_3 to ZrO_2 and subsequent decrease in oxygen vacancy sites could explain the loss in sensitivity. PLD samples were on average more stable and repeat-able and less process dependent, but had lower sensitivity towards humidity. This could be explained by the higher, more stoichiometric and less process dependent Ba concentration of these samples compared to the sputtered samples. At high temperatures (>550℃) only the PLD samples exhibit stable sensitivity towards humidity.rnThe resulting activation energies for the sputtered samples are only slightly larger than those of the PLD samples at lower temperatures (hole conduction) and much larger at high temperatures (oxygen ion conduction). Annealing of the PLD samples at 1000℃ increases the sensor response time by 10 times and the relaxation time by a factor of 8. Sputtered samples do not survive annealing at 1000℃. PLD samples also by far outlast sputtered samples in terms of lifetime.