(627e) Sensitivity of A Surface Acoustic Wave Hydrogen Fluoride Sensor to Quartz Substrate Etching

It has been proposed previously that a quartz surface acoustic wave (SAW) resonator can be sensitive to HF gas by etching the SiO₂ substrate making the sensor quite selective because of material removal as opposed to mass loading of the sensor. However, this paper shows evidence that during gas phase HF exposure to a pure SH SAW resonator, the dominant sensing mechanism reflects the detection of a condensed liquid layer on the SiO₂ surface, rather than material removal from the device's surface. At HF concentrations ranging from 1 ppm to 18 ppm, the resonator responds to the changing state of the condensed liquid layer with clear frequency shifts within minutes of exposure. Permanent frequency shifts in the SAW response are observed due to the etching of SiO₂ after the evaporation of the liquid layer when HF is removed from the test gas for longer than 3 hours. However those permanent shifts are smaller than the frequency shift response due to the presence of the condensed liquid layer. The reaction rate and development of the condensed water layer are a function of HF concentration, humidity, temperature and time, and are correlated to the frequency sensitivity of the two-port resonator with platinum electrodes. The SAW device was exposed to HF through a low-volume (≈1 cm³) test cell. An automated gas delivery system was used to vary HF concentrations, flow rates, and relative humidity.