6D-4 Generalized and Pure Shear Horizontal SAW Sensors on Quartz for Hydrogen Fluoride Gas Detection

Hydrogen fluoride (HF) is a hazardous compound used in a variety of industrial processes and is a decomposition product of many other fluorinated volatile organic compounds (VOCs), which are often environmental contaminants. Surface acoustic wave (SAW) resonators on quartz substrates are suited for HF sensing because the analyte can react directly with the sensor substrate to produce H2O and the volatile compound SiF4, which evaporates from the surface. This work shows evidence that during gas phase HF exposure to a generalized SAW (GSAW) resonator and a pure shear horizontal SAW (SHSAW) resonator, the dominant sensing mechanism is the detection of a condensed liquid layer on the substrate surface, rather than material removal via SiF4 desorption. The GSAW and SHSAW, fabricated on ST-X and ST-900 quartz, respectively, have been simultaneously exposed to HF through a low-volume (~1 cm ) test cell. The devices' responses were monitored, with data collected every minute. An automated gas delivery system was used to vary HF concentrations from 1-18 ppm, while maintaining a constant flow rate of 100 seem. While both resonators are sensitive to the formation of a condensed liquid layer, the frequency shift of the SHSAW resonator, due to this effect, is up to seven times greater than that of the GSAW device for the HF concentrations investigated.