Hydrogen Fluoride Gas Detection Mechanism on Quartz Using SAW Sensors

Hydrogen fluoride (HF) is a hazardous compound used in a variety of industrial processes and is a decomposition product of many environmentally harmful fluorinated volatile organic compounds. Surface acoustic wave (SAW) resonators on quartz substrates are suited for HF sensing because the analyte reacts directly with the sensor substrate, producing H
$_{2}$
O and the volatile compound, SiF
$_{4}$
. This work shows evidence that during gas phase HF exposure to a generalized SAW (GSAW) resonator and a pure shear horizontal SAW (SH-SAW) resonator, the dominant sensing mechanism is the detection of a condensed liquid layer on the substrate surface, rather than simply material removal via SiF
$_{4}$
desorption. The GSAW and pure SH-SAW resonators, fabricated on ST-X and ST-90
$^{\circ}$
quartz, respectively, have been simultaneously exposed to HF in a low-volume (1.3 cm
$^{3}$
) test cell. An automated gas delivery system developed under this project varied HF concentrations from 1–18 ppm. Both resonators are sensitive to the formation of a condensed liquid layer, but the frequency shift of the pure SH-SAW resonator, due to this effect, is up to 4.6 times greater than that of the GSAW device for the HF concentrations investigated. The measured sensor frequency response to potential inteferents, such as R-134a (C
$_{2}$
H
$_{2}$
F
$_{4}$
), isopropanol (C
$_{3}$
H
$_{8}$
O), propane (C
$_{3}$
H
$_{8}$
), acetone (C
$_{3}$
H
$_{6}$
O), and carbon monoxide (CO), is below the device's limit of detection, while its response to HF is as high as 7.5 times its limit of detection.