Choice of SAW humidity sensor coatings

Very thin sensitive layers deposited on the surface of SAW resonators were investigated (ST-Quartz was used as substrate). The thickness of the layers did not exceed several percent of the wavelength at the operating frequency of the device. The humidity sensitivity and hysteresis of several organic and inorganic sensitive coatings were studied at different temperatures, the reproducibility and stability of the results were analyzed. The following materials were applied by centrifugation and subjected to heat treatment: Nafion® polymer (Naflon®); polybenzoxazole polymer; silicate films of the general composition nP2O5 ∗ SiO2; single-layer and multilayered carbon nanotubes (CNTs); polymethyl methacrylate (PMMA); PMMA with CNT; polymer Nafion
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with CNT. Comparing resonators with three different thicknesses of the deposited Naflon® polymer film (50, 100 and 150 nm) has shown that the larger is the thickness, the higher is the initial frequency offset from the initial frequency of the resonator at 434 MHz. This required topology modifications to compensate for the frequency change due to film deposition. It was found that the resonance frequency sensitivity to humidity increases with decreasing coating thickness. It was also found that the hysteresis and the sensitivity of the relative humidity measurement varied with temperature — with the increase in air temperature the hysteresis decreased from ± 2 % RH at 0 °C to ± 1% RH at 60 °C, and the sensitivity to relative humidity increased from 4.5–5.0 kHz /% RH at 0 °C to 6.5–7.0 kHz /% RH at 60 °C for 434 MHz resonators. Measurements with Naflon® films thinner than 150 nm revealed that the change of the resonator S11 magnitude induced by humidity moderate increase started to return to its initial value with further humidity increase towards the values approaching 100%. This experimental feature has proven to be very useful for wireless sensor interrogation setup operation, as it allowed to maintain the impedance matching conditions in the electrical circuit of the antenna connection to the sensor and thus to reduce the readout and interrogation distance sensibility on the potential impedance mismatch induced by humidity changes. A conclusion was made that for Nafion® polymer, the recommended layer thickness should not exceed 100 nm for operation at 434 MHz.