SAW device for liquid vaporization rate and remaining molecule sensing

This paper presents an Aluminum Nitride (AlN) surface acoustic wave (SAW) device for liquid detection, based on remaining liquid molecules and vaporization rate. The sensing mechanism of the SAW device uses linear attenuation and frequency shift during the vaporization process of the liquid. As the device is meant to operate with liquids, the piezoelectric film and metal interconnects are protected by a silicon oxide layer. A change in insertion loss is measured when liquid drops (0.5-2 μl) of demi-water (DW), isopropyl alcohol (IPA) and ethanol (ETH) are applied on the surface of the SAW device. The selected liquids have different equilibrium vapor pressures and boiling points. The vaporization process of DW is slow enough for the network analyzer to measure linear changes in the insertion loss parameter. For IPA and ETH, there is a frequency shift caused by the appearance of remaining molecules on the surface after they vaporize. The center frequency is shifted by 0.25 MHz for ETH and 0.198 MHz for IPA.