High sensitivity Love-wave humidity sensors using glancing angle deposited thin films

The effect of SiO2 nanostructured thin films grown by glancing angle deposition (GLAD) on the sensitivity of Love-wave surface acoustic wave (SAW) sensors was investigated for the first time. High-surface area, vertical post nanostructures of varying thicknesses were grown at an angle of 70 ° relative to the substrate normal on top of the Love-wave devices. The SAW devices, operating at ∼120 MHz, were placed in an oscillator circuit and the frequency was monitored as a function of relative humidity (RH) in the range of 0-90%RH at room temperature. The effect of GLAD film thickness on sensitivity was also examined. The sensitivity increased exponentially with film thickness, and all devices with GLAD films exhibited higher sensitivities than any of the devices without. The device with the highest sensitivity had a 705 nm thick film and showed a minimum sensitivity of 125 ppm/%RH.