Acoustic wave liquid sensors enhanced with glancing angle-deposited thin films

The application of nanostructured thin films grown by glancing angle deposition (GLAD) to enhance the sensitivity of Love wave liquid sensors was investigated. The effect of high-angle GLAD films, with and without nanostructure modification by ion milling and clustering, on device response was studied. Ion milling was used to prevent clustering of individual nanostructured posts, and was compared to films that were intentionally clustered. Both sets of modified films were shown to eliminate signal losses due to damping from the non-rigid, as-deposited posts. Sensitivity enhancement was tested by viscous loading with varying mixtures of glycerol and de-ionized water. Frequency shifts were found to be non-linear with the square root of the density–viscosity product, (ρη)1/2, and were modeled with exponentials. The sensitivity was shown to increase with film thickness, but decreased with increasing values of (ρη)1/2. There was also an increase in signal loss with film thickness under a liquid environment. The balance between sensitivity enhancement and signal loss must be carefully optimized with consideration for the intended application.