Surface Acoustic Wave Sensors Deposited on AlN Thin Films

Abstract

Over the past few decades, there has been considerable research and advancement in surface acoustic wave (SAW) technology. At present, SAW devices have been highly successful as frequency band pass filters for the mobile telecommunications and electronics industries. In addition to their inherent frequency selectivity, SAW devices are also highly sensitive to surface perturbations. This sensitivity, along with a relative ease of manufacture, makes SAW devices ideally suited for many sensing applications including mass, pressure, temperature, and biosensors. In the area of biosensing, surface plasmon resonance (SPR) and quartz crystal microbalances (QCM) are still in the forefront of research and development, but advancement in SAW sensors could prove to have significant advantages over these technologies. This study investigates the advantages of using aluminum nitride (AlN) as a material for SAW sensors. AlN retains its piezoelectric properties at relatively high temperatures when compared to more common piezoelectric materials such as lead zirconium titanate (PZT), lithium tantalate (LiTaO₃) and zinc oxide (ZnO). AlN is also a very robust material making it suitable for biosensing applications where the sensing target is selectively absorbed by an active layer on the device which may attack the piezoelectric layer. AlN thin films of different thicknesses have been deposited on Si substrates by DC reactive sputtering. Rayleigh-wave SAW devices have been fabricated by the deposition of platinum contacts and interdigital transducers (IDTs) onto AlN thin films using standard photolithographic processes. Experiments have been conducted to measure Rayleigh velocities, resonant frequencies, and insertion loss. Experimental results are compared to theoretical calculations.