High performance piezoelectric AlN MEMS resonators for precise sensing in liquids

Piezoelectric silicon micro-machined resonators triggered in the two-dimensional roof-tile shaped mode exhibit exceptionally high quality factors even in highly viscous liquids. In combination with an all-electric readout, this enables precise measurements in liquid media with high output signals, if the electrode design and the anchor geometry are tailored to match the selected order of the roof-tile shaped mode. This paper provides an overview of this promising class of resonator modes for micro-electromechanical systems (MEMS) and highlights three application scenarios. In the food industry, the MEMS sensor can be used to monitor the fermentation process in wine by tracking the density and hence, the concentration of ethanol. In technical lubricants, the concentration of ferrous particles can be monitored by integrating a magnetic actuator on the resonator. Finally, even in highly viscous bitumen, the MEMS sensor is successfully used to measure the viscosity of this material.