AlN-based sputter-deposited shear mode thin film bulk acoustic resonator (FBAR) for biosensor applications — A review

A new type of biosensor device is here presented which is fabricated using the same processes used for the fabrication of integrated electrical circuits to enable tighter integration and further sensor/biosensor miniaturization. The device is a so-called thin film bulk acoustic resonator (FBAR) operating in shear mode. Here specifically AlN-based shear mode FBAR is addressed but also an overview of the shear mode FBAR development in general is presented. Developments are reported of a low temperature reactive sputtering process for growing wurtzite–AlN thin films with a close to homogenous c-axis inclination over a 4″ substrate wafer. This process enabled fabrication of shear mode FBAR sensors. The sensor operation is described along with how the design parameters influence its performance. Specifically, sensitivity amplification utilizing low acoustic impedance layers in the FBAR structure is demonstrated and explained. The resolution of the AlN shear mode FBAR sensor is demonstrated to already be comparable with the conventional quartz crystal microbalance (QCM) sensor, suggesting that shear mode FBAR may be a competitive and low cost alternative to QCM.