Abstract

Thin Film Bulk Acoustic Resonators (TFBARs) are currently widely employed as communications filters, duplexers and multiplexers in mobile telephones. They consist of a thin film of a piezoelectric material on the surface of a silicon wafer with electrodes above and below the thin film which are designed to overlap in a small area (_{100 × 100 µm). By applying an a.c. electrical signal to the electrodes, an oscillating mechanical deformation can be set up in the piezoelectric material to produce an acoustic wave. When the half wavelength matches the thickness of the piezoelectric film, a standing wave resonance is produced, typically at a frequency} 1 GHz. In order to achieve a high Q-factor, the acoustic energy must be confined in the resonant area, and this is achieved either by removing the silicon substrate underneath the resonant area to create a Film Bulk Acoustic Resonator (FBAR), or by using an acoustic Bragg reflector of multiple layers of high and low acoustic impedance materials between the resonator and the silicon substrate to create a solidly mounted resonator (SMR).

In addition to their use in mobile communications, the TFBAR can be used as a gravimetric sensor. Although the thickness of the piezoelectric thin film has primary control of the resonant frequency of the TFBAR , it is also sensitive to mass on the surface of the electrodes. By functionalising the surface of the TFBAR in a specific chemical or biological receptor, it is possible to detect particular chemical or biological species.