Thin film bulk acoustic wave devices : performance optimization and modeling

Abstract

Thin film bulk acoustic wave (BAW) resonators and filters operating in the GHz range are used in mobile phones for the most demanding filtering applications and complement the surface acoustic wave (SAW) based filters. Their main advantages are small size and high performance at frequencies above 2 GHz. This work concentrates on the characterization, performance optimization, and modeling techniques of thin film BAW devices. Laser interferometric vibration measurements together with plate wave dispersion modeling are used to extract the full set of elastic material parameters for sputter deposited ZnO, demonstrating a method for obtaining material data needed for accurate simulation of the devices. The effectiveness of the acoustic interference reflector used to isolate the vibration from the substrate is studied by 1-D modeling, 2-D finite element method and by electrical and laser interferometric measurements. It is found that the Q-value of reflector-based BAW resonators operating at 2 GHz is limited to approximately 2000 by mechanisms other than leakage through the reflector. Suppression of spurious resonances in ZnO resonators is studied in depth by modeling and measurements. It is verified that the approximate mode orthogonality is behind the suppression in boundary frame type ZnO devices operating in the piston mode, but also another narrow band mode suppression mechanism is found. A plate wave dispersion based 2-D simulation scheme for laterally acoustically coupled BAW resonator filters is developed and employed in designing of experimental devices, which show both good agreement with the model predictions and a remarkable 4.9 % relative bandwidth.