Switchable and tunable bulk acoustic wave resonators based on BaxSr1-xTiO3 thin films

Doktorsavhandling, 2012

The main focus of this thesis is the study of a novel microwave component, the tunable thin film bulk acoustic wave resonator (TFBAR), based on ferroelectric BaxSr1-xTiO3 thin films. Conventional fixed frequency TFBARs, widely used for filtering applications in wireless communication systems, are based on ordinary piezoelectrics. The tunable TFBAR on the other hand utilizes the dc field induced piezoelectric effect in the ferroelectric film. This allows dc field tuning of the resonance frequency as well as switching between resonator and capacitor response. A brief introduction to the BaxSr1-xTiO3 material system is given, followed by results from thin films grown by pulsed laser deposition. Effects of substrate temperature and ambient gas pressure during growth are discussed. Resonator design and fabrication issues are covered, including studies of electrode configurations and acoustic mirror structures. Measurement results are presented in wide dc bias and temperature ranges. The achieved room temperature tuning range for a 5.7 GHz Ba0.25Sr0.75TiO3 TFBAR is 3.8% and the effective electromechanical coupling coefficient at maximum bias is 7.1%, with a Q-factor of 130. A switchable bandpass filter is considered as a possible application of BaxSr1-xTiO3 TFBARs. Simulated filter characteristics and preliminary results from fabricated filters are presented.