High frequency thin film ferroelectric acoustic resonators and filters

There is a need for more compact filters that are able to meet the needs of the next generation of mobile phone operating in the l-2GHz frequency band. These filters are required to have low insertion loss, high Q, low sensitivity to temperature and in some cases wider bandwidths than those currently available. In this paper we review some of our recent work on the application of PZT (sol-gel PbZr _0.3Ti _0.7O ₃) as the piezoelectric layer in FBARs (thin Film Bulk Acoustic Resonators) and FBAR filters. The high electromechanical coupling coefficients in PZT offer the possibility of realising wide band-width filters. It is shown possible to incorporate PZT into a FBAR structure previously designed for piezoelectric ZnO by the simple addition of a TiO ₂ layer that avoids the crazing that occurs when PZT is deposited directly on SiN _x. PZT FBAR filters of extremely small area (<300 μ ²) can be produced. Preliminary measurements suggest that the resonance frequencies in PZT FBARs are only weakly temperature dependent. The advantages and disadvantages of using PZT in FBAR structures is discussed.