PZT bulk acoustic wave resonators for MMIC technology

Abstract

Thin films of the piezoelectric ceramic lead zirconate titanate (PZT) have been prepared using sol-gel processing techniques. PZT can be used as the dielectric material in bulk acoustic wave (BAW) resonator devices which can operate at microwave frequencies. Coupled with the relative ease of sol-gel processing, these structures have the potential of being readily incorporated into existing monolithic microwave integrated circuit (MMIC) technology. The thin films were prepared via a novel sol-gel route based on a 1,3-propanediol solvent with up to 1 μm thickness obtained by a single sol coating deposited on Pt/Ti coated oxidized silicon substrates. Films annealed in a conventional furnace at 700 ° C for 30 minutes were seen to fully crystallize into the required perovskite phase and showed excellent ferroelectric behaviour, demonstrated by reproducible hysteresis loops (P_r=25μC/cm², E_c=4.5MV/m). Microwave measurements were performed on wedge shaped samples obtained via a combination of etching and polishing procedures. This resulted in a BAW structure with approximately 10μm of silicon wafer left beneath the device. Fundamental series and parallel resonances were observed at 0.073GHz and 0.111 GHz respectively. The unloaded Q value for the parallel resonance was seen to be −1000. We have shown that BAW microwave devices composed of PZT are feasible.