High Frequency Thin Film Acoustic Ferroelectric Resonators

Both ZnO and PZT Thin Film Bulk Acoustic Resonator filters were fabricated, tested and modeled in this study. The development of an accurate Mason model allows the effect of particular parasitic components on the microwave s-parameters in the region of the series and parallel resonances to be identified. The parasitic components that limit the performance of our ZnO and PbZr0.3Ti0.7O3 Thin Film Bulk Acoustic Resonator filters are analysed. From an analysis of PbZr0.3Ti0.7O3 Thin Film Bulk Acoustic Resonator measurements values for the longitudinal acoustic velocity and electromechanical coupling coefficient can be derived. Measured PbZr0.3Ti0.7O3 Thin Film Bulk Acoustic Resonator filter responses confirm that the larger electromechanical coupling coefficients in this material compared to ZnO give wider filter band-widths. INTRODUCTION: There is a great commercial interest in decreasing the size of microwave 1-3 GHz filters to allow more functions to be incorporated in future mobile phones [1]. Presently there are two types of filters being developed to meet this need, ceramic filters based on electromagnetic modes and acoustic filters. The typical dimensions of both types of microwave filters are similar to the wavelength at the operating frequency. By using the piezoelectric effect to generate acoustic modes wavelengths and dimensions can be reduced by about four orders of magnitude compared to electromagnetic modes. There are two types of acoustic filters considered contenders for future generations of mobile phones, both based on piezoelectric materials: surface acoustic wave (SAW) devices and Thin Film Bulk Acoustic Resonators (FBAR). The piezoelectric effect has been widely used in bulk acoustic resonators, such as single crystal quartz for many years. Recently by careful thinning or etching the quartz plate operation up to 200 MHz can be achieved but the low acoustic velocity of quartz and the resulting fragility of thinned substrates means that this technology cannot progress to higher frequencies. In SAW devices that direction of propagation is in the plane of the wafer while for FBAR it is perpendicular to a substrate surface. For FBAR operation the piezoelectric film thickness must be of the order of the acoustic wavelength at the desired operating frequency. In this paper we compare two candidate thin film piezoelectric materials, ZnO and PbZr0.3Ti0.7O3 (PZT) that have different acoustic properties. Although there has been considerable previous work on ZnO FBAR [2] and ZnO FBAR filters [3] there has only been a few reports on PZT FBARs [4]. In particular, the electromechanical coupling coefficients Mat. Res. Soc. Symp. Proc. Vol. 655 © 2001 Materials Research Society