Novel Dual Mode SAW-BAW Device

In this paper preliminary work on a novel dual mode SAW-BAW, (surface acoustic wave- bulk acoustic wave), device is presented. BAW resonators exhibit very high Q and are ideal in oscillator circuits at lower frequencies, while SAW resonators are generally designed for higher frequency operation. Dual mode BAW resonators are used for highly accurate temperature compensation in OCXO's and TCXO's. In this new device, the TSM (thickness shear mode) is excited using a lateral field BAW resonator and a one-port resonator is used to excite the SAW mode. The device embodiment consists of a SAW resonator physically coincident with a BAW electrode pattern. The BAW electrodes are part of the SAW transducer excitation bus bars and the SAW resonator is located at the center of the BAW transducer. The BAW electrodes are separated by a narrow gap formed by the SAW transducer which allows lateral field excitation. The BAW mode occurs at a frequency of 3.27 MHz, while the SAW mode operates at a frequency of 157 MHz. The BAW and SAW devices are spatially coincidental and could potentially be driven simultaneously to operate in a dual mode oscillator. The devices were fabricated on ST-quartz wafers that were polished on both sides. Swept frequency measurements were obtained using a network analyzer and a wafer RF probe, and resonator device parameters including the quality factor have been extracted. The initial devices show good responses and a discussion of the design and analysis will be given. The BAW resonator shows a Q of approximately, 100K, while the SAW has a relatively lower Q of approximately 14K, which was close to the coupling of modes (COM) model prediction. The paper will show both theoretical predictions and experimental results for the SAW and BAW devices. This new device embodiment has potential applications in dual-mode TCXO's and sensors where different modes can be used for temperature compensation, surface sensing or similar applications.