1TriQuint Semiconductor, Apopka, FL, USA
2TriQuint Semiconductor, Chelmsford, MA, USA
Temperature sensitivity is a major criterion when choosing the substrate for a SAW filter. It is known that a better coupling factor /temperature sensitivity trade off can be obtained by burying the electrodes inside a SiO2 overlay. On lithium niobate substrates, velocities are larger inside the resonator than outside resulting in losses by energy leakage in the busbars . In practical devices, the electrode end gaps play an important role since they constitute a very fast region and reflect the acoustic waves creating multiple transverse modes. Usually, gap sizes are small and some energy is still leaking by tunneling effect. For apodized transducers, the transverse modes frequencies and shapes vary along the transducer cancelling their effect on the response, but also increasing the losses. Guiding conditions need to be created in SAW resonators to obtain good quality factors. It can be done by using large enough gaps sizes. Unfortunately, this leads to even stronger transverse modes. The proposed solution consists in tailoring the shape of the main mode. A "piston mode" shape is obtained by adding a slow region at the edge of the active region. The mode shape is matched to the rectangular excitation and the coupling to the higher order modes becomes negligible. This method is described and experimentally demonstrated for various devices. Results exhibit low spurious, lower losses and better effective coupling coefficients than apodized devices. Quality factors in the 1500 range are measured on resonators while CRF losses are significantly reduced.