A method to reduce losses in buried electrodes RF SAW resonators

Buried electrodes SAW resonators on lithium niobate suffer from transverse mode spurii even if the acoustic velocity inside the resonator is faster than the acoustic velocity outside the resonator (i.e. when a `waveguiding' condition is not established). 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. Guiding conditions need to be created in resonators to obtain good quality factors. This can be done by using sufficiently large 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 therefore, the coupling to the higher order modes becomes negligible. 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.