XBAR physics and next generation filter design

XBARs are laterally-excited bulk acoustic resonators consisting of interdigital transducers (IDT) on thin piezoelectric plates. Their low loss and excellent piezo-coupling are ideal for wideband 5G cellular and next-generation Wi-Fi filters. We present measurements of 5 GHz and 6 GHz Wi-Fi filters fabricated by Resonant with respective bandwidths exceeding 600 MHz or 1200 MHz respectively. Avoiding spurs over such large bandwidths while maintaining high power performance requires careful choice of the electrode thickness. FEM simulations demonstrate a minimum in spurious content at particular values of the metal thickness. This minimum can be understood by calculating propagating mode dispersion curves in two 2D structures: 1) an infinite lithium niobate (LN) plate which matches the XBAR structure between electrodes, and 2) an infinite composite plate with LN and thick metal which matches the XBAR structure in the electrode region. We show that the resonant XBAR mode for electrode thickness near the spur minimum can be approximately described as Al between electrodes, and A3 in and beneath the electrodes. The minimum in spur content occurs close to the metal thickness where the A1 mode of the open plate between electrodes is best matched to the A3 mode of the composite LN and thick metal plate at the electrodes.