XBAR

The micro/nano acoustic filters operating at 3 GHz – 7 GHz frequency range with large bandwidth and low loss are currently in demand. Such filters are usually designed as “ladder” circuits connecting resonators with large relative frequency distance between the anti-resonance and resonance and having a few orders impedance difference between them. Ideally, these resonators must be manufacturable with optical lithography, and the filters must tolerate an input power of a few watts. Essential progress been reached nowadays based on vibration modes in thin, submicron, crystalline platelet of strong piezoelectrics, such as lithium niobate (LN). Commercial availability of ion-sliced layers of different cuts of LN transferred on Si, SiC and other substrates opens wonderful possibilities for using materials with desired properties. Compared to SAW, in membranes it is easier to create a more uniform electric field better corresponding to stress distribution in an excited mode and thus providing higher piezo-electric coupling. Therefore, low order Lamb and plate modes, such as S0, SH0, S1, SH1 and A1 - providing the strongest piezo-coupling are of interest. In this paper I will review devices based on laterally excided Lamb mode A1 (so called XBARs) and shear plate resonances (SH1) excited by transversal electric field in periodic system of such resonators (YBARs). The use of suspended cavity membranes allows reaching excellent Q-factor and strong coupling. The device frequency is mainly determined by the membrane thickness and 3GHz–7 GHz can be reached, while maintaining electrode critical dimensions CD >0.5 µm. However, this technology is difficult. The devices are fragile, and their power handling can be limited because of low heat evacuation from the thin LN membrane. Therefore, we will also briefly discuss layered structures when such a membrane is mounted on a substrate - solving part of these problems at the cost of reduced coupling. This technology will inevitably dominate the area of micro-nano acoustic devices for frequencies higher than 3 GHz in the foreseeable future.