Composite Tunable Bulk Acoustic Wave Resonator Based on Lithium Niobate Thin Films

Changing the electrical boundary conditions applied to a piezoelectric film by means of an external tunable capacitor is one of the possible mechanisms to induce frequency tunability in acoustic resonators. Its application to Bulk Acoustic Wave (BAW) resonators requires the implementation of composite structures obtained by stacking two piezoelectric films, ideally of largely coupled materials, one of which being connected to a tunable capacitor which controls the operation frequency. Leveraging progresses in the development of BAW resonators based on lithium niobate thin films, we propose here a first demonstration of such tunable devices, operating in the GHz range, obtained by transferring in sequence two 500 nm-thick films by crystal ion slicing along with patterned electrodes. Similar to classical coupled resonator filters, the device exhibits several resonance modes, the two of which are located at respectively 1.1 and 2.9 GHz. They are respectively tunable over 2.4 and 13 MHz.