Lithium Niobate on Silicon Dioxide Suspended Membranes: A Technology Platform for Engineering the Temperature Coefficient of Frequency of High Electromechanical Coupling Resonators

This paper presents a new class of laterally vibrating resonators (LVRs) based on Y-cut ion-sliced lithium niobate (LN) thin films on silicon dioxide (SiO₂). The SiO₂ layer is used to engineer the temperature coefficient of frequency (TCF) of the LN resonator. The LN LVR is built on top of a SiO₂ layer and released from the underlying silicon wafer by dry etching in Xenon difluoride (XeF₂). For a given sample having an LN layer thickness of 420 nm and SiO₂ thickness of 1600 nm, this paper demonstrated resonators with TCF of +17 ppm/°C, and +18 ppm/°C for devices vibrating at 460 and 420 MHz, and, respectively, oriented at 10° and 30° to the x-axis. TCF of +24.1 ppm/°C and +27.7 ppm/°C were recorded for devices vibrating at 720 MHz, respectively, oriented at 40° and 50° to the x-axis. These results correspond to a 4-5 X reduction in the TCF of standalone LN resonators. The positive TCF clearly indicates the effect of the SiO₂ layer, and its value matches with what is predicted analytically by finite element method simulations and nonlinear analysis of the resonator amplitude-frequency response. This demonstration offers evidence that TCF engineering of LN LVRs is possible. Most importantly, these LN LVRs still exhibited high values of electromechanical coupling, k²_t , around 9% at 723.7 MHz, and Q in excess of 1320 in air at 419.3 MHz. By optimizing the relative values of the LN and SiO₂ thickness, it is ultimately possible to attain devices with zero first order TCF.