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, k2t , 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.