Ion-sliced Lithium Niobate on silicon dioxide for engineering the temperature coefficient of frequency of Laterally Vibrating Resonators

This paper presents a new Laterally Vibrating Resonator (LVR) based on Y-cut ion-sliced Lithium Niobate (LN) thin films on silicon dioxide (SiO₂). In this work, the LN LVR is built on top of a SiO₂ layer and released from the underlying silicon wafer by dry etching in XeF₂. For a given sample having a LN layer thickness of 420 nm and SiO₂ thickness of 1600 nm, this first demonstration yielded resonators with temperature coefficient of frequency (TCF) of + 17 ppm/°C, and + 18 ppm/°C for devices vibrating at 500 MHz, respectively oriented at 10 and 30 to the x-axis, and TCF of +24.1 ppm/°C, and +27.7 ppm/°C for devices vibrating at 750 MHz, respectively oriented at 40 and 50 to the x-axis. The positive TCF clearly indicates the effect of the SiO₂, matches with finite element method simulations and non-linear analysis, and offers evidence that TCF engineering is possible. Most importantly, these LN LVRs still exhibited high values of electromechanical coupling, k_t², around 9% at 723.7 MHz, and Q in excess of 1,320 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.