Development of Thin Film Lithium Niobate Shear Bulk Acoustic Wave Resonators

RF MEMS piezoelectric acoustic resonators are the essential building blocks for RF filters used in RF front-end modules for wireless mobile communication due to their compact size in the MHz-GHz frequency range, high Quality factors and large relative bandwidths. They are currently produced in the tens of billions per year, in order to serve the growing mobile phone industry. Piezoelectric filters have been part of mobile phones since the 1st generation (1G) starting in 1979. Two main options dominate the market: Surface Acoustic Wave (SAW) resonators made of lithium niobate (LiNbO3, LNO) or lithium tantalate (LiTaO3, LTO); and Bulk Acoustic Wave (BAW) resonators made of aluminum nitride (AlN) or aluminum scandium nitride (AlxSc1-xN). However, with the advent of 5G, new sub 7GHz bands at higher frequencies and larger bandwidths are required, and both "traditional" options cannot achieve the market requirements. SAW devices suffer from scalability of the frequency and BAW devices from a low electromechanical coupling of the material. Therefore, new solutions for piezoelectric resonators are required to meet market needs.This thesis focuses on addressing these challenges for future generations of wireless communication using suspended monocrystalline thin film LiNbO3 resonators. The thesis presents the design, fabrication and characterization of suspended thin-film LiNbO3 resonators to specifically address filter specifications for bands n79 and n78 in the 3-6 GHz frequency range. The thesis is divided into two main parts, one for each filter.The first section focuses on laterally excited A1-like mode resonators on a 375nm Z-cut suspended LiNbO3 membrane. Through design and optimization, a large electromechanical coupling of 10.5% in RaR (relative resonance-antiresonance distance) with Q of 340 is shown. In addition, a large tunability of up to 20% of the resonance frequency is demonstrated by reducing the pitch to dimensions close to the membrane thickness. By optimizing the design parameters, it is demonstrated that it is possible to suppress different spurious modes so that they do not affect the filter performance. The presented fabrication process shows a high yield of over 90% of functioning devices. As a conclusion of this first part, the presented resonators can be used to constitute band n79 filters.The second section focuses on development of vertically excited shear SH1 plate mode resonators on a 400nm Y-cut Lithium Niobate film. The resonator consists of a Lithium Niobate film with interdigitated electrodes (IDE) on top and a floating metal plate at the bottom. Through vertical electric field excitation, shear bulk acoustic waves are generated in each unit cell of the device functioning as a quasi-independent resonator. Measured devices at 3.7GHz show large electromechanical coupling up to 16.7% in RaR (750MHz), and ~2O impedance at resonance. Devices can show aggressive lateral spurious modes all-along the bandpass, but thanks to the etching of trenches in LNO and geometric optimization these spurious can be suppressed. The optimized resonators can be used to synthesize band n78 filters.