Novel Lithium Niobate Shear Acoustic Wave Resonator Architectures for Wideband Wireless Communication

Wireless devices, such as smartphones, are ubiquitous and essential tools in modern society. Cellular devices require dedicated hardware components that interface between wireless radiofrequency (RF) signals and the digital signals understood by processing units. Among the numerous components in front end modules, RF filters, based on microacoustic technology such as surface acoustic wave (SAW) or bulk acoustic wave (BAW) resonators, play a key role in ensuring seamless wireless communication. SAW filters, based on lithium niobate or lithium tantalate, have historically dominated the market for frequencies below 1 GHz, while BAW filters based on aluminum scandium nitride (AlScN) are prevalent in the 1-8 GHz range. The fifth generation (5G) wireless communication standard, marking the most recent step in pursuit of higher data rates and network capacity, calls for higher frequencies and wider bandwidths compared to previous generation standards. Current acoustic filter technology faces difficulty in meeting the complex catalog of filter specifications for 5G, which includes high frequencies (> 3 GHz), wide bandwidths, temperature stability, and high power handling capability. This thesis explores alternative acoustic resonator architectures enabling wideband filtering for 5G bands, with a focus on exploiting shear acoustic waves in lithium niobate thin films. The reported developments specifically target the wide n77-n79 5G bands, which are especially difficult to cover using conventional SAW and BAW technologies. The first two chapter focus on suspended SH0 and S0 plate acoustic wave resonators and address several key limitations that have so far hindered their use in practical filters. First, the origin of transversal spurious modes, which are detrimental for filter performance, is investigated using simulations, and several geometric design approaches for their suppression are proposed and validated experimentally. The fabricated SH0 resonators exhibit strongly reduced spurious responses, achieving effective electromechanical coupling factors of 29% and resonance quality factors on the order of 350. Subsequently, an improved transducer architecture based on embedded electrodes, is developed to facilitate scaling to > 3 GHz. Using a tailored fabrication process, SH0 and S0 resonators with embedded transducers in the 3-7 GHz range are demonstrated, achieving high coupling factors on the order of 30% and 15%, respectively. The improved transduction efficiency enables the successful realization of filters for the n77 and n79 band. Furthermore, an unconventional, non-planar bulk acoustic resonator is introduced. Instead following the conventional approach of confining an acoustic resonance between the boundaries of a film, this resonator uses the width of a patterned lithium niobate ridge as an acoustic cavity. In addition, the structure is solidly-mounted to provide mechanical stability and increase power handling capacity. This architecture suppor