dc.title: Toward high-performance compact RF acoustic filters in thin-film piezoelectric platforms dc.description.abstract: Continuing evolutions in mobile technologies have made wireless systems pervasive and tightly integrated into daily life. Such systems demand compact radio frequency (RF) front ends (FEs) capable of supporting ever-increasing data rates and functionalities within crowded spectrum and energy constraints. Among a number of components inside an RFFE modules, filters are critical for ensuring spectral selectivity, interference suppression, and reliable signal integrity in densely populated wireless environments. While achieving high efficiency necessitates low insertion loss (IL) and steep roll-off characteristics, multi-standard mobile devices further demand filter scalability toward much higher frequencies with wider fractional bandwidths (FBWs), all within an extremely small footprint that can be integrated at the chip scale. These stringent requirements are difficult to practically realize using conventional electronics. Acoustic resonators, which employ piezoelectric transduction between the mechanical and electrical domains, offer compelling opportunities to exploit mechanical vibrations with high quality factors (Qs) within substantially smaller cavities, owing to the much shorter wavelengths of acoustic waves compared to their electromagnetic (EM) counterparts. To translate these promises into practical technologies, advances in material preparation, device microfabrication, wave-phenomena modeling, and experimental characterization must be jointly considered throughout the design and implementation process. This thesis presents holistic and practical demonstrations of advanced, compact acoustic filters based on lateral-field-excited bulk acoustic wave resonators (XBARs) realized on thin-film lithium niobate (TFLN) platforms, encompassing fundamental wave and device physics as well as their design, fabrication, and experimental characterization. The first contribution presented in this thesis leverages in-plane anisotropic properties of TFLN and the nature of Lamb waves in a suspended thin film to practically customize the FBW and operating frequency of XBAR filters. An in-house fabricated three-element ladder filter prototype achieves an insertion loss (IL) of only 1.79 dB and a controlled 3 dB FBW of 8.58% at 20.5 GHz, with an out-of-band (OoB) rejection greater than 14.9 dB across the entire FR3 band, while featuring a compact footprint of 0.67 mm². Moreover, an eight-element filter prototype shows an IL of 3.80 dB, an FBW of 6.12% at 22.0 GHz, and a high OoB rejection of 22.97 dB, demonstrating the potential for expanding to higher-order filters. Furthermore, advanced measurements are conducted to investigate the temperature dependence of XBAR devices. [1] The second work proposes the first demonstration of lattice XBAR filters. Two filter implementations, namely direct lattice and layout-balanced lattice topologies, are designed and fabricated in periodically poled piezoelectric film (P3F) TFLN. By leveraging the strong electromechanical coupling of XBARs in P3F TFLN together with the inherently wideband nature of the lattice topology, 3 dB FBWs of 27.42% and 39.11% and low ILs of 0.88 dB and 0.96 dB are achieved at approximately 20 GHz for the direct and layout-balanced lattice filters, respectively, under conjugate matching. Notably, all prototypes feature compact footprints smaller than 1.3 mm². These results highlight the potential of XBAR-based lattice architectures to enable low-loss, wideband acoustic filters for compact, high-performance RF front ends in next-generation wireless communication and sensing systems, while also identifying key challenges and directions for further optimization. [2] Beyond the aforementioned original works, this thesis provides a broad range of essential backgrounds for enabling high-performance, compact acoustic filters, including the physics of acoustic waves in solids, advances in microelectromechanical systems (MEMS), key MEMS fabrication techniques, and state-of-the-art filter technologies. Furthermore, the thesis thoroughly discusses the challenges and opportunities in the future development of acoustic innovations, ranging from device design and electromagnetic-acoustic co-modeling to advanced device evaluation and applications beyond filtering.