|Abstract:||The fifth-generation (5G) communication has sparked great research interest in developing the next generation radio frequency (RF) front ends for more stringent requirements on performance, power consumption, and spectral utilization efficiency. More parallel RF bands and paths are added in the same form factor, along with which come more components and tighter integration. Designing portable systems faces the new challenge of reducing component size while still operating at RF, where the path attenuation is low and fading is readily manageable. Satisfactory size reduction is particularly difficult for passive components that rely on the principle of waveguiding and thus scale with electromagnetic (EM) wavelength at RF (typical ~10s cm). Hence, radical size reduction by several orders of magnitude can only be attained by resorting to a physical domain other than EM, namely acoustic waves with wavelengths 4~5 orders of magnitude smaller. In fact, acoustic devices at RF, such as surface or bulk acoustic wave devices, have been widely used for mobile phone applications. Other acoustic elements, such as couplers, correlators, and impedance matching networks, have also shown promising potential to outperform the state-of-the-art EM counterparts. However, the past developments often battled the challenge of efficiently accessing the acoustics over a sufficiently wide bandwidth and subsequently producing application-worthy performance, because of the fundamental limitations from the lack of high electromechanical coupling (k2) and low damping piezoelectric platforms. Recently, thanks to the advances in materials, design, and fabrication, LiNbO3 thin-film has been proved as a promising low-loss, wideband, and frequency-diverse acoustic platform for novel functions toward high-performance 5G front-end signal processing.
Based on acoustic devices in LiNbO3 thin films, this thesis aims to design and demonstrate several classes of novel RF microsystems that can enable conventional signal processing functions with better performance or new tasks for emerging applications. First, the acoustic systems are used as passive signal processing elements for the Internet of Things (IoT) applications. The high figure of merit (FoM) LiNbO3 resonator array is used as the impedance matching element for interfacing with the high impedance CMOS rectifiers in the IoT-inspired wake-up radio. The high FoM, adequately large static capacitance and spurious free performance collectively contribute to a high voltage gain over 20. Another type of microsystem for IoT applications is 1-dB IL acoustic delay lines (ADLs) on the S0 mode in thin-film LiNbO3, showing record-breaking low IL over a larger bandwidth, opening new horizons for low-power RF acoustic signal processing. Second, the miniature nonreciprocal component based on switched high-performance delay elements is demonstrated for full duplex radio. The wideband and long delay featured by the high-performance LiNbO3 ADLs significantly benefit the performance, including the dynamic switching induced IL and intra-modulations, while relaxing the requirements for synthesizing and synchronizing the control signals. The demonstrated 4 port circulator shows a highly symmetric performance across the 4-ports with 18.8 dB nonreciprocal contrast between the IL (6.6 dB) and isolation (25.4 dB) over an FBW of 8.8% at a center frequency 155 MHz, all of which are accomplished with a record low switching frequency of 877.22 kHz. Upon further optimizations, such circulators can potentially outperform ferrite-based devices in loss, bandwidth, and isolation while offering more compact size and reconfigurable operation. Third, low-loss wideband GHz S0 mode ADLs are explored for self-interference cancellation in full-duplex radio. The fabricated miniature acoustic delay lines show a fractional bandwidth of 4% and a minimum IL of 3.2 dB at a center frequency of 0.96 GHz. Various delays ranging from 20 ns to 900 ns have been obtained for digitally addressable delay synthesis. Multiple acoustic delay lines with center frequencies from 0.9 to 2 GHz have been demonstrated. The demonstrated ADLs can potentially provide wide-range and high-resolution reconfigurable delays for future SIC applications. Finally, design and measurement of 5 GHz antisymmetric mode acoustic delay lines for 5G enhanced mobile broadband (eMBB) applications are presented; the demonstrated ADLs significantly surpass the state of the art with similar feature sizes in center frequency. The implemented ADLs at 5 GHz show a minimum insertion loss of 7.94 dB and a fractional bandwidth around 4%. In addition to the remarkable device performance, these designs also point out the opportunities to advance the operation frequencies of acoustic devices toward the wideband and high-frequency signal processing functions required for future 5G applications.
RF acoustic microsystems demonstrated in this thesis have shown promising prospects for 5G front-end signal processing applications. Thanks to the simultaneously low damping and wideband performance at RF, acoustic devices based on LiNbO3 thin films are auspicious candidates to provide the design flexibilities and high performance required for various 5G application scenarios. Further development in high-performance RF acoustic devices may put on the horizon an RF front-end synthesized either purely or predominantly from an RF acoustic component kit.