High Performance Lithium Niobate Resonators for Passive Voltage Amplification in Radio Frequency Applications

Internet of Things (IoT) applications are fostering the development of near-zero power consumption and event-driven Radio Frequency (RF) sensors that can outperform traditional networks based on scheduling algorithms. In case of infrequent events, asynchronous Wake-Up Radio Receivers (WuRx) are the desired solution to attain high sensitivity (<100 dBm) and ultra-low-power consumption (~tens of nW). The strict available power budget led to drastic reduction of active components in the WuRx, which relies more extensively on passive voltage amplification and filtering in the RF Front-End (RFFE). High performance microelectromechanical (MEM) piezoelectric resonators are an ideal solution as Matching Network (MN) in the RFFE due to their compact footprint, high quality factor at resonance (Q_s) and range of operation (up to few GHz). Despite the availability of succesful commercial products, such as Aluminum Nitride (AlN) Film Bulk Acoutic Resonators (FBARs), WuRx are driving the demand for devices which can simultaneously exhibit large quality factor and large k_t² or, overall, a large Figure of Merit (FoM, defined as Q_s·k_t²). This dissertation focuses on the development of high performance X-cut Lithium Niobate (LN) MEMS Laterally Vibrating Resonators (LVRs) to be implemented as the matching network of novel WuRx architectures. The requirements of these devices in terms of operating frequency (fRF), static capacitance (C₀), and Figure of Merit are investigated according to the sensitivity optimization of the Resonant Micromechanical Receiver (RMR) envisioned at Carnegie Mellon University. Different established and innovative piezoelectric MEMS technologies are compared to identify the solution that offers the highest FoM while satisfying the matching constraints provided by the RMR numerical optimization. Preliminary work on X-cut Lithium Niobate resonators operating in the S₀ mode exhibited incredibly high k_t² (> 30%) combined with good Q_s (~ 1,500), which translate to FoM greater than 400 (similar to commercial FBARs performance). Since the operating frequency and the load condition of the matching network are usually fixed, the Figure of Merit, and in particularly the quality factor, is the only lever available to increase the sensitivity of the RMR. The resonators geometry is thus thouroughly investigated through Finite Element Analysis (FEA) to maximize Q_s and consequently the achievable voltage amplification. Fabricated devices showed Q_s as high as 8,000 in vacuum at 100 MHz, with k_t² = 28% and FoM > 2,500, the highest ever reported to date for piezoelectric MEMS operating in the frequency range of interest for WuRx. Devices at higher frequencies showed improved performance compared to the state-of-the-art, with Q_s greater than 2,500 up to 550 MHz. Damping mechanism in X-cut LN LVRs were investigated on fabricated devices. Air damping (Q_air) proven to be a significant source of quality factor degradation up to 200 MHz, with Qs reduction in the order of 20 – 30%, while becomes negligible above 550 MHz. Anchor losses were investigated on devices with different plate length (L) over width (W) aspect ratios. Shorter resonators exhibit lower Q_s due to energy leakage through the substrate, over the entire frequency range of interest. As a consequence, the quality factor of devices with limited aperture (L_e < 5 λ) showcased a stronger dependency from the anchor dimensions. The electrical loading introduced by the Interdigitated Electrodes (IDE) and the interconnects were identified as the main damping mechanism in resonators with high L/W ratios. Cryogenic measurements highlighted Q_s as high as 26,000 at base temperature (10 K), hinting that anchor losses play a limited role in devices with a slender geometry. A tradeoff between electrical loading and anchor losses was also identified.
The limited static capacitance (C₀) achievable for the optimized resonator geometries required the investigation of alternative solutions to match those devices to the typical input capacitive load of an envelope detector (~1 pF). Arrays of identical, parallel resonators and alternative matching network configuration were investigated for this purpose. In the first case, arrays with C₀ of 1 pF and quality factor greater than 2,000, with k_t² = 30% and FoM = 600 were demonstrated at 50 MHz. Frequency mismatch between the elements was identified as the main damping mechanism for reduction in effective Q_s and was modeled through a statistical Monte Carlo approach. An alternative matching network based on a combination of series and parallel resonators (L-network) with identical C₀ was also investigated, showcasing a gain 30% higher than a simple series configuration (46 V/V vs. 35 V/V for a capacitive load of 1 pF) and a higher robustness in regards to frequency variations.