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

Abstract

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 ( 30%) combined with good Qs (~ 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 Qs and consequently the achievable voltage amplification. Fabricated devices showed Qs as high as 8,000 in vacuum at 100 MHz, with kt2 = 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 Qs greater than 2,500 up to 550 MHz. Damping mechanism in X-cut LN LVRs were investigated on fabricated devices. Air damping (Qair) 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 Qs 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 (Le < 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 Qs 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 kt2 = 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 Qs 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.