Yaoyao Long’s research while affiliated with Georgia Institute of Technology and other places

Gyroscopes are crucial for accurate angular velocity measurements in navigation, stabilization, and control systems. MEMS gyroscopes offer advantages like compact size and low cost but suffer from errors and inaccuracies that are complex and time varying. This study leverages machine learning (ML) and uses multiple signals of the MEMS resonator gyroscope to improve its calibration. XGBoost, known for its high predictive accuracy and ability to handle complex, non-linear relationships, and MLP, recognized for its capability to model intricate patterns through multiple layers and hidden dimensions, are employed to enhance the calibration process. Our findings show that both XGBoost and MLP models significantly reduce noise and enhance accuracy and stability, outperforming the traditional calibration techniques. Despite higher computational costs, DL models are ideal for high-stakes applications, while ML models are efficient for consumer electronics and environmental monitoring. Both ML and DL models demonstrate the potential of advanced calibration techniques in enhancing MEMS gyroscope performance and calibration efficiency.

This paper describes prototype temperature compensated piezoelectric MEMS oscillators operating in the wide temperature range of -40 °C to 85 °C for RTC applications. The AlN-on-Si resonator is centrally anchored at one point and designed for low power operation with a wide frequency tuning range of 5000 ppm. The oscillators exhibit a stable sinusoidal output at about 497 kHz frequency for time keeping applications with an integrated phase jitter being 10× better than the best commercially available MEMS RTC oscillators for supplementary use in portable devices for clocking audio circuits. The measured oscillator performance remains relatively unchanged when comparing the wafer level packaged capped MEMS resonator with the uncapped one, showing great potential for a high performance low-power RTC oscillator.

Inertial navigation on a chip has long been constrained by the noise and stability issues of micromechanical Coriolis gyroscopes, as silicon, the dominant material for microelectromechanical system devices, has reached the physical limits of its material properties. To address these challenges, this study explores silicon carbide, specifically its monocrystalline 4H polytype, as a substrate to improve gyroscope performance due to its low phonon Akhiezer dissipation and its isotropic hexagonal crystal lattice. We report on low-noise electrostatic acoustic resonant gyroscopes with mechanical quality factors exceeding several millions, fabricated on bonded 4H silicon carbide-on-insulator wafers. These gyroscopes operate using megahertz frequency bulk acoustic wave modes for large open-loop bandwidth and are tuned electrostatically using capacitive transducers created by wafer-level deep reactive ion etching. Experimental results show these gyroscopes achieve superior performance under various conditions and demonstrate higher quality factors at increased temperatures, enabling enhanced performance in an ovenized or high-temperature stabilized configuration.

Precision time and frequency references are critical components in electronic devices, impacting sectors such as wireless communications, global positioning systems, and network synchronization. While quartz-based oscillators have historically dominated the market, micro-electromechanical systems (MEMS) resonators are emerging as potential successors, albeit with challenges related to thermal frequency drifts. This paper presents doubly-clamped beam resonators in monocrystalline 4H-silicon carbide (4H-SiC), showcasing a tunable local zero Temperature Coefficient of Frequency (TCF) across a wide temperature range. Our novel approach employs axial stress to counteract temperature-induced softening in the 4H-SiC beam, leveraging the unique attributes of a 4H-SiC on insulator (SiCOI) substrate with a silicon handle layer. By manipulating the beam’s geometrical dimensions, we demonstrate the capability to define the TCF turnover point from -20°C to 100°C and tailor the overall frequency shift. The fabrication process ensures strong covalent interlayer bonds in the 4H-SiCOI substrate, eliminating frequency hysteresis and enhancing yield and stability metrics. We conducted comprehensive short- and long-term stability tests, showing that our resonators exhibit negligible frequency hysteresis across temperature cycles and exceptional long-term stability. Our findings enrich the current understanding of 4H-SiC MEMS resonator thermal stability and pave the way for future innovations in timekeeping and frequency reference technologies. This study underscores the potential of stress-modulated 4H-SiC resonators as reliable, efficient, and versatile instruments for advanced precision timing applications.

Inertial navigation on a chip has been an unfulfilled dream limited by the noise and stability of micromechanical Coriolis gyroscopes. While silicon has been the mainstream material for microelectromechanical system (MEMS) devices, the performance of silicon MEMS gyroscope has come near the physical limit of its material properties. To overcome these limitations, we explore the potential of silicon carbide (SiC), specifically 4H-SiC, as a promising substrate for substantially enhancing MEMS gyroscope performance, owing to its small phonon Akhiezer dissipation and in-plane isotropic hexagonal crystal lattice. Here we report on low-noise electrostatic acoustic resonant gyroscopes with mechanical Q factors in excess of a few million fabricated on bonded 4H silicon carbide-on-insulator wafers. The reported gyroscopes use megahertz bulk acoustic wave (BAW) mode for wide open-loop bandwidth. The gyroscopes are actuated and tuned electrostatically using micron gaps defined by wafer-level deep reactive ion etching (DRIE). Experimental results indicated that these batch-fabricated SiC gyroscopes have very minimum inherent imperfections across wafers, and upon tuning, beyond-tactical grade gyroscope performance is achieved under various testing conditions. Moreover, we highlight the higher quality factor of the silicon carbide BAW resonators at higher temperatures, in contrast to conventional silicon devices. This characteristic enables temperature stabilization through ovenization while achieving better gyroscope performance. The 4H-SiC BAW gyroscopes demonstrated here are the first of their kind and pave the way for the use of 4H-SiC as a superior acoustic material for next-generation MEMS positioning, navigation, and timing (PNT) devices.

This article discusses the potential of 4H-silicon carbide (SiC) as a superior acoustic material for microelectromechanical systems (MEMS), particularly for high-performance resonator and extreme environments applications. Through a comparison of the crystalline structure along with the mechanical, acoustic, electrical, and thermal properties of 4H with respect to other SiC polytypes and silicon, it is shown that 4H-SiC possesses salient properties for MEMS applications, including its transverse isotropy and small phonon scattering dissipation. The utility and implementation of bonded SiC on insulator (4H-SiCOI) substrates as an emerging MEMS technology platform are presented. Additionally, this article reports on the temperature-dependent mechanical properties of 4H-SiC, including the temperature coefficient of frequency (TCF) and quality factor ( ${Q}$ -factor) for Lamé mode resonators. Finally, the 4H-SiC MEMS fabrication including its deep reactive ion etching is discussed. This article provides valuable insights into the potential of 4H-SiC as a mechanoacoustic material and provides a foundation for future research in the field.