Piezoelectric MEMS resonators possess favorable properties such as strong electromechanical coupling, high Q, and polarized linear transduction, making them ideal for various applications including timing, sensing, and RF communication. However, due to process nonidealities and temperature variations, these resonators characteristics may deviate from their designed frequency and resonant eigenmode, requiring careful compensation for stable and precise operation. Furthermore, certain devices like gyroscopic resonators have two eigenmodes that need to be adjusted for frequency proximity and cross-mode coupling. Therefore, mode shape manipulation can also be important in piezoelectric resonators and will be another focus of this paper. Techniques for frequency and eigenmode control are classified into device- or system-level tuning, trimming, and compensation. This paper will compare and discuss the effectiveness of these techniques in specific applications to provide a comprehensive understanding of frequency and eigenmode control in piezoelectric MEMS resonators, aiding the development of advanced MEMS devices for diverse applications. 

A Review of Eigen-Mode and Frequency Control in Piezoelectric MEMS Resonators

6 GHz lamb wave acoustic filters based on A1-mode lithium niobate thin film resonators with checker-shaped electrodes

Wireless local area network (WLAN) has gained widespread application as a convenient network access method, demanding higher network efficiency, stability, and responsiveness. High-performance filters are crucial components to meet these needs. Film bulk acoustic resonators (FBARs) are ideal for constructing these filters due to their high-quality factor (Q) and low loss. In conventional air-gap type FBAR, aluminum nitride (AlN) is deposited on the sacrificial layer with poor crystallinity. Additionally, FBARs with single-crystal AlN have high internal stress and complicated fabrication process. These limit the development of FBARs to higher frequencies above 5 GHz. This paper presents the design and fabrication of FBARs and filters for WLAN applications, combining the high electromechanical coupling coefficient (Kt2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${K}_{{\rm{t}}}^{2}$$\end{document}) of Al0.8Sc0.2N film with the advantages of the thin film transfer process. An AlN seed layer and 280 nm-thick Al0.8Sc0.2N are deposited on a Si substrate via physical vapor deposition (PVD), achieving a full width at half maximum (FWHM) of 2.1°. The ultra-thin film is then transferred to another Si substrate by wafer bonding, flipping, and Si removal. Integrating conventional manufacturing processes, an FBAR with a resonant frequency reaching 5.5 GHz is fabricated, demonstrating a large effective electromechanical coupling coefficient (keff2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{k}}_{{\rm{eff}}}^{2}$$\end{document}) of 13.8% and an excellent figure of merit (FOM) of 85. A lattice-type filter based on these FBARs is then developed for the Wi-Fi UNII-2 band, featuring a center frequency of 5.5 GHz and a −3 dB bandwidth of 306 MHz, supporting high data rates and large throughputs in WLAN applications.

5.5 GHz film bulk acoustic wave filters using thin film transfer process for WLAN applications.

The film bulk acoustic wave resonator (FBAR) is one of the most popular devices in the radio frequency field. Numerous researchers are simultaneously working to develop an effective electromechanical coupling coefficient (keff2) tuning technology, aiming to meet the diverse bandwidth requirements of the 5 G era. Based on a traditional FBAR process, this work prepares several different devices and then analyzes the four factors that influence keff2 from the perspective of process, material, and design. The pillar structure achieves the largest keff2 tuning range of 2.84% (33 MHz). The composite piezoelectric film can tune the overall resonant frequency in a wide range, and its keff2 tuning range is 1.75% (12.5 MHz). The area effect tunes the fs in a small range, ultimately achieving a keff2 tuning range of 1.16% (11 MHz). Film stress regulation achieves keff2 tuning range of 2.73% (30 MHz), but it has the greatest difficulty. The integration of various keff2 tunable methods has important guiding significance for the design of FBAR filters in the future.

Advanced technologies of FBAR for tuning effective electromechanical coupling coefficient

The inferior film quality of sputtered AlN has hindered the further development of high-performance film bulk acoustic wave resonators (FBAR). In this work, the ZnO buffer layer was introduced into the FBAR to improve the performance of FBAR for the first time. Compared with its counterpart without the buffer layer, the devices with a buffer layer exhibited a far higher average quality factor (2π times the maximum energy stored in the FBAR/energy dissipated per period) of 2575 ± 136, which was about 64 % larger than that without a buffer layer. Benefiting from the enhanced crystalline quality of AlN, the device also showed an improved effective electromechanical coupling coefficient. 

Highly improved quality factor of the film bulk acoustic wave resonator by introducing a high quality ZnO buffer layer

Laterally excited bulk acoustic resonators (XBARs) using interdigital electrodes are currently limited in potential 5G applications due to the accompanied spurious modes and small capacitance. This Letter presents an XBAR with scattering vias in electrodes, named SV-BAR. The non-parallel boundaries thoroughly disrupt the coherent formation of standing waves in the lateral direction, thereby suppressing the higher-order spurious modes. The duty factor of SV-BAR is optimized as 0.5 using COMSOL Multiphysics finite element analysis for the 50 Ω impedance matching. The SV-BAR is fabricated using a 390 nm Z-cut LiNbO3 on an insulator substrate and exhibits an impressive ratio of Zp/Zs near 64.5 dB, a Kt2 of 21.80% at 4.762 GHz, as well as a large value f · Bode_Qmax · Kt2 (3.43 × 1011). The measured temperature coefficient of frequency is −69.8 ppm/°C, which can be compensated using SiO2 thin films. The SV-BAR provides an effective solution for high-performance radio frequency filters for 5G communication. 

A laterally excited bulk acoustic resonator with scattering vias in electrodes

Film bulk acoustic resonators (FBARs) with a desired effective electromechanical coupling coefficient (Keff2) are essential for designing filter devices. Using AlN/AlScN composite film with the adjustable thickness ratio can be a feasible approach to obtain the required Keff2. In this work, we research the resonant characteristics of FBARs based on AlN/AlScN composite films with different thickness ratios by finite element method and fabricate FBAR devices in a micro-electromechanical systems process. Benefiting from the large piezoelectric constants, with a 1 μm-thick Al0.8Sc0.2N film, Keff2 can be twice compared with that of FBAR based on pure AlN films. For the composite films with different thickness ratios, Keff2 can be adjusted in a relatively wide range. In this case, a filter with the specific N77 sub-band is demonstrated using AlN/Al0.8Sc0.2N composite film, which verifies the enormous potential for AlN/AlScN composite film in design filters.

/pdf/demonstration-of-thin-film-bulk-acoustic-resonator-based-on-1c2zv0hl.pdf

Demonstration of Thin Film Bulk Acoustic Resonator Based on AlN/AlScN Composite Film with a Feasible Keff2

Improving the quality factor (Q) is a hot topic in research of laterally-excited bulk acoustic resonators (XBARs). We proposed a high Q XBAR with reflective grids which are formed by etching LiNbO3 films, named an RG-XBAR. From theoretical analysis and simulations based on COMSOL, the reflective grids are set on both sides of the busbar which suppress the acoustic energy dissipation in the anchor effectively. The steady-state temperature and thermal stress increase as the etching depth increases. Considering the attenuation of Q and mechanical stability of the film, the etching depth is designed at 130 nm. Finally, the fabricated RG-XBAR operates at 6.203 GHz with a $K_t^2$ of 21.69%, exhibiting a high impedance ratio of parallel resonance (Zp) to series resonance (Zs) 88 dB and a Qmax of 3383. The RG-XBAR exhibits a remarkable Q over 6 GHz, showing its great potential for high-performance 5G/6G filters.

A High Q 6.203 GHz Laterally-excited Bulk Acoustic Resonator with Reflective Grids

A piezoelectric MEMS microphone with proof mass and its array

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A laterally excited bulk acoustic resonator with scattering vias in electrodes

Demonstration of Thin Film Bulk Acoustic Resonator Based on AlN/AlScN Composite Film with a Feasible Keff2

A High Q 6.203 GHz Laterally-excited Bulk Acoustic Resonator with Reflective Grids

A piezoelectric MEMS microphone with proof mass and its array