An overview of microelectromechanical systems (MEMS) resonators for frequency reference and timing applications is presented. The progress made in the past few decades in design, modeling, fabrication and packaging of MEMS resonators is summarized. In particular, the state-of-the-art technologies for improving the overall performance of MEMS resonators, such as quality factor (Q), motional impedance, temperature sensitivity, and initial frequency uniformity, are reviewed in detail. The challenges and opportunities during the commercialization of MEMS resonators are also stated, and future development trends driven either by technology or market are outlined. This paper intends to provide an outlook for possible research directions of MEMS resonators in frequency reference and timing applications. With outstanding reliability, unique multi-frequency functionality on a single chip, and high accuracy, MEMS resonators show great potential for replacing the quartz crystal resonators which have been dominating the timing market since 1920s. [2020-0106]

In this paper, we present an experimental investigation of a water pipeline leak detection system based on a low cost, tiny sized MEMS hydrophone sensor with chip size of 3.5 $\times$ 3.5 mm $^2$ . The hydrophone device is packaged with a customized on-board pre-amplification circuit using an acoustic transparent material. The packaged hydrophone size is $\Phi1.2$ cm $\times$ 2.5 cm, and achieves an acoustic sensitivity of -180 dB, a bandwidth from 10 Hz to 8 kHz, and a noise resolution of around 60 dB (re: 1 $\mu$ Pa/ $\sqrt$ Hz) at 1 kHz. A section of ductile iron water pipeline with an internal diameter of 10 cm, wall thickness of 0.73 cm and length of 30 m is constructed as the test bed for the water leak detection. Two different leak sizes with leak flow rates of about 30 L/min and 180 L/min are designed along the pipe, which is pressurized at 3.2 Bar. Analysis of the transient signals and spectrograms shows that the MEMS hydrophone can capture the key acoustic information of the water leak, i.e. identifying the leak and locating the leak position. The measurement results demonstrate the feasibility to construct an affordable, highly efficient, realtime, and permanent in-pipe pipeline health monitoring networks based on the MEMS hydrophones due to their high performance, low cost and tiny size.

In this article, we present a high-performance aluminum nitride (AlN)-based microelectromechanical system (MEMS) hydrophone sensor, in which the sensing cells are designed and arranged as an innovative honeycomb architecture for achieving large acoustic pressure sensitivity and high fill-factor. An 8×9 array of 360 μm in characteristic size, 0.92-MHz MEMS hydrophone sensor is developed based on an AlN-on-cavity silicon-on-insulator (CSOI) platform. The size of the MEMS hydrophone sensor is 3.2 mm ×3.2 mm. The MEMS hydrophone sensor and its preamplification circuit are integrated on a printed circuit board and packaged together with an acoustically transparent material, in order to meet the stringent requirements of underwater applications. The packaged MEMS hydrophone is fully characterized through using an industry-standard hydrophone calibration instrument. The MEMS hydrophone achieves an acoustic pressure sensitivity of -178 dB (re: 1 V/ μ Pa), with a maximum nonlinearity of 0.1%, and a noise resolution of 58.7 dB (re: 1 μPa/Hz). The measurement results show that the reported MEMS hydrophone sensor with bioinspired honeycomb structure is compared favorable with advanced commercially available bulky hydrophones.

A high quality factor (Q) and low-impedance aluminum nitride (AIN) on silicon piezoelectric resonator is reported in this paper. The piezoelectric resonator is fabricated based on the AIN-on-cavity-Silicon-on-Insulator (SOI) platform and vacuum encapsulated using the waferlevel chip-scale packaging by aluminum-germanium eutectic bonding approach. The fabricated resonator achieves a loaded Q of 9520 and a motional impedance of 32.64 Ω at its series resonant frequency of 27.19 MHz. Thermal cycles (TC) and highly-accelerated stress test (HAST) are performed to evaluated the packaging reliability of the fabricated devices. No obvious impedance change caused by the TC and HAST is observed. The TC results indicate that the packaged devices have an equivalent life of more than 11.65 years based on the Coffin-Manson model. The piezoelectric resonator-based programmable oscillators demonstrate an overall frequency drift of ± 3 ppm over the temperature range of -20°C to 70°C thanks to the passive and active temperature compensations. With this stable frequency reference, the programmable oscillator produces a 75.01-MHz frequency output. The measured integrated phase jitter is 2.02 ps (rms) from 12 to 10 MHz.

This letter reports a spurious mode free GHz aluminum nitride (AlN) lamb wave resonator (LWR) towards high figure of merit (FOM). One dimensional gourd-shape phononic crystal (PnC) tether with large phononic bandgaps is employed to reduce the acoustic energy dissipation into the substrate. The periodic PnC tethers are based on a 1 μm-thick AlN layer with 0.26 μm-thick Mo layer on top. A clean spectrum over a wide frequency range is obtained from the measurement, which indicates a wide-band suppression of spurious modes. Experimental results demonstrate that the fabricated AlN LWR has an insertion loss of 5.2 dB and a loaded quality factor (Q) of 1893 at 1.02 GHz measured in air. An impressive ratio of the resistance at parallel resonance (R_p) to the resistance at series resonance (R_s) of 49.8 dB is obtained, which is an indication of high FOM for LWR. The high R_p to R_s ratio is one of the most important parameters to design a radio frequency filter with steep roll-off.