This paper reports on glass frit wafer bonding, which is a universally usable technology for wafer level encapsulation and packaging. After explaining the principle and the process flow of glass frit bonding, experimental results are shown. Glass frit bonding technology enables bonding of surface materials commonly used in MEMS technology. It allows hermetic sealing and a high process yield. Metal lead throughs at the bond interface are possible, because of the planarizing glass interlayer. Examples of surface micromachined sensors demonstrate the potential of glass-frit bonding.

Glass frit bonding : An universal technology for wafer level encapsulation and packaging

Micromechanical resonators are promising replacements for quartz crystals for timing and frequency references owing to potential for compactness, integrability with CMOS fabrication processes, low cost, and low power consumption. To be used in high performance reference application, resonators should obtain a high quality factor. The limit of the quality factor achieved by a resonator is set by the material properties, geometry and operating condition. Some recent resonators properly designed for exploiting bulk-acoustic resonance have been demonstrated to operate close to the quantum mechanical limit for the quality factor and frequency product (Q-f). Here, we describe the physics that gives rise to the quantum limit to the Q-f product, explain design strategies for minimizing other dissipation sources, and present new results from several different resonators that approach the limit.

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Quantum Limit of Quality Factor in Silicon Micro and Nano Mechanical Resonators

The importance of thermoelastic damping as a fundamental dissipation mechanism for small-scale mechanical resonators is evaluated in light of recent efforts to design high-Q micrometer- and nanometer-scale electromechanical systems. The equations of linear thermoelasticity are used to give a simple derivation for thermoelastic damping of small flexural vibrations in thin beams. It is shown that Zener’s well-known approximation by a Lorentzian with a single thermal relaxation time slightly deviates from the exact expression.

Thermoelastic Damping in Micro- and Nano-Mechanical Systems.

·In this paper, the design, implementation and characterization of a continuous time transimpedance-based ASIC for the actuation and sensing of a high-Q MEMS tuning fork gyroscope (TFG) is presented. A T-network transimpedance amplifier (TIA) is used as the front-end for low-noise, sub-atto-Farad capacitive detection. The T-network TIA provides on-chip transimpedance gains of up to 25 MΩ, has a measured capacitive resolution of 0.02 aF/√ Hz at 15 kHz, a wide dynamic range of 104 dB in a bandwidth of 10 Hz and consumes 400 μW of power. The CMOS interface ASIC uses this TIA as the front-end to sustain electromechanical oscillations in a MEMS TFG with motional impedance greater than 10 MΩ. The TFG interfaced with the ASIC yields a two-chip angular rate sensor with measured rate noise floor of2.7°/hr/√Hz, bias instability of 1°/hr and rate sensitivity of 2 mV/°/s. The IC is fabricated in a 0.6-μm standard CMOS process with an area of 2.25 mm 2 and consumes 15 mW.

A 104-dB dynamic range transimpedance-based CMOS ASIC for tuning fork microgyroscopes

Phononic crystals (PnCs) control the transport of sound and heat similar to the control of electric currents by semiconductors and metals or light by photonic crystals. Basic and applied research on PnCs spans the entire phononic spectrum, from seismic waves and audible sound to gigahertz phononics for telecommunications and thermal transport in the terahertz range. Here, we review the progress and applications of PnCs across their spectrum, and we offer some perspectives in view of the growing demand for vibrational isolation, fast signal processing, and miniaturization of devices. Current research on macroscopic low-frequency PnCs offers complete solutions from design and optimization to construction and characterization, e.g., sound insulators, seismic shields, and ultrasonic imaging devices. Hypersonic PnCs made of novel low-dimensional nanomaterials can be used to develop smaller microelectromechanical systems and faster wireless networks. The operational frequency, compactness, and efficiency of wireless communications can also increase using principles of optomechanics. In the terahertz range, PnCs can be used for efficient heat removal from electronic devices and for novel thermoelectrics. Finally, the introduction of topology in condensed matter physics has provided revolutionary designs of macroscopic sub-gigahertz PnCs, which can now be transferred to the gigahertz range with advanced nanofabrication techniques and momentum-resolved spectroscopy of acoustic phonons.

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Progress and perspectives on phononic crystals

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]

MEMS Resonators for Frequency Reference and Timing Applications

In this paper, we present an experimental investigation of a water pipeline leak detection system based on a low-cost, tiny-sized hydrophone sensor fabricated using the microelectromechanical system (MEMS) technologies. A 10 × 10 element arrayed MEMS hydrophone device with chip size of 3.5 × 3.5 mm $^2$ was used in the experiment. The hydrophone device is packaged with a customized on-board preamplification circuit using an acoustic transparent material. The overall package size of the MEMS hydrophone is $\Phi$ 1.2 × 2.5 cm. The packaged MEMS hydrophone achieves an acoustic sensitivity of −180 dB (re: 1 V/ $\mu$ Pa), a bandwidth from 10 Hz to 8 kHz, and a noise resolution of around 60 dB (re: 1 $\mu \text{Pa/}\sqrt{\text{Hz}}$ ) 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 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, real-time, and permanent in-pipe pipeline health monitoring network based on the MEMS hydrophones due to their high performance, low cost, and tiny size.

Low-Cost, Tiny-Sized MEMS Hydrophone Sensor for Water Pipeline Leak Detection

A wafer-level vacuum package with silicon bumps and electrical feedthroughs on the cap wafer is developed for a microelectromechanical systems (MEMS) resonator device. A MEMS resonator wafer and a cap wafer are bonded together in a vacuum chamber using glass frit bonding. The cap wafer not only provides a vacuum chamber to protect the movable resonator structure and improve the resonant performance but also realizes the redistribution of the electrical feedthroughs by using the silicon bumps. The silicon bumps provide vertical interconnections between the cap wafer and the resonator wafer, which realizes the bonding pads “transferring” from the resonator wafer to the cap wafer. A gold-aluminum eutectic is used to ensure electrical contacts between the cap wafer and the device wafer. The device fabrication and glass frit hermetic bonding process as well as the packaged MEMS resonator characterization are presented in this paper. Experimental results show that the wafer-level vacuum-packaged MEMS resonator results in over 100× higher quality factor (Q) than the resonator vibrating in atmosphere pressure, which confirms the transmission performance improvement due to vacuum packaging. Vacuum inside the package is measured indirectly by measuring the Q of the MEMS resonator inside the package. The experimental results indicate that vacuum about 1 mbar can be sealed in this approach.

Wafer-Level Vacuum Packaging for MEMS Resonators Using Glass Frit Bonding

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 (Rp) to the resistance at series resonance (Rs) of 49.8 dB is obtained, which is an indication of high FOM for LWR. The high Rp to Rs ratio is one of the most important parameters to design a radio frequency filter with steep roll-off.

GHz spurious mode free AlN lamb wave resonator with high figure of merit using one dimensional phononic crystal tethers

Micromechanical resonators must be clamped to the substrate via anchors to support the suspended microstructure. However, these anchors will introduce anchor loss, and decrease quality factors (Qs) of the micromechanical resonators. To reduce the anchor loss, one dimensional phononic crystal based strips are employed as anchors of the microresonators in this paper. The dispersion relations and eigenmodes of the phononic crystal strips are presented. Flexural mode ring resonator and Lame mode square plate resonator are designed to verify the effect of phononic crystal strips. The calculated results and finite-element simulations indicate that the leaky energy could be effectively reduced by the phononic crystal strip anchor design. Resonators with different anchor designs are also fabricated and characterized. The measured Qs of the microresonators show that the phononic crystal strips could reduce the energy dissipated through anchors, and with increasing the number of phononic strip periods, Qs of the resonators could be further enhanced. Furthermore, the experimental result also shows that compared with the Lame mode square plate resonator, the flexural mode ring resonator is more susceptible to the anchor loss.

Guoqiang Wu

Papers

MEMS Resonators for Frequency Reference and Timing Applications

Low-Cost, Tiny-Sized MEMS Hydrophone Sensor for Water Pipeline Leak Detection

Wafer-Level Vacuum Packaging for MEMS Resonators Using Glass Frit Bonding

GHz spurious mode free AlN lamb wave resonator with high figure of merit using one dimensional phononic crystal tethers

Phononic crystal strip based anchors for reducing anchor loss of micromechanical resonators